FWIW, used to sell battery testers. There are basically three;
1) A volt meter. All that does is establish that there IS a charge and what it is. (One CAN check for voltage drop using the starter, but not conclusive.)
2) "Load" tester. These are usually a carbon pile or other heat dissipating controlled load. These will hit the battery with a small load (hand held 75-100 amp unit) or a controlled higher load (adjustable/programmable carbon pile) to simulate a maximum load (starter plus accessories). VERY dependable test method, and can be used to check alternator output.
3) "Impedance" battery tester. These check for internal resistance and voltages. These have performance maps that calculate what the battery is doing and its condition. PROPERLY used, these are very good tools.
There are all sorts of "shade tree" methods to test batteries and a lot of them DO generate good data. For instance, using a volt meter a properly charged battery will show 12.8 - 13.4 volts and should not drop below 9.8-10 volts when the starter is engaged. That said, it is not wise to use a carbon pile load tester on glass mat, gas mat, or other dense compact batteries as they can (will) damage them. Small, high output batteries generally fall into that category (like our cars). But, there is another very important aspect to this - that battery terminals and clamps. In the case of the Kappa terminal clamps - they SUCK !!!! Tighten that little 10mm nut a bit too much and it WILL break. ALWAYS start with a physical inspection before testing stuff.
School is out !!!!!
