battery testing 2

Using the small automatic tester to check car sized batteries takes much longer but gives useful sorting information.

Continuing with the theme of battery testing but on a different scale, I have changed the loading of the battery tester to draw more current. The three large resistors are in series across the load terminals of the tester. The resistors are the three rectangles with my handwriting on them.

[click any pic to enlarge]

The handwriting is the measured resistance in ohms. Since the resistors are in series, the three total 15.7 ohms. Each are rated at 22 watts so the three theoretically could dissipate over 60 watts. With only 12 watts used in this test, the resistors get very hot, too hot to touch, hot enough to boil water. The metal screen helps to cool them.

I would first charge the battery fully with a separate charger and let it rest overnight. A good battery would measure in the range of 12.4 to 12.8 volts. The tester was then set to discharge the battery to 11 volts and the test started.

The tester checks the battery voltage and offers to start with a default of 10.5 volts but I did not want to discharge the batteries that low.

With a freshly charged battery the current drawn at the start of the test is about 850 milliamps (0.85 amp), or about 12 watts. This is a modest rate for a car battery. The tester is specd at a maximum current of 3 amps so this is well within its range. A load of 12 watts might simulate one charging cell phone for example or a bright LED light.

The test takes some time. I managed to check four batteries in the past couple weeks including an almost dead battery that was then returned for recycling.

I expected this battery to test weak. It had been removed from my car three years ago. It would start the car ok in the summer but was not reliable in the winter. This particular battery had continued in service here in a small solar UPS in the shop and true to form, it would last only one night in the dead of winter powering a 12 watt LED overnight. If it was cloudy the next day, the LED would not last till morning.

This battery took almost 14 hours to discharge to 11 volts and gave a result of 10.3 amp hours. I charged it again, repeated the discharge test and got 10.7. So there was some indication of repeatability to the test.

The new battery I purchased (the one in the first picture) is now installed in my car. As expected, the factory fresh battery scored well at 54.5 amp hours. It took three days to discharge!

So I now had a range for good to bad.

Continuing with three other lead acid batteries, I managed to get clear, unambiguous ratings of the battery capacities, all at the same rate.

The "old" battery that I just just removed from my car will now go to the UPS. It tested 37.2 AH.

Video is here.

Thanks for your interest.

George Plhak
Lions Head, Ontario, Canada

[update Oct 6] Great comment from Len Warner:
This is very sensible testing but one should not deep discharge automotive batteries unless there is a real need to know. However, all this seems valid: new battery tested to establish a reference; replaced car battery tested to qualify it for ups; and old ups battery tested to confirm it's ready to scrap. But the other two tests have cost you a cycle life.
Another test you haven't done is charge retention: ancient automotive batteries tend to have lost plate material, which accumulates at the bottom of each cell and shorts it out so the battery won't hold its charge long-term. It also tends to affect cells unequally, leading to cell imbalance and overcharging.
Traction batteries and leisure batteries should be more resistant to deep-cycling because of stronger plates and sumps for debris.