RV batteries and solar: match demand, storage and charging
Size RV off-grid power from daily demand, the installed battery’s permitted operation and the charging available on the trip. Batteries store energy; solar replaces energy over time. A chemistry label, panel wattage or nominal amp-hour figure alone cannot establish usable runtime.
A discharge recommendation is not a physical half-capacity cutoff
Trojan’s lead-acid FAQ recommends 20–50% discharge for life and performance while recognizing cycling to 80%. Its AES AGM guidance separately recommends not exceeding 80% discharge. Do not apply a universal “AGM only has half its capacity” rule to every product.
Assume a 12-V 100-Ah battery and a chosen 50% planning discharge: nominal energy is 1,200 Wh and the chosen allocation is 600 Wh before losses. At an assumed steady 100-W load, the simple allocation lasts 6 hours. Temperature, discharge rate, aging and equipment limits can reduce that result.
Cold charging is a model-specific boundary
Victron’s Lithium Smart manual permits charging from +5°C to +50°C, with matching BMS and charger control requirements. That is more restrictive than merely avoiding temperatures below freezing. Other batteries require their own operating limits and any approved heating arrangement.
| Part | Evidence needed |
|---|---|
| Battery | Voltage, permitted discharge, current and temperature limits |
| Converter and alternator charger | Approved charging profile and current |
| Solar controller | Battery compatibility plus panel voltage/current limits |
| Protection and cables | Correct installation for the actual currents and routes |
| Monitoring | Configured capacity, current measurement and alarm response |
Solar production needs explicit assumptions
Assume 400 W of panels, 4 equivalent full-output hours and an assumed 0.75 overall factor. Multiplication gives 1,200 Wh for that assumed day. The 0.75 factor and four hours are scenario inputs, not measured output or a universal RV derating rule.
Compare that assumed generation with the 600-Wh daily allocation above, but include all actual loads and charging losses. Shading, weather and seasonal sun can change production; having more panel watts does not prove that the battery will recover before the next night.
Use battery selection, solar-array sizing and charging and monitoring for those details. Enter measured loads in battery and solar energy budget before ordering an upgrade.