I had a conversation with a fellow EV enthusiast about how the Battery Management System (BMS) that I am building works. I am planning on building a 76.8 volt, 40 Ampere-Hour battery pack as the first pack for my EV. Our discussion was really educational so I’ve decided to post it here to share with other people.
— snip
Q: So here you have 72 volt x 40 a-hr. Cell count is 24 x 4 = 96 Cells Don’t you need 4 of the x24 BMS boards, for the above configuration?
Nope, only need one of the x24 BMS. If the cells were Nickel then the answer would be different. Check out the attached diagrams. One shows the preferred cell connections for Nickel systems, the other for LiFePO4 systems.
Battery Diagram LiFePO4
Battery Diagram - Nickel
The BMS doesn’t connect to each individual cell – it connects to each parallel set of cells. LiFePO4 is different from Nickel chemistries in that it prefers that cells in a pack be parallel connected before each parallel block is then series connected. Nickel requires the opposite. Nickel heats up rudely when in use if one cell in a parallel set has more capacity than it’s neighbor. LiFePO4 don’t have that problem – they politely balance to the same voltage without much heat dissipation.
When building parallel packs of LiFePO4 care has to be taken to make sure the cells are not at different charge levels before connecting them. If one cell reads 3.45v on a volt meter and the other reads 2.2v then you should fully charge the 2.2v cell before connecting them to build a parallel block of those cells. The best practice is to fully charge each cell on a single cell charger before connecting them. This is how I built my 5p blocks of 18650s.
The BMS in these situations is balancing the charge voltage across these parallel sets of cells. I can use my BMS on a string of 24 single cells ”(1p)x24s”, a series string of 24 of pairs of cells ”(2p)x24s”, a series string of 24 quartets of cells ”(4p)x24s”, or octets ”(8p)x24s”.
The trade off for the larger packs “(4p)x24s” and ”(8p)x24s” is how long the BMS takes to balance the cells at the end of the charge cycle. There are two variables here: how out-of-balance the blocks got while in use (because of physical differences in the cells that make up the different blocks) and how much current the BMS can bypass for a fully charged block when it is bringing the other blocks up to the same voltage. The BMS I am using does a maximum of a 1/2-amp bypass current.
Let me explain how the BMS balancing works with a hypothetical. Suppose I have a 12v battery pack made up of 4 blocks (exactly like in my LiFePO4 diagram). Suppose the third block of cells in the batter is “weaker” than the other 3 and after use has a lower voltage. If I read the voltage across each block I might read values like: 3.31, 3.30, 2.87, 3.29. The 2.87v reading is clearly the weaker cell. When charging, the BMS system makes sure that each block is charged up to a “full” 3.69 volts before turning off the charger. Blocks 1, 2, and 4 will get there first, while block 3 will need more time and current to get up to the 3.69v charge level.
When the strong blocks are “full” at 3.69v the BMS cuts the charging current to 1/2 amp and routes the power around the “full” blocks and keeps applying power to the weak block. If the weak cell only takes 1/4 an amp-hour to fill up then the BMS will finish balancing the pack in 30 minutes. If the weak block takes 2 more amps-hours of current to fill back up to 3.69v then the BMS will take 4 hours to finish balancing the pack. With larger packs the BMS will take longer to maintain a perfectly balanced pack because the potential for each block to be slightly different in capacity is higher.
People in the field who are using this BMS report that after the initial balance they rarely see long balance cycles. Here is a link to a discussion thread by a guy who is running the BMS with LiFePO4 cells (different cells than I am using, however) on his e-motorcycle. It has great photos of his assembly process and wiring diagrams. He is using a product called a “PakTrakr” that is basically a set of volt-meters that record to a file so he can graph the volts from each cell under load. He also has great graphs of the voltage during charging with the BMS.
— snip
–adam
