Starting and deep cycle battery testing

This article outlines ‘health checks’ for the main (starting) and auxiliary (deep cycle) batteries in a dual battery system. These two batteries have different applications and require different test procedures. The procedures are from Car and Deep Cycle Battery FAQ with examples from testing real batteries.

I assume that have a a basic understanding of electrical systems and know how to measure voltage with a multimeter. I have outlined multimeter basics for the novice in a preceding post.

Disclaimer: Electrical systems and batteries can be dangerous. I am not responsible for any losses, damages or accidents you may incur by following these instructions. If you don’t understand then don’t do it!

Step 1 Measure State of Charge

Measure the voltage and temperature of both batteries. Car and Deep Cycle Battery provides State of Charge (SoC) tables for different battery chemistries. It is recommended to let a battery rest (no discharge, no charge) for two to eight hours before measuring SoC. SoC should be more than 75%, else charge the battery before proceeding.

Main Aux
Rested voltage (V) 12.58 12.46
Temperature (degC) 21.4 23.2
SoC 92% 66%
State of Charge (SoC) measurements. The main battery was a wet low maintenance battery. The aux was a flat plate AGM and not fully charged.

Step 2 Cranking test (main battery)

Disable the ignition or fuel supply and turn the engine over for 15 seconds with the starter motor. Record the minimum voltage during cranking.

I can disable my ignition by pulling out a multi-contact plug at the distributor. Alternatively, I can remove the fuel injection fuse. Removing the fuel pump relay would also stop the engine from starting. Diesel engines and other cars are different. Take care you don’t damage the computer or other sensitive electronics.

I measured a minimum cranking voltage 10.63 V and a rebound voltage 12.56 V. Cranking voltage should usually be greater than 10 V although it can approach 9 V in freezing conditions. It is better to ‘stress test’ a starting battery in cold weather because a battery that is adequate during summer might not work in winter. Car and Deep Cycle Battery FAQ provides a table of minimum cranking voltage versus temperature.

A low voltage could be caused by:

  • Flat or bad battery.
  • Undersized battery (insufficient Cold Cranking Amps).
  • Very cold temperatures.

Step 3 Capacity test (aux battery)

Connect a steady, moderate load and monitor voltage and current as the battery discharges. The recommended load is battery capacity (Ah) capacity divided by 8 or 20.

The capacity of my aux battery is 100 Ah and the recommended testing discharge rate is 5 to 12.5 A. I discharged the battery with four 21 W automotive bulbs wired in parallel: combined load 84 W or about 7 A at 12V. Later, I performed another discharge test at 3.6 A, which is closer to average current for my applications.

Battery discharge testing set-up.

Battery discharge testing set-up with 10 A DC ammeter.

Voltage can be measured with a multimeter at the battery terminals. To measure current, connect a DC ammeter in series with the load. Cheap panel meters can be purchased on ebay and they seem accurate enough. Low-current ammeters (less than about 50 A) do not require an external ‘shunt’ and are very easy to hook up.

Here are two discharge curves for my aux battery, where voltage and current were measured every hour or so. Beware at high discharge rates that voltage drops quickly when the battery is completely flat. Never discharge a 12 V battery below 10.5 V.


Ritar RA12-100 AGM deep cycle battery discharge plots. Solid curves starting from SoC = 66%, average current = 6.9 A, duration = 9 h, capacity = 62 Ah. Dashed curves starting from Soc = 79% (maximum for this old battery), average current = 3.6 A, duration = 23 h, capacity = 83 Ah.

Capacity can be calculated as average current times discharge hours. The above graph shows that battery capacity is reduced at higher discharge rates. This is known as Peukert’s effect. Capacity testing results should only be compared for the same testing conditions (starting voltage, final voltage, discharge rate, ambient temperature).

My aux battery test extracted 62 Ah in 9 hours, which is much lower than the manufacturer’s rating of 100 Ah at 10 hours. Less than rated capacities can be caused by:

  • Starting with less than 100% SoC (as in this example).
  • Capacity loss with age (my battery is four years old and has been poorly treated).

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