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bronsonj · 11-27-2018, 05:07 PM

Please forgive/correct anything that is way out in left field. This is my first lithium powered project for anything bigger than my cordless tools...

I get what you're saying. I looked for BMS systems that transferred energy and the cheapest one I found was over $800 (before all the add-ons) for a BMS that would handle 14 cells. Seems 14S is an odd size, I found many 12S and 16S in many places but 14S was different. I'm new to the BMS world so if the BMS didn't state they could handle fewer cells I have to assume it can't, I know some can handle fewer cells but I don't know if that's standard practice among all BMS modules.

FWIW, I bought the 58.8v lithium and 16.8v lithium aux battery for around $700 total (I can look up actual amount, but close enough for now). I figure spending more on the BMS than I spent on the battery didn't make much sense. I also wanted a BMS I could set the max Vpc to be lower than 4.2v and most like to fully charge the battery and don't actually balance the individual cells until very close to full charge. So I'd see a BMS that would charge to 4.2vpc and not start the balancing until 4.18vpc. I didn't like that because I want to terminate the charge at 4.1vpc.

Since the max is 58.8 and the cells are all within 0.01v of each other I set my mental cutoff to 57.6v (4.1vpc) and in practice I normally cut the voltage at 56.6v with the new charger. If I took the cells to 3v each, that'd be 42v pack. I set my controller to cutoff at 45.5v. This gives me a pretty good margin of safety on the charge and discharge.

I routinely check the cell variation and they all are staying within 0.01v thus far.

So I do agree with you that a more expensive BMS would solve these issues. I haven't seen one that I consider to be a reasonable price yet so I give myself a buffer at the charge and discharge cutoffs.

11-27-2018, 05:07 PM	#8
bronsonj Gone Wild Join Date: Jan 2013 Location: Saint Petersburg, FL Posts: 2,089	Re: Another Voltage question.....Lithium Please forgive/correct anything that is way out in left field. This is my first lithium powered project for anything bigger than my cordless tools... I get what you're saying. I looked for BMS systems that transferred energy and the cheapest one I found was over $800 (before all the add-ons) for a BMS that would handle 14 cells. Seems 14S is an odd size, I found many 12S and 16S in many places but 14S was different. I'm new to the BMS world so if the BMS didn't state they could handle fewer cells I have to assume it can't, I know some can handle fewer cells but I don't know if that's standard practice among all BMS modules. FWIW, I bought the 58.8v lithium and 16.8v lithium aux battery for around $700 total (I can look up actual amount, but close enough for now). I figure spending more on the BMS than I spent on the battery didn't make much sense. I also wanted a BMS I could set the max Vpc to be lower than 4.2v and most like to fully charge the battery and don't actually balance the individual cells until very close to full charge. So I'd see a BMS that would charge to 4.2vpc and not start the balancing until 4.18vpc. I didn't like that because I want to terminate the charge at 4.1vpc. Since the max is 58.8 and the cells are all within 0.01v of each other I set my mental cutoff to 57.6v (4.1vpc) and in practice I normally cut the voltage at 56.6v with the new charger. If I took the cells to 3v each, that'd be 42v pack. I set my controller to cutoff at 45.5v. This gives me a pretty good margin of safety on the charge and discharge. I routinely check the cell variation and they all are staying within 0.01v thus far. So I do agree with you that a more expensive BMS would solve these issues. I haven't seen one that I consider to be a reasonable price yet so I give myself a buffer at the charge and discharge cutoffs.