Battery balancing question

[Electrobob]

I have four 12v 100 ah batteries wired in series parallel, connected to a 24v mppt. I know when you have multiple batteries wired together they can become unequally charged through charging/discarging cycles. People say you should charge each battery to 100% individually once in a while, but I know alternatively people will let batteries sit wired in parallel so they balance against each other and therefore stay in sync. My question is: since I have only 2 batteries in each series and then both sets of two are connected in parallel, won’t they all balance out together after a charge just by sitting there wired in parallel and avoid the issue of getting out of sync with each other? Is separate battery charging really necessary when wired in this configuration?

 

[MrSparkle]

I am guessing those that are connected in series do not communicate with each other in any way. (I think most cases.) If so, then they charge and discharge individually. Hence, over time, there will probably grow a difference between those connected in series.

 

[timselectric]

I have four 12v 100 ah batteries wired in series parallel, connected to a 24v mppt. I know when you have multiple batteries wired together they can become unequally charged through charging/discarging cycles. People say you should charge each battery to 100% individually once in a while, but I know alternatively people will let batteries sit wired in parallel so they balance against each other and therefore stay in sync. My question is: since I have only 2 batteries in each series and then both sets of two are connected in parallel, won’t they all balance out together after a charge just by sitting there wired in parallel and avoid the issue of getting out of sync with each other? Is separate battery charging really necessary when wired in this configuration?

The two sets in parallel, will remain in balance with each other.
But each sets individual batteries in series will drift away from each other in SOC.
You can take them apart and seprately charge them (be the balancer), or put a balancer on each set. (Buy the balancer)

 

[robbob2112]

I am putting together my system exactly like this... one series set with a balancer and one set without... victron shunt with midpoint aux connection on each set.... once I am up and running a few months I will KNOW if I need to connect the other balancer I got at the time.

 

[trb]

The two sets in parallel, will remain in balance with each other.
But each sets individual batteries in series will drift away from each other in SOC.
You can take them apart and seprately charge them (be the balancer), or put a balancer on each set. (Buy the balancer)

Do the batteries have to be separated from the series to be charged with a 12v charger? If no, can the system be operating while charging the one 12v battery?

 

[timselectric]

Do the batteries have to be separated from the series to be charged with a 12v charger? If no, can the system be operating while charging the one 12v battery?

If you choose to be the battery balancer (and not buy one), you will have to shut down the system, while balancing. You don't have to separate them from each other, but you will probably have to isolate the series set, from the rest of the system.

 

[trb]

Thanks for your reply. I would like to know the reason isolation is necessary through. I guess it's not a big deal disconnecting one cable.

 

[robbob2112]

If you want to be the balancer you need to be able to turn off each string with a switch. At that point you would be able to connect a charger to each and fully charge them. If you make a mistake and try to charge them while using the results would be completely unpredictable.... a $25 balancer per string seems like a cheap insurance policy. I am only leaving one off to test.

 

[timselectric]

Thanks for your reply. I would like to know the reason isolation is necessary through. I guess it's not a big deal disconnecting one cable.

Because your charger isn't fully isolated. There's going to be some bleed through on the AC side, between systems.

 

[trb]

If you make a mistake and try to charge them while using the results would be completely unpredictable....

Has this never been done before? I'm sure it must have been. The results should be known. Not trying to argue, just trying to find out why.

 

[timselectric]

Has this never been done before? I'm sure it must have been. The results should be known. Not trying to argue, just trying to find out why.

I tried it, once.
My inverter shut down and rebooted.
I don't recommend it.

 

[trb]

I tried it, once.
My inverter shut down and rebooted.
I don't recommend it.

Thanks, that's what I wanted to know. Better to isolate then.

 

[Electrobob]

I am putting together my system exactly like this... one series set with a balancer and one set without... victron shunt with midpoint aux connection on each set.... once I am up and running a few months I will KNOW if I need to connect the other balancer I got at the time.

Any results on your test yet? If you have 2 sets of series connected in parallel and you put a balancer on one set, won’t the parallel connection involve the other set by default?

 

[robbob2112]

It is almost installed, just a few more wires to add, then pull wires from the solar panels.... got side tracked with other projects

Post in thread 'My build thread - robbob2112'

Dec 28, 2024

OK, everyone guess who didn't leave enough room on the top shelf for the battery to clear the inverter? Go ahead and guess... So, I jacked the rails with the inverter still attached verse dismounting and remounting everything. Pushed everything up as far as the travel on the jack would go, so about 6 inches. I thought about bolting down, moving the top shelf brackets and doing it another 6 inches - may still do that, but for today I am done playing..

NOTE - had a 3 week break while I installed a new gas line and humidifier for the furnace. The old POS flooded the furnace for the 3rd...

• robbob2112

 

[EPicTony]

Just get two of these (one per set of series connected) and call it a day. I found out the hard way as the batteries will definitely get out of balance 100% guaranteed without the balancer. Also the parallel connection won't do anything for a set with out the balancer.

 

[chateau_nomad]

Since I have a similar issue I'll post it here. I have three SOK 100Ah 12v batteries cabled parallel in a motorhome. For clarity, the third battery has a terminal mounted shunt, then goes to ground.

The first battery has a terminal mounted fuse, then goes to a battery disconnect switch.

I'm using a 3k Sungoldpower inverter/charger for charging.

I installed this setup about three years ago. I initially fully discharged all three batteries to BMS cutoff, then fully charged all three batteries individually to cutoff. I then programmed the shunt for 300Ah = 100% state of charge.

The cells in each battery stay well balanced. However, each battery BMS state of charge gradually drifts. Battery #1 might be 100%, battery #2 60%, and battery #3 35% as reported by each battery Bluetooth BMS. This is with the shunt displaying 100%.

I don't know which to believe! It's a royal pain removing the batteries, discharging/recharging, etc. ... which I've done at least twice. All's well for a couple months, then they drift out of whack again!

Consider that the batteries are directly cable in parallel... the classic post to post. I was VERY careful to make all cables the exact same length.

Would it make any difference if I changed the cabling to run each battery to master positive and negative busbars instead of chaining in a string??

I made the assumption that by just using the batteries they would eventually balance between themselves... all receiving charge until each BMS cutoff. It almost seems like they're charging at different rates, then when the first one cuts off, the rest in the string are also cut off. ???

 

[MrSparkle]

I don't know which to believe!

AFAIK, in many (most?) cases, the built in shunt inside most BMSes is far less accurate compared to a separate, dedicated shunt. I guess that might explain discrepancy?

 

[mikefitz]

when the first one cuts off, the rest in the string are also cut off.

That won't happen, there is no communication between batteries.

What is possible occurring is that you are charging the batteries to BMS protection due to a too high charge voltage and/or cell inbalance. Another factor that may exist is a sleep mode in the BMS
When you start discharging, only one of the three, recovers from protection/sleep, and initally powers the load, eventually no2 then later no3 join in.
You should be able to see this via the battery bluetooth.

The other factor that may be confusing the battery BMS reading is that low current discharge/ charge may not be recorded by the BMS leading to display errors.

Since the batteries are in parallel they must all be at the same charge voltage when under charge, so must all be 'full' at the end of a charge cycle.

All you are doing by seperatly discharge and charge is resetting each BMS

Since you have a shunt , once all the batteries are charged in parallel disconnect all but the one with the 35% SOC and load through the shunt to see the actual capacity.

 

[timselectric]

I made the assumption that by just using the batteries they would eventually balance between themselves.

They will.

all receiving charge until each BMS cutoff.

BMS cutoff is a safety feature. It should not be used for a daily charge limiter.
If you are hitting cutoff, your charger is set too high.

Would it make any difference if I changed the cabling to run each battery to master positive and negative busbars instead of chaining in a string??

Yes

 

[chateau_nomad]

That won't happen, there is no communication between batteries.

What is possible occurring is that you are charging the batteries to BMS protection due to a too high charge voltage and/or cell inbalance. Another factor that may exist is a sleep mode in the BMS
When you start discharging, only one of the three, recovers from protection/sleep, and initally powers the load, eventually no2 then later no3 join in.
You should be able to see this via the battery bluetooth.

The other factor that may be confusing the battery BMS reading is that low current discharge/ charge may not be recorded by the BMS leading to display errors.

Since the batteries are in parallel they must all be at the same charge voltage when under charge, so must all be 'full' at the end of a charge cycle.

All you are doing by seperatly discharge and charge is resetting each BMS

Since you have a shunt , once all the batteries are charged in parallel disconnect all but the one with the 35% SOC and load through the shunt to see the actual capacity.

The only charge current the batteries have received for at least 6 months is from three 200 watt (600 watts total) solar panels. According to my charge controller they output at best 23 amps (winter day in Ohio). SOC recommends 20 amps... 50 amps max charging.

The only time the inverter/charger is active is when the RV is plugged in to power... which is a rare occasion.

Looking for a SPECIFIC strategy here... what's recommended to get all three batteries TRULY fully charged? Secondly, what strategy should I use to prevent this situation? IF I re-cabled to busbars, why is that any different than the direct parallel I have now? The batteries are NOT in an easily accessible area, so any reconfiguration is a major project!

 

[mikefitz]

Looking for a SPECIFIC strategy here... what's recommended to get all three batteries TRULY fully charged? Secondly, what strategy should I use to prevent this situation?

Leave in parallel, set up the charger to deliver 13.8 volts fixed voltage for 24 hours.
This will fully charge and help cell balance.

Set normal charge to 14.2 volts with 2 hours absorption. 13.4 volt float

 

[chateau_nomad]

The options I have are specific to battery chemistry with the solar charge controller. The inverter/charger has battery chemistry, plus the current can be adjusted. I'll have to check what settings I have them set to.

 

[chateau_nomad]

I plugged in to AC power and adjusted the charger to as close to 20 amps as possible... it jumps around somewhat. I also disconnected the solar to avoid muddying the waters. The following screenshots are from each battery BMS... Thoughts ?? Also... after about two hours of charging, the current dropped to ZERO, and the little charge indicator in each BMS display switched to OFF, "grayed out". What's going on here???

 

[timselectric]

I plugged in to AC power and adjusted the charger to as close to 20 amps as possible... it jumps around somewhat. I also disconnected the solar to avoid muddying the waters. The following screenshots are from each battery BMS... Thoughts ?? Also... after about two hours of charging, the current dropped to ZERO, and the little charge indicator in each BMS display switched to OFF, "grayed out". What's going on here???
View attachment 295049View attachment 295050View attachment 295051

They are still taking current, so they aren't full.
Keep charging until all are full.
Hopefully their SOC will reset when full.

 

[chateau_nomad]

They are still taking current, so they aren't full.
Keep charging until all are full.
Hopefully their SOC will reset when full.

I just checked them again. AA01523 was 7% and charging indicator OFF. Zero current. The other two were still enabled and still accepting current. I'm going to leave them on charge overnight and see what happens. Something's going on with that one battery. If still problems, looks like removing it an individually charging it to 100%.

This is getting frustrating... I've already removed all three batteries... discharged them all to cutoff, then recharged to cutoff. Even so that calculated capacity was still way off.