On 3/11/19 7:40 PM, Dan Baker via EV wrote:

I have two really nice 17amp 48v Sevcon lead acid chargers,  would I be
able to use them with these Lithium packs (connected to match 48v of batts
of course)?


You can certainly hook up a Lead Acid battery charger to a Lithium battery bank, and it will charge the cells as long as it's voltage is above the battery bank voltage. Doing so is probably not safe however.

The trick is stopping the charge right when the cells are full, but not overcharging them. If you overcharge a flooded Lead Acid battery, it bubbles a bit of Hydrogen out and the next time you top it up with distilled water everything is good. (This is how they equalize all the cells in a Lead Acid battery...they overcharge them all until they all bubble....)

If you did the same to a lithium ion battery, it may burst into flames very energetically, so your charger must be able to detect the voltage level of every cell and stop charging the pack as soon as any cell reaches it's "maximum" voltage.

Typically a "12 volt" charger will go up to 14.6 volts for a while to equalize all six of the 2 volt cells in a lead acid battery, and then drop down to 13.6 volts to maintain (float) the charge.) In the same way, a "48 volt" charger may actually go up to 58.4 volts.


You need to make sure that your "48 volt" lithium ion batteries can safely be charged up to 58.4 volts (and that there is some way to make sure that none of the cells are out of balance, because if even one cell is too high, it may burst into flames....igniting other cells.....)

So, if you have a BMS that can turn a relay to turn off your chargers...you probably could use those chargers to charge your batteries safely. As long as the relay/bms worked every time.

Typically, a charger dedicated for LiIon batteries allows you to program a specific stopping voltage (or comes pre-programmed for the appropriate voltage of your pack.) They sometimes also have other safety features such as automatically stopping after a set time limit, or a set number of amp hours, or watt hours have been delivered, even if the voltage hasn't reached the stopping point. They also usually have some way to interact with a BMS (or a built in BMS) that allows them to stop charging if any single cell within the pack goes above the max voltage per cell.

Jay
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