Walk into any caravan supply shop in Australia, and you will see the same three numbers stamped across every battery on the shelf: 12V, 100Ah, and whatever the manufacturer wants to put next to the word deep cycle. Those numbers are doing almost none of the work most buyers think they are. The figure that actually predicts how long your fridge will run, how many lights you can keep on, and how many nights you can spend off-grid is one that most spec sheets do not even print. It is the usable watt-hour figure, and learning to calculate it for yourself is the single most useful skill a 12V battery buyer can develop.
This guide walks through how to read a 12V deep cycle battery spec sheet the way an electrical engineer would, not the way a marketing department wants you to. By the end, you will be able to glance at any battery label, in any chemistry, and within thirty seconds know roughly how much real energy it will deliver into your fridge, your inverter, and your camp lights.
Why Amp-Hours Are the Wrong Question
The amp-hour figure on a battery label is a capacity rating, not a delivery rating. It tells you how many amps of current the cells can theoretically supply for one hour, under laboratory test conditions, at a specific temperature, drawn down to a specific cut-off voltage. None of those conditions matches what happens in the back of a four-wheel drive heading up the Oodnadatta Track at the end of summer.
There are three things the amp-hour number does not tell you. It does not tell you what voltage the battery holds while it delivers those amps, which matters because watts equal volts multiplied by amps. It does not tell you how deep you can safely discharge the battery without permanently damaging it. And it does not tell you what proportion of the rated capacity the chemistry actually delivers in real use, as opposed to in the manufacturer’s test rig. Miss any one of those, and you can buy two batteries with identical 100Ah labels that give you completely different amounts of usable energy.
VoltX 12V 100Ah Lithium LiFePO4 Battery
The Watt-Hour Conversion Every Buyer Should Memorise
Watt-hours are the right unit because they describe energy, which is what fridges, lights, and inverters actually consume. The conversion itself is simple enough to do in your head.
Multiply the amp-hour rating by the nominal voltage to get the gross watt-hour figure. A 12V 100Ah battery like the VoltX 12V 100Ah Lithium battery has a gross capacity of 1,200 watt-hours, regardless of chemistry. That is the number every brochure quietly hopes you will accept at face value.
Here’s what one of our customers said:
“I have 4 of these (version with battery monitor) in my caravan to provide 400ah of power. They are honestly brilliant. They do actually provide the rated power, I can hammer these and they just hold up. Running them for over a year now. I highly recommend these batteries as I have the confidence in them when off grid, they run a 3000W inverter at full tilt (Pulling +280A) no problems at all. There may be cheaper and more “premium” expensive batteries out there, just get these, you won’t be disappointed.”
Depth of Discharge: The Hidden Multiplier
Depth of discharge, abbreviated DoD, is the proportion of a battery’s capacity you can safely pull out before the chemistry starts to suffer permanent damage. It is the multiplier that turns the gross watt-hour figure into the usable watt-hour figure, and it is wildly different between chemistries.
A quality absorbent glass mat battery, the sealed lead acid variant most caravans were sold with for the last twenty years, is conventionally rated to a safe working DoD of 50 per cent. Drag it deeper than that on a regular basis, and the lifespan figure on the brochure collapses. So, a 100Ah AGM, at 50 per cent DoD, gives you 600 usable watt-hours.
A lithium iron phosphate battery, abbreviated LiFePO4, runs the other way. The chemistry tolerates being drawn down to 100 per cent DoD on every cycle, and the internal battery management system protects the cells from going further. A 100Ah LiFePO4, such as the VoltX 12V 100Ah Bluetooth Daly LiFePO4 Battery, gives you the full 1,200 watt-hours every time. That’s twice what the AGM delivers from an identical amp-hour label.
Queens 12V 95AH Lithium Iron Phosphate Battery
C-Rate and the Voltage Sag Problem
The next missing variable is the C-rate, which describes how quickly the battery is being discharged relative to its capacity. A 100Ah battery drawing 100 amps is being discharged at 1C. The same battery drawing 50 amps is at 0.5C. The higher the C-rate, the more usable capacity the chemistry typically loses.
AGM suffers badly at high C-rates. Pull 100 amps from a 100Ah AGM to run a kettle through an inverter, and you might see effective capacity drop by twenty to thirty per cent. The voltage sags hard under load, which then causes a downstream problem: most 12V appliances and almost every inverter have a low-voltage cut-off. The fridge does not stop running because the battery is flat. It stops because the voltage dropped below where the appliance will accept it, while there is still energy chemically present in the cells you cannot reach.
LiFePO4 holds a remarkably flat voltage curve from about 95 per cent state of charge down to about 15 per cent, which means appliances see something close to a constant supply right across the discharge window. That flat curve is doing real work that the spec sheet rarely quantifies, and it is one of the largest practical reasons LiFePO4 outperforms AGM in real touring.
VoltX 12V 100Ah Bluetooth Daly Lithium LiFePO4 Battery
Temperature: The Variable Hidden in the Footnote
Battery test conditions are almost always reported at 25 degrees Celsius. Australian touring is not. A battery box on the rear bar of a four-wheel drive in February sits at well above 50 degrees Celsius on a sunny afternoon, and an alpine touring rig in winter sees the battery at zero or below overnight.
Capacity drops at both extremes, but the chemistries react differently. AGM tolerates cold reasonably well but loses capacity quickly above 40 degrees Celsius. LiFePO4 holds up superbly in heat thanks to the management system intervening before damage occurs, but should not be charged below zero degrees Celsius. None of that appears on the front of the spec sheet. It hides in a footnote, or in a separate technical data document that most buyers will never see.
Any honest reading of a spec sheet has to ask, at minimum, what the rated capacity drops to at the temperatures the buyer’s installation will actually see. If the manufacturer cannot produce that figure, treat the published amp-hour number with suspicion.
Cycle Life Is Not What the Front Page Says It Is
The cycle life figure on a battery label is the number of complete charge and discharge cycles the cells can complete before capacity drops to a defined percentage, usually 80 per cent of the original rating. The number is published under specific test conditions, and changing those conditions changes the number dramatically.
A 100Ah AGM rated at 500 cycles is usually being measured at 50 per cent DoD, the safe working window of the chemistry. Push the same battery to 80 per cent DoD on each cycle, and the realistic cycle life drops to perhaps 200 cycles. The label is honest. The application of the label, again, is what trips buyers up.
A 100Ah LiFePO4 rated at 4000 cycles is typically measured at 100 per cent DoD. Run the same cells at 80 per cent DoD, and the figure climbs to 5,000 or 6,000 cycles. The published number is a floor, not a ceiling. The two cycle life numbers are not directly comparable across chemistries without knowing the test conditions under which each was measured.
VoltX 12V 100Ah Blade Lithium LiFePO4 Battery
What the Real Numbers Look Like on a Typical Caravan Setup
Take a working example. A 60-litre 12V camping fridge averages around 45 watts of continuous draw across a 24-hour cycle in moderate Australian conditions. Across one day, that is roughly 1,080 watt-hours of energy consumed.
The 100Ah AGM delivers around 600 usable watt-hours at safe DoD. The fridge alone empties it inside thirteen hours, well before the next morning. In hot conditions, with a higher fridge draw and the high C-rate losses described above, that figure drops further.
The 100Ah LiFePO4, like the Gentrax 12V 100Ah LiFePO4 Battery, delivers 1,200 usable watt-hours at 100 per cent DoD. The same fridge runs for slightly over twenty-six hours, on a single charge, before the battery management system intervenes. In hot weather, with a higher draw, that drops to perhaps eighteen to twenty hours. Add a 200-watt solar panel into the loop, and the system is genuinely self-sustaining across a multi-day trip.
None of that maths is visible on the front of the label. All of it is sitting in the gap between the amp-hour figure and the usable watt-hour figure.
How to Read Any 12V Battery Spec Sheet in Thirty Seconds
With everything above in mind, here is the sequence to run through whenever a new battery lands on the bench.
- Multiply nominal voltage by amp hours to get gross watt-hours.
- Apply the chemistry’s safe depth of discharge multiplier. AGM at 0.5, LiFePO4 at 1.0, gel and flooded lead-acid at 0.5, unless the manufacturer specifies otherwise.
- Subtract roughly 10 to 20 per cent if the application involves high current draw through an inverter, to account for voltage sag and C-rate losses, particularly on AGM.
- Check the temperature footnote and discount further if the installation runs hot or cold compared to the test condition.
- Read the cycle life figure together with its depth of discharge condition, not in isolation.
The number left at the bottom of that exercise is the usable watt-hour figure. That is what a buyer should actually be comparing across two batteries with identical front-of-label specs.
The Buyer’s Real Question
The question to ask when buying a 12V deep cycle battery is not how many amp-hours it claims. It is how many usable watt-hours it will reliably deliver, in the conditions the application actually runs in, across the cycle life the chemistry can honestly support. Once that becomes the question, the comparison gets much sharper, the brochures get much shorter, and the decision gets much easier.
Read the label, do the maths, and trust the watt-hours. Ready to buy your battery pack? Head straight to Outbax for a premium range of LiFePO4 batteries, all designed to provide reliable power outdoors and off-grid.
