Testing Cells
 

Hey Guys,

As you can tell from my signature I have a lot of questionable cells and I was wondering how to test them.

I took apart 1 fully charged pack and voltage is the same on each. Is there a way to test the capacity.

The packs still work, but VERY short run time.

Thanks,

Some chargers allow you do discharge cells/packs. The Duratrax ICE is one that has a LCD display that could show you the capacity, and there are lots of other chargers that can do it.

Thanks Metalman
 

Bummer my charger is not digital. I wonder if radio shat has something cheap that might do the same thing? I definitely do not want to buy another charger. I could use that money for a Lipo charger and head down that path.

If anyone knows of an inexpensive <$30 please let me know.

Cheers!

:eek:

More than likely you won't find what you are looking for at Radio Shack. If you have a way to discharge cells right now, you could hook a watt meter inbetween the cell(s) and the discharging device.
http://www.powerwerx.com/product.asp?ProdID=3809 It tells capacity, voltage, amps, and watts.

Instead, you could buy a Duratrax ICE, which is a Lipo charger as well as NiMh and NiCd.

Thanks Metalman! I will be getting the ICE after next pay day!

I am pretty happy with my ice biga bike. You need a power supply to run it though. Just to let you know. Mike has some in the store as well. it is not an ice though.

Well, a cheap option would be to measure each cell unloaded (which you did), and then get like three or four RadioShack 0.47ohm resistors and wire them in parallel. Four resistors (0.1175 ohms) on a 1.2v cell will draw a little over 10A. Measure the cell voltage again under load. Make sure you test each cell individually! Putting that load on a whole 6 cell pack at once would heat up those resistors pretty quick and can burn you.

Once you get all the loaded vs unloaded measurements, you can make sure one cell isn't dropping more than the others under load.

Like I said, it's cheap - well under your $30 limit. A little tedious though.

I have a pack that is showing resistance on my charger. Is this a sign of a bad cell.

All cells will show some resistance. The lower the better. The higher the resistance on a cell means less voltage will be available to the motor under high currents since more will be dropped across the cell itself (and will heat up). You generally want resistances at, or lower than, .005 ohms per individual cell. If all your cells show about the same resistance, then you're fine. If one cell has a substantially higher value than the others, then you might have a bad cell. It depends on how much higher the resistance is. .001 ohm difference it OK. .01 ohm difference is not. Since cells in a pack generally are series connected, you'll just see the summed resistance from all the cells of the pack and could be affected by one bad cell.

With the simple method I stated in my previous post, you can calculate how much resistance there is on a cell:

CellResistance=(Vunloaded-Vloaded)\loadedCurrent

To get a better (more accurate) calculation, you really don't want to use a truly unloaded measurement. You'd want to load it with about 1-2 amps to get some type of drop. Then:

CellResistance=(vLightLoad-vHeavyLoad)\HeavyLoadCurrent

To get the "HeavyLoadCurrent", you'd simply take the measured voltage at the heavy load and divide by the test load resistance.

Unfortunately, the loading curve of a cell is not truly linear. You'd have to measure and calculate values at several increments of load - like at 5A, 10A, 20A, etc.