Two batteries sit on the same shelf. Both are labelled 100Ah. One runs your fridge through a hot night with charge to spare. The other is flat by breakfast, and will need replacing three years sooner.
The number on the label is not the number you get. The specification that explains the gap is depth of discharge, and it is the most useful figure available to anyone building a 4WD dual battery setup for Australian touring. It decides how much power you can take out, how often, and what each cycle costs you.
Here is what it means, how it converts a rated amp hour figure into power you can use, and how it should change the capacity you buy. Prices quoted are from the Outbax 4WD lithium battery range at the time of writing.
What Does Depth of Discharge Actually Mean?
Depth of discharge is the percentage of a battery’s rated capacity that has been drawn out of it. A 100Ah battery that has delivered 80 amp-hours is sitting at 80% depth of discharge.
Depth of Discharge and State of Charge Are the Same Figure
Your battery monitor almost certainly displays state of charge, which is the percentage remaining. The two always add up to 100. A display reading 20% state of charge means you are at 80% depth of discharge. Reading one as the other makes every specification sheet easier to interpret.
Why the Amp-Hour Rating Tells You Very Little on Its Own
An amp-hour rating is a laboratory measurement. It describes total capacity under controlled conditions, not the portion you can safely and repeatedly draw in a vehicle working through 40-degree heat. Chemistry decides how much of that total is available.
Lead-acid and AGM chemistry degrades quickly when drawn down hard, which is why manufacturers recommend keeping an AGM battery above 50% state of charge. LiFePO4 is far more tolerant. Most lithium iron phosphate batteries are specified for routine cycling to 80% depth of discharge, with the battery management system protecting the cells below that.
That single difference is where the entire buying decision sits.
How Much of a 100Ah Battery Can You Actually Use?
Usable capacity is the rated capacity multiplied by the depth of discharge the chemistry tolerates. It is the only capacity figure worth comparing across two batteries.
What a LiFePO4 Battery Gives You
At 80% depth of discharge, a 100Ah LiFePO4 battery delivers 80 amp-hours per cycle. The VoltX 12V 100Ah LiFePO4 Battery sits in this bracket at $479. It is the most reviewed unit in the Outbax range, with close to 300 customer reviews and three Best Lithium Battery awards behind it.
Here’s what one of our customers said about:
“I have 4 of these (versions 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.”
What an AGM Battery Gives You
At the recommended 50% limit, a 100Ah AGM battery delivers 50 amp-hours per cycle. You can pull more. You will get the power once and pay for it in lost cycles.
The Comparison in One Table
| Measure | 100Ah LiFePO4 | 100Ah AGM |
|---|---|---|
| Recommended depth of discharge | 80% | 50% |
| Usable capacity per cycle | 80 amp-hours | 50 amp-hours |
| Typical weight | Under 10kg | Around 30kg |
| Typical cycles at that depth | 2,000 to 5,000 | 300 to 500 |
| Voltage under heavy load | Holds steady | Sags noticeably |
Read across, and the familiar claim that one lithium battery replaces two AGM batteries looks conservative. You get 60% more usable capacity from the same nameplate rating, at roughly a third of the weight, for several times the working life.
Depth of Discharge and Cycle Life: What It Costs You Over Time
Cycle life is the number of charge and discharge cycles a battery completes before capacity falls to a set percentage of the original, usually 80%. It turns a purchase price into a running cost.
Shallower Cycles Buy You More of Them
Cycle life moves inversely with depth of discharge. A cell cycled to 80% daily completes fewer total cycles than the same cell cycled to 40%, though each cycle delivers twice the power. The only practical way to cycle shallower is to own more capacity than you need, so this is really a sizing decision.
Working Out Cost Per Cycle
Take the VoltX 12V 100Ah at $479. At a conservative 4,000 cycles, that is about 16 cents per cycle for 80 usable amp-hours each time. A comparable 100Ah AGM at roughly $300 and 400 cycles works out at about 75 cents per cycle for 50 usable amp-hours.
Per usable amp-hour across the life of the battery, the AGM costs several times more. Step up to the VoltX 12V 300Ah Pro LiFePO4 Battery at $1,499, and the arithmetic improves again: roughly 50 cents per cycle for 240 usable amp-hours, the lowest cost per amp-hour in the range.
Here’s what one of our customers said about the VoltX 300Ah Battery:
“I converted my caravan from 3 x 100ah lithiums to this. So much easier and has run 2 large waeco fridges and van lights continuously for the last week and dropped to 99% only overnight. Ordering was easy. Product as described and delivery on time. I highly recommend it. Great service.”
What a Five-Year Warranty Is Really Telling You
Some of the batteries in the VoltX range carry a five-year warranty. No manufacturer underwrites five years of Australian touring on a battery it expects to fade in two. Warranty length is one of the few cycle life claims a buyer can hold someone to.
Sizing a Lithium Battery 4WD System to Your Real Touring Load
Your touring load is the total amp-hours your accessories draw in 24 hours. Work this out before you look at capacities, not after.
Add Up Your Daily Draw
Typical figures for an Australian touring setup in summer conditions:
- 50L 12V fridge: 30 to 45 amp-hours, and more in sustained heat
- LED lighting: 4 to 6 amp-hours
- Phone, tablet, and camera charging: 6 to 10 amp-hours
- Fans or a diesel heater: 5 to 8 amp-hours
- Laptop or satellite internet: 10 to 20 amp-hours
Most setups land between 60 and 80 amp-hours a day. The fridge is usually more than half of it.
When a 100Ah Lithium Battery Is Genuinely Enough
If you drive most days and run a fridge plus lighting, 80 usable amp-hours covers a full day with a sensible margin, and a DC-DC charger replaces it on the next leg. Where space rather than budget is the constraint, the VoltX 12V 100Ah Slim LiFePO4 Battery at $599 fits under a seat or against a canopy wall where a standard case will not.
When to Step Up to 200Ah or More
Stay put for two or three nights, add an inverter or a coffee machine, and 80 amp-hours disappear fast. The VoltX 12V 190Ah Pro LiFePO4 Battery at $1,049 and the VoltX 12V 200Ah Pro LiFePO4 Battery at $999 both clear two full days without touching the bottom of the pack.
One Outbax customer running two 200Ah Pro units in parallel with 650W of solar reported four days off-grid with air conditioning, an air fryer, and a coffee machine, finishing the week still at 100% state of charge. Surplus capacity means the batteries barely cycle, so they last.
Protecting Depth of Discharge in a 4WD Dual Battery Setup
Buying the right capacity is half the job. The charging and monitoring around it decide whether you ever see that capacity.
Fit a DC-DC Charger, Not a Relay
An alternator cannot deliver the charging profile a lithium battery needs, and a lithium battery will draw more current than most alternators are built to supply. A DC-DC charger, such as the VoltX SRNE 12V 30A DC-DC MPPT LiFePO4 Battery Charger, regulates current and follows the correct profile. That is the difference between reaching 100% on a long drive and stalling at 85%.
Fit a Battery Monitor So You Can See What You Are Doing
A shunt-based battery monitor counts amp-hours in and out and reports state of charge directly. Voltage alone is close to useless on lithium, because LiFePO4 holds a flat voltage across most of its range. Without a monitor, you are guessing at your depth of discharge.
Know What the Battery Management System Will Not Do
The BMS is a protection device, not a management strategy. It disconnects the pack before the cells are damaged, but a battery that regularly hits its low voltage cutoff has been undersized. Heat matters too: the VoltX range is tested to 45 degrees ambient, yet an engine bay will still shorten a pack’s life. A rear canopy, cabin, or under-tray box is a better home. Where the whole build has to fit a budget, the Gentrax 12V 300Ah LiFePO4 Battery at $899 delivers 240 usable amp-hours and leaves more for charging gear.
Buy for Usable Capacity, Not the Number on the Label
Depth of discharge turns a marketing figure into a working one. Size your battery on usable amp hours against a real daily touring load, and the choice between 100Ah, 200Ah and 300Ah stops being guesswork.
Cycling shallower buys more cycles and a lower cost per cycle, which is why stepping up a tier usually costs less over the life of the setup than buying tight and replacing early. Compare usable capacity, temperature ratings and warranty terms across the Outbax lithium battery range before committing.
