Hi All,
Some time ago, I decided to build my own UPS. Simple reason for this was that all my devices can run off 12V and 12V UPS are either expensive or crap.
Anyway, along the way I knew I’d need a BMS. On researching the topic I discovered DIYBMS and then found someone selling a 6S kit fairly cheap, so jumped at it.
That was a long time ago, but today I had time to build a 10P6S pack and realised I might have made a fatal error.
So the basic setup is following the block diagram below. (I know it’s terrible but it gets the idea out).
This would basically act like an always on UPS. The DIYBMS could dictate when it would charge from the mains and could shut down the output if one of the cell packs gets too low.
Apart from this being overly complex (in my opinion) for a little UPS, I think this should have worked fine (assuming I’ve understood how the DIYBMS works).
However, I completely forgot the DIYBMS itself needs to be powered! This leads to needing to source 5v from somewhere. So now I’ve ended up with this:
Yup, the only way I can see this work is to add a secondary BMS that feeds a Buck regulator to switch off the DIYBMS if the voltage gets too low…
This also has the added issue that I’d need an additional switch to override Relay 1 if DIYBMS gets kicked off due to low voltage. This would be needed so I could get the battery voltage up high enough to re-enable the output of the undervolt BMS…
Do I need to rethink this whole thing, perhaps removing the DIYBMS and going for a simpler solution? Any input on this is welcome. I started this whole thing over year ago and never actually stopped to think about how this system would work, just picking up parts as I found them cheap. I now think this whole thing is just a bad idea…
Thanks for any input.
J