Basic Power Supply Question: +12v, -12v, & 24v from 2 lead-acid 12v batteries?

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Scott Monaghan

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Apr 21, 2023, 12:38:50 PM4/21/23
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I came to robot building via programming so I am self-taught (and missed a lot of steps) for electrical engineering.

This is a basic question.

Right now for my larger bot I have 2 12v lead-acid batteries in series to have 24v off my power supply.

But would it be possible for me to get 12v & 24v without stepdown by creating 3 power rails:
  1. +12v
  2. - 12v
  3. Combined for 24v?



camp .

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Apr 21, 2023, 12:44:34 PM4/21/23
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Yes, you can tap off the center for 12V. Watch the polarity!  :- ] 

- Camp

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camp .

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Apr 21, 2023, 12:47:38 PM4/21/23
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    Let me be more clear. This is for sealed lead acid, alkaline batteries, etc. NOT LITHIUM! Also, the batteries will eventually become unbalanced, but yes, for homebrewed purposes, you can tap off the center.

Thanks,
Camp

Chris Albertson

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Apr 21, 2023, 12:59:44 PM4/21/23
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Yes this can work but there will NOT be a common ground for all three.

what you want is    

1.  0v (ground)
2. 12v
3. 24v

Then they both reference the same ground.   I suspect this is what you want and you want need any power that is 12 volt below your zero reference.

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Chris Albertson
Redondo Beach, California

Scott Monaghan

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Apr 21, 2023, 1:13:20 PM4/21/23
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Thanks all!
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Alex Sy

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Apr 21, 2023, 1:27:45 PM4/21/23
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Hi Scott,
You can do it with the two batteries in series so you get ground, +12V and +24V.
The batteries should not be Lithium type batteries because they will likely get unbalanced loading.  I assume you will have the motors on the 24V and other electronics like laptop (most take 19V nominal) or Raspberry Pi etc getting their voltage from a DC-DC converter at 5V.    If you do not have anything that specifically needs 12V, you will be better off using a 24V input DC-DC converter to get your 5V or 3.3V to avoid the unbalance.  The unbalance can lead to a lot of issues in charging and battery life even for lead acid batteries.
 
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James H Phelan

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Apr 21, 2023, 4:25:47 PM4/21/23
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Team

For the Open Source Rover I'd like to have more power for longer runs.  I have a couple LiPo batteries.

I have somehow in the back of my mind that I can't just wire them in parallel.

What do I need to avoid trouble?

RoverDoc

James H Phelan
"Nihil est sine ratione cur potius sit quam non sit"
Leibniz

"Here am I, the servent of the Lord;
let it be with me, according to your Word"
Luke 1:38

Stephen Williams

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Apr 21, 2023, 6:53:47 PM4/21/23
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In general, and specifically for Lithium-Ion batteries, you can put all the batteries in series to get the highest voltage, then use voltage regulators to tap off for each lower voltage you need.  There is some power overhead to stepping down, but the voltages should stay precise and everything is simplified.

I bought these for direct battery to 12v, 5v:

https://www.amazon.com/gp/product/B078Q1624B

https://www.amazon.com/gp/product/B08R6337QY

Even better, using large PD batteries, you can use USBC adapters programmed for the voltage you need, up to 20v anyway.

And these for PD batteries USBC to certain voltages:

https://www.amazon.com/gp/product/B0953G14Q2

If the amperage & voltage is high enough, these seem great.  Putting the output of these in series to get over 20V and/or in parallel to get over 5A may be a problem, but I may try it at some point.  The USBC regulation seems very good.

Stephen

Chris Albertson

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Apr 21, 2023, 10:08:41 PM4/21/23
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With LiPo batteries, you need to actual design the power system.  You can lace them in series or parallel but you did not give us enough information.  

What exactly is required?  What voltages are needed and how much current for each and how long should the run time be.    Finally, how are you controlling the motors?

Then you might tell us about the batteries you have.   

What is your plan for charging?

The direct answer to you question is that if LiPo batteries are placed in parallel, you must be careful that the batteries are identical and have identical state of charge.   When I say Identical, this means same capacity and internal resistance.

Many times it is better to place the batteries in series and get more volts.   But we'd need to know more

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James H Phelan

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Apr 22, 2023, 10:16:19 AM4/22/23
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Team,

I have 2 different Li batteries.  Both 14.8V.  The OSR has a voltage regulator to step down to 12V for the motors and 5V for the RPi.  I don't need more voltage, just more amp-hrs.

The older smaller one is:

https://www.batteryspace.com/li-ion-18650-battery-14-6v-5-2ah-75-92wh-7a-rate-inr18650m26-for-diving-light.aspx

The newer larger one is:

https://www.amazon.com/Zeee-9000mAh-Battery-Connector-Traxxas/dp/B07YP73LMX

This is the charger:

https://www.batteryspace.com/smart-charger-3-0a-for-14-8v-li-ion-polymer-battery-pack.aspx

James H Phelan
"Nihil est sine ratione cur potius sit quam non sit"
Leibniz

"Here am I, the servent of the Lord;
let it be with me, according to your Word"
Luke 1:38

Alex Sy

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Apr 22, 2023, 3:39:46 PM4/22/23
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To get more current you would need to parallel them.  That means that each all the cells at each voltage level are in parallel, then in series with the next voltage level etc.  This requires that the cells be matched and balanced.  This is easier to do if you buy the cells from a reputable vendor as a batch so they are likely to be in the same condition and charge level.   Unfortunately, you are using two different battery packs which means they cannot be paralleled.  Also I think most of those packs are likely made in China from Shenzen and are not going to be matched.  If you are going to build battery packs yourself, use Lithium Iron Phosphate batteries which are much less likely to have battery fires, although their capacity is less than Lithium Ion or Lithium Poly batteries.  After that you need to add a battery protection board which as a minimum has an under/over voltage protection and over current protection.  This needs to match the battery chemistry (voltage threshold), total voltage and max current.  Unless you are prepared to do a lot of work it may be better to buy a larger capacity battery pack and charger.

James H Phelan

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Apr 22, 2023, 10:33:01 PM4/22/23
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Alex,

"Unless you are prepared to do a lot of work it may be better to buy a larger capacity battery pack and charger."

I definitely hear that!

RoverDoc

James H Phelan
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Leibniz

"Here am I, the servent of the Lord;
let it be with me, according to your Word"
Luke 1:38

Stephen Williams

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Apr 22, 2023, 11:08:38 PM4/22/23
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This is a complete explanation of the series / parallel / charging / management area:
https://www.synopsys.com/glossary/what-is-a-battery-management-system.html

Perhaps they might be interested in giving a presentation at some point.

I've purchased BMS (battery management systems) for sets of small cells.  Interesting to think about what is needed to use those battery packs in larger series / parallel uses.


These (randomly / top of search chosen) are larger, heavier batteries, but interestingly they say you can use them in series and/or parallel without a problem.  Perhaps they coordinate battery management?

https://www.power-sonic.com/blog/how-to-connect-batteries-in-series-and-parallel/

https://www.power-sonic.com/batteries/psl-sc-series/

Similar, with cute diagrams: https://www.lithium-battery-factory.com/wire-lithium-batteries-parallel/


Stephen

Chris Albertson

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Apr 23, 2023, 12:28:05 AM4/23/23
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Those 14 volt LiPo batteries are actually a package that contains some lipo battery cells that are wired in series.    It is NOT good to parallel a pair of series wired batteries unless they are PERECTLY matched.  You can bet they are not.

But if you need 12V for a DC motor and you are using PWM on the 12 volts.  You do NOT need a 12 V regulator.  The PWM'd h-bridge can serve that function.   Well it can if you have an encoder on the motor.    You can run the raw batter power to the h-bridge.   Yes, this means a software error can send the full 14 volts to the motor,   This might be a problem or not.   You can always hard-code a "speed limit" to keep the motor within its limits.      Using a regulator in this case is redundant.

Dan

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Apr 23, 2023, 12:44:19 AM4/23/23
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"It is NOT good to parallel a pair of series wired batteries unless they are PERECTLY matched"
Hmm.
Well Tesla seems to have done a good job of it. See model S battery design.

[Each module has a 6S 74P configuration, i.e. 6 cells are connected in series and each of these series has 74 cells connected in parallel. ]





Alex Sy

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Apr 23, 2023, 2:55:41 AM4/23/23
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I think it would be 74 cells in parallel, then 6 of those 74 parallel cells are in series for about 25V.   Then each pack has a BMS to balance the 6 sections.   This is how I built KW level 480V battery packs.  This is different from paralleling series connected cells, which will be a problem.
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Mark Johnston

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Apr 23, 2023, 5:40:18 AM4/23/23
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I am late to this thread. I have read the prior threads, lot of content there.    

With the recent great advances in the use of very efficient buck converters be sure you cannot run from one main rail before you try to do heavier loading on the 'first battery'.   I agree with the comments on it is best to try to load the cells evenly..

So be sure you cannot put into use buck converters to deal with step up and step down with enough current (especially surge currents) before you unevenly load your cells for 24V systems where you use the lower 12V for greater 'suds'.   

Be watchful of your ground return currents if large and perhaps consider a 'star ground' for higher current needs. 

A 'home brew' project is not as much to worry about as a 'real product'  For a home brew project, try not to over-think the power distribution except to be very aware of high current ground differential issues can lead to signal problems due to shift in ground from processor to motor drivers.   This sort of problem can be minimized by use of differential signal systems such as rs-422 or rs-485 or even CAN but CAN carries with it a lot of 'learning curve' and over-complexity for small systems. 

Mark

Dan

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Apr 23, 2023, 1:23:02 PM4/23/23
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The spec in the square brackets is from:

It is a bit confusing taken out of context. Read the article for a detailed explanation.

It will help if you want to design a ~24 volt module that uses both parallel and series connections.
Depending on the cell chemistry you use your voltage may vary.



Chris Albertson

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Apr 23, 2023, 2:33:55 PM4/23/23
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Tesla has complete control of the entire process and designs from the basic calls upward.  They can specify the internal resistance tolerance of each cell ad they can test and reject defects.  They have temperature sensors and a computer to limit charge and discharge rates.  You don't have this control if you simply buy two 14.7 Volt batteries and connect them in parallel.   Almost 100% certainly, the two batteries are not well matched.

But before connecting the two 14.7 batteries check the maximum discharge rate.  It might be so high that it already can supply far more current than you need.   Some of mine can output 100+ amps.     

Connecting them in series force the current through each to be identical.  So long as you limit the current to what the samllr battery can handle you are OK.   You get the same power (as measured in Watt Hours) out of the system if use series wiring.  Then you limit the voltage to the motors with PWM.

This brings up another topic, fuses.   The purpose of a fuse is to protect the wire.  I had a 6-wheel drive robot and got this wrong and a short caused a section of 8 gauge wires to instantly vaporize.  #8 wire is not so thin and it make a loud noise when al that solid copper is turned to hot melted metal and fly into space.     The rule is to place a fuse INSIDE a box that contains the battery and the fuse.  Then the on/off switch can be anyplace, even outside of the box.

When I owned a sailboat, it had a small diesel engine.  I watched to diesel mechanic training videos and in one they showed someone dropping a crescent wrench across the terminals of a engin-start battery, it blew the wrench apart. (it was filmed through a big plexiglass safety shield.)

If you look at how Tesla does this, there are a lot of safety features, I think (?) they use 500 Volts to the motors.  My Prius uses 400v.  THere are a few different auto disconnects.   But our little robots don't have lethal voltages so at least it is safe as long as we have fuses inside a battery box.

To view this discussion on the web visit https://groups.google.com/d/msgid/hbrobotics/427392276.3860654.1682225052905%40mail.yahoo.com.

Alex Sy

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Apr 23, 2023, 10:38:46 PM4/23/23
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That article was quite good and specific.  I think the original question was whether you can parallel two battery packs where the battery pack had series connected cells.  I was trying to say that you should never wire series connected cells in parallel, even if they are matched, but instead you can wire parallel connected cells in series.   The parallel connections of the cells is used to balance the cells to the same voltage level, then the BMS balances the parallel cells that are in series so that each parallel cell is approximately the same voltage.   The cheap passive ones use a resistor to drain the strongest parallel section wasting energy, while the active ones use upconverters to use the stronger parallel cells to charge the weaker one, so it is much more efficient.
In the picture is a 3S8P 250WH using 2160 LiFePO4 cells.  It include thermistors and balance wires.  The picture does not show the balancer.  I use this for robotics since it uses safer LiFePO batteries which has less likelihood of Lithium fires.
Another picture shows the BMS that protect and balances the 60 18650 Lithium Ion cells on the bottom to produce 240VDC.  Several of these are used to build up 480V for several kilowatts to power server farms.  This is not good for robotics since it uses higher capacity but less safe Lithium-Ion cells that can cause Lithium fires.
20230423_154347.jpg
20230423_110550.jpg

Scott Monaghan

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Apr 24, 2023, 11:10:26 AM4/24/23
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Hi James, 

I'm late on this, but for anything more than s1 (3.3v) lipos, you've got to deal with balanced charging of lipos. I've seen many many videos of these exploding when charging. When balance charging these s2+ high-c batteries you need to take a lot of safety precautions--more than I wanted to worry about.

That's why for my larger robots that can handle the weight but do not need high speed, I use classic lead-acid batteries that don't have the same possibly-burn-your-house-down risk.
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Dan

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Apr 24, 2023, 5:14:32 PM4/24/23
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Yes Alex you are 100% correct...Thanks for great feedback for this thread.

Chris Albertson

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Apr 24, 2023, 11:46:39 PM4/24/23
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What you are saying is that a good charger for LiPo batteries has a computer inside.  Yes they do.   The charger will have taps for each cell in the stack.     Look at any common tool battery for a drill and it has more then two contacts that connect to the charger.   Yes, you have to do that.   The better quality battery packs have temperature sensors inside and the charger monitors the internal temperature.   

The price you pay for the high power density and lightweight is added complexity.   But "computers" cost under $1 now so that can put them even inside a low-cost charger.

A robot with big wheels can haul around a lead acid battery but I've built one that flies and another that walks.   That would be impossible with lead acid.  From notebook PC to cell phones, to electric cars, all modern electronics runs on lithium batteries, this battery technology has enabled a revolution.    But at the price of high complexity.

I do have on robot in my pipeline that is using a huge lead battery.   It is a ride=on "kiddie car" with a 80 Kg payload made to drive on sidewalks.   It's designed for two 4-year old kids to ride in.   Thay make great robot chassis.






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Scott Monaghan

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Apr 25, 2023, 9:55:08 AM4/25/23
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Good points all around Chris! 

For me, I don't want to take the safety responsibility of DIY handling of raw high discharge-rate LIPOs like these:https://a.co/d/1orb7eO

When I do use LiPos, I like to have them either be teeny-tiy like the 3.5v lipos available from Adafruit, or have lots of built in safety features like this 65w power brick with USB-C power delivery that I use for Robud: https://a.co/d/8qmDmSY
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