Batteries have become an essential part of any serious camper and 4wding setup . Find out how to calculate the best battery for your 4wd, camper trailer or weekend warrior setup.

How is a battery rated? What is 100Ah?

To understand how a battery is given a particular Amp Hour rating, first you need to know how many hours that rating is tested at!

Adventure Kings Deep Cycle AGM Batteries (and many other leading brands) are tested by completely discharging them down to 10.5 Volts over a 20 hour period. This is commonly written as C20 which means the Capacity over 20 hours.

The testing involves finding a constant current (in Amps) that over a 20 hour period will discharge the battery right down to 10.5V. In the case of a 100Ah battery, that figure will be 5 Amps.

Manufacturers test their batteries with many different levels of current draw and to varying voltages to find out exactly what current is required to discharge any given battery to 10.5V.

A 120Ah battery is able to constantly discharge 6A over the 20 hour period, before it reaches 10.5V, 6 Amps x 20 hours = 120Ah

How does battery capacity vary up and down due to current load?!

What that might lead people to assume is that a 100Ah battery could cope with a 100A load for 1h (or a 50A load for 2 hours etc)

But remember: the 100Ah rating only applies over 20 hours or C20! If you increase the current, the time it takes to discharge is not linear! This is called Peukert’s Law and it shows the relationship that if you increase the discharge current on a battery, you decrease its overall capacity.

So let’s look at an example 100Ah battery.

As you can see! If you were drawing 68A constantly from the battery, it would only last 1 hour before reaching 10.5V – that means a capacity at C1 of 68Ah!

However, the good news is if you reduce the current the capacity actually increases! (Again this is all related to the C rating of the battery!)

TIP: When you’re comparing two batteries, ensure they use the same rate (whether it’s C20 or C100) or you’re not comparing apples and apples! Most leading companies including Adventure Kings use a C20 rate, but some sellers may use C100 without saying so their batteries appear to have a much higher capacity.

HOW TO ROUGHLY CALCULATE YOUR BATTERY CAPACITY AT ANY RATE WITH PEUKERT’S LAW

As you’ve learned, battery capacity is calculated through testing, but you can estimate it thanks to a formula based on Peukert’s Law:

H is the rated discharge time in Hours (eg 20 Hours)

C is the rated Capacity at that rate (eg 100Ah)

k is Peukert’s constant which is either given by the manufacturer or can be calculated using two given C ratings. As a guide use 1.17 for AGM batteries, 1.25 for Gel and 1.45 for flooded *(older batteries and batteries outside of the range 0-30°C will have higher Peukert figures)

I is the current you’re actually discharging from your battery 

T is the time in hours that will take to discharge the battery at your actual discharge current

Using our info above if you wanted to know how long your battery would last when constantly discharging 2.5A you could put it in as

= 45 Hours

2.5 Amps x 45 hours = 112.5Ah

Therefore a 100Ah battery with a constant discharge rate of 2.5A will offer around 112.5Ah of capacity!

FINAL TIP: Remember you shouldn’t discharge your AGM battery below 50% to get the best out of it

So, a 100Ah battery gives you around 50 usable Amp hours.

A rough way to check this is to keep an eye on your battery voltage. An Adventure Kings deep cycle AGM battery will be around 50% discharged once it reaches 12.3V – that’s 12.3V while nothing is connected to the battery and you’ve given it a good amount of time to settle to its actual voltage. Or, fit a battery monitor for a more accurate idea!

To see the full range of batteries and battery accessories, head to www.4wdsupacentre.com.au or call us on 1800 88 39 64 to pick the brains of our expert staff or drop in to one of our stores to see the full range in person and get the best possible setup for your next trip away.