Different Capacity Batteries

[Anteclansing]

Hello all. I was wondering if you could help me with a question. I have 3 Lifepo4 batteries 48v: 100ah, 100ah, 280ah. They are all connected to a common bus bar with MRBF fuses for each battery and T-class fuse between bus bar and inverter. They are working so far for about 1 year now. However, I notice that the two 100ah always get charged/ discharged before the big one. Why is that? All have same length same gauge wires. Maybe loose connections on the big battery? Please help.

 

[rodrick]

If it’s pulling equal amounts of power from each you should deplete the 100Ah battery and still have 180Ah left in the larger battery this is exactly what should happen you should treat it as a 300Ah bank

 

[Anteclansing]

If it’s pulling equal amounts of power from each you should deplete the 100Ah battery and still have 180Ah left in the larger battery this is exactly what should happen you should treat it as a 300Ah bank

I understand that but isn’t it supposed to pull based on voltage? I see the voltages of the smaller ones get to max charging volt much before the larger one. And vice versa when discharging, the smaller ones drop much faster than the bigger one. Maybe I’m wrong but don’t they should reach 100 SOC almost at the same time or 0 SOC almost at the same time. Now the amp being pulled should not be the same I understand that.

 

[underdog5004]

If they're paralleled, I'd think that the 280ah would provide (and take) about 30% more power than the 2x100ah units. Seems like the voltage of all batteries would direct the charge more into (and from) the larger battery. Pretty sure i saw a 10ah and 100ah cell being paralleled, (off grid garage?) And the larger cell provided 90% of the current b3ing drawn from that paralleled pair of cell.

 

[Bananaman321]

LFP has a flat charge curve meaning 45% SOC and 80% SOC can have the nearly the same voltage that is why cell balance start voltage is 3.45v x 16=55.2v this is referred to as the knee of the curve. You can set cell disconnect voltage to 3v in BMS of 100Ah battery to prevent it from fully discharging.

 

[rodrick]

I’m guessing that depending on how low you pull the batteries down your getting a voltage difference and the 280Ah bank is passing power to the smaller batteries and powering the load also so it ends up with more Ah depleted

 

[Anteclansing]

It seems that I’m doing some wrong then? My 2 smaller ones get charged discharged first with all the power going to them, then when they get about 80% power starts to go to the bigger one. Then when the small ones get fully charged all the power now goes to the bigger one. Any idea? I measured with a tool and I think it’s because the bms from each one isn’t doing 100% accurate voltage measure and bms dictates how the power goes to the battery. Right?

 

[Bananaman321]

It seems that I’m doing some wrong then? My 2 smaller ones get charged discharged first with all the power going to them, then when they get about 80% power starts to go to the bigger one. Then when the small ones get fully charged all the power now goes to the bigger one. Any idea? I measured with a tool and I think it’s because the bms from each one isn’t doing 100% accurate voltage measure and bms dictates how the power goes to the battery. Right?

Upload some pics. Cable length size and quality of made connections can affect which batteries get discharged/charged_first/last my batteries have excessive cable length and they lag behind but they still top off eventually daily. In the end it doesn't really matter as long as there all getting fully charged.

 

[Bananaman321]

It seems that I’m doing some wrong then? My 2 smaller ones get charged discharged first with all the power going to them, then when they get about 80% power starts to go to the bigger one. Then when the small ones get fully charged all the power now goes to the bigger one. Any idea? I measured with a tool and I think it’s because the bms from each one isn’t doing 100% accurate voltage measure and bms dictates how the power goes to the battery. Right?

Another possibility you're 100Ah batteries may have cells that have a lower internal resistance this would cause them to charge and discharge at a faster rate than a battery that contains cells of higher internal resistance.

This could be offset by shortening a battery cable to the larger pack.

 

[Ellcon123]

If your connections are ok then I wouldn't worry about it. Charge and discharge will come down to resistance, both internal and all cables/connections. Parallel batteries have to have the same voltage so they will self balance in the knees.

As your smaller batteries enter the lower knee their voltage will want to go lower than the larger battery but they can't. The larger battery, or the one with higher state of charge, will then feed the others to maintain constant voltage. So all batteries should reach empty and full at similar times.

 

[TimE]

If it’s pulling equal amounts of power from each you should deplete the 100Ah battery and still have 180Ah left in the larger battery this is exactly what should happen you should treat it as a 300Ah bank

That is not correct. Capacity drop is controlled by the common voltage

 

[TimE]

It seems that I’m doing some wrong then? My 2 smaller ones get charged discharged first with all the power going to them, then when they get about 80% power starts to go to the bigger one. Then when the small ones get fully charged all the power now goes to the bigger one. Any idea? I measured with a tool and I think it’s because the bms from each one isn’t doing 100% accurate voltage measure and bms dictates how the power goes to the battery. Right?

If you are using the same size cables of the same length on all three batteries then you have created a lower resistance path to the two smaller batteries. The cables on the smaller batteries will be carrying less amps therefore have a lower resistance.
I wouldn't worry about it. I have a 280Ah and 105Ah battery in parallel. I see the smaller battery get charged first but it backs off the amps going in to zero when the larger battery is still charging. It doesn't last long. Since I rarely fully charge my batteries, its not an issue at all.

 

[timselectric]

Hello all. I was wondering if you could help me with a question. I have 3 Lifepo4 batteries 48v: 100ah, 100ah, 280ah. They are all connected to a common bus bar with MRBF fuses for each battery and T-class fuse between bus bar and inverter. They are working so far for about 1 year now. However, I notice that the two 100ah always get charged/ discharged before the big one. Why is that? All have same length same gauge wires. Maybe loose connections on the big battery? Please help.

What you are seeing is normal.
I'm sorry, but You're going to have to find something else to worry about.

 

[TomC4306]

If it’s pulling equal amounts of power from each you should deplete the 100Ah battery and still have 180Ah left in the larger battery this is exactly what should happen you should treat it as a 300Ah bank

Wrong

 

[740GLE]

What kind of load is the bank supplying? 50a or 300a?

Do you need the combined BMS current of the three batteries to meet the load?

 

[wpns]

Wrong

"That's not even wrong".

Listen to @Ellcon123 , if they all discharge fully you'll get the 480AH, and it'll take 480AH to charge it back up. The individual batteries SOC may vary on the way up or down, but it'll all work out. Just make sure you spend some time fully charged so all the cells can balance occasionally.

 

[TomC4306]

Actual testing...

 

[LydMekk]

Post a photo of your physical battery cabling.
Between all batteries and to the inverter(s).

 

[Anteclansing]

Thank you everyone! You are all great and I really appreciate it. Here are some pictures of my set up at the busbars. The cable is 1/0 welding cable. My load is usually around 5-6kw and inverter is split phase 10kw. Most I pulled is about 8kw but for a short time only. As you can see the different bms display vastly different voltage. It gets worse when solar charging kicks in with 7kw of power. I just used a meter to check the voltage at each battery terminal and it seems that they are the same bms just display the wrong voltage on the big battery. I guess I shouldn’t worry too much? The lowest voltage is the big battery bms. I can’t upload the big on the positive it says image size is too big but it’s the same just with MRBF fuses and t fuse and battery disconnect.

 

[740GLE]

Thank you everyone! You are all great and I really appreciate it. Here are some pictures of my set up at the busbars. The cable is 1/0 welding cable. My load is usually around 5-6kw and inverter is split phase 10kw. Most I pulled is about 8kw but for a short time only. As you can see the different bms display vastly different voltage. It gets worse when solar charging kicks in with 7kw of power. I just used a meter to check the voltage at each battery terminal and it seems that they are the same bms just display the wrong voltage on the big battery. I guess I shouldn’t worry too much? The lowest voltage is the big battery bms. I can’t upload the big on the positive it says image size is too big but it’s the same just with MRBF fuses and t fuse and battery disconnect.

Is that a washer under that non tinned copper lug?


Do you have washers under all the lugs?

 

[Anteclansing]

Is that a washer under that non tinned copper lug?


Do you have washers under all the lugs?

Yes. There is a washer underneath the lugs. Is that a bad idea?

 

[Lt.Dan]

Yes. There is a washer underneath the lugs. Is that a bad idea?

If it's a copper washer it's not that bad, if it's stainless, brass, steel or anything else, then you are creating a HUGE resistance bottleneck, which will generate a LOT of heat. We have seen it start fires here on the forum.

 

[740GLE]

What bus bars are those? Do you have a link?

Even if it was copper washer, I wouldn’t run it due to reduced surface area of the full lug to a reduced area of a washer.

 

[underdog5004]

What bus bars are those? Do you have a link?

Even if it was copper washer, I wouldn’t run it due to reduced surface area of the full lug to a reduced area of a washer.

BlueSea is a reputable brand for bus bars.

Victron makes a bus bar with fuses (go for t-class fuses), limited number of lugs, but they're made to be added onto easily. Think it's called "Power in".

 

[Anteclansing]

If it's a copper washer it's not that bad, if it's stainless, brass, steel or anything else, then you are creating a HUGE resistance bottleneck, which will generate a LOT of heat. We have seen it start fires here on the forum.

Thank you! Will switch them out very soon.