Old Mate's campsiteCamping power, worked out

A battery is a box of cells. Each cell makes electricity from a chemical reaction between two different materials (the electrodes) sitting in an electrolyte. The materials decide the cell's voltage:

• Lead-acid, including AGM: about 2.1 V a cell when full (2 V nominal)
• Lithium iron phosphate (LiFePO4): about 3.2 V a cell
• Other lithium-ion, like phones and power tools: about 3.6–3.7 V a cell
• An AA battery (alkaline): 1.5 V

To get the voltage you want, cells are joined in series — end to end, positive to negative — and their voltages add up. Six lead-acid cells make a "12 V" battery (about 12.7 V full); four LiFePO4 cells make 12.8 V. Joining cells, or whole batteries, in parallel — side by side, positive to positive — keeps the voltage the same, and the capacity (amp-hours) adds up.

Voltage is the electrical potential — how hard the battery can push, its ability to do work. Strictly, one volt is one joule of energy for every coulomb of charge that moves through.

Current is the flow — how much charge moves past a point each second, measured in amps (one amp is one coulomb a second). Like water through a pipe: the voltage is the pressure, the current is how much water flows.

What sets the current is the thing you plug in. Its resistance (in ohms) decides how much it lets through: amps = volts ÷ ohms.

Power is the two together — how much work is being done right now, in watts:

watts = volts × amps

A fridge pulling 4 A from a 12 V battery is working at about 48 W. The same 48 W from a 240 V power point is only 0.2 A — more push, less flow, same work.

Watts are how hard something's working this second. Energy is how much work it does over time, so you multiply by the time:

energy = power × time

48 W for 10 hours = 480 watt-hours (Wh).

The proper scientific (SI) unit of energy is the joule: one watt for one second. An hour has 3,600 seconds, so 1 Wh = 3,600 J, and the kilowatt-hour on your power bill (1,000 Wh) is 3.6 million joules. Old Mate sticks to Wh because the numbers are friendlier.

Batteries are sold in amp-hours (Ah): amps × hours. That's an amount of electric charge (the SI unit is the coulomb; 1 Ah = 3,600 C) — not energy. To get energy, multiply by the volts:

Wh = Ah × volts

100 Ah × 12.8 V = 1,280 Wh. A 100 Ah battery at 24 V holds twice the energy of a 100 Ah one at 12 V, so Ah only compares batteries of the same voltage — and a 240 V kettle can't be added to a 12 V fridge in Ah without converting first. Old Mate does every sum in Wh and shows Ah alongside.

An AGM does its best work with a small, steady draw over a long time, like a fridge ticking over all night — that's when it gets closest to the Ah on its label. Lithium handles that just as well. The difference shows when you ask for big current:

• The voltage sags. Run a kettle through an inverter and an AGM's voltage can drop past a fridge's or inverter's low-voltage cut-out while there's still charge left.
• You get less out. An AGM's Ah rating is measured over 20 hours (a 100 Ah one at 5 A). Draw it faster and you get less — the Peukert effect. Makers' own sheets show it: a quality 105 Ah AGM gives 86 Ah drawn down over 5 hours and 59 Ah over 1 hour; pull 100 A and it's about half. Lithium barely notices.

Energy is volts × amp-hours, so the area under each line is the energy that battery hands over. Same 100 Ah on the label; the lower, sagging line delivers less — and with a big load on, a lot less.

Old Mate's take: the decision point is big amps. Up to that point — a fridge, lights, phones, a fan — a good AGM is the value option and does the job. Once you want to pull big amps — an induction cooktop, coffee machine, kettle or air fryer through an inverter — high-current gear just doesn't sit well with an AGM: it gives about half its rating and its voltage sags toward the cut-outs. That's where lithium is the standout: close to all of its rating at any rate, and it holds its voltage.

Old Mate's tip: on an off-grid setup, set the fridge's battery protection (low-voltage cut-out) to its lowest setting. The higher settings are there to stop a fridge flattening the vehicle's starting battery. On a separate camping battery you want the fridge to stay on, and a big load sagging an AGM's voltage shouldn't switch it off. If the fridge does run off the starting battery, leave it on high so the car still starts.

Lithium holds about 13 V until it's nearly empty, charges faster, weighs about half as much and is rated for thousands of cycles. Its built-in protection (the BMS) switches it off before it's flat.

Old Mate's take: got an AGM? Use it — all of it if you need to. No need to baby it.

You'll hear "never take an AGM below 50%". It's true the deeper you go each time, the fewer cycles it lasts: quality AGM makers rate theirs at around 1,000 cycles to 50%, 550 to 80%, and 300 to 500 run completely flat. But that's a lot of nights. At 40 nights a year, even running it flat every time is about 7½ to 12½ years of camping — and AGMs tend to age out with time and heat before then. What hurts them more is sitting part-charged, so get it back to full when you can. The calculator lets an AGM go down to 10% unless you change it.

When an AGM still makes sense:

• You've already got a good one — use it till it's done.
• Charging below freezing: lithium generally can't be charged under 0°C unless it has a built-in heater. An AGM can.
• Under the bonnet: engine-bay heat is generally beyond what lithium is rated for. AGMs are made for it.
• An older vehicle charging straight off the alternator through an isolator, no DC-DC charger: an AGM copes; lithium generally wants a DC-DC.

Lithium takes close to full charging current until it's nearly full. AGM slows right down for the last 20% or so, so on solar it often doesn't quite reach 100% before the sun goes. The chart shows that slow-down.

Charging isn't free either: roughly 95 Wh of every 100 Wh put into lithium comes back out; AGM, more like 85.

An inverter turns the battery's 12 V DC into 240 V AC for things with a normal plug. It costs you twice:

• About 10% of the energy going through it is lost as heat (cheaper ones lose more).
• Just switched on with nothing plugged in, it draws a few watts. 10 W left on all day is 240 Wh — more than half what a 12V fridge uses.

Running the 12 V version of something (or USB-C for a laptop) skips the inverter altogether.

Current from the battery = watts ÷ volts. A 2,000 W kettle through a 90% inverter on a 12.8 V battery:

2,000 ÷ 0.9 ÷ 12.8 ≈ 174 A

Heat in a cable goes up with the square of the current (heat = I² × R): double the amps, four times the heat. That's why big inverters need short, thick cables and a fuse close to the battery — and why big rigs run 24 V or 48 V. The same power at double the voltage is half the current, and a quarter of the heat in the same cable.

One 6-minute boil takes about 220 Wh from the battery — about what a 12V fridge uses in 14 hours.

A panel's watts are measured in a lab: full sun of 1,000 W per square metre, cells at 25°C. Outside:

• Hot panels make less — roughly 0.3–0.5% less for every degree above 25°C, and a panel in the sun runs well above that.
• Angle, dust, cloud and cable losses all take a bit. Shade on even part of a panel can cut its output a lot, because the cells are wired in a chain.

Sun hours: a day's sunshine squeezed into hours of that full-strength lab sun. A summer day might give 6–7; a southern winter day 2–3; an overcast day about a third of the usual.

panel watts × sun hours × ~80% (real world) × controller

PWM vs MPPT: a standard "12V" panel does its best work at about 18 V. A PWM controller connects it straight to the battery at about 13.5 V and the difference is wasted — about a quarter. An MPPT controller converts the extra voltage into extra current, at about 95%.

Old Mate's testbed is 28 panels of 440 W (12.3 kW), facing north on a tilt, near his campsite in country NSW. It logged what it made every 15 minutes for a year, October 2025 to October 2026. Here it's scaled down to a single 100 W panel.

• A clear summer day made power from about 6am to 8pm (daylight saving time); a clear winter day from about 7:15am to 5:15pm.
• The flat top on the summer day is the testbed's inverters at their limit, not the sun. A camping panel wouldn't stop there.

• An average summer day made about 560 Wh for each 100 W of panel; an average winter day about 310 Wh. Summer's about 1.8 times winter.
• The weather matters as much as the season. The dullest fifth of days made about half the usual in summer, and about 40% in autumn and winter; the worst winter days, a fifth or less. The calculator's sunny, cloudy and overcast buttons use these figures.
• In the whole year, the longest run of days making under half the usual was 2 days.
• Tilt counts in winter. The testbed averaged about 3.1 sun hours after its losses in winter, where Old Mate's flat-panel figures for inland NSW come to about 2.2. In winter, prop a portable panel up facing north, angled at the sun.

A DC-DC charger takes power from the vehicle's alternator and charges the second battery at the voltage it wants — many newer vehicles' "smart" alternators run too low to charge a second battery properly on their own. Its rating is the amps it puts in:

25 A × 12.8 V × 2 hours ≈ 640 Wh

Old Mate's not a sparky. These are rough numbers for planning, not an electrical design.

• In Australia, fixed 240 V wiring (like power points in a caravan) is licensed-electrician work.
• Fuse every positive cable close to the battery, sized to suit the cable.
• Lithium batteries generally shouldn't be charged below freezing unless they're built for it — check the spec sheet.