The BMS Was Supposed to Protect my Batteries?

[Davidasailor26]

Except that's NOT what Maine Sail describes in the first few sentences of his YouTube.
He says "...over 100 volts into a 12V LP battery..."

Yep, definitely 100V from a failed MPPT into a 12V battery, or paralleled group of them, would be bad news. My question is more about the middle of the video (starting around 11:30) where he talks about 4 series batteries charging at 3.65V per cell, totaling to 58.4V, which is concerningly close to the 60V. My thought is each 12V battery’s BMS is still only exposed to the normal 14.6 volts, not the threatening 60V the whole bank could reach.

When the charge FETs cut off, even with four in parallel, you still have 85V AFE chips that control the typically 60V FETS. Batts in parallel are all at the same voltage. Some batts use 80V FET's but I've yet to see a BMS, with n-channel FET's, that exceed 100V. Most budget priced batts use 85V AFE's & 60V FET's. Even if you found a batt that had high voltage FET's you'd need to find high voltage AFE's too and drop-ins just don't use high voltage AFE's.

Thanks for the clarification. Maybe I was caught up in the Series aspect. It makes sense that even in parallel the FETs would be vulnerable if they’re rated for 60V and exposed to more than that.

 

[Maine Sail]

IMO, LFP is way past the development and economic stage of replacing LA batteries. It is always difficult to explain the advantages of LFP to people who are using LA, but every single person who eventually converts has a "holy crap" moment and wonders why they didn't do it much earlier.

Mark

Way past! This same issue applies to lead acid, which have zero built in protection, except they can explode and spew acid all over your boat. Lead makes a fine starting batt but even that is going the way of the Dodo....

 

[Maine Sail]

Yep, definitely 100V from a failed MPPT into a 12V battery, or paralleled group of them, would be bad news. My question is more about the middle of the video (starting around 11:30) where he talks about 4 series batteries charging at 3.65V per cell, totaling to 58.4V, which is concerningly close to the 60V. My thought is each 12V battery’s BMS is still only exposed to the normal 14.6 volts, not the threatening 60V the whole bank could reach.

In a 4S config of 12V batts all BMS's must be 60V rated minimum..

Thanks for the clarification. Maybe I was caught up in the Series aspect. It makes sense that even in parallel the FETs would be vulnerable if they’re rated for 60V and exposed to more than that.

The series aspect only serves to define the max safe BMS voltage. Eg: if you had 12V batts that are only safe/rated for 2S /24V and you put four in series you will ruin the BMS's..