Different age and capacity in parallel is generally a non-concern provided they are connected with best practices (link #6 in my sig).
A new shunt is only needed if you want to know now much each battery is delivering. If there is a single shunt through which all current passes, it should work for the whole bank.
Good morning,
I'm looking to do something similar and figured I should post here before creating a new thread. I'm using LiFePO4 batteries.
I read link #6 and went down the rabbit hole of information, it was awesome.
If I understood the issue of expanding a battery bank with newer batteries correctly: the issue is the older batteries will likely have higher internal resistance and less capacity than the newer batteries.
Best practices to expand the banks:
- obviously ensure same voltage batteries and preferably from the same manufacturuer, model, etc.
- do not wire in series first, then parallel, e.g. 3S2P: here the
same current flows through each battery in a series connected sub-bank. When discharging, the older battery's voltage will drop faster and the voltage of the whole sub-bank drops below a useful level, before the newer battery is fully discharged; the newer battery cannot 'assist' the older battery as in a series connection. When charging, the voltage of the older battery may rise faster than the newer battery and will at the least cause the charge cut-off to occur before the newer battery is full charged, and at the worst go into over-voltage damage (though the BMS should protect the battery). Again to compare to a parallel connection when charging, the newer battery will continue to absorb current until the whole sub-bank arrives at the SOC cut-off. In essence when connected in series, the sub-bank takes on the charaecteristics and limitations of the weaker battery and the weaker battery risks being damaged even further.
- wire in parallel first, then series. e.g. 3PS2, put one old battery in each sub-bank: The benefits when discharging current is that the newer batteries will 'do more work' and provide more current and help out the older battery. Benefits when charging is the newer battery will absorb more current as the whole parallel bank's internal resistance rises as they achieve full charge.
In essence, it prevents imbalances with mixed age batteries.
- wiring: this one seems to be the simplest to understand for me personally if one understands 'Ohm's law i.e. not suggestion'.
* Basically ensure sufficient wire gauge for the current that'll flow between
each connection;
* ensure wire gauges are the same; ensure length of current travel remains the same;
* use best practices when connecting to ensure current draw is as even as possible from multiple parallel connected batteries, there's alot of info in your link #6.
Have I got it down pretty well? I'm particularly interested if my understanding of connecting parallel first then series for mixed age batteries is correct, and that I am not missing something.
Thanks and have a great day!
