Field tested through the 2026 Australian touring season
Ask a caravan owner halfway through their third year with an absorbent glass mat battery how much capacity they have left, and you will get a shrug. Ask them what the label said when they bought it, and they will tell you confidently: one hundred amp-hours. Both are true at the same time. The battery had 100Ah of label and roughly 50Ah of working capacity from the day it left the warehouse, and that gap is not a defect, a marketing trick, or a manufacturing flaw. It is the most basic property of chemistry. The phrase that describes it is depth of discharge, and understanding it is what separates buyers who get value from their deep-cycle battery from buyers who quietly replace it every eighteen months and wonder why.
This article is the deep-cycle buyer’s guide to depth of discharge. It explains what the figure actually means, why different chemistries treat it differently, how it determines both the usable capacity and the working lifespan of any battery on the market, and what to do about it when you are sitting in front of two spec sheets trying to make a buying decision.
What Depth of Discharge Actually Measures
Depth of discharge, almost always abbreviated as DoD, is the proportion of a battery’s total rated capacity that has been pulled out at any given moment. A fully charged battery sits at 0 per cent DoD. A battery that has had half its capacity drained sits at 50 per cent DoD. A battery drained completely flat sits at 100 per cent DoD.
Its inverse is the state of charge, abbreviated SoC, which describes how full the battery currently is. State of charge and depth of discharge always add up to 100 per cent. A battery at 30 per cent state of charge is at 70 per cent depth of discharge. The two terms describe the same thing from opposite directions, and both appear in technical documentation, so it pays to be fluent in both.
The reason DoD matters is that every battery chemistry has a working window inside which it can be cycled repeatedly without losing capacity faster than the lifespan figure on the brochure suggests. Push the chemistry past the edges of that window, and the brochure number becomes optimistic. Stay inside it, and the chemistry delivers what the manufacturer promised.
Gentrax 51V 100Ah Lithium LiFePO4 Battery
The 50 Per Cent Rule on AGM, Explained Honestly
The conventional rule of thumb for absorbent glass mat batteries is a safe working DoD of 50 per cent. It is the figure printed on most reputable AGM spec sheets and quoted by most caravan retailers. The reason it exists is rooted in the chemistry, not in conservatism on the part of the manufacturer.
AGM is a sealed variant of lead-acid. The active material on each plate is lead and lead dioxide, suspended in a sulphuric acid electrolyte held inside a fibreglass mat. When the battery discharges, the active material on both plates converts to lead sulphate. When it charges, the reaction reverses. Each time the cycle runs all the way through, a small amount of lead sulphate fails to convert back cleanly, and the working capacity of the battery drops a fraction of a per cent.
The deeper the cycle runs, the more aggressively that conversion damage accumulates. At 50 per cent DoD, a quality AGM will give somewhere between 300 and 500 cycles before noticeable capacity loss. Run the same battery to 80 per cent DoD on every cycle, and that figure can drop below 200. Run it to 100 per cent DoD regularly, and the working life can collapse to under a hundred cycles, which is a year of weekend touring at best.
So a 100Ah AGM is not really a 100Ah battery in any practical sense. It is a 50Ah usable battery with a 100Ah safety reserve that the chemistry cannot tolerate the owner using often. The label is honest. The application of the label is what trips people up.
Why LiFePO4 Runs the Other Way
Lithium iron phosphate, abbreviated LiFePO4, sits at the other end of the depth-of-discharge spectrum. The chemistry tolerates being cycled to 100 per cent DoD without the kind of structural damage that lead-acid suffers, because the underlying reaction is fundamentally different. Lithium ions move between the cathode and anode through an electrolyte, and the conversion is far more reversible than the lead sulphate reaction inside an AGM.
That tolerance is one reason every LiFePO4 battery on the market ships with an internal battery management system, abbreviated BMS. The BMS sits between the cells and the outside world, monitors voltage and temperature on every cell, and cuts the circuit before the chemistry can be pushed past safe limits. The published 100 per cent DoD rating is genuine because the BMS will not allow a deeper discharge to occur in the first place.
The practical result is that a 100Ah LiFePO4 battery, like the VoltX 12V 100Ah LiFePO4 Battery, delivers something extremely close to 100Ah of usable capacity on every cycle. The label and the working figure are within a few per cent of each other, whereas on AGM they are out by a factor of two.
Here’s what one of our customers said:
“Well priced and have had no issues with it. Now have 2 of these VoltX batteries in battery box setups, charged with solar panels and running fridge/freezers with no issues.”
Cycle life follows the same logic. A LiFePO4 cell rated at 4,000 cycles is typically rated at 100 per cent DoD. Run the same cells more conservatively, at 80 or 90 per cent DoD, and the cycle figure climbs further. That is the inverse of how AGM behaves, where running closer to full DoD shortens cycle life dramatically. Buyers who try to apply lead-acid intuitions to lithium often end up oversizing their banks unnecessarily, which costs money and tow weight for no gain.
Gentrax 12V 200Ah Lithium LiFePO4 Battery
Where Gel and Flooded Lead-Acid Sit
Two other deep-cycle chemistries appear in the Australian market. Gel batteries use a thickened electrolyte that allows slightly deeper cycling than AGM, with most manufacturers quoting a safe DoD of 60 to 70 per cent. Flooded lead-acid, the traditional liquid electrolyte deep cycle, is normally rated at 50 per cent DoD, the same as AGM, though some marine and industrial variants tolerate deeper discharge at the cost of accelerated water loss.
Neither chemistry approaches LiFePO4 for usable capacity per kilogram of label. Both remain in service in specific applications, particularly long-term off-grid solar installations where the buyer has space, weight tolerance, and a preference for a familiar service routine, but neither rewrites the depth of discharge maths in a way that materially changes the comparison with lithium.
The Maths That Explains the Headline
Take the headline figure that gives this article its name. A 100Ah AGM, at a nominal 12V, holds a gross capacity of 1,200 watt-hours. Applied to the chemistry’s safe DoD of 50 per cent, the working capacity is 600 watt-hours, or 50Ah of usable energy at the battery’s nominal voltage.
A 100Ah LiFePO4 battery, such as the VoltX 12V 100Ah Blade LiFePO4 Battery, at the same nominal 12V, holds the same 1,200-watt-hour gross capacity. Applied to the chemistry’s safe DoD of 100 per cent, the working capacity is the full 1,200 watt-hours, or 100Ah of usable energy.
Here’s what one of our customers said about the Blade Battery:
“Have installed 2 x these batteries under drawers in my LC300, running a 96L Fridge/Freezer. All good so far, thanks.”
Two batteries, identical labels, double the usable energy from one chemistry to the other. None of that calculation appears on either spec sheet. The buyer is expected to know the safe DoD figure for each chemistry and apply it themselves, and almost nobody does.
VoltX 12V 200Ah Slim Lithium LiFePO4 Battery
Depth of Discharge and Cycle Life Are Not Independent
The second thing the depth of discharge governs is how long the battery will last. The relationship is not linear, and it is steeper for lead-acid than for lithium.
A 100Ah AGM run consistently at 30 per cent DoD might deliver 700 or 800 cycles before noticeable capacity fades. The same battery at 50 per cent DoD gives 300 to 500 cycles. At 80 per cent DoD, it gives perhaps 200 cycles, and at 100 per cent DoD, the figure collapses below 100. That is roughly an order of magnitude difference between gentle and aggressive use of the same chemistry.
A LiFePO4 battery run at 100 per cent DoD delivers around 4,000 cycles on quality cells. The same cells run at 80 per cent DoD deliver perhaps 5,000 to 6,000 cycles. The cycle life curve is shallower, the absolute numbers are far higher, and the BMS removes most of the user error that wrecks lead acid prematurely.
The two figures, depth of discharge and cycle life, have to be read together. A spec sheet that quotes one without the other is hiding the relevant variable. A spec sheet that quotes both, with the test condition clearly stated, can be trusted to within the usual tolerances of battery manufacturing.
How to Use Depth of Discharge to Size a Battery Bank
With the chemistry rules in mind, sizing a battery bank for a real application becomes a straightforward exercise.
Start with the daily load. Add up the watt-hours the system will consume in a typical twenty-four-hour period, including the fridge, lights, water pump, USB charging, and any inverter loads. Add a safety margin of 20 to 30 per cent on top to absorb hot days, high-draw appliances, and the gradual capacity fade every battery experiences over its life.
Divide the resulting figure by the safe DoD multiplier for the chemistry. For AGM, that is 0.5; for LiFePO4, it is 1.0; for gel, use 0.65 unless the manufacturer specifies otherwise. The resulting figure is the gross watt-hour capacity the bank needs to deliver. Divide by 12 to get the amp-hour rating to look for on the label.
- AGM bank sized for a 600-watt-hour daily load: 600 watt-hours divided by 0.5 equals 1,200 gross watt-hours, or 100Ah at 12V.
- LiFePO4 bank sized for the same 600-watt-hour daily load: 600-watt hours divided by 1.0 equals 600 gross watt hours, or 50Ah at 12V.
- Same daily energy demand, half the rated amp-hours required on lithium, with no shortened lifespan as the trade-off.
That last calculation is why a 100Ah LiFePO4 replaces a 200Ah AGM in most caravan installations without any reduction in real-world performance. The label looks smaller. The deliverable energy is identical.
VoltX 48V 100Ah LiFePO4 Golf Cart Battery
The Practical Takeaway
A 100Ah AGM is genuinely a 50Ah usable battery, and that is not a problem with the battery or the manufacturer. It is a property of chemistry. A 100Ah LiFePO4, such as the VoltX 24V 100Ah Pro LiFePO4 Battery, is genuinely a 100Ah usable battery, and that is a property of its chemistry too. Buyers who learn to read depth of discharge as the single most important number on a spec sheet stop overspending on lead-acid and stop oversizing on lithium, and the system they end up with works the way the brochure said it would.
Read chemistry. Apply the multiplier. Trust the maths. Visit Outbax today and check out the full range of high-quality 100Ah lithium batteries perfect for your next getaway.
