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Jul 24, 2001, 06:38 PM
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Battery Internal Resistance

Can someone tell me how to get a rough measurement of battery internal resistance.
The Rint of most of my batteries seem to me to be much higher than advertised. Voltage loss seems excessive.

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Jul 24, 2001, 09:09 PM
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You may be interested to learn that Sanyo and Panasonic both rate the battery impedance by a measurement made at 1000Hz AC. Annoyingly, if you measure it at DC (where it counts), it will be higher than the spec sheet. The impedance is also lowest when the battery is charged. You'll need a DC voltmeter and a resistor that draws somewhere in the neighborhood of your load in actual use. (Sorry, you did not state the nature of the battery or the load, so I can't help you here). To get a feel for the impedance, charge the battery then set it aside for a few hours or even a day to allow the voltage to settle. Measure and record the voltage with no load. Then, measure the voltage of the battery with the resistor across it. divide the loaded voltage by the value of the resistor (in ohms). This yields the current. Record this value. Still with me? Now subtract the loaded voltage from the unloaded voltage. This is the voltage difference (or delta) from unloaded to loaded. Divide this value by the current that you previously computed and voila! This is the impedance. Some my argue that the value is best computed from testing at two different load levels and not from unloaded to loaded. They would be correct in this argument, but this simplified method will get you close enough.

Jul 24, 2001, 10:06 PM
Registered User
Thanks Quacker,
I'm using a R/S curve tracer for voltage readings, a Astro WattMeter for current, and either 2 or 4 12v 50W bulbs for a load. This gives me about 7 or 14 amps which I can cut in or out.
I calculated R using your method but for an old 8-pack of 1400SCR's it starts with a resistance of about 9mohms but increases to 35 or 40 as the pack is discharged. The current stays relatively constant as the voltage drops, making the R change radically.
I guess ist's best to just use 'em and not worry about the details too much.

Thanks again

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