After building and using the battery pack for a little while, I started to think of some improvements.
For example, the switch I installed has an ampere rating close to the maximum expected amperage of the load. Rather than a bigger switch, I added a 60A DC solid state relay, controlled by the existing switch. The switch now only needs to carry a few milliamps.
I also wired it so this relay turns everything off except for the charge input.
I noted, however, that the BMS itself imposes a small load on the cells, even when it is not in use. I suspect that it will deplete the cells over a period of time.
And I added a 30A resettable breaker on the battery positive side. You know, just in case. I consider it a check valve for the magic smoke…
As mentioned in the build article, there is a boost converter so I can take a typical 12V supply and have it charge at 14.5v or so.
Well, in practice, that didn’t work all that well. Boost converters will indeed increase the input voltage to the set output voltage, however, if you input a higher voltage, it is simply passed through. Do when I tried an old laptop power supply I had laying around, the 16v did a nasty thing to the BMS. No actual magic smoke was observed escaping the device, but it has definitely failed. Perhaps is contains a smoke-absorbing filter inside :-O
BMS and Boost Converter Replacement
So the better solution is a buck/boost converter, which outputs the preset voltage regardless of the input voltage, whether to high or too low.
The other advantage to arise out of this failure is that I was disappointed with the charge current from the previous converter. It was delivering way less than its rated current. So now I have a reason to find a better converter to deliver higher charge current.
I’ve installed a buck/boost converter rated at 10A. I’ve tested it from 5V to 30V in, and I see a stable 14.5V out. This provides the desired charge voltage to the cells from a variety of sources. I could charge this from a USB battery pack if I had to.
I repurposed a laptop power supply for charging, and its 16v output that is ‘bucked’ down for charging. It is capable of charging at 5A.
I’ve given this a good charging workout and it is looking solid so far.