• Materials Required/Recommended:
• Volt meter with load tester optional;
• Car battery charger;
• Manuals for batteries, volt meter and car battery charger; Read them and follow their instructions for your own safety. We are not responsible if you have problems.

Batteries are not simple. The basics of a battery are simple but determining how well a battery performs requires significant research, temperature adjustment calculations and time and retesting!

Manufacturers provide information about each battery they sell and also how they test the battery. You simply will not have the tools to test the way manufacturers test. For purposes of teaching simple methods we will consider sealed 12 volt batteries. If you have a car battery with water refill caps you can get a hydrometer from an auto parts store and in pretty short order know if your test battery is accepatable or not.

Get your volt meter set to DC current at the lowest setting above 12 volts, usually 20.
Connect the leads and read the meter. Worst case scenarion: If it is 0 then you have an open cell and attempts to solve the problem are a general waste of effort. Chances are your car battery charger if equipped will also tell you the battery has a fault. Connect the charger and attempt to charge battery then retest.  If this is your first time using a meter make sure you are using it correctly.  Go test the battery in your car. If it reads 0 then you are the problem and need to read up on the meter you bought or phone a friend to show you how to use your new meter.  Don’t be embarrassed.

If you have a charged battery you hope is useful then a load test is your next logical step.
With a battery load tester, apply a load equal to one half of the rating of the battery for 15 seconds.
DURING load testing, watch the voltage on a good battery. It will NOT drop below the the indicated voltage for the electrolyte at the temperatures listed in this chart used as a guide.  The chart is not applicable if you have one of the newer gel cell batteries.
             

Room Temperature °FRoom Temperature °F	Room Temperature °C	Minimum Voltage Under LOAD
100°	37.8°	9.9
90°	32.2°	9.8
80°	26.7°	9.7
70°	21.1°	9.6
60°	15.6°	9.5
50°	10.0°	9.4
40°	4.4°	9.3
30°	-1.1°	9.1
20°	-6.7°	8.9
10°	-12.2°	8.7
0°	-17.8°	8.5

Go inside if it’s any colder!  

If you don’t have a load tester start connecting equipment with known usage rating which may provide you very specific usage based on your real world usage.
Test a deep cycle battery by applying a known load and measuring the time it takes to discharge the battery until measures 10.5 volts.  For example, if you have an 7 ampere-hour rated battery, then an average load of one amp would discharge the battery in approximately 7 hours.  Manufacturers say the battery should generally be replaced if it is at only 75% of original capacity but since you already have the battery can still get some use from it.

After the test wait 10 minutes and see if the voltages bounces back up. If you don’t see it return to normal then you have a weak battery.  If the battery bounces back to less than 75% State-of-Charge recharge the battery and load test again. If the battery fails a second time and bounces back to less than 75% State-of-Charge, then go ahead and replace the battery.  There are devices called battery reconditioners but we just turn batteries in and get new ones.

In Conclusion, buying a new battery means you will get a warranty.  Pro rated battery replacement warranties tend to be marketing gimmicks by some battery manufacturers. A better warranty is the free replacement warranty which means the manufacturer has committed to providing a high quality battery.  A longer free replacement period is better.

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