The Following article is written by Dave Newell of Dave Newell Leisure Vehicle Services. A motorhome enthusiast and long time sponsor of MotorhomeFun; Dave provides a high quality service to leisure vehicle owners, whether the vehicle is a car, caravan or motorhome. Dave fits all sorts electrical equipment is also able to source most components for leisure vehicles.
Lots of people ask me about leisure batteries and charging them and there is an awful lot of misinformation about regarding this so I’ll try and clarify a few points for you. Think of the battery as a fuel tank and it becomes obvious that you have to “fill” it before you can use it. There are several ways of charging a leisure battery, it should take charge from the alternator when the engine is running, it can be charged via a mains powered charger, it can be charged via a solar panel or from a 12 volt generator such as the two stroke Gasperini unit or a fuel cell. How long it will take to charge will depend on three main factors, the capacity of the battery, the present state of charge of the battery (how full or empty it is) and the available charging rate (how many amps the charging system can supply).
Charging from the engine driven alternator on the face of it should be the fastest method because the alternator is often capable of 90 amps or more, if the battery capacity is 90 Amp Hours and it is completely flat then the alternator could recharge it in about one hour yes? Unfortunately not as the alternator output may well start out at a high level but it will very quickly drop off to a low level, effectively a trickle charge. One way to improve this situation is to fit a Battery to Battery (B2B) charger such as the Sterling or CTEK offerings, these devices fool the alternator into producing higher current for longer periods to recharge the battery faster. The most popular Sterling unit can pass 45 amps while the CTEK unit does 25 amps.
The on board mains charger fitted in most European motorhomes is generally rated at around 20 Amps so that would take 4.5 hours to recharge our battery, many on board chargers are so called “intelligent” units in that they constantly monitor the battery condition during charging and once it is full they will cut off and go into a maintenance mode, older more basic chargers were quite capable of overcharging a battery because they were a simple unregulated voltage transformer often capable of up to 17 volts output.
Solar charging can be very effective but it does need sunlight (obvious I know). Solar panels have a large surface area for a relatively low output, a 130 watt panel has a surface are of nearly two thirds of a square metre and at best can be expected to give about 7-8 amps, at this level it would take about 11 hours to fully recharge our battery although in reality it will take longer (probably at least twice as long) because the panels output will vary considerably as the sun travels across the sky.
12 volt generators are a good solution although they do burn expensive fuel and produce noise and fumes. The Gasperini EG20 unit has a two stroke motor driving a dynamo which can give up to 20 amps so in theory at least this would take 4.5 hours to recharge our battery. Efoy fuel cells are an alternative type of 12 volt generator, they produce much lower current but are near silent and can therefore run overnight to recharge the battery. The downside of fuel cells is their cost.
At the recent New Years meet at Wrenbury I gave my leisure batteries a serious workout. In our Iveco Turbo Daily camper we have 3x 110AH leisure batteries (330AH total capacity), there is a 155 watt solar panel on the roof and a CTEK 25 amp B2B unit. Before we left home I had the van on mains charge (CTEK 7 amp charger) for 24 hours to make sure we had the maximum life in the batteries. I was a little surprised to find that by teatime on Thursday (2nd day) the batteries were down to 12.2 volts ( fractionally over 50% state of charge)and as its not a good idea to repeatedly discharge batteries below 50% I dug out my little generator and hooked up my workshop CTEK 25 amp charger. Four hours later we were back to around 80% state of charge. Two days later I again needed to run the generator to recharge the leisure battery bank.
This makes our batteries look as if they are not in the best of health but in fact they are fine, you need to look at how much power you use before you condemn the batteries. We have a 100 litre compressor fridge, we watched TV for an average of three hours a day and listened to the radio for an average six hours a day. Our Propex heating was running for an estimated 10 hours a day plus there’s the lights and water pump to consider. I measured the current consumption of each item individually then multiplied the current by the hours used in a day to arrive at a reasonable estimate of power used, the fridge runs for about 15 minutes per hour hence the six hour figure against it.
As you can see the bigger users are the fridge and the heater but even the two relays controlling the fridge and lights add up to 9.6 AH per day. You can see that after two days we had used close to 50% of our capacity. As for the solar panel well all I can say is its performance was dismal but so was the weather and with the sun so low in the sky its hardly surprising, the highest current I saw from the panel was just 1.4 amps at about 11 am on the brightest day we had. I would estimate the panel managed to produce approximately 2 AH per day on average. One area I intend to improve on is the control relays, I’m presently using ordinary automotive 30 amp relays which take 0.2 amps each to keep on. The circuits they control don’t carry anything like that current so I can switch to smaller relays that will draw a lot less current themselves.