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Old 01-10-2010, 03:31 PM   #1 (permalink)
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Maroon Ballon - '98 Chrysler Town & Country LXI
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VOM as instrumentation??

This is a question for people who are knowledgeable about DC electronics and EVs.

I just installed a hub motor on my bike, and the throttle has a 3 LED meter that is supposed to gauge battery level, but so far, as long as the power is on, all 3 lights are lit. The way it's supposed to work is that the green shows full charge, the yellow means the battery is getting weak, and the red indicates empty. They're not all supposed to be lit at the same time. The motor and controller came as a kit, and the battery was a separate purchase, but everything is 36V. I'm wondering if the problem has something to do with the fact that I'm running a 36V 20AH lifepo4 instead of lead-acid.

I would eventually like to get a Cycleanalyst, but it's not in the budget for a while. In the meantime, Harborfreight is going to be putting their cheap volt-ohm meters on sale next weekend for $1.99. That I can afford!

My question is; how can I rig up a VOM as a dedicated "fuel gauge" to monitor the state of my battery, as opposed to idiot LED lights that don't function properly anyway? Would I want to monitor voltage or amperage, or could I get 2 VOMs and monitor both?

Thanks in advance for your shared genius.

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Old 01-10-2010, 09:30 PM   #2 (permalink)
Master EcoModder
 
Join Date: Nov 2009
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Maroon Ballon - '98 Chrysler Town & Country LXI
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After yesterdays shakedown ride of 20 miles on a battery that hadn't been charged in about a year and a half, it still read 38V. I put it on the charger overnight and I didn't pay attention to how many times it went on and off balancing the cells. I forgot to take a reading before I went riding today, but I rode for 23 miles, and this time the motor was on most of the time. I'm pedaling all the time so the motor isn't working as hard as if I were just being lazy. I'm not pushing the motor that hard - I'm just trying to maintain a speed a little above 15MPH. I can do that unassisted on the flats most of the time, but the terrain is more rolling than flat around here.

Anyway, the battery read 39V after all that riding. I sure would like to have a way to know how much time I had left in the pack before it just drops off and I have to pedal this heavy beast back home without any help. I'll keep doing trial and error, but it sure would help if there was a way to just to test how much juice was left, even if I can't figure out a real time monitor. I understand how lifepo4 gives you all or nothing, I'm just trying to stay ahead of the nothing.

Does anybody knowledgeable have any ideas short of paying a bunch of money for a Cycleanalyst or a Wattsup? I just don't have the money for that, and I won't for a while.
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Old 01-11-2010, 12:54 AM   #3 (permalink)
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Just beware that the cheap ones are likely to be affected by noise.
camerahacking :: View topic - Why you should not buy a cheap multimeter

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