Battery runtime guide
For a steady load, divide the amp-hours available before your planning reserve by the net current drawn from the battery. Start with the calculation below, then try a live circuit to see how appliances, charging and starting charge change the answer.
Checked Oct 21, 2026 · 12 V DC planning · your manual wins
Battery runtime calculation
This example uses one 100 Ah lithium battery, starting at 100% charge, with Crimp's 20% planning reserve. Loads and charging stay constant for each row.
Start full · 100 Ah lithium example
| Constant condition | Net battery draw | Time to reserve |
|---|---|---|
| 10 A loads, no charging | 10 A | 8 hours |
| 20 A loads, no charging | 20 A | 4 hours |
| 10 A loads, 5 A delivered charging | 5 A | 16 hours |
Runtime is the available amp-hours divided by the net discharge current. Net draw means the current the battery must supply after charging has helped with the loads. The 5 A in the last row is current delivered to the same battery bus, not a charger's nameplate rating. If that charging stops, the estimate returns to 8 hours from the same full starting point.
Crimp starts an unset battery at 70% charge. With the same 100 Ah lithium battery and 10 A draw, 70% down to 20% leaves 50 Ah, or about 5 hours. Set the slider to the charge you actually expect before comparing an overnight stop with a full-battery example.
The display estimates when you reach Crimp's planning floor: 20% for lithium or unspecified chemistry, and 50% for AGM, lead-acid or gel. Those defaults are not a battery maker's discharge limit or a BMS cutoff. Reaching reserve does not mean the battery is physically empty; check your battery manual when choosing how much capacity to rely on.
Try it here
This is the same interactive circuit as the front page. It opens in Simulate: switch equipment, change the sunshine or move the battery slider and watch the runtime update.
Live Crimp simulation. The battery starts at 70% charge, solar is Cloudy, and the fridge and lights are on. Try the controls in the drawing and results panel.
View full screenRed = positive · dark = return. Crossings are not junctions. Read the assumptions before adapting a circuit.
Set the solar panel to Off. Then turn the cabin lights or fridge Off and compare Time to empty. Switch them back On to restore the load.
Keep the MPPT and solar isolator On and change solar from Cloudy to Sunny. Watch Net power now and whether the battery is discharging or charging. Turn the isolator Off to disconnect the panel, or move the battery slider to compare starting points.
This live circuit uses the components' modeled draws, so its results differ from the simplified 10 A example above. The Battle Born battery has a built-in BMS, and the solar isolator disconnects both PV conductors. Fuse protection still needs verification against the battery's fault current and the chosen fuse and holder; a BMS does not replace that check. Treat this as a runtime demonstration, not an installation plan. The estimate holds the selected conditions constant and stops at the planning reserve.
Try your own system
Use Simulate on your connected drawing to compare the conditions you expect on the water or road.
Choose a battery with the right capacity and chemistry, and enter realistic appliance draws. Connect both the positive feed and negative return. Include the charger and converter paths you expect to use. Resolve missing capacity or operating-current data before relying on the estimate: unknown load current is omitted from energy totals, and an isolated battery cannot supply an unconnected load.
Choose Simulate in the top bar. Click a blank area of the canvas to show the system summary, then use the charge slider under Batteries. The slider sets your assumed state of charge (SoC); it is not a reading from a physical battery.
Use the controls on the component cards. Ordinary loads have On and Off; other devices may offer their own operating modes. Solar panels offer Sunny, Cloudy and Off. Engines offer Running and Off, while charger controls depend on the device. Set every relevant source to the condition you want to compare.
Watch Net power now, Time to empty and Time to full update. Check each battery card as well if your drawing has separate banks. These estimates assume the selected conditions continue; changing a control recalculates them immediately.
Useful comparisons
Change one condition at a time so you can see which appliance or charging source makes the difference.
Overnight
Cloudy stop
Under way
At the dock or campsite
Reading the estimate
Amps (A) describe current; amp-hours (Ah) describe capacity. At a consistent battery-side voltage, Ah divided by A gives hours. Watts (W) are voltage times current, and watt-hours (Wh) are voltage times Ah. Use Wh divided by W when comparing energy and power across different voltages. Do not subtract a solar panel's input amps from battery-side load amps, or compare an AC appliance's amps directly with a 12 V battery's amps; conversion and losses matter.
It depends on its starting charge, your reserve, the fridge's draw and how often its compressor runs, plus every other connected load. First compare the fridge On and Off in Simulate. Treat the On result as the selected draw held constant. Use the appliance's measured or documented consumption to assess a whole night; an instantaneous estimate does not model compressor cycling or changing ambient temperature.
Live means that the results update when you change the drawing or its controls. Crimp does not advance a clock, reduce the charge slider automatically, read battery telemetry or forecast tomorrow's solar. It estimates runtime if the current state holds. Actual capacity also changes with discharge rate, temperature and battery age; those effects are not simulated here.
A dash can mean there is no finite discharge estimate for the current state, such as when the battery is idle or charging. Missing capacity can also prevent a runtime estimate. Check the battery capacity, wiring and selected controls before interpreting it. It does not mean unlimited autonomy. With separate banks, check their individual battery cards: a charging bank does not necessarily help an isolated bank that is discharging.
References and next steps
These manufacturer references explain battery capacity and runtime estimates. Crimp uses the component data and conditions in your drawing; a physical battery monitor measures the installed system.