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If you have a camper trailer, caravan or other RV and you like to spend time away from 240V mains power, this article will help you estimate the size and number of batteries that suit your camping.
The aim is not to make the calculation complicated. You only need a practical estimate of how much power you use each day, how much of the battery you can safely use, and how quickly you can put the power back in through mains charging, DC charging, solar or generator charging.
A blank calculation sheet can be downloaded near the end of this article, but it is worth reading the explanation first so the numbers make sense.
See also our related guides: Lithium vs AGM Batteries, Caravan & Campers Guide to Battery Chargers and Complete Guide To Installing Solar Panels.
The simplest way to size an RV battery is to work out your daily usage in amp hours. Once you know that, you can decide how much battery capacity you need and how much charging you need to replace it.
The basic process is:
That last point is important. A battery that looks big enough on paper can still disappoint if it is not fully charged, is old, is discharged too deeply, or is being asked to run a heavy inverter load.
The amount of electricity used to run a device or appliance may be defined as watts or amps. If you know one, you can calculate the other, provided you know the voltage.
We want to deal in amps and amp hours, because deep cycle batteries are rated in amp hours. It is widely accepted that deep cycle batteries are the best option for RV house battery use. In the past we recommended AGM as the best general RV choice, but as lithium batteries have become more affordable, lithium is now often worth the higher initial setup cost.
A 100Ah battery should theoretically provide 1 amp for 100 hours, 2 amps for 50 hours, 3 amps for 33 hours and so on. It would be nice if this equation held true all the way up to 100 amps for 1 hour, but there are limits to maximum current draw and how much of the rated capacity you can use without shortening battery life.
A 10 amp appliance running from a 100Ah battery gives the simple estimate of 100 / 10 = 10 hours of usage. If you used it for 5 minutes a day, that sounds like a long time. In real life, high draw loads and battery limits reduce the usable result, so keep reading before taking the simple number too literally.
If a device specification does not include amps but does include watts, use this formula:
| Formula | Watts / Volts = Amps |
|---|
For 12V appliances running from a 12V battery, divide watts by 12V. For example, a 1.2 watt LED light running on 12V uses 1.2 / 12 = 0.1 amps. A 100Ah battery could theoretically run it for about 1000 hours.
For 240V appliances running through an inverter, calculate the load back at the 12V battery side. This is where many people get caught. A 240V microwave may only draw a modest current on the 240V side, but the inverter has to pull a much larger current from the 12V battery to make that power.
As a rough guide for inverter loads:
| Step 1 | Watts / 12V = approximate 12V amps |
|---|---|
| Step 2 | Add an allowance for inverter losses |
| Step 3 | Multiply by hours used to get Ah per day |
For example, a 400 watt appliance through an inverter can pull around 33 amps before losses. If it runs for 1 hour, that is roughly 33Ah plus inverter losses. From a 140Ah AGM battery, you would not want to run that sort of load continuously for long.
High-current inverter installations need correct cable size, fusing, isolation and ventilation. If you are wiring a large inverter, use an auto electrician or qualified installer.
A 60 watt fridge running on 12V uses 60 / 12 = 5 amps while the compressor is running.
The fridge motor does not usually run all the time. If it runs 15% of the time, the average use is 5 amps x 15% = 0.75Ah per hour. Over 24 hours that is about 18Ah per day.
Fridge use varies heavily with temperature, ventilation, thermostat setting, how often the lid or door is opened, and how warm the food or drinks are when they go in. The same fridge may perform very differently at Thredbo in winter compared with Darwin in summer.
Three-way absorption fridges on 12V are usually intended for use while driving and can draw heavily. Compressor fridges are normally much more practical for battery and solar setups, but they still need enough battery capacity and charging input.
Amp hours are the rating used by deep cycle battery manufacturers as a way to compare batteries, but you cannot simply use 100% of that rating in normal camping and expect long battery life.
If you have a 12V battery system, one of the most useful items you can have is a voltmeter. It plugs into a cigarette lighter socket and shows the voltage of your batteries. A multimeter will also work.
When you have finished charging your battery you may see a reading well over 13V. This can be surface charge and will settle to a more realistic reading. If you have a small load on the battery, such as a couple of lights, the voltage reading can be compared with the chart above to estimate remaining charge.
A moderate or heavy load will distort the voltage reading until the batteries have rested and the charge equalises.
Looking at Figure 1, 12.1V on an AGM or wet cell battery is roughly around 50% capacity remaining. Once you get closer to 30% remaining, the voltage will not be sufficient for many appliances. A light may still work, but a water pump may not perform well. Repeatedly using lead-acid batteries below these levels also reduces the number of recharge cycles you get.
Most battery manufacturers indicate you will get the longest life from a lead-acid battery by discharging only to around 50%. In an RV, it may be realistic to occasionally discharge lower during longer stays, but you should understand that it can shorten battery life.
Lithium batteries are different. Many lithium batteries can safely use a deeper percentage of their rated capacity than AGM or wet cell batteries, provided the battery management system, charger, DC-DC charger, solar regulator and cabling are suitable for lithium. Do not just swap chemistry without checking the whole charging system.
The time it takes a battery to equalise is something you get used to. Do not panic if voltage drops while using a microwave or another inverter load for a couple of minutes. A low load that draws power no faster than the battery can equalise will give a more useful voltage reading.
Be aware that a battery under no load at all may still show 12V but have little useful capacity left. A proper battery monitor with a shunt gives a better estimate because it measures actual current in and out.
Other more economical voltmeters are shown here:
Battery type affects usable capacity, voltage readings, charging requirements and service life.
Wet cell, Gel, AGM and lithium batteries can all behave differently. Voltage is useful, but it is not a fuel gauge. Petrol in a tank drops in a fairly predictable line. Battery voltage does not.
AGM deep cycle batteries accept charge easier and faster than wet cell batteries and recover better from lower discharge. They cost more than wet cell batteries, but with correct usage they can last longer.
Not everyone will benefit from using large deep cycle batteries. If your predicted usage is much lower than the capacity supplied by a larger AGM or lithium setup, a simpler battery system may suit. If you leave 240V only one or two days at a time and use only LED lights and a water pump, you may not need a large bank.
CaravansPlus does not sell wet cell batteries, as they are harder to ship and are readily available in most locations. Wet cell batteries need to be upright and located in a ventilated area that is not in the living area. AGM batteries are sealed and more flexible for RV installation. Lithium batteries need compatible charging and protection equipment.
If adding a second AGM battery to an old existing AGM battery, do not assume the result will behave like two new matched batteries. Mixed age, mixed capacity or mixed chemistry batteries can cause poor performance. For a reliable bank, use matched batteries and ask an auto electrician if you are unsure.
Battery capacity decreases as the discharge rate increases. Many 100Ah batteries are rated at a 20 hour discharge rate. That means 5 amps for 20 hours: 5 x 20 = 100Ah.
If you exceed that discharge rate, you can see a significant drop in usable capacity. If you need more than about 5 amps per hour from a 100Ah lead-acid battery for long periods, consider a larger battery bank or a different setup.
The good news is that if you use less than the rated discharge rate, you may get more usable capacity than the simple rating suggests.
Inverters deserve special attention. Just having an inverter switched on uses power, even with no 240V appliance running. Switch it off when not required.
Large 240V appliances such as microwaves, kettles, pressure washers, power tools, coffee machines and hair dryers can draw heavy current from the 12V side. A big inverter is not useful unless the battery bank, cable, fuses, charger and installation are sized to suit it.
Diesel heaters also need to be included in your calculation. They may draw a higher current for a few minutes while the glow plug starts the heater, then drop to a lower running current for the fan and fuel pump. Check your heater specifications and include both start-up and running use if you camp in cold weather.
One of the best customer comments on this article was that battery capacity is only half the story. You also need the ability to recharge efficiently and in time.
Two 100Ah batteries may sound like plenty, but if the only charger is a small 7 amp 240V charger, it can take a long generator run to replace a large discharge. Solar can also be overestimated if the panels are shaded, flat, dirty, undersized, or used in poor weather.
When charging your batteries from a good battery charger over a number of days, you can get to 100% charge. However, vehicle alternator charging can be limited by voltage drop, smart alternator behaviour, cable size and the charging equipment fitted.
Voltage drop is one of the biggest problems in getting the battery in your caravan or camper trailer fully charged from your vehicle. You can never charge the battery more than the voltage that reaches it through the copper cables. The longer the cable, the more the voltage drops. This can be reduced by increasing cable size, but even that has limits.
With a voltmeter, you can check what voltage your alternator is producing by starting your vehicle and connecting your voltmeter to the battery terminals. Ensure electrical accessories are not switched on. This should typically read from about 13.8V to 14.8V.
Next, test voltage at the rear of the vehicle with the engine still running. This may be at an Anderson plug or the auxiliary wire in the trailer connector. Record this voltage and note the drop. Then take a reading where the auxiliary cable reaches the RV battery or battery charger.
There are two common solutions. First, increase cable size so voltage drop is lower. Second, use a suitable DC-DC charger or voltage booster between the vehicle battery/alternator and the RV battery, so the battery receives the correct charge profile.
After allowing for charging limits and a sensible discharge limit, a practical rule of thumb is that lead-acid battery capacity should often be around twice the daily amp hours you calculate. This gives a buffer for lower solar input, longer stays, battery ageing or higher use than normal.
Use the following chart to calculate the number and size of batteries you need. You can download a blank calculation form and enter your battery size to estimate how long it will last.
Notes from the example:
When the amp hours for all devices are added up in the example, the daily use is around 13.5Ah per day.
Anything that exceeds the battery's comfortable discharge rate can reduce usable capacity and battery life. In the example, inverter use pushes the setup towards two batteries. For many people, a single AGM 100Ah to 120Ah battery may be enough if they have a gas fridge, LED lighting, modest water pump use and no heavy inverter loads.
You can download an Excel sheet to fill in your own values.
See also: Complete Guide To Installing Solar Panels.
Because the battery is supplying the power on the 12V side of the inverter. The appliance may be 240V after the inverter, but the battery still has to supply the watts from 12V, plus inverter losses.
Possibly for a short time, but it is a heavy load. 400W / 12V is about 33 amps before inverter losses. A 140Ah lead-acid battery will not deliver its full rated capacity at that draw, and you should avoid deep discharge. A lithium system may handle this better if the battery, BMS, inverter, cables and fusing are rated for it.
That points to a problem beyond normal LED lighting. Possible causes include a battery failing under load, a hidden load, poor connections, incorrect meter reading point, or charging that is not actually reaching full capacity. Have the batteries load-tested and check current draw with everything switched off.
Sometimes, but it is best to add matched batteries of the same type, age and capacity. Adding a new battery beside an older tired battery can give poor results. Also check the charger and solar regulator can correctly charge the larger bank.
Solar sizing depends on daily amp hours, climate, shade, panel angle, regulator type and how many days you want to stay without driving or mains power. Battery capacity and charging ability must be sized together. A big battery bank with too little charging will still run flat.
Use an auto electrician or qualified installer for high-current wiring, large inverters, lithium conversions, DC-DC charger installation, battery relocation, or anything you are not confident sizing and fusing correctly. Incorrect 12V wiring can overheat and start a fire.
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