Britbike forum Forums Thread Like Summary - Battery testing

This is a follow-on to the "electrical problems make my head hurt" thread. I did some testing, and realized that I had posted some erroneous information in that thread, so I want to set the record straight.

This test is of a healthy 11AH AGM battery after charging, with a voltmeter across the battery terminals. Static voltage (nothing on) is 12.8 volts. When I turn on the ignition, the voltage drops to 12.3, but then immediately climbs back up to 12.5. (This is apparently the result of the ignition module energizing the coil and then de-energizing when no signal is received from the trigger mechanism.) Now I turn on the headlight, which is a sealed-beam automotive headlamp, I believe 45-55 Watts. Note, the taillight is also on, and of course the ignition is still on. Voltage drops to and remains steady at 12.1.

When everything is disconnected (turned off), battery voltage recovers to 12.7.

An unhealthy battery will read lower when loaded, to a varying degree, and won't recover into the high 12's when unloaded.

Automotive batteries "recover" due to their chemical nature. Batteries do not behave like other electrical devices in that they have more than a simple ON and and OFF state. They develop what is called a "surface charge" which can give a VERY misleading, higher voltage reading. Because of this phenomenon, the surface charge must be bled off before a typical VOM can make sense of a battery's state of health. Most shops would use a device called a Battery Load Tester. The BLT is nothing more than a calibrated resistor that can knock off the surface charge in a couple of seconds, combined with a volt meter.

About 15 years ago, we here on BB.C developed what we thought was a workable equivalent to test battery life in a standard home shop. It consisted of charging the battery, turning the headlamp ON for a timed 2 minutes, and THEN immediately taking the voltage reading.

If you don't use one of the profession battery testing devices, or the BB.C method, then all bets are off. Despite very good intentions, you're just fooling yourself with a hand-held VOM.

Hope this helps.