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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 12-12-2010, 05:50 AM   #1
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Default Measuring Battery DC Resistance Procedure

I've placed this article in another www.wattflyer.com location. It might be useful under the battery thread.

Many top of the line Lipo chargers provide an indication of the internal resistance of the attached battery. The A123 2300 Milliampere Hour batteries I've been using rate their cells under an AC (Alternating Voltage) test, something that may not be the same as a DC Test.

As it turns out, calculating the battery DC resistance is really not hard to do. All it requires is an accurate DC Voltmeter, and DC Ammeter. Many of the commonly available all in one meters such as the Astroflight Whattmeter will do very well.

What is required for this test is to load your battery at two different current levels, while measuring the batteries voltage at these two different currents.

This test will require a resistive load, such as a bank of 12 volt automotive lights, or a dedicated resistor bank. I prefer the resistor bank, since they can be subjected to load currents and wattages far above their continuous rating for several seconds without damage. Or, another option would be to use your wattmeter, and measure the Volts/Amps on your motor when it is powered up. So, just run your motor at say, 30% power, write down the Volts and Amps, and run it up to 80% or so, and again write down the Volts and Amps.

What I use is a few one ohm, 50 watt power resistors. To guestimate the current one resistor will pull, the formula is I = E/R (or current equals voltage divided by resistance). For a single one ohm power resistor, an 11 volt battery will be I = E/R or I = 11/1 or 11 Amperes. The power output is P=E*E/R or 11 Volts times 11 Volts divided by one Ohm, or 121 watts. That 50 watt resistor can easily handle 121 watts for periods of 5 or 10 seconds or so. (If it gets to hot, put it into a small container of water) If you are using smaller batteries, put three of these resistors in series. Then the current pulled by loading that 11 volt battery will be 11/3 or 3.66 Amps.

Picking up 5 of these resistors, and connection them up in series, or parallel, (or maybe both) will allow load tests from low current values, up to 25 amperes or more. A quick way to do these tests is to hard wire the resistors to a connector compatible with your battery, so it can just be plugged in. (Note that you can't leave it connected, you'll kill your battery! And don't do it with a nearly dead Lipo, that could also kill it before you can pull the plug )

I use big power resistors for this type of test, available from places such as www.digikey.com, and their part number http://search.digikey.com/scripts/Dk...e=FVT50-1.0-ND. Digikey has resistors available up to 150 or 200 Watts, but they get very expensive.

This is a one ohm, 50 watt resistor that will handle 7 volts DC continuously. You can put one, two or more in series or in parallel with your battery for load testing. These resistors will easily handle overloads for short periods of time, like 100 watts or more. Do not drop these resistors. They are ceramic, and will break if dropped.

Note that these resistors will run VERY hot when running at their maximum wattage rating. Very hot means several hundred degrees, you can burn your fingers on them.

Load the battery for 5 or 10 seconds or so at about five amperes. Write down both the voltage and the 5 amp current from your wattmeter.

Repeat the load test at about 20 amps (Depending on what your battery is rated for, don't exceed its maximum rating!). Again write down both the voltage and current.
Now, the calculate internal DC resistance will be the difference in voltage divided by the difference in current.

If you measure 3.62 Volts DC at 5 Amps, and 3.51 Volts at 20 Amps, that is (3.62-3.51)/(20-5) or 0.0073 Ohms.

This is something that can very easily be completed, and once you have the equipment set up, can be performed in far less time than it took to read this thread.
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