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2 X 24V self build JK 7S40P Frankenstein Batterys, Can they run in Parallel?

ADi-HiDef

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Jan 29, 2022
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Hi there Y'all,
I'm ADi-HiDef, Long time devourer of info, first time poster!
I built 2 x 24v batterys 7s40p with JK B2A8S20P 200A smart bms's controlling parameters.
one BMS was bought just over a year ago and the other about 6 months ago, (the new one seems to have more options in the settings including "Device Address")which the original one does not despite using the exact same App).
My problem being when I connect them in parallel the recent built battery starts charging the original despite being a lower voltage ( voltage was exactly the same when I first tried to parallel them.)
It doesnt seem to matter which is turned on first the same thing happens, I have tried setting the "Device Address" to 1 and 2 and 0, same outcome in all cases.
Am I missing something in the BMS settings ?
Has anyone else encountered a similar problem?
I am attempting to run a PIP4024MT charge controller / inverter with the batterys, individually they work great I have tested them up to 2500w but I have yet figured out a way to have them
work together without the above problem happening.
Any ideas on how to proceed will be much appreciated.
Thanks in advance for any advice,
and for running an awesome forum, thank you Will !
Mucho thank-yous
ADi!24v ADHD9240.jpg
 
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My problem being when I connect them in parallel the recent built battery starts charging the original despite being a lower voltage ( voltage was exactly the same when I first tried to parallel them.)
...
Since it is impossible for a battery of lower voltage to charge a battery of higher voltage* than your older battery is of lower voltage once connected in parallel. Are you reading each batteries voltage with the same meter (DMM and not using BMS indications)? Have you tried placing a small load on each batteries before reading voltage?

* This is without some form of Boost device that raises the lower voltage up to above the higher voltage battery.
 
Last edited:
Thanks a lot Mattb4, your spot on, I tried measuring with the same meter and turns out the first battery I built hed a display that is way out, at least .4v so I thought it was the most charged battery but was in fact a fair bit lower, so as I write I'm draining the higher one and will reconnect when I'm pretty close to same voltage. I have placed 2500w load on both for an extended period so individually they're fine.
Thanks a lot for the response I'll post the results soon, fingers crossed.
 
Problem seems to be solved! The Current wasn't flowing upstream, the JK display and the app values are far from acurate.
so magic can seem to be happening, when measured with a meter (accurate or not) at least I'm getting a consistent reading that makes sense.
Both batteries now connected in parallel and running small loads without a complaint.
now I can get on with testing.
oh yeah and lesson learned, don't go posting questions on the forum before checking everything at least four or five times!
Thanks for your help Mattb4!

I guess now the question is, can they be calibrated,? I'll read a manual or 2 before asking that one.
 

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