The short answer. Run time is the usable amp-hours divided by the amps the load pulls. A 100Ah lithium battery used to 80% runs a 60-watt load on 12V for about 16 hours. The same label in lead-acid, used to half, gives about half that, and less again if you drain it fast.
Enter the bank size and the load.
Batteries in series: enter one battery’s Ah. Two 100 Ah batteries in series make a 24V bank of 100 Ah.
An estimate from the maker’s ratings. A shunt battery monitor shows the real number on your own boat or rig. Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
How the math works
Watts divided by your system voltage gives amps: 60 W on 12V is 5 amps. Then the tool takes the battery’s label, cuts it to the share you plan to use, and divides by those amps.
Two maker numbers change the answer, and the tool uses both.
Lithium in the cold. Victron lists its 12.8V 100Ah Smart battery at 100Ah at 25°C, 80Ah at 0°C and 50Ah at -20°C. Pick the temperature closest to where your battery lives. That 100Ah battery at 0°C, running the same 5 amps, gives about 12 hours 48 minutes instead of 16.
Lead-acid drained fast. Lead-acid labels are rated over a slow 20-hour discharge. Victron’s AGM sheet says you get 85% of the label if you drain it over 5 hours and 65% over 1 hour. So for any load that empties the battery faster than 20 hours, the tool shows a range instead of one number. A 100Ah AGM used to 50% at 5 amps comes out at 8.5 to 10 hours.
Lead-acid also loses capacity in the cold. Victron shows that as a graph only, with no figures to read off, so the tool leaves it out and tells you so.
Why “use down to” isn’t 100%
You can run a battery flat. It just won’t last. Victron rates its lithium battery’s 2,500-cycle life at 80% depth of discharge and gives its AGM 600 cycles at 50%. The tool starts there. Change it if your battery maker says otherwise. Their manual wins.
Then check it for real
The tool works from the ratings. A Victron SmartShunt 500A counts the amps your gear actually pulls, and after a weekend you’ll know your real run time. If the answer is too short, size a bigger bank with the Battery bank size calculator, or see how the two chemistries compare in the Lithium vs lead-acid batteries for boats, vans and RVs. Running AC gear? Work out the load with the Inverter size calculator first.
Bundles for this job: RV battery monitor kit
Quick answers
How long will a 100Ah lithium battery run a fridge? Divide 80Ah (80% of the label) by the fridge’s average draw in amps. A fridge cycles on and off, so its average is lower than the label; a battery monitor shows the real figure after a day.
Why does my lead-acid battery die faster than the math says? Fast drains cut lead-acid capacity. Victron’s AGM sheet gives 85% of the label over 5 hours and 65% over 1 hour, and cold makes it worse.
Does a lithium battery run shorter in winter? Yes. Victron lists its 100Ah Smart battery at 80Ah at 0°C and 50Ah at -20°C. It also shouldn’t be charged below 5°C, per the same datasheet.
Can I run an inverter off this? Yes. Enter the inverter’s DC draw in amps, or its AC watts plus a bit for the inverter’s losses. The Inverter size calculator shows the DC draw for each inverter on our list.
Sources
- Victron Lithium Battery Smart datasheet: https://www.victronenergy.com/upload/documents/Datasheet-12,8-&-25,6-Volt-lithium-iron-phosphate-batteries-Smart-EN.pdf
- Victron Gel and AGM batteries datasheet: https://www.victronenergy.com/upload/documents/Datasheet-GEL-and-AGM-Batteries-EN.pdf
