Battery Testing can be done in more than one way. The most popular is measurement of specific gravity and battery voltage. To measure specific gravity buy a temperature compensating hydrometer (a couple of quid from halfords) and measure voltage, use a digital D.C. Voltmeter. A good digital load tester may be a good purchase ( I use an old cigarette lighter from a car and an old headlight depending on what I'm testing) if you need to test batteries like sealed batteries.
You must first have the battery fully charged. The surface charge must be removed before testing. If the battery has been sitting at least several hours (I prefer at least 12 hours) you may begin testing. To remove surface charge the battery must experience a load of at least 20 amps for 4/5 plus minutes. connecting a car headlamp should do the trick. After turning off the light you are ready to test the battery.
State of Charge Specific Gravity Voltage
12V 6V
100% 1.265 12.7 6.3
*75% 1.225 12.4 6.2
50% 1.190 12.2 6.1
25% 1.155 12.0 6.0
Discharged 1.120 11.9 6.0
Hydrometer readings should not vary more than .05 differences between cells.
Digital Voltmeters should read as the voltage that is shown . The sealed AGM and Gel-Cell battery voltage (full charged) will be slightly higher in the 12.8 to 12.9 ranges. If you have voltage readings in the 10.5 volts range on a charged battery, that indicates a shorted cell.
If you have a maintenance free wet cell, the only ways to test are voltmeter and load test. Most of the maintenance free batteries have a built in hydrometer that tells you the condition of 1 cell in 6. You may get a good reading from 1 cell but have a problem with other cells in the battery.
So you can see battery testing isn't as easy as you may be led to belive it is, its much simpler to get it tested by someone with the corect test gear.
If you use the wrong test on the wrong battery, you run the risk of irreparable damage.
A battery tester is a discharge tester, so before you use it make sure the fully charged battery has been standing for at least 12 hours to gain an accurate reading when you test it.