Parallel batteries and long term storage?

[Rednecktek]

So quick question, but since all batteries will self discharge and you don't want to connect batteries in parallel that are different voltages, what's the easiest way to balance the charges on parallel batteries without having to disconnect, charge one up somehow, then reconnect, and repeat?

Example:

I buy my mid life crisis homestead in Montana and build out a shipping container power room with a pair of NHX 12k's and 4 of the Docan 15kwh batteries. When I go back to sea, I kill the solar coming in, fire up the air compressor and blow lines, nuke up leftovers on paper plates, basically drain some of the battery SOC down out of the knee. Then I go secure the breakers on each battery and lock the door.

8 months later I get back to land and go to fire everything back up. If I'm smart I turned the "balance on charge" to OFF so each battery should still be pretty well balanced on its own cells. However, I'm assuming that the self discharge of the 4 batteries through a Montana winter has left all the voltages a bit out of whack from each other and if I just threw on all the breakers, the batteries woukd surge and trip and bad things would happen, correct?

I know, in a perfect world I'd disconnect each battery from the bus bar, fire up a generator, charge each one, then bolt them back to the bus bar. Repeat 4 times so they're all the same, THEN go ahead and turn on the inverters and start getting the water going and so on.

Problem is, that's a lot of time after 30 hours of flying home in the dark and all I want is something to eat and sleep. There's GOT to be a better way.

My thought: since we use pre-charge resistors to slow charge the inverters, what would happen if I were to just install some permanent resistors on the Pos+ from each battery to the other?

Pos+ -> 40ohm -> Pos+ -> 40ohm -> Pos+ -> 40ohm -> Pos+

My sleep deprived brain tells me that with all the Neg- lines connected to a bus bar, the resistors would balance out the voltages on the other end so if one battery were to self discharge a little faster, the slower ones would top it up, correct?

Or is the easy answer just to leave the batteries on when I leave and just open the DC breaker between the bus bar and the inverters?

 

[upnorthandpersonal]

Then I go secure the breakers on each battery and lock the door.

Any chance to make a modification so you can turn the BMS off? Self discharge over 8 months is not that bad, but the BMS increases this because it needs power to keep going.

However, I'm assuming that the self discharge of the 4 batteries through a Montana winter has left all the voltages a bit out of whack from each other and if I just threw on all the breakers, the batteries woukd surge and trip and bad things would happen, correct?

I don't think so, you should be fine. The batteries will be pretty close to 3.2V nominal (in the flat part of the curve), so there won't be a surge.

 

[wpns]

So quick question, but since all batteries will self discharge and you don't want to connect batteries in parallel that are different voltages, what's the easiest way to balance the charges on parallel batteries without having to disconnect, charge one up somehow, then reconnect, and repeat?

Example:

I buy my mid life crisis homestead in Montana and build out a shipping container power room with a pair of NHX 12k's and 4 of the Docan 15kwh batteries. When I go back to sea, I kill the solar coming in, fire up the air compressor and blow lines, nuke up leftovers on paper plates, basically drain some of the battery SOC down out of the knee. Then I go secure the breakers on each battery and lock the door.

8 months later I get back to land and go to fire everything back up. If I'm smart I turned the "balance on charge" to OFF so each battery should still be pretty well balanced on its own cells. However, I'm assuming that the self discharge of the 4 batteries through a Montana winter has left all the voltages a bit out of whack from each other and if I just threw on all the breakers, the batteries woukd surge and trip and bad things would happen, correct?

I know, in a perfect world I'd disconnect each battery from the bus bar, fire up a generator, charge each one, then bolt them back to the bus bar. Repeat 4 times so they're all the same, THEN go ahead and turn on the inverters and start getting the water going and so on.

Problem is, that's a lot of time after 30 hours of flying home in the dark and all I want is something to eat and sleep. There's GOT to be a better way.

My thought: since we use pre-charge resistors to slow charge the inverters, what would happen if I were to just install some permanent resistors on the Pos+ from each battery to the other?

Pos+ -> 40ohm -> Pos+ -> 40ohm -> Pos+ -> 40ohm -> Pos+

My sleep deprived brain tells me that with all the Neg- lines connected to a bus bar, the resistors would balance out the voltages on the other end so if one battery were to self discharge a little faster, the slower ones would top it up, correct?

Or is the easy answer just to leave the batteries on when I leave and just open the DC breaker between the bus bar and the inverters?

I’m with @upnorthandpersonal, turn off the BMS and even if the SOC isn’t identical between batteries, the voltage will be very close.

 

[chrisski]

It’s a tough call. I would just store the batteries in parallel at 50% for 8 months, but my temps go from a low of 30f to a high of 120f.

I recently stored two 280 ah 24 volt batteries each with an Overkill BMS connected on parallel through the AZ summer for four months and the discharge may have been a couple of percent. Overkill had something in their documentation about how little the BMS uses.

Some math at 24 volts has two 40 ohm resistor in series bleeding off .3 amps, so could be a while to balance. You would not be able to balance batteries quick enough to put on a meal on the microwave.

 

[Texas-Mark]

However, I'm assuming that the self discharge of the 4 batteries through a Montana winter has left all the voltages a bit out of whack from each other and if I just threw on all the breakers, the batteries woukd surge and trip and bad things would happen, correct?

Do the batteries each have their own breaker or are they always connected in parallel? If always connected in parallel, they should all be at the same voltage.

 

[Rednecktek]

I figured on each battery having its own breaker, but the more I think about it, just leaving them on and securing the inverters at the bus bar might be the way to go.

Short of unplugging the harness, how would one shut off a BMS anyways?

 

[wpns]

Short of unplugging the harness, how would one shut off a BMS anyways?

My batteries have a power button, not sure if yours will.