Deep Cycle Batteries - Understanding Usable Power Capacity

When we go shopping for a deep cycle battery (AGM or Lead Acid), we get told how much power capacity a battery has. The manufacturer will indicate this by way of Amp Hours (Ah). So what does this mean you may wonder. 

Ampere-hours is a unit of measurement used to quantify the amount of electrical charge a battery can hold and deliver. The amp-hour rating of a deep cycle battery provides an estimate of how long the battery can sustain a given current draw. For example, a battery with a rating of 100Ah can theoretically deliver a continuous current of 1 amp for 100 hours, or 10 amps for 10 hours, before reaching its fully discharged state.

In my Land Cruiser my  deep cycle has a capacity of 120Ah which powers my 40l fridge, some LED camp lights and charging of camera batteries. In terms of amps drawn it is around 5 amps per hour when all these accessories are running at the same time. However, the lights and battery charging takes place only for a few hours. So it is the fridge which pretty much runs all night long once I am camped for the night. This means my average amp draw would actually be around 3 amps per hour. So theoretically my battery should be able to power my consumption for 40 hours before the battery if fully discharged (120Ah / 3amps).  Going by these numbers you would think if the battery was charged every second or third day that would sufficient.  Well, this is not quite correct because capacity is one thing, depth of discharge is another. 

It's important to note that the usable capacity of a deep cycle battery is typically less than its rated amp-hour capacity. To maximize battery lifespan, it is generally recommended to avoid fully discharging the battery. Discharging a battery below 50% state of charge can have negative effects on its longevity. Therefore, the usable capacity is generally 50% of the rated amp-hour capacity to maintain battery health. In other words, the useable capacity of my 120Ah deep cycle battery is actually 60Ah. A stark contrast to what the stated capacity is. 

It's also important to note that practical usage may differ from theoretical calculations due to various factors. The actual capacity delivered by a deep cycle battery can be influenced by the discharge rate, temperature, age, and battery chemistry. Higher discharge rates may slightly reduce the usable capacity compared to lower discharge rates. For example charging camera batteries will be at a much lower discharge rate as compared to say running a 12v hair dryer or even a 12v oven. 

Another fact important noting is deep cycle batteries  don't like heat. So ideally having the battery away from the engine bay would be good. This would increase battery lifespan as well as useable capacity. However, due to space constraints this is not always possible. So in most cases the engine bay is where the battery ends up. Like in my Land Cruiser. In this case make sure to get a deep cycle battery which has been built for high heat environments. Your battery retailer should be able to advice you on this. 

So the next time you go shopping for a deep cycle battery make sure you are aware of what the consumption of your 12v accessories are and select a suitable battery applying the 50% rule to the manufacturer stated capacity.

I hope this has been of use to you. If you have any questions (or comments) please write to me.  

Cheers

Duncan Udawatta

This article is in a series of related articles which can be found in the EDUCATIONAL section of the website. These are:

-Why a DC-DC Charger is a Must-Have for Deep Cycle Batteries

-What Is The Difference Between AGM and Lead Acid Batteries

-How to chose a deep cycle battery for your consumption.

-How to maintain batteries when parked for a prolonged period of time.