New 12v Electrics – Total Charging Solution

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.

IMG_1290    smartcompic

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.

28 comments on “New 12v Electrics – Total Charging Solution

  1. Hi,
    I’m currently doing a split charge system in my vivaro (more for work purposes than camping) I am now looking for a 240 charger, that I can plug in when Van parked on drive to charge my two 110 leisure batteries. Which charger would you recommend as I know you have done a lot of research on this matter.

    • Hi Craig. If your looking for a very good charger then the two I would recommend are the Numax Connect & Forget 20a which is about £95 or if you can stretch to it the CTEK MXS 25 which is about £180. I’ve tested both of these chargers and both are great. The Numax charges at up to 20a per hour and the CTEK up to 25a per hour. You could look at a lower amperage version of either of these to save some money but they would charge much slower. Hope this helps, David.

      • Thanks David,
        I have done alot of research today and I think the MXS 25 is expensive but most likely worth it! So think I will give one a go.
        How is your camper coming along? Do you mind me asking what you do as you are hoping to use the camper for work too if I am correct?

  2. The MXS 25 is what I have fitted, as you say it costs a little more but it’s a great charger. My van is coming along really well. I have quite a few articles almost ready to publish and a few more videos nearly finished, should have them on here within the next week.
    My work is a little varied. I own an online replacement kitchen doors company that pays the bills and I write articles here and there on a couple of subjects. I prefer the writing tbh but it doesn’t pay enough to do full time.

  3. Hello David, congrats on your work!!!! It is actually very professional…
    Just a short question for my understanding. By using the great ctek stuff means that you do not need an MPPT for your solar panels anymore????

  4. What size/rated busbars did you use? I understand the nasa bm1 comes supplied with the correct shunt but my knowledge isn’t great so don’t know what busbars I should us, thanks. This is a genial set up!

  5. Awesome stuff. I love the sound of this and might go this route. Just to clarify on parts:

    x 1 ctek d205s
    x1 dtek smartpass
    x 1 mxs 25
    and x 1 Nasa BM1 for monitoring?

  6. I really like your set up, and am considering using the Ctek D250S + Smartpass for my own (similar) set up. I can’t find many reviews for the Ctek product online – perhaps you could fill me in on how well the set up is working? It seems that I can get the D250S and Smartpass for the same price as a nice 40A MPPT Charge Controller, so I’m a bit hesitant, and have a feeling it’s “too good to be true”. The only real difference I can see between this set up and a nice MPPT Charge Controller is that the max. input voltage on this is 23V, while some MPPT controllers allow upwards of 150VDC… but charging voltage and amperage seem to be similar. How is yours working out so far?


    • Hi Doug, mine has been absolutely flawless – true fit and forget. My (cheap) batteries are still fine and it keeps my starter battery nicely topped up. If I were building another van I’d use exactly the same setup.

  7. hi david
    looks like your system works really well and largley due to you thourough research and thinking!
    Im embarking on our 07 sprinter conversion , started 2 years ago with windows fitted and a roof light.
    I now have time to continue and want it up and running asap.
    Trying to decide on charging solutions and electrics is as you say tricky….
    I like the look of the d250 s , we only want to run 12v lights, a 50 litre compressor fridge,(weaco crx50) , and have ability to charge phones , and a laptop. Also have ability of ehu charging (v occassional or at home use)
    I have 2 x 100 ah leisure batteries. plan to fit d250s , .and 2x 100w solar panels.
    In your opinion would this provide enough amps to keep the system running? basically im hoping i dont need the smartpass as our consumption will be minimal , (really mainly the fridge).

    Would welcome your thoughts based on your experience with this system.
    Many thanks

  8. Hi Dave. This info has been great. I have a vintage Citroen H Van converted to a camper and I installed a split charging system and leisure battery myself using a voltage sensitive relay. However, i keep getting flat batteries and can’t work out where the issue is. So I had decided to go back to the drawing board and I am going to base it on your system but with 1 leisure battery rather than multiple. Your wiring diagram is great. Can you clarify for me, should the negative bus be grounded to the chases at all? If not, is the only ground in your system that from the negative of the starter battery? Thanks again for the help.

    • Hi Chris, in my setup the system is grounded from the starter battery and the leisure batteries. I didn’t ground the negative bus bar – from memory this was something to do with the Nasa monitor. Could be wrong on that, just from memory. Good luck sorting it out, David

  9. Hi. I am very pleased that I stumbled on this post as I have been looking for a system for charging my leisure batteries. I do a lot of wild camping in the UK mainly because of my love of wildlife and especially at night time. I have been using a split charging system plus a 100W solar panel to keep my batteries charged and a smart charger to top them up when back at home. My two 110A leisure batteries have just given up on me after 3-4 years so I am looking for a different system. Whilst watching wildlife I use my laptop and night vision cameras with pan and tilt functions and I draw about 5-6 A for about 5-6 hours and on top of that during winter I have an Eberspacher diesel heater (I forget how much current it draws). What sort of system would you recommend to suit my needs especially for the winter months as it is a time where I use the most current. What type of batteries do you use and long do they last? I am planning to go away shortly and need to replace the present ones. I have read many articles on batteries and it seems traction batteries are more suitable, but this comes at a great cost. Any advice is greatly welcome. I am looking forward to take early retirement next year and build a bigger van so add a few extra AHr for a compressor fridge. Les

  10. Hi David. Massive thanks for the website. I was already half way through my build when I found it and coincidentally we seem to have gone with virtually the same setup with the same experience – even down to having a rather large and expensive Sterling B2B charger that I decided against using! So it was great to pop on here and cross-check my plans against yours.

    One thing I can add for interest is that I have found the CTEK can handle more than 23v solar input. My panel is rated at 23v max output but in practice I have found it consistently puts out 28v in direct sunlight. After 5 months of use with the CTEK it’s still working brilliantly. I may be shortening the life of the unit of course, only time will tell, but I think this is very unlikely because it is usually current that kills electronic components, not the voltage.

    I did cynically wonder if they only limit it to 23v because they want to encourage people to buy the more expensive heavy-duty devices from their range. 23v is incredibly low compared to equivalent solar regulators on the market.

  11. Hi David,
    Could you clarify something for me? I would like to use the d250s/smartpass system, but my van is a 2013 boxer and therefore Euro 5 compliant. I have been advised that this will mean it has a smart alternator. Your van is post sept 2009 (euro 5 inception) so I guess this is the same? The CTEK d250s does not work with the smart alternator, and a modification is required, which is outlined on the CTEK website. However no mention is made of using the d250s/smartpass combo with a modern alternator. Does this mean that the smartpass works fine with the smart alternator? There is no mention that I can find anywhere in the specs for the smartpass that mentions this issue, yet it is clearly mentioned that the d250s is incompatible. I am worried that the smartpass will also be incompatible, as the recommended fix for the d250s is NOT the addition of a Smartpass….. In which aces why is the issue not mentioned anywhere in relation to the smartpass?! I would value your thoughts. Thanks Paul

    • For anyone who reads this blog and needs to use the d250s/smartpass combo with a variable voltage alternator, I have managed to get some advice from an Oz supplier (CTEK are popular down under). Refer to the CTEK website for basic info, they have a wiring diagram for the d250s alone. You will see that an automotive relay is used between the sources and the charger. If you are using the smartpass as well, the only difference is that switch contact 87 is wired to the alternator-in post on the smartpass as well as to the positive starter battery. Note that it is NOT connected to the alternator-in post on the d250s.
      This is not the best system for modern vehicles as there are alternative battery to battery chargers available which have up to date technology that work seamlessly with smart alternators without the need to add extra relays to make to product work and mess about splicing into the ignition loom on your expensive new van.

  12. Hi David,
    Thank you for these gems of information. I’m wearing my fingers out on the keyboard researching van conversion electrics. I can see this system would work well but would it work with a ‘smart alternator’ which under braking can increases voltage to 16v+ and down to 10v under acceleration? Thanks. Rob

      • Hello Rob and David,
        Since my original post(s) I have had the opportunity to chat to CTEK and done some research on my own charging system.
        To be honest I did not originally understand quite how the Smartpass works. The D250s is a 20a charger, which is inadequate for a 400ah system, as David says. The intesteresting thing is that the Smartpass is just a relay. It claims it can charge batteries at up to 100a rate by boosting the D250s, but what in fact it does is just allow alternator direct charging to leisure batteries until absorption phase, in other words it’s just a ‘smart’ switch, switching from alternator direct (just like a very simple and cheap VSR system) to the D250s. When the bulk charging has been done via alternator, the D250s takes over, so in fact when the difficult part of the charging cycle is operative, the D250s is working alone.
        I had a chat with CTEK and they state that the Smartpass with D250s combination is not compatible with smart (variable voltage) alternators, but they are bringing out a new product in 2017 which will meet that need.
        I ran my van with a voltmeter on the battery and discovered that in fact it held a steady alternator charging voltage of 13.7v – therefore my 2013 Euro 5 van does NOT have a smart charging system. Hope this is of use to someone else. Great news for me as charging leisure batteries from a smart system is a nightmare. Although there are products available that claim to cope with this, the reality is that the alternator is off for much of the time and therefore is producing no charge at all.
        If you do have a smart charging system on your van the only option is to get a B 2 B charger designed to cope with low alternator input voltage, such as the Redarc systems (Australian) which are available in the Uk, and have inbuilt mppt solar trackers. Another option is the Sterling products, but they do not include in-built solar controllers. This is not necessarily a bad thing, as the CTEK controller in the D250s is very poor according to test results I’ve seen, but more expensive doing it all separately of course.
        Good luck with it!

    • If you have a smart charge system that controls the alternator output via the ecu the D250s/Smartpass combo will not work. This is because the ecu monitors the voltage requirement on the main vehicle systems and makes sure this is supplied via the alternator (it is a common misconception that it is the vehicle battery that supplies this charge), such as current draw for headlights. Once the starter battery is charged (within a minute or two of setting off) the alternator output drops off, sometimes to zero, depending upon what you are running. Even the smart chargers designed for variable voltage alternators generally only boost voltage from a minimum alternator supply of 9v or so. So for a lot of the time whilst driving a modern vehicle will supply insufficient voltage to a battery to battery charger to allow secondary battery charging. This is true even for so a called smart chargers. This is especially true for the CTEK units as they are designed to cut out if the input voltage drops below about 12v (11.5v?) in order to prevent battery discharge. I think this sort of technology is going to catch up with the emissions bandwagon soon, but in the meantime it’s a pain in the arse.

  13. Hi THere,
    i have bought a motorhome recently and bought 230ah battery with 2x100w solar panel , would you say this combination of products will work or i could do with something else as well ?
    x 1 ctek d205s
    x1 dtek smartpass
    x 1 mxs 25
    and x 1 Nasa BM1 for monitoring?

    MPPT is enough?

    Thank you so much

  14. Hi,

    An excellent article and you have provided some very useful information!

    Just to clarify, I have 3 ways of charging: Solar, Split Charge, EHU.

    The control module you have recommended climates the issue of overcharging and maintains the battery health (TEK-D250S).

    My question is how do you switch the 12v off all together and change to EHU ? I assume you have a smart charger on the EHU side to also maintain the battery health.


2 Pings/Trackbacks for "New 12v Electrics – Total Charging Solution"

  1. […] If your looking at possibly getting a B2B charger and also possibly solar it might be worth looking at the CTEK D250S Duel and Smartpass. The D250S is a great battery to battery charger that you can also connect solar panels directly too (it has a MPPT solar charger built in) but only charges at a maximum rate of 20a. If you add the CTEK Smartpass it charges at up to 100a. It also keeps your starter battery topped up. I have this system going into my own van (fitting 4 x 100ah batteries) and did a ton of research before deciding on this set-up. I've written a small blog post which has a video showing how the units work here if your interested:…ging-solution/ […]

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