|Aug 08, 2002, 11:11 PM|
United States, CA, Hidden Valley Lake
Joined Aug 2002
Battery Testing (Loading)
The best way to test a battery is to put a load on it and see if it maintains its voltage.
How much load?
My first guess would be so that it draws at its rated load. A 10 volt 1000mAh battery pack would use a 10 ohm resistor, r = e / i. Or would this be to much load?
Radio Shack sells battery checkers that put a load on the tested battery but I don't know how much load. What if your voltage isn't listed - a 4.6 volt receiver pack, useing the 6 volt setting could introduce a 25% error. The problems with these testers is the varied size, voltage and current, packs that are used by modelers.
This is brought about by my receiver battery going dead on the second flight Sunday (fortunately we were on approach when it happened). First test said fair, loaded test said DEAD.
I'm thinking about building a loader to put between my VOM and the battery. What are some of your thoughts and expieriences?
Thanks for your help,
|Aug 09, 2002, 02:33 AM|
N. Staffs, UK
Joined Jan 1997
Batteries don't have "rated loads" in the way you're describing. 1000mAh doesn't mean 1000mA is the "correct" load it just means it will last 1 hr at that current or 10 hrs at 100mA or 1/10th hr at 10A etc.
Ideally the load would draw the current you're going to be using but that's not easy to calculate. For standard Rx/servo setups it seems to be conventional to use a load of around 250mA but it really doesn't matter very much. What's important is that the battery is doing something.
For a 4.8V Rx battery something like 22ohms is fine. Get one of at least 2W rating or it will get very hot very quickly. You can use this value with other voltages too, it will just draw a different current. For higher voltages you may need to increase the wattage.
|Aug 09, 2002, 12:18 PM|
A load of 2C will discharge the battery in something less than 1/2 hr if it was "full" and all cells are good. That is, a 10 cell pack will drop to less than 9V, a 4 cell pack to less than 3.6V (0.9 x 4 = 3.6). Sooo, with a 1000mah battery, a 2C load would be 2A. This would be 20W that you need to handle. Get a resistor that will handle at least that much.
A battery with problems is going to show a distinct voltage drop well before the 1/2 hour passes when the "bad" battery poops out. If you see a sharp drop in V before 25min, measure the V of each cell while under load to find the bad cell.
|Aug 09, 2002, 12:42 PM|
If you expect to see the rated mAH of the cell, then the load should be C/5 with a 1.00V per cell cutoff.
If you wish to test for practical performance then the load should be the same as what the cell would experience while being used in your application.
If your needs are best met with the latter, then I suggest you do both measurements.
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