Leaf / hybrid voltage conversion

[Jason McCreary]

 No idea if anyone will see this as the group seems rather dead but hello anyway!

After we built a kart in a vacuum more or less for 2019 we got to the makerfaire and realized that a lot of teams had leaf batteries (unlike out lead acid setup).
We asked a lot of questions but what we failed to ask at the time was how people were convertering the relatively low voltage of the single leaf cells to the voltages needed by the boma scooter motors they were running.

Anyone have info on that?
Muuuch appreciated.


Thanks!

[Randy Farmer]

They're 7.5V nominal... string em together. 

[Randy Farmer]

BTW, that's from a powerwall setup, probably one pack too many to be legal in PPPRS as a 48V pack.

[Brian Silverman]

Another note: Those packs are 2S2P internally (there are a total of 4 cells, hooked up with 2 in parallel and 2 pairs in series). Some people (like me) open the packs up and rewire them to 1P internally to get up to reasonable voltages with less cells on the kart.

Not sure how familiar you are with batteries, but keep in mind that you will need a charging system that monitors the voltage of each cell. Battery cells in series will never be perfectly identical (although manufacturers try to match them within a pack), so you have to somehow finish charging some of the cells even though others are already full, or they will get progressively more out of balance. Lead acid cells dissipate excess energy fairly harmlessly, so you can just charge them in series and the full cells will produce hydrogen. There's a limit to how much of this a cell can take over its lifetime, and how fast it can safely do it, so you can only do this with matched cells (which means the charger will have ramped down to a low current by the time any cell is full, and the amount of balancing necessary is small). However, li-ion cells (any chemistry, including all generations of leaf packs) don't do that. They are fully sealed, so they puff up and then vent instead. You need to use either a BMS rated for the size of your pack, or a charger with balance connections to each of the cells. Any number of cells hooked up in parallel will be at the same voltage by definition, so you only need a single connection (for the BMS or the charger) to each parallel set.

The vast majority of li-ion chemistries are charged to 4.2V, so multiplying that by the number of cells gives you the maximum voltage. They all stay around 3.4V - 3.8V for the vast majority of the discharge cycle, with the details depending on the specific chemistry. Typical minimum safe voltages are 2V - 3V. Most of the energy in a li-ion battery is delivered at a voltage towards the middle, but most of the cumulative damage which determines the lifespan happens at the extreme voltages, so some applications won't fully charge or discharge the cells to maximize lifespan (and some numbers you can find online will take those deratings into account, so you may see a variety of numbers for the same battery). Different chemistries also have differing nominal voltages, which is what PRS fuse classes are based on.

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[jesse brockmann]

Is there a guide on conversion this conversion to 1p?  Seems like interesting idea

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