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Battery Capacity

It’s important to understand Battery capacity when choosing what type and how may batteries required to adequately power your Caravan or Motorhome.  Battery capacity is measured in Amp Hours (Ah). Using this capacity, you can roughly work out how long your battery will last by finding out the Amp Hour draw of whatever is going to run off the battery.

Eg: a 100Ah Deep Cycle Battery running a fridge with a 4Ah draw would last about 100/4 = 25 hours before going completely flat.

However, it is recommended not to drain the battery that low and there are factors like low voltage cutouts that may limit the actual run hours you get from a battery.  An AGM Battery at approx. 12.7 volts is considered 100% fully charged, while an AGM battery at 10.5 volts is considered entirely discharged at 0%.

Ambient Temperature

All batteries are rated at a standard 25℃.  An often-ignored factor influencing battery capacity is the batteries ambient temperature.  Temperature is a major factor in battery performance, shelf life, charging and voltage control. At higher temperatures there is dramatically more chemical activity inside a battery than at lower temperatures. Therefore, the capacity of a battery is lower when the ambient temperature drops.

Capacity Factors with Different Temperature

AGM 12V             -20℃ is 46%    +45℃ is 109%

GEL  12V             -20℃ is 50%    +45℃ is 105%

As most of us in Australia live in a worm temperate climate the change in Battery Capacity with different in temperature may not be as apparent compared to someone living in Antarctica.

High temperatures will reduce the service life of VRLA batteries dramatically and can, in extreme cases, cause thermal runaway, resulting in high oxygen or hydrogen gas production and battery swelling. Batteries will not recover from this condition and must be replaced.

Typical reduction in design life against temperature

Capacity Hr Rate, Hour Rate, C Rating

While battery capacity is in Ah, it is also rated at a certain hour rate. This is because the capacity of a battery changes depending on how quickly it is discharging.  The implication of this phonon cannot be underestimated when comparing battery capacities.

Example: Discharging a battery with a steady 5 amps/hr draw, the battery lasts 20 hours. This means that the battery is rated to 5x20 = 100Ah at 20hr rate.

Using the same battery with a higher draw of 8.5 amps/hr, the battery only lasts 10 hours. This means that the battery is rated to 8.5x10 = 85Ah at 10hr rate.

To accurately compare capacities of battery they must be measured at the same hour rating. Most deep cycle batteries in Australia follow a standard 20hr rate (sometimes shown as C20).  This is particularly relevant when comparing batteries manufactured overseas, for example Lead Crystal batteries have the 10hr rating stamped on the casing which is an understated value at the C20 rate.

Nominal Voltage

Simply put, this means “named” voltage, and is basically an easy way to categorise any batteries within a standard voltage range. This is because the voltage of deep cycle battery actually fluctuates depending on its construction and use.

Example: A 12V Nominal voltage battery could actually be measured at 11V, 12.2V, 14V, etc. but to easily categorise them, we just call them all 12V.

Charge voltage

This is the recommended voltage to charge the battery at. Different batteries charge at different rates, such as flooded batteries charging at a higher voltage than AGM. It is important not to charge a battery above its charge voltage, as this will overcharge the battery and cause damage.

Discharge Rate

The discharge rate of a battery is the amount of power being draining at any one time (usually measured in amps). If a high amount of power is to be drawn in a short amount of time, it’s important to make sure that the battery being used can support this rate.

Self-Discharge Rate

All batteries slowly lose power over time, even when they aren’t being used. The self-discharge rate is the percentage of capacity that the battery will lose when it is in storage. Typically batteries like AGM or Tubular Gel will only lose a couple percent capacity per month, while other batteries can lose over 10%. This is important when working out how often to provide a maintenance charge on batteries when they aren’t in use. A 100Ah battery with a 3%/month 25 C.

CCA (Cold Cranking Amps), MCA, CA

Cold cranking amps is the maximum amount of amps that a battery is able to release on a cold start. This is typically used for starting engines in vehicles, as the engine will draw a high rate of amps over a short period of time to start the vehicle.  

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