ElectroDacus -- everything I always wanted... except....

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Derek Wolfson

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May 20, 2021, 7:27:54 PMMay 20
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Wow Dacian -- this is AWESOME.  I wanted something clean, stylish and fully-featured for my next RV build... and this is DEFINITELY it.

Plan is to do either 12V or 24V with 280Ah prismatics.  Probably will go with 24V and use this as the main BMS...

I have a BUNCH of 500A contactors lying around and was planning to use these as my LOAD disconnects... 

Looks  like the coil is rated at 3.14Ohm -- so I think at either 12V or 24V -- I exceed (by a lot) the 50mA otuput of the EXT_IO outputs.   

Definitely could rejigger the plan and use a Victron BP to disconnect DC loads -- and an inverter with remote on/off... but given you can get these contactors on eBay for like $30 -- is there a way to control the coil via the Electrodacus?  

I don't know if beefing up the octocouplers is reasonable or even possible (in fact, I didn't even know what an octocoupler was until today).... perhaps an additional low current relay to trigger the contactor?    if you know of a relay that would work for this application (and isn't insanely expensive)  -- that'd be awesome.  

It seems adding more "umph" to that output could allow people to use these cheap and readily available relays to disconnect... well... anything. 

In my opinion -- this is the best thing I've seen in a long time.  Clearly designed by someone with an attention to detail that I can relate to.

Well done Dacian -- I will be buying one of these for my latest project -- and I'll even pony up for components with remote on/off (if I have to) -- but it seems beefing up those IO outputs could be useful to people!

Excitedly, 
-Derek

Derek Wolfson

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May 20, 2021, 7:36:19 PMMay 20
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welll... maybe I'm just a big dumb-dumb and it already can handle this?  I've seen others mention control of "relays" from the EXT_IO! 

Would be awesome-sauce if so -- provided the components I've selected can handle being disconnected from the battery directly.

Oberon Robinson

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May 20, 2021, 7:38:08 PMMay 20
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Derek, there is a Beginner's Guide that covers a lot of this:

Dacian Todea

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May 20, 2021, 7:40:03 PMMay 20
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Derek,

EXT IOx are small solid state relays the Toshiba TLP172GM so is either a close or open circuit no voltage and that 50mA is the current limit for continues use. This was not designed to control a coil so for that you will need to add externally a larger SSR.
Still there should not be a need for any relay/contactors as if you have an inverter then that will have a remote ON/OFF typical less than 10mA needed to close the contact on the main ON/OFF switch and if is for small DC loads then something like the Victron BP65 is an inexpensive a good solution again I think just around 2mA is need to control that ON/OFF and this BP65 is around $40
So most people use an inverter if is a home setup not many use the DC directly and there just control the ON/OFF of the inverter directly with the EXT IOx and for those with DC loads the BP65, BP100 are a better option as it will not use energy just to be enabled (as mentioned just about 2mA)
For charging the DSSR20 require also just around 2mA so quite a few DSSR20 soon DSSR50 can be controlled by a single EXT IOx port. There are also grid chargers if that is needed with remote ON/OFF control or if both inverter and grid charging is needed the Victron Multiplus is the best option I know having separate remote ON/OFF for inverter and for charging  https://www.victronenergy.com/upload/documents/Manual-Connecting-other-lithium-battery-systems-to-Multis-and-Quattros-EN.pdf

Derek Wolfson

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May 21, 2021, 12:54:36 AMMay 21
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JUST FABULOUS!  Thank you Oberon!  

This answered all of my questions -- I think the "TYPE" section should be in the documentation of the unit on the webpage -- I was so confused reading that the first, second and third time, but now it all makes sense!  

Dacian -- any chance you can allow different temperature cut-offs for charging and discharging?  OR even better -- multiple temperature readings, even if via an add-on board? 
  • 5.) XT1 — this is a battery temperature sensor circuit. It is optional but highly recommended that you connect a 10k Ω   thermistor on this circuit, so the system can prevent damage to  your battery if it is outside of safe operating temperatures. If your battery is in a conditioned living space or your climate never goes outside a safe range, you can leave this off. See Section 9.4 for thermistor sourcing info.  

It would be nice to have separate discharge and charge temp cutoffs profiles -- as this is something many of my RV customers desire -- sometimes it gets below 0C -- but rarely will it drop below like -10C and it is pretty OK to discharge the battery to turn on the heater until charging is safe.

I reckon I can just put a NC 0C thermoswitch inline with the charge EXT_IO outputs -- this way charging is interrupted under 0C but discharge can keep happening until the cut-off within the ElectroDacus as measured with the 10KΩ Thermistor to shut down both charge and discharge below say -5C -- or pick your own temps.

Sure this is COLD weather -- but I would assume this change to allow separate cutoffs is a software/firmware change and shouldn't affect the hardware -- but I'm not a electrical engineer or PCB designer :)


Awesome work everyone -- this is a very, very cool product.

-Derek

Derek Wolfson

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May 21, 2021, 12:56:03 AMMay 21
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RE: Dacan - All sounds good -- I'm probably going with Victron for most builds anyways -- but the Beginner's guide showed me how to use the EXT IOx to power relays that can power subsequent relays... :)  WOOT!

Dacian Todea

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May 21, 2021, 12:47:38 PMMay 21
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Derek,

The EXT IOx type is explained in the manual at different points and the on screen help (on the SBMS screen) will also give a short definition.
The temperature sensor is read and dealt with directly by the BMS IC (ISL94203) not by the microcontroller so protection is done in hardware. A over or under temperature limit is considered a fault so both charge and discharge will be disabled.
In an RV when battery is exposed to ambient temperature and in a cold climate the only proper option is to have the battery in an insulated battery box and heated with a 10 to 15W heating element controlled by a thermostat set to maintain the battery always above +10C
Once the battery drops below freezing it will take forever (hours to heat the battery back up to temperature so not a good idea).

If you use Victron equipment then you do not need any relays as all that equipment has remote ON/OFF directly compatible with SBMS0 EXT IOx

Peter

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May 21, 2021, 1:42:31 PMMay 21
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Dacian,

why exactly +10C?

CEO of Battle Born explains that they are charging their LiFePo4 even at -3,89C, meant for 1C charging, he explains that lower charge rate is better in low temperature conditions. (https://www.youtube.com/watch?v=ywn-vBjKblI)
In my RV I have 125 kg of LiFePo4 and regulated heating power 0-150W (10m heating wire, 5ohm driven by ESP+PWM/mosfet) .
I was going to set target temperature to +5C. My solar can charge max 0.05C so I think I could be probably even around 0 Celsius.

Dne pátek 21. května 2021 v 18:47:38 UTC+2 uživatel electr...@gmail.com napsal:

Oberon Robinson

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May 21, 2021, 2:03:56 PMMay 21
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Derek, if you want different thermal cut-off temperatures for charge and discharge, you could simply wire thermostats in series with your Type 1 & Type 2 EXTIOs.  You would set XT1 for a fault condition, i.e. if your heater or thermostat has failed, it will prevent battery damage at out-of-temperature conditions.

My understanding is that minimum charging temperature depends on the specific electrolyte in the cells, so unless you know for a fact that your cells have the same electrolyte as Battleborn's, you may want to stay on the conservative side for charging.  There's also no way to guarantee that every part of every cell is at the same temperature as your thermostat is measuring.  So there are good reasons to keep a temperature buffer above 0°C.

Oberon Robinson

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May 21, 2021, 2:04:51 PMMay 21
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Sorry, just noticed I was replying to Peter, not Derek.

Dacian Todea

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May 21, 2021, 2:05:43 PMMay 21
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The lower the battery temperature the higher the internal resistance and 10C is a good enough compromise to keep the power consumption low for heating and have decent enough internal resistance as increase in cell internal resistance is fairly dramatic with lower temperature.
Lower charge rate will do a bit less damage but you can not control that with solar charging.
Is the battery in an insulated box ? If not then even your 150W element will do nothing if you have -10C ambient.
Batteries are expensive and I do not think you want to risk Lithium plating due to low temperature charging.
Having an insulated battery means you need very little energy to keep the battery warm as you need to cover just the insulation losses and thus 10 to 15W heating element is more than enough to manatian +10C while there is -10C even -20C outside.
Battery also has a large thermal mass so say you battery gets to -10C overnight as there is no heating and then you try to heat the battery in the morning (if ambient is still -10C and battery is not insulated then you may never be able to do that even with 150W of heating) and even if you can heat the battery it will take time and you are not sure if inside of the battery is above freezing or just the outside thus there will still be damage done.
Battery is the most expensive part of the system so it is worth taking extra precautions not to damage the battery.

Derek Wolfson

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May 21, 2021, 2:26:36 PMMay 21
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I understand the need to be conservative -- but if I told some of my customers they need to keep their batteries above 10C and need a heating element and etc. and all of this has to be controlled via analog thermoswitches (which may fail) inline with EXT outputs... why not just allow the Electrodacus to have the logic built in to toggle the EXT_IOS directly at different temperatures?  I certainly trust the octo-couplers and internal logic more than a suite of thermoswitches.    It's not often that people go camping in -10C... but in a pinch when it happens -- it is important they can use their battery to discharge.

I understand these are edge cases -- and low temperature use like this is REALLY rare -- but it is kind of like "range anxiety" when people buy electric vehicles -- many people "fear" running out of charge even though their daily commutes only use like 10-20% of the range of an EV!!  

A lot of my customers *think* they are going to camp in sub-zero temperatures -- when in reality they aren't -- but when they are choosing their system -- they *really* fixate on this topic.  Telling them they'll lose charge and discharge at 5C versus they'll lose charge at 5C and discharge at -5C ends up becoming an important distinction.  

I understand that a single temp probe may give misleading readings -- but if the unit is already reading the temp -- I think it is a reasonable feature to add different charge/discharge shutdowns based on temperature -- given that the discharge and charge limits are very different.  

I certainly would rather the Electrodacus take multiple temperature inputs versus having to put a thermoswitch inline with each EXT_IO output -- but understand this changes the PCB layout and calls for additional inputs, which is a bigger change.   Heck -- I'd pay extra for an add-on unit that has 8 temp inputs and sends the "minimum" temp to the Dacus.

Dacian - I get your "protect the cells" at all costs mentality -- but it would be nice to have flexibility in these parameter sets for my application.  I'd write the code myself if I knew how -- but, alas, that's beyond my skillset. 

Then again -- adding a rocker/momentary switch in parallel at the load-side EXT_IO for a discharge temperature override is quite easy too... given how freakin' clean the 'Dacus is -- it'd be nice to just have this flexibility built in and avoid that analog solution and additional components.



Derek Wolfson

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May 21, 2021, 2:28:15 PMMay 21
to electrodacus
 As a note this statement: 

"given that the discharge and charge limits are very different."

is related to the constraints of the LifePo4 chemistry.

Peter

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May 21, 2021, 3:16:03 PMMay 21
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Dacian, thanks for the point about higher internal resistance.
You think solar charge rate ( 0-0.05C/28A in my case) can be noticeably slower at +5C with higher internal resistance than at +10C?

My bat is in an insulated box. The heating wire is quite long and I think it distributes the heat quite evenly + heating goes nonstop even during the night.
I have 2 temp sensors on my ESP that monitor 2 different spots in battery bank. And SBMS0 has its own temp sensor inside the bat pack as a failsafe.
Even as a winter camper, the battery should be quite comfortable. :-)

Oberon, well, there are some differences between Battle Born and my 8s100p 32700 cylinder cells pack. But it is still LiFePo4.. You think there could be significant differences in low temp usage?

Yes, bat is the most expensive part of the system so for me, conservative way is the way to go. I was thinking that +5C is fine. After this discussion I probably try Dacians recommendation +10C. Why not.. :-)
Thank you guys! I love this community.
Dne pátek 21. května 2021 v 20:28:15 UTC+2 uživatel derekw...@gmail.com napsal:

Dacian Todea

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May 22, 2021, 1:21:19 PMMay 22
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Derek,

I do not think you understood my earlier comment.  The temperature protection limit is done in hardware by the ISL94203 so I can not write any software to change this the logic is done in hardware.  I can of course disable the hardware protection (as it is anyway when I deliver the SBMS0) and then add a software protection that can differentiate between charge and discharge. That will be some extra settings that someone can misunderstand an set incorrectly. :)
In any case you can set the limit to +1C so just above freezing and have no battery insulation or heating element if it is sure that camping will not be done below freezing.
There are plenty of people that do camp below freezing and for those the only proper option is an insulated battery box as without insulation you can not keep the battery warm with -10C ambient no matter how much energy you waste and once battery drops to those sort of temperatures it will take to long to get them back to normal operating temperatures as the internal of the cells will also need to be above freezing not just the outside case of the cell.
So a battery box that is properly insulated similar to how a small 12Vdc fridge is will require just a small 10 to 15W heater (can be a power resistor and a small fan to keep the inside of the battery box at +10C) and the advantage of this is that will use just at most if is super cold around 0.25kWh in 24h so any battery should handle that and two 60 cell panels should be able to produce that even in cloudy days.
As for the control circuit an electronic thermostat is less than $10 to control the small heater and if you are worried that can fail just add a bimetal thermostat in series as backup set to disconnect in case temperature gets above +25C

All types of Lithium batteries need heating if used outside and ambient temperature can be below freezing. All EV's have a battery heater even if they do not have a cooling system.

Peter,

It depends on the battery but for some internal resistance may increase with 30 to 50% when temperature drops by 5C and yes 0.05C is a fairly slow charge rate so any battery should handle that at +5C
+5C is OK but say average outside temperature is -10C then delta is 15C and if you were to increase that to +10C then delta will be 20C and so power consumption to maintain +10C will be just 25% higher not that significant.
The +10C allows for the case where at night you battery is empty thus battery heater is disabled and in that case the +10C and battery thermal mass may be sufficient that battery is still above 0C in the morning and solar can start charging the battery.

Derek Wolfson

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May 24, 2021, 3:53:16 PMMay 24
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Hi Dacian,

yeah -- I missed that point -- didn't realize it was hardware driven. 

I hear you --- the right way in insulation and heating -- but I still think we as users should have control over separate charge/discharge shutdowns -- especially given that the lifepo4 has different limits for charging and discharging.

Whether a customer misunderstands something and screws up their cells is largely on the consumer -- especially if you ship the unit with good conservative settings and warn about changing the settings unless they really know what is going on.  Some customers may misunderstand having a single temperature shutoff and set it to the discharge limit not the charge limit -- and then again, they misunderstand the settings and destroy their expensive cells.

I appreciate your commitment to protecting idiots against themselves -- but I would rather have the flexibility and configurability.

If the change isn't hard -- I'd be sold on the Dacus for all my future builds if I can have separate temperature limits for charge and discharge.  It's such an awesome piece of hardware -- but I think forcing us into using a single temp shutoff isn't aligned with the limits of the chemistry -- regardless of use case and the actual limits chosen.

Sure bimetal thermostats will work -- but it's just one more piece of hardware to fail AND the end-user will not be able to reconfigure those without changing hardware.  It's much easier IMO to see what your temperature cutoffs are on the screen... instead of depending on the bi-metal thermostats and install documentation.  I'll use them if I have to -- but as an end-user, I'd rather have this information on the display than buried in a piece of hardware that the end-user (in my case) probably doesn't understand.

Great work on this unit... I'm a persnickity customer and this thing is perfect for me, besides this issue.   I'm all for this feature expansion, and more flexibility in general and appreciate your active involvement with end-users in the design of this product!

Cheers Dacian -- appreciate you and all the hard work you've put into this unit.

-Derek

Dacian Todea

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May 24, 2021, 4:46:26 PMMay 24
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Derek,

I do not see how a bimetal thermostat can fail as they are usually rated for 5A and will need to handle just 10mA or so.

The thing is that having a separate temperature control will not help with anything. You either have a proper insulated and heated box or you do not and in that case the separate temperature controls will not help with anything.

Say you have an non insulated battery and is -10C overnight with maybe 0C max during the day and it is a sunny day. Say you have -15C set as limit for discharge and +5C for charge
Then you can discharge the battery if there is anything left but you are unable to charge it so you lose all that available PV energy.
How will that be something someone will want. By next day you are sure to have an empty and cold battery if conditions are the same.
If you have a large heater say maybe 150 or 200W you will still not be able to maintain or heat the battery to +5C if battery is not thermally insulated and all you will do is discharge the battery even more attempting to heat it to +5C.

Now if you have as I recommend and insulated and heated box with a 10 to 15W heater as that is all it will be needed with good battery thermal insulation then battery can be set to be above +10C at all times and thus you do not waste solar PV energy as you can charge in the morning in that cold sunny day and the SBMS0 temperature limit can be set to +5C just in case that something happens and the heating fails so battery is protected.

The point I try to make is that in cold environments where battery is outside of climate controlled living space (as my battery that is inside the house just less than 2m from me now) you need to have an insulated battery box and small thermostat controlled heater to maintain battery at reasonable temperature.
You can not skip the insulated box as then when battery cools down there is no chance for you to bring it back to normal operating temperatures. I'm guessing that is what you think that having a higher power heater you can bring the battery to usable temperature in reasonable amount of time one or two hours but that is not the case.
You may falsely think the battery is at good working temperature as with 150W or 200W heater may heat the outside surface of the battery but inside battery will still be below acceptable levels and still get damaged.

Derek Wolfson

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May 24, 2021, 5:44:16 PMMay 24
to electrodacus
Dacian,

I'm not pitching this option as the end-all solution for cold weather.  In fact, in builds that people are using for skiing or winter boondocking I do require them to insulate and add a heater.  But most people aren't boondocking for extended periods in -10C.. and if they are -- I agree -- they MUST have a insulated and heated box...but if you are unexpectedly or periodically find yourself in a cold weather situation having different set points can be very helpful.  

Think something like this: 

I am camping in the summer and end up a high altitude lake and even though is was 35C in the valley -- it is 5C at the lake during the day .... and -5C overnight.  I go for a nice long hike and come back to and find my my van (and my cells) are at -5C.  My batteries are fully charged.  I want to turn on my diesel heater to warm up the cabin before I go to sleep -- but the Electrodacus won't let me discharge them because the temp sensor has turned off both charging and discharging.  What do I do?

This is probably the MOST common cold-weather use for RV customers -- not people who want to boondock in low temps... but people who either through bad-luck or choice -- end up at places that get awfully cold but don't plan to stay there very long.  There is absolutely nothing about this use case that will lead to long-term damage to the cells, so I don't understand why the ElectroDacus needs to be configured this way.

So let's say I use a bi-metal thermostat... let's say I set the temp limit on my ElectroDacus to -10C and then have a +5C NC bimetal thermostat on the EXT_IO for charging.  Now I have to run the EXT_IO output into a bi-metal thermostat that I need to place somewhere in very close proximity to my cells and then run the wires out of that bi-metal thermostat back to the charge enable/disable pin on my charger(s).  It would be a lot cleaner to use the pre-existing temperature signal to trigger this logic - especially since it already has a temp input!

If someone wants to spend a week up at that lake and will regularly see mornings at -5C/-10C -- yes -- they should have a heated/insulated box.  But what about people like me -- who spend one night at that lake -- and decide then to move to a lower altitude and warmer place tomorrow.  Does it squander some early AM solar?  Sure.  Does it matter?  Depends on how you design your system and your use case. 

 In fact -- many of my builds have zero solar panels and rely entirely on DC-DC charging.  I see no need for a heated/insulated box for this very common use-case style among the overlanding crowd.  Heck some of my clients have no DC-DC alternator charging and rely on AC charging only (i.e. fill it up before the weekend and fill it up when they get back).  Should they have to keep their cells above 5C with an insulated box/heater to use them while on the road if there is legitimately zero possibility of charging -- or should they set their ElectroDacus to -10C while on the road?  What if they forget to reset it back to +5C and then plug it in to charge?  here are lots of use cases that make this feature desirable in my line of work. 

RE: the bimetal thermostats -- I'm not saying they are going to fail -- but the minute you do add more components, there are more failure points -- regardless of how simple or over-rated they are.  I'm all for parsimony in electrical circuits -- especially when it seems feasible to add this as a software driven change.  

Also -- how does the end consumer know what that setting is other than looking at the build sheet?  My customers (and me) love data/GUIs/logging and seeing the parameters... as a product designer to I want to show my customer that the shutoff is set to +5C on the GUI vs. telling them "trust me -- there is a bimetal thermostat in a piece of wireloom behind that wall panel that'll turn off your charger at low temp".

Regarding cell temps vs. measured temp -- I hear you on that -- and given that the ElectroDacus only runs with a single temperature sensor -- I'd do my best to sandwich it the middle of the cells and probably set the charge cutoff to +5C (or even +10C) to provide a safety margin given the deviation between inside/outside cell temp.  Regardless of the deviation from measured temp to cell temp -- it still seems rather draconian to force users into having the same temperature cutoffs for charge and discharge.

I want to reiterate -- I think everything you are saying about cold weather/etc. above is correct--but I'm struggling with your opposition to this setting.  I really do not see what there is to lose offering this functionality -- ship it with the factory settings of both charge and discharge being cut off at +5C (or whatever you deem the relevant safety margin).  You already warn people not to mess with the advanced settings unless they know what they are doing -- so why not give users that know what they are doing this added functionality?  I'm not making this argument on the basis of thinking I am the expert on cold-weather management -- I am making this argument on the basis that the lifepo4 chemistry allows different limits for charge/discharge so why doesn't the ElectroDacus?


Dacian Todea

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May 25, 2021, 1:35:38 PMMay 25
to electrodacus
Derek,

I sure noted your feedback and I will think about implementing separate temperature control in software. The current control is in hardware so that can not be changed.
If you remember this year even Texas had a long very cold period and while is not that common a battery without thermal insulation will have been unusable or damaged if used in that scenario.
I'm on the other way in Canada and here you can have weeks with less than -20C and lows at night around -40C are not uncommon.
 
In your example that is one off case for one night what can be done is to disable the charging manually (can have a switch in series with EXT IO4 or just set that to type 0 in the SBMS0 menu) then for discharge to be enabled you can either set the temperature at a different level and power cycle or replace the temperature sensor with a 10KOhm resistor then you can use whatever is left in the battery (likely less is available due to low battery temperature) and then you will need to way for at least 10h with battery above +5C to allow charging as battery may be +5C on the outside but internally battery may still below freezing for many hours until temperature equalizes.

You can have all this prepared and all that will be needed is that 10KOhm resistor next to the temperature sensor, with sensor and resistor having the XT1- common and on the XT1+ have simple switch to select between the Thermistor and the 10kOhm resistor basically disabling the temperature measurement and showing a false +25C reading. Then you just flip that switch and go in to menu and disable charging by setting EXT IO4 (and/or other EXT IOx used for charging) to type 0 then when conditions are back to normal (battery temperature above +5C for more than 8 to 10h) you can enable the charging.
 
Next time I will work on the software (maybe by the end of the year) I will look at the best way to do this separate charge and discharge temperature control.

Derek Wolfson

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May 25, 2021, 11:08:19 PMMay 25
to electrodacus
Sounds good -- I've enjoyed this exchange and appreciate your commitment to this lovely product.  It only galvanizes my interest in using this unit in all of my builds moving forward.

One question -- given the hardware/software distinction -- if I were to buy an Electrodacus now to start prototyping -- would it be possible to gain this functionality down the line via a firmware/software flash/update?

Dying to get my hands on one -- this is definitely the best thing I've seen in the DIY space and I absolutely love that you embrace the open-source movement.

Thanks much for the back and forth -- and if there's anything I can do to convince you to *think* about this feature set sooner than the end of the year -- let me know!!

Thanks Dacian.
-Derek

Jhon

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May 26, 2021, 6:59:52 AMMay 26
to electrodacus
The thermostat and heating seems to be a common feature request and I can think of other features I would like myself.  I think Dacian is right by not adding too many extra's as it would complicate things and an add on board, like an Arduino or Raspberry Pi might well be the better option, since it can be programmed for each individual setup.  (I am reluctant to play with the SBMS0 firmware)

The only question I have is this, what would "adequate" insulation be for different temperatures and a 15W heater, R5 to R10 for -10C?

Dacian Todea

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May 26, 2021, 3:43:47 PMMay 26
to electrodacus
Derek,

Yes the firmware can be update on the SBMS0 you will just need a $5 StLink v2 programer.
I'm just to busy with production of SBMS0 and DSSR20/DSSR50 so I will not have time to work on the software now.

Jhon,

The better the thermal insulation the lower the energy consumed to keep the battery at set temperature.
The US system is fairly hard to use and non intuitive so I will give you the example in metric.
Say you have R10 insulation (US units) that will convert to  R 1.76 in international units.
Then U value again international will be 1/1.76 = 0.57 W per square meter per degree Kelvin or celsius the same thing.
That means that a wall that has 1 square meter (about 10.7sqft) with US R10 insulation will lose 0.57W for each degree celsius difference.
So say you want battery at +10C and outside temperature is -10C then there is a 20C delta between one side of the box and the other and if total box outside surface is 1 square meter then 20C x 0.57W = 11.4W will be needed to maintain the battery temperature at +10C while outside is -10C

So let me use a more real example and say you want a 24V battery made with 8 of those popular 280Ah cells.
One of those cells is 205 * 174 * 72 mm  so with 8 of them in series say connected on two rows of 4 cells to have a more square battery then size of that will be 205 * 348 * 288 mm
There is some space needed for clamping plates and connections so will round that to 300mm * 400mm * 400mm for the inside of the box (fairly generous I think just to have a bit more rounder numbers).
So there will be 4 sides that are 300 * 400mm and top and bottom 400 * 400mm
0.3m * 0.4m = 0.12m2 * 4 = 0.48m2   then top and bottom   0.4m * 0.4m = 0.16m2 * 2 = 0.32m2
Total area of the box (on the inside but we ignore the thickness for simplicity) is 0.8m2

So from the above example with R10 (US) insulation assuming no thermal bridging  and assuming 20C delta in temperature  11.4W * 0.8 = 9.12W
So this box for this 280Ah 24V battery (or maybe 12V 560Ah) with R10 insulation will need 9.12W to maintain +10C inside while there is -10C outside
So say you had a 15W heater with thermostat control that heater will be ON and OFF so that average will be around 9.12W and thus consumption over a 24h period (assuming temperature outside remains -10C for those 24h) will be 218.88Wh
The battery is 25.6V * 280Ah = 7.168kWh so if fully charged at beginning and there is no charge source it can keep itself warm for a full month 32 days.

Now say the battery box is made of of 1" thick wood with about R1 insulation value then heat loss will be 10x larger than with R10 thus you will need 90.12W to maintain temperature so 2.188kWh in 24h and if there is no insulation then with 150 or 200W heat pads it will still not be able to maintain the battery temperature and battery will be drained from the heating element in maybe a day or so assuming it is full to start with.

Derek Wolfson

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Jul 26, 2021, 2:24:48 AMJul 26
to electrodacus
Hey Dacian --

I'm turning back around to this project soon!  Glad the firmware update will be easy! 

Looking forward to the update and getting my hands on one of your units!

Best,
-Derek

Derek Wolfson

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Aug 31, 2021, 2:32:02 PMAug 31
to electrodacus
Hey Dacian!!

Any chance this'll be configured in the new release BMS units?

I know you're busy - but I *really* desire this functionality and I think it'd be a nice addition for my upcoming snow-bird customers (which always pour in this time of year -- given the proximity to ski season!!)

Thanks a bunch!

-D

Dacian Todea

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Aug 31, 2021, 2:40:41 PMAug 31
to electrodacus
Derek,

What exactly are you referring to ?  In original post you asked about EXT IOx having higher current capability to control a large relay/contractor.
I think I explained that a contactor / relay is just not necessary in a typical setup and will just get damaged. If you want to switch ON something like an inverter that is a huge capacitive load there will be many kA (like a short circuit) and relay will be damaged (contacts will be welded together).
If you still wanted to use a large relay despite the problems mentioned maybe just for disconnect and no automatic reconnect then you can just add externally any current capability like for example using this thing that I just reviewed a few days ago 

Derek Wolfson

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Aug 31, 2021, 7:34:06 PMAug 31
to electrodacus
Realized I sent a "private" reply instead of a "reply all"... here's what I wrote: 
-----

Hey Dacian -- appreciate the quick response. 

I'm referring to the firmware/hardware update to have separate temperature cutoffs for charging/discharging.

I know we can do this with additional hardware -- but seems unnecessary given the dacus already reads temp inputs and, well, parsimony rules.

 


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