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When we were converting our van, I was keen to make sure that we would not routinely need mains hookup to keep the leisure battery charged. Indeed, I didn’t really ever want to need mains hookup as we normally stay on aires and wild camp, meaning that mains electricity is usually unavailable.
After reading around and seeing what other people had done (the SBMCC is a great source of information on this subject), I decided to splash out a little and purchase a Sterling Power Battery-to-Battery Charger, which promised to provide fast, efficient charging for our van’s leisure battery whenever the engine was running. As we rarely stay in one place for more than one or two nights, this sounded perfect for us.
What Is A Battery-to-Battery Charger?
Sterling Power makes a range of 12V and 24V power devices that are mostly aimed at the marine market but that also work well in motorhomes. Sterling equipment has a very good reputation and the company’s founder, Charles Sterling, is a very experienced and passionate engineer.
The Battery-to-Battery (B2B) charger is a sophisticated, multi-stage charger* that connects between the vehicle battery and the leisure/auxiliary battery. It charges the leisure battery to the maximum capacity possible as fast as possible, using surplus power taken from the alternator, via the vehicle battery.
B2B chargers do not need to be connected to the vehicle’s alternator, nor do they need mains connections. The aim is to make the vehicle completely self-sufficient, electrically.
*To charge a battery fully, it must be charged in several stages. Each stage uses a different combination of charging voltage and charging current. Doing this enables the battery to be charged as fully as possible.
Installing the B2B Charger
One of the attractions of the B2B charger was that installing it is very simple, especially if, like me, you have a van with the vehicle battery under the driver’s seat (Mk.6 Ford Transit). The first job, before installation, is to set the jumper switches on the charger for your type of leisure battery – AGM, gel, sealed lead acid and open lead acid batteries all have different charging programs.
Installation is as simple as running one suitably fused cable from the vehicle battery to the charger, and another from the leisure battery to the charger (plus a ground connection to the van’s body). I also opted to fit a battery isolator switch between the vehicle battery and the charger, so that I could completely isolate the motorhome electrics from the vehicle electrics, should I want to. I think this is a sensible precaution in case of problems with the vehicle or the motorhome electrics.
The charger also comes with one or two optional temperature sensors. These can be connected to the alternator and/or to the leisure battery. If either temperature gets too high, the B2B charger will go into standby for a while to allow the temperature to fall. I would recommend installing these, despite the hassle of fitting a temperature sensor to the alternator.
The quality of the charger seems excellent – the casing and fittings are very well made and should last well in automotive use. The instructions are also good and are comprehensive, assuming you know basic 12V electrics.
In use, the B2B charger is designed to be a ‘fit and forget’ piece of equipment. It automatically goes into standby when the engine is not running and it is programmed to make sure that the engine battery gets enough charge to keep it healthy. It then devotes the rest of its time to charging the leisure battery as fast and fully as possible. It does this by ‘tricking’ the vehicle’s alternator into thinking that the vehicle battery is flat. The alternator then works flat out to provide as much power as possible to the vehicle battery. Instead of charging the vehicle battery, which doesn’t need it, this power is siphoned off by the B2B charger and used to charge the leisure battery.
Once the leisure battery is charged, the B2B charger will go into maintenance mode, just keeping it topped up as necessary.
It Sounds To Good To Be True – Is It?
The B2B charger is a serious piece of kit that does a good job. However, it is not cheap and it does need to be installed as part of a balanced, well-designed system. Its fast charging regime also means that open lead acid leisure batteries (and starter batteries?) will need to be topped up fairly regularly, due to losses from gassing.
Price: At the time of writing, Sterling Power is selling the BB1250 model that most motorhomers would choose for £349. It was considerably cheaper when I bought ours but it still cost us around £200, a couple of years ago. This makes it a pretty expensive battery charger.
12V System Design: The second consideration when choosing a B2B charger is that it is a powerful piece of equipment that can place a heavy load on your van’s electrical generation system.
What this means is that if your leisure battery is too heavily loaded for too much of the time, the B2B charger will transfer that load onto your van’s alternator and starter battery by maintaining a very high rate of charge for long periods of time. Based on my experience with the charger, I can envisage situations where this might cause problems. This is one reason I would recommend that you fit the alternator temperature sensor.
Of course, none of this will happen if you have a balanced 12V system – one in which the demands of the electrical equipment (e.g. fridge and inverter) can comfortably be met by the leisure battery(s).
The Sterling Battery-to-Battery Charger can be a good solution for motorhomers who are keen to reduce or eliminate their need for mains hookup.
If you are not too bothered about this – if you regularly stay on campsites, for example – or if your electrical needs are quite modest, then the B2B charger is probably overkill, considering its cost. Your motorhome’s mains-powered battery charger will do what is necessary, combined with a simple split-relay charging setup from the alternator (standard in most professional motorhome conversions) for when you are driving.
Model Reviewed: BB1250 Sterling Battery-to-Battery Charger 12V-12V 50A (£349.90 direct from Sterling)