I’ve been struggling with 12v electrics right from the start of planning my camper conversion. This is the final system that’s being installed.
The problem was my knowledge of 12v electrics was close to nothing. I would say after a few months research and learning, I now have a good understanding of the subject.
12v electrics are probably the single most important part of my van. As I’ve mentioned before I work online. If I can get a PC working reliably in the van with a good internet connection I can work from the beach instead of my office. This means instead of using the van for maybe a weekend a month I can use it for say three days a week. I won’t be on EHU (electric hook up) so my sole source of power is the 12v system.
The first thing to decide on was the batteries. After working out how many amp hours I will need (See Here) the next thing is the best way to charge them and keep them in good condition. The three ways I will be able to charge my leisure batteries are:
1. From the van alternator whilst driving
2. Via a battery charger when at home or on a site with EHU
3. Solar Panels
I’ll start with the solar panels.
From the start I was advised I would need solar panels along with a MPPT (Maximum Power Point Tracking) controller to supply the charge from the solar panels to my leisure batteries.
After a lot of research I decided on two 100w Monocrystalline panels with the option of fitting a third at a later date if I found I needed it. I went with Monocrystalline as they supply more power than Polycrystalline in lower light which is important with UK weather.
Photo shows my two solar panels fitted with space at the front for a third if needed.
For the MPPT I bought a 25a model from China that was recommended to me. It was quite cheap and can take 300w of solar panels. There is a good video review of this unit here:
So that was the solar set-up completed, time to look at charging from the van alternator.
There seems to be two main ways of charging from the van alternator. The cheaper way using a VSR and the more expensive way, using a Smart Charger.
A VSR is a Voltage Sensing Relay. It works by connecting the starter battery to a high amperage relay, and then to the Leisure battery(‘s) with a heavy duty battery cable (size of cable depends on how long the cable is and how many amps are going through it). A VSR is then connected to the high amperage relay. The VSR senses when the vehicle is running and opens the connection on the high amperage relay to allow the leisure battery(‘s) to be charged. When the engine is switched off the VSR senses the voltage drop and switches the high amperage relay off. This stops the engine battery from being flattened by the leisure batteries. This is known as a Split Charge System.
High amperage relay on left with VSR on right
Some VSR’s can also be connected to the 12v input of a 3 way fridge so that the fridge is on whilst the engine is running but switches off when the engine switches off, again to stop the starter battery from being flattened. This is what I originally bought.
The more expensive way is to use a Smart Charger. A Smart Charger is connected between the starter battery and leisure battery(‘s) the same way as the high amperage relay above. I’ll come back to this later in the article.
The third way I will have to charge my leisure batteries is via a battery charger on EHU
For this I just need to decide what the best charger for my set-up will be. After doing a lot of research and getting advise from others I decided the CTEK MXS 25 would be the best choice for me. It is a Smart Charger that charges the batteries in multiple stages that enable the battery to be FULLY charged.
The CTEK MXS 25 is a 25a charger and is suitable for battery banks from 40ah – 500ah.
I bought all of the above and was ready to install them. The thing is, I wasn’t happy with my choices
As I mentioned at the beginning of this article, a few months ago my knowledge of 12v electrics was bad and I had started to buy the above items without understanding what I really needed. After learning all I could I now have a very good understanding of 12v electrics. I understand how batteries work, their characteristics and how they like to be charged. It was time to rethink my system.
The first problem was with the Split Charge System. Whereas a system like I had bought will charge my leisure batteries it wont charge them fully. A Split Charge System will only charge the leisure batteries to about 85% due to the way it works.
The solution was to buy a Smart Charger. A Smart Charger charges the battery(‘s) in different stages that enable a battery to be fully charged. Now I had to choose which Smart Charger to buy.
Initially I was looking at a Sterling B2B charger that looked great. It charges at a rate of up to 50a per hour and came well recommended by most.
I say most because a few people who have the unit have had trouble with it conflicting with their MPPT.
I did more research and found that CTEK make a unit called the D250S Dual.
The CTEK D250S Dual is a combined Smart Charger and MPPT. This means it will not only charge the leisure batteries from the alternator when the vehicle is running but also charge them from the solar panels whenever there is light. This unit charges at a maximum rate of 20a per hour which would be fine for a single 100a battery, but not very effective on my 400ah of batteries. I found CTEK also make another unit called a SmartPass. The SmartPass is a separate product that can combine with the D250S Dual and boosts its charging all the way up to a maximum output of 100a per hour.
I was very interested in these two units so started to find out more about them. I found that as well as keeping my leisure batteries in tiptop condition they would also charge my vehicle starter battery from my solar panels whilst the vehicle was parked up. As I’ve had problems with the starter battery discharging this is a big plus point. It seems to be a common problem with motorhomes as they can be sat without running for long periods of time and just an alarm system can drain the battery. This video shows how the units work together:
This left charging the battery whilst on EHU, either at a camp-site or at home.
I had already decided to use a smart charger (CTEK MXS 25) to charge the leisure batteries when 240v was available. This was still the best option. The difference was now I could combine it with the D250S Dual and Smartpass so my 240v charger will also charge the vehicle starter battery.
These items together will keep all of the batteries in my vehicle, leisure and starter, in optimal condition. They will charge the leisure batteries fast whilst driving, they will keep all the batteries topped up if the van is not in use for a while and they will optimise solar power. Best of all, once these are fitted I can forget about them as they all work automatically with nothing to switch on or off if I say, switch to EHU.
this was the answer. the most efficient charging system that i can find commercially available.
The CTEK D250S Dual with Smartpass and a CTEK MXS 25 giving a total charging solution. A system that I am, at last, very happy with.
I have all of these units here now ready to fit along with a Nasa BM1 to monitor 12v batteries.
I hope to get it all fitted at the end of this week (I’m still waiting for a few cables and fuses).
I’ll write an update to this post once it’s all fitted on how I fitted it all and why I did it the way I did. For now I’ll finish with a diagram of the whole 12v system that’s being installed. Thanks for reading and if you have any questions just post in the comments below.
Click drawing to view full size.