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Verification on NiMH pack

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damo:
Hi EEV peeps,

I just need a quick verification on the internal impedance of a NiMH pack. Its a 14.4V 3Ah pack made from 12 cells about 1/2 the size of an 18650 form (not sure on the exact form, short and stumpy lol). I calculated the internal resistance to be 5 ohms using a 20ohm 20W resistor using the standard voltage divider formula (drop of around 3V between measurements). This figure seems to me to be very high. Can someone verify that this is high as I thought the typical internal resistance of a NiMH was in the tens to hundreds of milliohms region? The device is exhibiting some of the signs of a dead pack, but before I fork out the \$70 for a replacement... you get the gist.

For reference, it's an iRobot Roomba a device I have never worked on.

amspire:
Are these Sub-C batteries, like ebay item 180652839961?

Anyway 5 ohms is way too high, but if you have just charged them, the voltage can drop significantly when you first apply the load.A more reliable test would be to start with the 20 ohms load, and then add another 50 ohms load and measure the voltage change. If you have 15V out and adding the 50 ohm resistor causes the voltage to drop by 0.05 volts, then the resistance is 0.05 volts / (15v / 50) = 0.05/0.3 = 0.01666 ohms.

When you said the voltage dropped by 3 V, what did it drop to? If the batteries were charged, then with a load, the voltage should be over 15 volts with the load.

Now the battery will behave very poorly if one of the cells is shorted, as all the other cells will be overcharged, and the voltage will typically drop very quickly from the charged voltage to maybe 14.5V. If one of the cells has failed, you need to replace the whole battery.

Richard.

damo:
Thanks ;D Thats what I needed.

IanB:
When you measure the internal resistance of a battery with a single resistor, the best technique is to alternately connect and disconnect the resistor while watching the voltage. Connect the resistor for a few seconds, wait for the voltage to stabilize (record it), disconnect it, wait for the voltage to stabilize (record it), and repeat. Continue with the connect/disconnect procedure until you obtain a fairly stable and repeatable step up and down between voltages.

As Richard mentioned, freshly charged NiMH cells should deliver above 1.3 V per cell under a moderate load, although this will drop towards 1.25 V per cell before too long if the load is maintained. 12 x 1.25 V would give you 15 V. Lower than this after a fresh full charge and you have signs of a fault.

NiMH cells almost never fail shorted, they just tend to go high resistance. In good condition a sub C cell should have an internal resistance somewhere in the 10-100 milliohm range.

damo:

--- Quote from: IanB on February 20, 2012, 01:13:53 am ---When you measure the internal resistance of a battery with a single resistor, the best technique is to alternately connect and disconnect the resistor while watching the voltage. Connect the resistor for a few seconds, wait for the voltage to stabilize (record it), disconnect it, wait for the voltage to stabilize (record it), and repeat. Continue with the connect/disconnect procedure until you obtain a fairly stable and repeatable step up and down between voltages.

--- End quote ---
Thanks Ian. This is exactly how I measured it.

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