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Motorhome batteries explained



MMM is occasionally offered products to Review. So after a visit from CTEK, which makes battery chargers, we were offered a D250SC Battery-to-Battery charger.

RoadPro, one of CTEK’s official dealers, installed the charger in MMM’s long-term test at the time, the Bürstner. Here, the RoadPro team helps explain how the electrical system in a motorhome works.


It is worth spending the extra money to get a good battery. This benefits not only the overall battery performance but also the length of time the unit will last.

Paul Chamberlain of Northampton Motorhome Services, who works for RoadPro and installed the charger said: “I learnt the hard way. I’ve never kept a battery longer than a year, but I buy cheap ones.”

The advice is also to get two batteries if you have the space, As we’ll explain later, the less stress you put a battery under the longer it will last.

Batteries are constantly discharging, so if you leave your motorhome for longer periods, a solar panel could keep the battery charged. Batteries only have so many charge and discharges, called cycles. The deeper the cycle (the more power you use before charging the battery up again) the fewer cycles the battery will provide.

They are rated at a certain amount of amps at a certain rate of discharge. It is therefore obvious to most that withdrawing lots of volts at the same time (like using a microwave on an inverter) can kill the battery quicker.

So if you can keep your battery constantly topped up (by solar or other means), it should last longer.


Traditional flooded lead-acid (FLA) batteries are commonly known as starter batteries. Similar to car batteries, they provide massive currents for short periods of time (to start a vehicle) and to be re-charged almost immediately afterwards at quite high charging currents.

Leisure batteries are different from car batteries though, with much thicker plates to better withstand more frequent and more intense charge/discharge requirements.

Sealed lead-acid (SMF) batteries offer the same technology as flooded lead-acid batteries but the battery is sealed. These batteries have a recombination lid, with cells to trap the hydrogen and oxygen gases and condense them back to liquid within the battery.

In AGM (absorbed glass mat) batteries, the electrolyte is stored in an absorbed rather than liquid form. The battery acid is soaked up in a form of fibreglass blotting paper and sealed to guarantee no spillage and no maintenance.

The plates are of deep-cycle construction (thicker). They are often termed VRLA or valve-regulated lead-acid batteries, which refers to a safety valve feature to stop any emission of gas from the battery.

Gel batteries are similar to AGM, but the electrolyte is stored in a gel rather than absorbed form, enabling them to withstand more extreme conditions and temperatures. The only drawback to gel batteries is that they require a specific charger and care is required when charging. If gel batteries are charged at too high a rate, you’ll actually lose some of the electrolyte through gassing and drying out of the battery (shorter life).


All 12V batteries are composed of six 2.1V cells each made up of plates. Thin plates charge quickly, but can buckle with overcharging. So leisure batteries need thicker plates which are more resistant to the abuse from a typical motorhome power system.

Vehicle starter batteries last for years because they have a much easier life. The constant deep discharge and recharge cycles of a leisure battery shorten its life considerably.


Most motorhomes are fitted with a standard charging system. The Elektroblock fitted to the Bürstner controls its electrics. Some devices will charge both the starter and leisure battery depending on your vehicle, but this is one question you should ask when buying a new ‘van, especially in the handover.

When driving most older ‘vans, the leisure battery is charging at a very basic input. What you need to be aware of is that a fully charged battery should read with an output of about 14.1V. If the meter says anything between 11.5V and 12V, the battery is essentially flat and there is no power.


There is a host of options and upgrades that will improve power supply and maintain a battery. Peter Rosenthal (June, p197) fitted a mains battery charger recently, which will ensure a battery is kept well charged when hooked up.

These devices control the charge into the battery, to improve a battery’s performance. If you spend a lot of time wild camping, consider battery-to-battery chargers like the one that we had fitted.

If you are electrically minded, you may be able to fit this device yourself. The process is simple, but may take longer depending on where the leisure battery is in relation to the vehicle battery.

Solar panels on motorhomes are a contentious issue, some would never be without a panel while others claim they are a waste of time.

Andy, the owner of RoadPro, said: “I’ve changed my mind about solar panels and think they actually do well.” This was after RoadPro had fitted one onto a Niesmann+Bischoff A-class and the meter showed real variation when the sun was out, although the panel was still supplying charge when the clouds rolled in.

Solar panels are ideal if you have the roof space, the budget (decent panels are not cheap) and want your batteries always charged to optimum.

And then there are devices that provide MPPT (maximum power point tracking) for solar panels. These devices are like intelligent battery chargers for solar panels, monitoring the solar input and maximising the output accordingly.

Devices like these can mean that you may get between 25 to 30 per cent more power from your panel, especially on days without direct sunlight.

Fuel cells are another option, but they are expensive and offer a similar level of charge as solar panels. You don’t need to rely on the sun, but you do need methanol, which powers the most common ones – like EFOY. Essentially fuel cells are battery chargers, offering 12V top-up for your leisure battery.

Click here to download the feature, originally printed in the August 2011 issue of MMM.

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