T-Pain Is On A Boat: [Electric Boats] Battery diagnostics

Tuesday, June 23, 2009

[Electric Boats] Battery diagnostics

I'd like to add a section on my site about troubleshooting - particularly battery/range issues. But this seems like it's pretty much dark magic.

To make it particularly tough I have a complex pack for both space and charging issues. It's 4 sets of (3 parallel) batteries cross-connected in series for 48V. From reading it seems this is sub-optimal but I'm not clear exactly on why (charge imbalance is the stated reason). I charge each set as a if it were a single 12V battery. One issue I suspect I face is sulfation because my charger output is so low (6amp) in comparison to the size of the set (345amp) - the batteries are sitting around in a discharged state for probably 3-4 days.

I recently have been having trouble with greatly reduced range, and rapid voltage sag (like 40 down to 30 volts within 5 minutes). Reduced range I could expect due to sulfation, but the voltage sag was more troubling.

First I checked water - I found a dry cell in one of my batteries a few weeks ago and thought that was the cause, but I dropped 1/3 of my pack (so now I am 4 sets of 2), that cell included, and it didn't help.

Next I ran a load test after charge - couldn't see anything. (Sears has a nice 130amp load tester for about $50)

Then I ran a load test after dropping the batteries pretty low. Bingo - 1 battery showing a dramatic difference. Pulled that battery and now I've got a range much closer to the predicted value (about 10AH down from "ideal"). And that's a string that has the "dry" cell battery in it (after refilling)

Other than "divide and conquer", load testers, specific gravity and voltage meters - what other tools, strategies and processes can be used to diagnose battery issues?

For example, I hear about "shorted" cells - what does that look like?

Finally - let's say you find one bad battery as I did. Are there any strategies for adding it into an existing "aged" pack. All the stuff I've found basically says "don't do it"

Thx

-Keith