In parallel you get one larger ah battery at the same voltage, in use they should both decrease in voltage at a similar rate and within 0.01-0.03v of each other (all cells being equal). If one battery for any reason shuts down the other will still work. Charging after use as you say the voltage should be the same occasionally worth checking though as it only takes 2 secs to test. But remember charge each battery independent of the other disconnected from the Y lead.Maybe I didn't explain to them properly what I meant.
So, all you are doing is joining two batteries together with a Y piece? When the first one runs flat, the second kicks in....or they just make one big battery? If you then charge them both to full after each cycle, they should be at the same voltage?
I use lipo with out bms and will connect either 6.6ah or 5.2ah packs together 3,4 or 6 at a time to have a larger discharging pack all connected using parallel triple/double Y leads for the most part they stay within 0.3v of each other and only balance them if they are over 0.05v out.
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