24v battery bank question

[solarnoob22]

Hello experts, I have 2 Rich Solar 100 ah 12v in series, and 2 Eco Worthy 260ah batteries on bus bars and I have a couple questions:

1.) Does the power pull evenly from both strings?
2.) Will they balance themselves since the bus bars complete a 24v circuit?

Terrible visual representation:

 

[robbob2112]

So i was wrong and hadn't thought through it well.

The ir of the two batteries will determine the proportion of current from each battery. They both will run out of power at about the same time


The internal resistance of the larger batteries is smaller than that of the larger batteries.

So the 260ah batteries will be used first then ans the ir increases as they are discharged the 100ah will be pulled from as well. The 100ah will hit zero before the 260ah ones. When charging happens the 260ah batteries will get charged first and then equally in the middle and then at the end.

Net result both sets will be used but unevenly. The downside is the 100ah batteries will hit low voltage cutoff... and if they don't self wake then you will have to disconnect them, wake them up, then reconnect them....or put a switch in that side. The other problem is if your inverter demands more current it will be supplied unevenly. So you need to use fuses on the most positive of each set.

The hard part comes because the 260ah will provide an uneven amount of the current...but if one of those shuts down it will all come from the 100ah...and the kick will probably blow that fuse.

2400w / 25.8v = 93a
So at some point all current will come from the 260ah batteries and at some points half will come from the 100ah pair. If you graph the usage you can get an idea how much current is coming from each.

So, if the 100ah batteries have a 100amp BMS in them the simplest is to fuse both at 125amps... and use wires sized for 125amps. Use the MRBF style fuses on the most positive post of each set.

If you use oversize wire on the 100ah the smaller amount of resistance will help balance out the load a bit so the 100ah is pulled from earlier but it will hit zero earlier as well.

 

[robbob2112]

Played with the numbers and some sample data - made the assumption that the 260ah battery has a proportionate less resistance than the 100ah battery.. used a constant load of 100amps.


EVERYTHING IN THIS POST IS THEORETICAL AND PARTIALLY BASED ON GUESSES.


If you graph the percentage contribution from each battery -

The contribution of the 100ah would be roughly 27% of the total amperage for however long it lasts.

Here are some graphs to show it...

NOTE - the 260ah battery is the left axis and the 100ah battery is the right axis - in reality the 100ah battery never provides more than 29% of the current so the two lines are separated

Bigger note - I used the artifical idiot to model these and coached it into doing the correct thing - using one can be a challenge unless you know kinda what the graphs should look like and can tell it what is wrong





This shows the true relationship between the batteries and their SOC

 

[mikefitz]

The 100ah will hit zero before the 260ah ones.

Why? They are at the same voltages, it's unlikely the low voltage protect level will be the same in both the BMS , so either could cut off first.
Roughly the current share will be proportional to their capacity, 2.6 to 1, but the state of charge will be almost the same as they discharge.
Total capacity 360 Ah, if 180 amp hours is taken by the load the batteries end up at 50 Ah, 50% SOC and 130An 50% SOC.

 

[robbob2112]

Why? They are at the same voltages, it's unlikely the low voltage protect level will be the same in both the BMS , so either could cut off first.
Roughly the current share will be proportional to their capacity, 2.6 to 1, but the state of charge will be almost the same as they discharge.
Total capacity 360 Ah, if 180 amp hours is taken by the load the batteries end up at 50 Ah, 50% SOC and 130An 50% SOC.

After the post with the graphs I agree with you --- it is not intuative and I don't like it but the math can show me interesting things even if I am still in bed dreaming

 

[HRTKD]

Well I sure as heck wouldn't put BMS-based batteries in series.

 

[robbob2112]

Well I sure as heck wouldn't put BMS-based batteries in series.

??? Kind of out of the blue there.... no we don't like series of batteries...sometimes it just happenes... if. It does you put an active balancer on it or you pop the tops and replace the 2 bms with one. Neither is ideal and one depends on the confidence level of the person.

 

[HRTKD]

Well I sure as heck wouldn't put BMS-based batteries in series.

Was meant as a reply to the OP. I opened the thread, wrote an answer, then got distracted.

 

[mikefitz]

Well I sure as heck wouldn't put BMS-based batteries in series.

The topic was covered here,

Post in thread 'BMS to maintain balance 12v batteries in series'

Jan 26, 2025

It's no problem with lead acid batteries,

AGM, GEL and sealed lead acid don't tollerate boiling.

• mikefitz

Post #10 suggested,
"For resistor dump balancing, having four separate 4 cell BMS's in series connection is no different than having a single 16s BMS balancer"
Unless I have misunderstood the statement, it seems there is no problem in connecting batteries in series.

 

[HRTKD]

I still wouldn't do it. If you need a 48 volt battery, build or buy a 48 volt battery.

 

[solarnoob22]

So just to recap:

The batteries will discharge at the same rate?

Batteries with separate BMS's in series is bad.

 

[Q-Dog]

Batteries with separate BMS's in series is bad.

Not bad, just not ideal. If you are willing to deal with the issues, go for it. You will likely have to break down the pack and fully charge/rebalance the individual batteries every few months. OR, you can use balancers between the batteries (additional expense) to keep individual batteries balanced within the pack.