Bus Bars, Parallel, Dissimilar Capacity

[BroomJM]

I know when you connect two batteries in parallel, they should be the same voltage, and you should verify they are at roughly the same state of charge so there isn't a huge inrush of current from one battery to another. In my research on this, it seems that the voltage is crucial, but state of charge isn't necessarily a big problem?

My question concerns using bus bars and connecting batteries with a dissimilar capacity.

If you have quality external bus bars, rated for 500 amps, and you connect two batteries of the same voltage, (say they're both at 13.2v) but one of those batteries has a capacity of 100ah and the other has a capacity of 200ah, will they still attempt to balance out their state of charge? Is the common advice to only connect batteries in parallel if they are the same voltage and similar capacity still applicable, if those batteries are connected to external bus bars and not directly to one another? Is that a distinction without a difference?

 

[littleharbor2]

I wouldn't mix battery sizes on bus bars because you are essentially connecting them together, in parallel. Now you have charging imbalances being the smaller batteries will fill up sooner, the larger ones later. Your charging source(s), working on voltage will decide on SOC when the larger batteries aren't full and, possibly, the smaller batteries are overcharged.

 

[DIYrich]

Batteries should be same chemistry. I have connected six 5kWh batteries and one 14kWh battery. To the busbar, the six are in sets of 3, so at the busbar they are close. In use, they seem to draw proportional.

When connected to busbar, they will balance to same voltage. That is slightly different than SOC.

 

[BroomJM]

I wouldn't mix battery sizes on bus bars because you are essentially connecting them together, in parallel. Now you have charging imbalances being the smaller batteries will fill up sooner, the larger ones later. Your charging source(s), working on voltage will decide on SOC when the larger batteries aren't full and, possibly, the smaller batteries are overcharged.

This has always been my understanding, as well, but if the battery with lower capacity is topped off, and the BMS cuts off charging, wouldn't the current supplied to the bus bar continue charging the higher capacity battery until its BMS stops allowing it in? If your solar array is still live, and your solar charge controller is still presenting current at the bus bar, wouldn't the large battery keep using it as long as it was needed? We know that when both batteries are full (irrespective of capacity) the SCC still presents current, but neither battery utilizes it, right?

Is there something I'm missing?

 

[Danke]

 

[BroomJM]

"(Safe to do, but not the best)"

I guess that about explains it.

 

[TomC4306]

I wouldn't mix battery sizes on bus bars because you are essentially connecting them together, in parallel. Now you have charging imbalances being the smaller batteries will fill up sooner, the larger ones later. Your charging source(s), working on voltage will decide on SOC when the larger batteries aren't full and, possibly, the smaller batteries are overcharged.

Not true with a constant voltage charger.
Which any decent mppt is.

 

[littleharbor2]

This has always been my understanding, as well, but if the battery with lower capacity is topped off, and the BMS cuts off charging, wouldn't the current supplied to the bus bar continue charging the higher capacity battery until its BMS stops allowing it in? If your solar array is still live, and your solar charge controller is still presenting current at the bus bar, wouldn't the large battery keep using it as long as it was needed? We know that when both batteries are full (irrespective of capacity) the SCC still presents current, but neither battery utilizes it, right?

Is there something I'm missing?

Ok first of all I didn't know you were using lfp batteries. The BMS shouldn't be used as a charge controller. Its the last line of defense to protect your battery. Again the voltage at the bus bars will be an average of different battery voltages, not necessarily the true voltage of any one battery. I don't think you are going to kill any of your batteries doing this but I wouldn't recommend doing so. With lead acid I would definitely say NO.

 

[BroomJM]

Ok first of all I didn't know you were using lfp batteries. The BMS shouldn't be used as a charge controller. Its the last line of defense to protect your battery. Again the voltage at the bus bars will be an average of different battery voltages, not necessarily the true voltage of any one battery. I don't think you are going to kill any of your batteries doing this but I wouldn't recommend doing so. With lead acid I would definitely say NO.

Yeah, I wouldn't even think of doing this with lead acid...I should have specified LFP and individual BMS's for each battery in my original post. Per the video posted of Will doing exactly what I asked about, it seems that you can do this, but you really want to be precise with your cable runs, to/from the bus bars, AND make sure you have proper fuses or breakers, for a worst-case scenario. He said if you aren't working any one battery really hard, this should not be an issue, to use batteries of the same chemistry and voltage, but somewhat different capacity.

I was asking about this because I'd like to build a 48v system and wanted to understand how you would go about adding more capacity at a later date, should you decide you don't have enough for your current or future loads. This alleviates my concerns.