Bill Best wrote:Justin, how fast can you charge your batteries? What type of charger are you using?
Hi Bill. Good question. My original goal was to be able to recharge the ebike from dead flat in about 2 hours. A couple years ago I scored a large stash of these Vicor DC-DC converter circuits (model VI-264-20/F1) off of ebay:
The input can vary from 200 to 400volts DC and the output is factory set to 36V at 6 amps, all in a tiny package. I mean really tiny when you compare it to most ebike chargers which only have like a 2A output. By putting a 1MOhm resistor between the Trim and the +S leads on the output side the output voltage was tweaked up to 41.5V. So the original plan was to connect a voltage doubling rectifier out of the wall, rig it up to the input of this DC-DC module, and then feed the output adjusted to 41.5V straight into the batteries, using the internal current limit of the device to make it behave like a 6A 41.5V CC/CV charger.
Unfortunately that arrangement would overheat and go into thermal shutdown within just a minute or two of charging. Apparently even though the output is current limitted to 6A, it is also not supposed to exceed 170 watts, which at 41.5V is more like 4 amps. So, I had to rethink things a bit and wire up a feedback circuit using an op-amp and a current sensing shunt resistor in order so that the Trim pin on the DC-DC converter would be governed to run a constant current output rather than a constant voltage. I don't have time to sketch the schematic right now, but in short it ended up with a potentiometer with which one can dial in the desired current, from 1A up to 5A, as well as orange and green LED's to indicate which charging state it is in.
At 4A it still gets hot and shuts down after about an hour, so either I need to pour water on the heatsink on a regular basis or tweak it down to 3A to make good use.
Anyways, So I have two of these homemade units, one commercial 36V 4A lithium charger, as well as a 36V 4A NiMH charger. So in the case with all four chargers plugged in I can have 14 amps or so of charging current. But having 4 chargers to wire up and connect is a bit of a pain, so I usually just connect 2 or 3 of them. What's the rush eh?