VOM as instrumentation??

Joined
Oct 9, 2008
Messages
11
I just installed an ampedbikes 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 pingping 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.
 
You can indeed use two of those meters to measure volts and current, but you better not draw more than 10A thru it when doing it, and that for not very long:
dsc01729.jpg
dsc01733.jpg
That's what happened to mine after a while; the solder melted from the shunt heat, then the shunt fell out at one end. ;)

Using just a voltmeter to determine state of charge (SOC) on Lithium is difficult, because it has a relatively flat discharge curve, usually. So it will appear to stay at around the same voltage for most of it's charge, then suddenly drop off at the end. When it drops off, you're done using the motor and need to charge it at that point.

I doubt that the lights not working as described has anything to do with the difference in battery chemistry, as long as it is the same voltage it shouldn't matter.
 
thatguitarguy said:
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?

Well, if you hook it up as a straight voltmeter, you will get a lot of useful info. Voltage will vary with load, but when you are off the throttle (i.e. zero load) you will see the resting voltage of the battery. This is a good indicator of charge level. For example, with lead-acid 12 volt, 12.5 is close to fully charged, 11.5 is close to empty.

If you want to use two, I'd recommend using a shunt resistor (for example a .01 ohm resistor.) Put that in series with your motor and then measure voltage across it. 1 amp =.01 volts, so for example a reading of .22 volts would mean 22 amps. This will tell you what kind of loads you are putting on the battery.
 
You can use that voltmeter to measure the pack voltage. The change in voltage is not great between full and cutoff, depending on your controller and BMS cutoff volatage it might range from 38 to 32 volts. Under load it will drop near the end of the range. You can get experience and see how it reads and get a pretty good idea. Don't try to run any current through meters like this, they are made for low currents only. For current you have do what was mentioned, measure the voltage across a low ohm resister or better yet a shunt made for a panel meter, these are not too expensive at electronics supply places. That can get messy though, so you might want to wait on the current measure until you can afford a Turnigy (25+ship) meter, a stripped down cycle analyst. Also, you can't power those hand held meters (even with a DC to DC) on the power source you are measuring, it has to run off it's own battery.
 
Although you would want to put that shunt in series with the *battery* to the controller, not in series with the motor, since motor currents can be way higher than battery currents, depending on throttle / duty cycle. :)
 
Well, I found a good deal on a used Watts-Up meter, and though it's not in my budget, I splurged. I certainly can't afford to buy a new battery pack, and this one has been sitting waiting for it's chance to shine.

What do I need to know about wiring up the meter? Is there a shunt that I need somewhere in the circuitry? Or come other kind of protection?

I had basic electronics many years ago, so I'm not that knowledgable, but I can use a soldering iron, and do fairly decent job if things aren't too small.
 
Wire your main battery power, after the fuse and switch, to the In side of the WU, + to + and - to -. (red to red, black to black, most likely).

Wire the WU Out side to your controller's power input, + to + and - to -. (red to red, black to black, most likely).

That's it. The WU will monitor everything that happens once the power is turned on at the battery.

If you want to monitor charging as well, then put Andersons (or your favorite type of connector) on the WU, your battery, controller, and charger, so that you can do this:

Unplug WU from battery and controller.
Plug WU In end to charger.
Plug WU Out end to battery.
Turn on charger, WU will monitor power flowing from charger into battery.
 
Back
Top