BatteryBeak: Utility of making internal resistance and state of charge measurements

[Ether]

Attached is a graph of the discharge curves at 77F for the Yuasa NP18-12B battery, taken from the Yuasa datasheet for the battery.

That battery is rated at 17.2 AH. So the 0.2 CA discharge curve would correspond to roughly 3.4 amps discharge rate. According to that curve, the battery voltage should remain above 11.5 volts at that discharge rate for 2 hours. That corresponds to roughly 6.8 AH discharged.

Now take a look at this CBAIII curve for a battery with a weak cell¹. Note the sudden drop-off in voltage at about 3.2 AH into the test, caused by a weak cell.

125 amps for 15 seconds is only 0.5 AH, so it's not obvious that such a test would identify the bad cell. In fact, I have a lawn tractor battery with a bad cell that passes the 125A test when freshly charged.

If you don't have the budget for a CBA tester, there is a home-brew solution. Go to the auto junkyard and find a nice sealed beam² 36 watt 12V headlight. Hook it up to the battery and monitor the voltage under load for 2 hours³. That's roughly 6 AH. If the loaded voltage drops below ~11.5 volts, stop the test. You may want to mark that battery not for competition use.

BTW, the lawn tractor battery mentioned above (which passes the 125A test) fails the 2 hour test.




¹ reproduced from this post, for those who don't have PDF reader installed

²sealed beam for safety (do not use a replaceable bulb). no cracks or chips or signs of damage. competent adult supervision required. always use safety glasses.

³digital voltmeter with sufficient accuracy can be had for $5 (or less, on sale) at Harbor Freight

[Ether]

Quick pic of poor man's battery discharge tester.

Battery voltage is on the left. Battery amps on the right^*. Meters can be had for less than $5 each at Harbor Freight.

Sealed beam headlight is from my 68 Camaro, I think. Found it in a pile of old auto parts. Not quite 36 watts.

Battery is from my lawn tractor.

For actual use, the wiring should be cleaned up, and the headlight secured safely on a fireproof surface.

sealed beam for safety (do not use a replaceable bulb). no cracks or chips or signs of damage. competent adult supervision required. always use safety glasses.



^*The ammeter is just to confirm the lamp's wattage. You don't need the ammeter for the test if you know the wattage.