Re: [electricboats] LifePo4 build questions

Great advice.

As for a single BMS being "the only way to balance cell voltage across cells", it's not exactly the case.

Let's use my boat's pack as an example.

Battery configuration: 20P(12S2P) for 700ah @ nom. 42v

That's 480 cells, 240 cell pairs.

BMS: 20 remote BMS cards, one for each of the paralleled strings.

Each of these manages its own series string of cells.

Alone, these BMS subsystems do not ensure against over- or under-voltage, but they do ensure (if powered), that the cells in any given string track each other, leaving cells balanced.  And given the 20 modules are paralleled, the cell voltages for the entire pack end up all tracking, even without having a master controller watching constantly.

For automatic charge/discharge over/under voltage protection, a system controller would be used to monitor these BMS boards and manage charging/discharging.

And yes, then a single system would be used to manage things.

 

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of Randy Cain
Sent: Sunday, May 8, 2022 4:29 AM
To: electricboats@groups.io
Subject: Re: [electricboats] LifePo4 build questions

 

Definitely build a single 36 volt pack with a single BMS. That's the only way to balance cell voltage across all the cells. To determine the the number of cells, divide the charger's open circuit max voltage by 3.5 and then round up to the whole number. I recommend a JK-BMS with active balance. It's a-ok if it supports more cells than you need (eg, a 16S BMS for a 36v 11S BMS battery). I suggest going over your anticipated current requirements by 100% so that the cells and BMS is not stressed during heavy loads.