RV and marine banks: house versus starting
Two jobs, two batteries, and the compromise sitting in most RVs is the source of most disappointment. A starting battery wants thin plates and a burst of current; a house bank wants thick plates and slow, deep discharge.
Why "dual purpose" is a compromise
A dual-purpose battery is engineered to do both adequately rather than either well. If your rig has a single battery running the fridge, lights and inverter as well as cranking the engine, it is being asked to do a job it was not designed for — and the failure shows up as a fridge that stops overnight or a bank that lasts 18 months.
Sizing a house bank around daily loads
Add up a day's consumption in amp-hours at 12V: fridge 30-60Ah, lights 5-10Ah, inverter loads divided by ten (watts ÷ 10 = amps at 12V, roughly), charging devices 5-15Ah. Multiply the total by the days of autonomy you want, then divide by the depth of discharge you are willing to use.
Example: 80Ah per day, two days of autonomy, 50% DoD on lead-acid = 320Ah of lead-acid capacity, which is four 6V golf-cart style batteries in series-parallel or two large 12V units. The same duty on lithium at 80% DoD needs about 200Ah — smaller, lighter, and typically cheaper per usable amp-hour over life.
| Use | Lead-acid | LiFePO4 |
|---|---|---|
| Weight for 200Ah usable | ~250 lb | ~90 lb |
| Charge acceptance | Slower, needs long absorption | Fast, accepts most of what you give it |
| Venting | Flooded needs venting | No venting |
| Cold weather | Works, capacity drops | No charging below 0°C without heating |
| Cost per usable Ah | Lower up front | Lower over 5 years |
Charging hardware decides satisfaction
A 30A converter/charger will take a long weekend to refill a 300Ah bank from half-empty. Lithium tolerates higher charge rates and will absorb them, which is why an upgraded charger often transforms a bank more than extra capacity does. Solar sizing follows the same logic: match the array to the bank, and favour MPPT controllers for anything above about 200W.