Start with the arithmetic
Take a common setup: a 100 Ah AGM battery, a compressor fridge, a few LED lights and phone charging, with no solar overnight. Lead-acid batteries are usually planned at about half their rated capacity, so the battery offers roughly 600 Wh before you start shortening its life. The loads below use about 295 Wh in a night.
- Usable energy
- 600 Wh
- Daily use
- 295 Wh
- Daily charging
- 0 Wh
- Autonomy
- 1.7 to 2 days
Fridge running 5 hours overnight, lights 4 hours, phones 3 hours. Starting assumptions, not measurements.
Open in the calculatorOn paper that night uses about half of the usable energy. If your battery is flat by morning with loads like these, the loads are probably not the whole story. If your own figures come out close to 600 Wh, the battery is simply too small for how you use it, and the fix is a planning one.
Three causes and how to tell them apart
| What you notice | Likely cause | How to check |
|---|---|---|
| Flat after a night with heavy use, fine on quiet nights | Loads are larger than planned | List every load with watts and hours. Compare the total with usable capacity. |
| Drops even when parked with everything switched off | Standby draw | Read the current on a battery monitor, or with a DC clamp meter, with all switches off. |
| Was fine last year, now fades fast with the same use | Lost capacity | Age, cold and past deep discharges. A controlled capacity test or a specialist check confirms it. |
| Inverter fan or a light on the inverter at night | Inverter left on | Switch it off and compare the next morning. Idle draw runs all night. |
Standby draw adds up quietly
Alarm systems, gas detectors, control panels, USB sockets with indicator lights and some radios draw current all the time. A draw of 0.3 A sounds harmless. Over 12 hours at 12 V it is about 43 Wh, and the nominal estimate for the same battery falls from about 2 days to about 1.8 days.
An inverter left switched on is worse. With an idle draw of 8 W for 12 hours plus one hour of laptop charging, the night's use rises to about 444 Wh. The inverter's own consumption is 96 Wh of that, more than the laptop.
Measure, do not guess from voltage
- A battery monitor with a shunt counts the energy going in and out. It is the most useful single tool for this problem.
- Voltage alone is a poor fuel gauge, especially for LiFePO4, whose voltage stays nearly flat across most of its charge.
- Read the current with everything off. Then switch one circuit on at a time and note what each adds.
- Write down the state of charge at bedtime and on waking, with the outside temperature. Three nights of numbers beat any estimate.
When the battery itself is the problem
Batteries lose capacity with age and with cycling. Lead-acid batteries also suffer when they are left partly discharged for long periods. Manufacturer cycle-life figures show the effect of depth of discharge: Victron rates its LFP Smart range at about 2,500 cycles to 80% depth of discharge and about 5,000 cycles to 50%.
If the monitor shows the battery reaching its low limit after far fewer watt-hours than its rating suggests, the battery has likely lost capacity. That is the point where a replacement makes sense, and the monitor data tells you what size to buy.
Write it down before you buy anything
- Battery type, rated Ah, age and date code.
- Every load with watts and hours per night.
- Standby current with everything off.
- State of charge at bedtime and on waking, for three nights.
- Outside temperature on those nights.
- Every charging source and whether it ran that day.