How to Size an RV Lithium Battery Bank for Off-Grid Camping

An off-grid RV takes every load from its battery bank. Solar, the alternator and a generator put energy back, and none of them do it reliably. Campsite use is the opposite case: shore power carries the load for as long as the vehicle is plugged in, and the battery only bridges the gap between hook-ups. Two different duty cycles, and most sizing advice is written for the second one.

Sizing for the first is not a choice between 100Ah, 200Ah and 300Ah. Four variables set the number — the loads the vehicle actually carries, the days it has to run without a charge, the charging sources that can be counted on across those days, and the discharge current its BMS will allow. The order matters. Each one consumes the figure produced by the one before it.

Get it wrong on the low side and the bank reaches low-voltage cut-off during ordinary use. The customer reports a system that keeps switching itself off, and the dealer spends a season replacing parts that were never at fault. Oversizing fails more quietly: weight and cost go up, the compartment loses volume the floor plan needed, and the charging equipment is asked for more than it was ever sized to give.

1. Daily Energy Consumption

Start with the energy that leaves the bank across 24 hours, not with the appliance list. Two vehicles carrying identical equipment can differ by 30% or more in daily consumption once usage patterns are taken into account, and that difference propagates through every later step.

For a DC load, energy is rated power multiplied by the hours it actually runs.

Daily energy (Wh) = Power (W) × Operating time (h)

A 60W compressor refrigerator running twelve hours draws 720Wh, which already assumes a 50% duty cycle. In mild weather the compressor cycles closer to a third of the time and the figure drops to roughly 430Wh. In sustained heat it holds the rated draw for longer stretches, and 720Wh becomes optimistic. Measured consumption from a comparable vehicle beats any assumption here. Whichever figure you end up using, record it next to the result — the assumption is the part that gets forgotten when the specification comes back for review.

AC loads have to pass through an inverter, and the inverter takes its cut.

Battery energy (Wh) = AC load energy (Wh) ÷ Inverter efficiency

1,000Wh at the socket needs about 1,111Wh from the battery at 90% efficiency. That 90% is a nameplate best case. Efficiency falls at light load and at high temperature, and a unit sitting in standby keeps drawing: 10W of idle current is 240Wh over a day, more than the water pump in Table 1 gets through in a week.

1.1 A representative load list

Table 1 is a load list for a medium-sized motorhome. Read the values as order-of-magnitude. Appliance specifications, ambient temperature, duty cycles and how the occupants behave all move them.

About 1.96kWh a day. It is an input, not a battery specification. Inverter losses, discharge limits, temperature derating and reserve all still have to be applied, and the specifications that come out wrong are usually the ones that stopped here.

The most common error at this step is multiplying every appliance’s rated power by 24 hours, which assumes it runs continuously. Most loads do not. The microwave in Table 1 draws 1,000W for fifteen minutes, not for a day, and the gap between 250Wh and 24kWh is two orders of magnitude. Spread that assumption across a load list and the requirement inflates several-fold. Weight, space and purchase price all follow.

2. Autonomy

Autonomy is the number of days the vehicle runs without meaningful charge. It comes from the itinerary and from what charging infrastructure exists along it, which makes it a commercial decision as much as an engineering one.

l Weekend use: 1–2 days

l Extended off-grid camping: 2–3 days

l Remote touring and overlanding: 3–4 days or longer

Those three bands describe applications, not standards. What matters is the longest stretch the vehicle will realistically go without a reliable charge, and that number should be agreed with the customer before anyone opens a battery catalogue.

Required energy = Daily energy consumption × Autonomy days

1.96kWh × 3 = 5.88kWh. That is usable energy, not nameplate.

2.1 Usable capacity and rated capacity

Capacity is quoted in amp-hours, which do not measure stored energy. A 12.8V 200Ah battery and a 25.6V 100Ah battery hold the same 2.56kWh, so comparing them by Ah rating gets you nowhere. Any comparison across voltages has to be made in watt-hours.

Nominal battery energy (Wh) = Nominal voltage (V) × Rated capacity (Ah)

Nominal energy is not what the vehicle can use. Depth of discharge, BMS cut-off settings, operating temperature and the service life the specification has to deliver all take a bite. For preliminary LiFePO4 work, 90% usable DoD is a reasonable planning figure. The number that governs the warranty is the one the cell and BMS suppliers state.

Required nominal energy = Required usable energy ÷ Usable DoD

5.88kWh ÷ 0.90 = 6.53kWh, which at 12.8V is 6,530Wh ÷ 12.8V ≈ 510Ah.

510Ah does not exist as a product. Table 2 brackets it. 400Ah gives 5.12kWh nominal and about 4.6kWh usable, short of the 5.88kWh required. Two 280Ah units in parallel give 560Ah, 7.17kWh nominal and roughly 6.45kWh usable. Which side of that bracket you land on is decided less by the arithmetic than by how the packs fit the compartment, which is section 5.

The calculation assumes no charging and no variation in load, and neither holds in service. Add the reserve deliberately, against the conditions the vehicle will meet, and put a number on it. Rounding the result upward does the same job worse, and it is how a 510Ah requirement turns into a 600Ah purchase order nobody can justify at review.

3. Charging You Can Count On

Most RV banks do not work alone. Solar, alternator charging and generators all contribute during off-grid operation, and how those contributions are treated decides whether the specification is realistic. Ignore them and the bank comes out larger than the vehicle needs. Take rated output at face value and it comes out too small. The rule that works is to subtract only the energy that will actually arrive, then size the bank for the deficit.

3.1 Solar: nameplate versus production

Daily production depends on peak sun hours, panel orientation and tilt, season, cloud, shading from roof equipment, and losses in the controller and the wiring. None of that appears on the nameplate.

Daily solar energy (Wh) = Array power (W) × Peak sun hours × System efficiency

400W at 4 peak sun hours and 80% overall system efficiency: 400 × 4 × 0.80 = 1,280Wh a day.

Against 1.96kWh of consumption, that is a deficit of 0.68kWh a day. Four peak sun hours is a summer number at mid-latitudes. At 50° north in winter it is closer to one, and a flat-mounted panel in partial shade sits below that. Over three days the deficit accumulates to about 2.04kWh, which brings the usable requirement to 3.84kWh, nominal to 4.27kWh, and capacity to roughly 333Ah.

Both answers are correct and they are not interchangeable. A summer-only vehicle with 400W of panel can be specified near 400Ah. One that will be occupied in winter, or left standing for days at a time, needs the 560Ah figure. Solar contribution has to be calculated for the season the vehicle is used in, which for most European and North American customers is not the season the data sheet was collected in.

3.2 Alternator charging

Alternator charging works while the vehicle is moving. Most installations put a DC-DC charger between the alternator and the house bank to regulate voltage and current; with a smart alternator and its variable set point, leaving it out is not really an option.

Charging energy (Wh) = Charging voltage × Charging current × Charging time

40A at an average 14V for two hours: about 1,120Wh. Energy actually stored depends on the charger’s rating and thermal behaviour, on the battery’s acceptance rate at its current state of charge, on temperature, and on charging losses. Even at the illustrative figure, 1,120Wh is roughly half a day of the load in section 1. Alternator charging buys back hours, not days.

For a vehicle that moves between stops regularly, it justifies a smaller bank. For one that stands for several days at a time it cannot be counted as a daily source, and the specification should not lean on it.

3.3 Generator and shore power

Where solar and alternator charging fall short, some platforms carry a generator or an AC charger. Either one reduces the capacity required. The price is fuel, noise, site restrictions and one more maintenance item, and both change the charging architecture of the vehicle. Whether that trade is acceptable depends on the application. Rarely for a compact campervan, often for an expedition vehicle or a rental fleet.

4. Discharge Current

Capacity says how much energy the bank holds. Discharge current says how much power it can deliver at one moment. They are independent constraints, and a bank that passes the energy calculation in section 2 can still fail on current.

Air conditioning, microwaves, induction hobs and coffee machines set the peak. The inverter turns that peak into battery current.

DC current (A) = AC output power (W) ÷ (Battery voltage × Inverter efficiency)

2,000W from a 12.8V battery at 90% efficiency: 2,000 ÷ (12.8 × 0.90) ≈ 174A. Current climbs as the battery discharges and voltage falls; the same load at 11.5V pulls about 193A, and that is before the inverter’s surge for a compressor or motor start.

A 12.8V 300Ah battery is not automatically right for a 2,000W inverter. The capacity is sufficient. The BMS may not be, and a continuous rating below the calculated current will trip on overcurrent in normal operation. Not in a fault condition — in normal operation, which is the part that gets missed in review.

Check the following as a set:

l Continuous discharge current, quoted at the temperature the battery will see rather than at 25°C

l Peak discharge current and how long it is permitted

l BMS overcurrent protection settings and recovery behaviour after a trip

l Inverter surge requirement

l Cable cross-section, length and connection resistance

l Fuse and circuit protection ratings

Where several high-power appliances are installed, evaluate 24V or 51.2V at this stage rather than later. Doubling system voltage halves the current for the same power, which simplifies cable sizing and cuts resistive loss. The change reaches further than the battery: inverter, chargers, DC-DC converter and distribution all have to match, so the decision belongs here, before the configuration is frozen.

5. Configuration

12.8V LiFePO4 suits conventional 12V house systems. Larger and higher-power platforms generally move to 25.6V or 51.2V — for the current reasons in section 4, not for extra capacity.

5.1 Single battery or parallel bank

A single high-capacity battery means fewer interconnections, one BMS, one set of terminals, and less documentation to get wrong. It suits platforms with a fixed compartment footprint, provided the pack meets the continuous discharge requirement and physically goes through whatever opening the vehicle provides.

Parallel connection raises capacity and holds voltage. Two 12.8V 200Ah units in parallel: 400Ah at 12.8V, about 5.12kWh.

Parallel operation comes with requirements that are easy to state and easy to skip:

l Identical model, specification and BMS firmware

l Consistent cable lengths and cross-sections across strings

l Correct connection practice and overcurrent protection for each string

l Verified current sharing between units under load

l BMS coordination, including what happens when one unit disconnects

l The manufacturer’s approved parallel limit

Not every battery is designed for unrestricted parallel operation. The permitted number of units is a specification item, not an installation preference, and it should be obtained in writing before the configuration is finalised.

5.2 Installation space

Available space decides the configuration more often than the electrical calculation does. A single large battery is preferable where the compartment has a fixed footprint; several smaller units give more freedom in an awkward layout. Low-profile formats and customised enclosures widen the options in campervans and compact motorhomes, where height is usually the binding constraint rather than floor area.

For OEM projects, dimensions, terminal positions, mounting and cable access should be confirmed and frozen before the vehicle layout is signed off. A dimensional change found after tooling, or after the floor plan is released, is expensive to absorb.

6. Monitoring and Communication

An off-grid system depends on knowing how much energy is left. Voltage cannot tell you. The discharge curve of a LiFePO4 cell is flat across most of its working range, so a pack at 13.0V may be at 40% or at 85% state of charge. The BMS has to report, at minimum:

l State of charge

l Battery voltage

l Charging and discharging current

l Battery temperature

l Protection and fault status

l Remaining capacity, where the BMS supports coulomb counting

CAN bus and RS485 are the usual interfaces to inverters, displays and energy management systems. Sharing an interface is not the same as being compatible. Protocol, message structure, data definitions and the supported function set all have to match, and two devices on the same connector at the same bit rate can still fail to read each other.

For OEM and integrator projects, get the communication documentation first, then run an integration test with the intended inverter, display and charger before the battery is approved for production. Testing after the first shipment turns a design question into a field failure, and it lands on whoever placed the first order.

7. Temperature and Environment

RV systems run across a wide band of conditions. A vehicle can go from a warm coast to a cold mountain inside a week, and the compartment runs hotter than ambient in summer and colder at night. Both ends change what the battery will accept and what it will give.

Low-temperature charging needs the most attention. LiFePO4 cells have a manufacturer-defined minimum charging temperature, and charging below it causes lithium plating — irreversible, and it surfaces as capacity loss long after the event that caused it. Protection logic may block charging until the cells warm up. That is correct behaviour, and it leaves the customer with no charging on a cold morning. Self-heating batteries address this, and are worth considering for Canada, northern Europe and other cold markets.

Where a heating option is specified, put the following in the specification rather than the brochure:

l Heating activation temperature

l Heating power consumption

l Heating time under stated conditions

l Charging permission logic

l Low-temperature discharge limits

l Thermal protection settings and sensor arrangement

Heating draws from the bank it protects, so its consumption belongs in the energy budget. A 40W heater over eight hours is 320Wh, 16% of the 1.96kWh daily load — and it draws that in the season when solar output is at its lowest.

At the top end, cycle life is rated at 25°C, and a compartment on a sun-facing wall in summer is well above that. Every LiFePO4 chemistry loses cycle life as temperature rises, so the useful comparison between suppliers is the derating curve, not the headline cycle count. Ask for the figures at 35°C and 45°C, and for the maximum charge temperature.

Mechanical conditions belong in the same document. Locker floors and chassis rails transmit vibration and shock, and compartments see dust and water. Enclosure construction, terminal torque, mounting arrangement and ingress protection should be defined on the drawing rather than left to installers to work out on site.

8. Worked Example: A Three-Day Off-Grid Motorhome

A medium-sized motorhome, specified for three days without charging.

Step 1 — usable energy required: 1.96kWh × 3 days = 5.88kWh

Step 2 — nominal energy required: 5.88kWh ÷ 0.90 = 6.53kWh

Step 3 — capacity in amp-hours: 6,530Wh ÷ 12.8V ≈ 510Ah

Preliminary requirement: 12.8V 510Ah. A practical specification is 12.8V 560Ah, which gives 7.17kWh nominal and, at 90% usable DoD, about 6.45kWh usable. The margin over the 5.88kWh requirement is roughly half a day of consumption, which covers load variation without paying for capacity the vehicle will never draw down.

Three things qualify that result.

First, the energy calculation does not check power. Two 280Ah units with adequate continuous ratings will run a 2,000W inverter. Units rated below the 174A in section 4 will not, whatever their capacity says. Run the current check separately.

Second, the daily load in section 1 already includes inverter losses. Applying an efficiency factor again at step 2 counts them twice and buys capacity the vehicle does not need.

Third, the example assumes no charging. With a 400W array and a realistic winter sun-hour figure, the requirement falls toward 350–400Ah, as section 3.1 shows. Which case applies is a question about how the vehicle will be used, and the assumption should be stated in the specification so it can be reviewed if the intended use changes.

9. Selecting a Supplier

For a manufacturer, conversion company or distributor, choosing a supplier is not a comparison of nominal capacity and purchase price. What is being bought is a partner who understands RV electrical systems and can support integration. That capability shows up in what a supplier can produce on request.

A supplier with its own manufacturing and engineering can adapt dimensions, enclosure, BMS parameters and communication interface to the platform, instead of picking the nearest item in a catalogue. Ask what the factory can change, and how long a change takes to reach production. The second answer is usually the more informative one.

Conclusion

Off-grid sizing comes down to five inputs: daily consumption, autonomy, charging that can be counted on, discharge current, and the conditions the vehicle will operate in. Each one moves the result. None can be assumed away without moving the risk onto the customer.

Capacity should land where the vehicle needs it, not at the top of the range. Enough usable energy, enough current for the peak load, a pack that fits the compartment and the weight budget, and behaviour that holds up at the temperatures the vehicle will actually meet — those four define a correct specification. Anything above is capacity the customer pays for and never uses.

For manufacturers and conversion specialists, a bank sized this way delivers the off-grid performance the vehicle was sold on, without adding cost the platform cannot justify.

ODIPIE manufactures LiFePO4 battery solutions for RV, campervan, marine, golf cart and residential energy storage applications. The RV range covers 12.8V, 25.6V and 51.2V configurations, with OEM and ODM options for battery dimensions, enclosure design, BMS settings and communication interfaces.

Whether you are developing a new motorhome platform, upgrading an existing electrical system or sourcing lithium batteries for your own brand, we can support the project from the load list through to a production specification.

Contact ODIPIE to discuss your RV lithium battery requirements and OEM/ODM opportunities.