maanantai 14. huhtikuuta 2014

Adventures in bottom balancing

Here's my bottom balancer - take one. The 50W 0.2ohm resistor is attached to a rather large CPU heatsink from a Dell 2U server, complete with heatpipes and thermal paste. Looked sort of nice and ready rock.

The first issues were the little clips heating up rather considerably, so I took them out. Next it seemed like the resistor was getting pretty hot too, so I added a little fan to cool the heatsink. It seemed ok. Also the voltage measured by the device was a lot off compared to a separate voltmeter. Turns out some the connections and the cell fuse were heating up and thus contributing to the error in measurement.

When I got back a bit later I found out that the resistor had given up the ghost. The device was activated, but no amps were drawn. The resistor would have needed more cooling than the little fan was able to produce. Luckily I remembered the rather large heating element from a Citroën C5 which I had purchased earlier to heat up the cabin, but which didn't give enough heat. I gave it a go and what do you know, it ended up consuming about 12 amps from a single cell. Prefect!

Also with extra fuse removed, heavier cables and unnecessary connections taken away the device also now reports correct voltages. I've set it to deactivate the relay at 2.7 volts, which means that it will drain the cell until 2.7 volts is reached and then starts to go on/off in an increasingly rapid manner as it's trying to get the cell to stay at 2.7 volts.

I'm doing this to prepare these old 40 Ah cells for marrying them to the new 60 Ah cells which should be arriving this wekk.

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