Automatic inverter pre-charge?

[Peter]

Hello guys,

I am considering automatic inverter pre-charge in my RV.

My inverter is eating over 30W with no load so its necessary to keep it off.

I would explain my motivation and goal with this example of using induction hob:

1) user pushes special button to turn the induction hob on,

2) ESP turns on the inverter using automatic pre-charge,

3) ESP turns on the induction hob,

4) when amp meter on the output of inverter notices that there is no load on the inverter, after some time delay ESP turns the inverter off.

I would prefer to include automatic pre-charge, but I dont know about any usable big relay with minimal resistance which could handle continuous 135A.

Would you know about such "super relay" which could be used like this?

Or do you have any ideas around this automatic pre-charge idea/setup?

Thank you!

[Will OBrien]

Yeah, I'm doing this with my arduino mega that coordinates everything.

For the precharge, I'm using a relay + big resistor.

You don't need a huge relay, just an appropriate resistor for your battery voltage.

[Peter]

Yes, pre-charge relay can be small.

But you when you are pre-charging, you need to have your main power line between battery and inverter disconnected, right? There I think I need that big relay..

Dne středa 19. května 2021 v 18:50:42 UTC+2 uživatel Will OBrien napsal:

[Oberon Robinson]

Peter, are you saying that your inverter draws 30W when it's turned off?  Or just when it's turned on with no load? 

Victron Battery Protect has models that go up to 220A.  They are not supposed to be used to switch inverters, but having a pre-charge resistor may make it ok.

[Peter]

Oberon,

its cheap chinese Edecoa 3500/7000 24V. My measurements: off: 10 mA ~ 0.27W, on without load: 1.16A ~ 31W.

But I presume every high power inverter takes quite a lot when its on without load.

My complete RV setup is here: https://groups.google.com/g/electrodacus/c/ioJeZUEUuLA, I am grateful for every opinion.

Dne středa 19. května 2021 v 19:24:52 UTC+2 uživatel Oberon Robinson napsal:

[Oberon Robinson]

So why not just use the inverter's built-in ON/OFF switch function to turn it off after a delay?  You don't need a power relay.  If you can control it with a SBMS0, the same control function should work with your ESP32.  There's no point trying to save 0.27W.

[Oberon Robinson]

You do need a pre-charge resistor for your inverter.  But you generally should never have to disconnect your inverter from battery unless you are doing maintenance or shutting down your system for storage.  Check out the attached guide by FilterGuy, for a manual inverter disconnect with built-in precharge.

[Peter]

I have already studied this guide for manual pre-charge.. I was rather thinking about automatic pre-charge.

But maybe now I got your point.

When the inverter is under power and off while consuming idle 0.27W, does it mean that its capacitors are still charged?

It would be so nice.. pre-charging would not be needed in this situation.

Dne středa 19. května 2021 v 20:32:20 UTC+2 uživatel Oberon Robinson napsal:

[Richard Bewza]

I have been working on a similar project and some of the concepts are transferable.

See attached 

[Peter]

Richard,

wow.. thank you for the tip for EV200AAANA relay! There are offers of this piece for only $27: https://www.aliexpress.com/item/1005002582721052.html.

Did you try to measure real internal resistance?

Thank you!

Dne středa 19. května 2021 v 23:17:42 UTC+2 uživatel Richard Bewza napsal:

[Richard Bewza]

My design is theoretical. I do not own any of these parts yet. My plan is to plug in my 5th wheel while traveling. My victron multiplus 3000-24 will be set to limit incoming AC to 1800 watts. This will keep the truck alternator and inverter to below 50% of their capacity. This will charge my 7Kw LIFEPO4 battery bank in about 4 hours.

[Dacian Todea]

Peter,

As Oberon already mentioned it makes no sense to have a relay to disconnect the inverter DC input when you have a toggle ON/OFF switch on your inverter that can be directly controlled by the SBMS0.

[Dan Karau]

There is zero need to limit charging from the 7 pin plug, the tow vehicle will have a 10 - 15a fuse on this line (because of the long distance and small wires used).   The whole "Blow up your alternator" issue with LiFePO4 is only an issue in motorhomes where the wires from the alternator are much larger and the distance is significantly shorter. 

[Dacian Todea]

If wires are to long and thin (high resistance) then you can not control the charging current. It may be to high with a deep discharge LiFePO4 that it will blow the fuse or if battery is closer to full charge current may be so small that it will take forever to charger.

Typical alternator will be set around 14.4V so say LiFePO4 is at 12.4V so there will be a 2V delta and say there is 0.1Ohm total cable resistance (including contact resistance and fuse resistance) then current will be 2V / 0.1Ohm = 20A fairly sure that will blow the 10 to 15A fuse relatively fast.

The same conditions 14.4V 0.1Ohm say 13.4V thus 1V drop will provide you with 10A and that current will continue to drop as battery charges and if battery is say made with the popular 280Ah cells it may just take forever to charge the battery.

Having a battery to battery charger will ensure charge current will be the same no matter the input voltage.

[Jim Niemann]

Been using my inverters on/off switch for years.   The inverter is always connected to the battery,  no need to precharge.