Charging from vehicle alternators

 Every vehicle has an alternator, we know that it is responsible for keeping the starting battery from going flat.  But can we use the alternator to charger our motorhome house batteries too?  What is the best way to do this?


Before we look at the options, let’s look at the issues:

  1. Crank batteries differ in their construction and charging requirements when compared to the deep cycle batteries that we use in our house systems.
  2. Flattening the crank batteries must be avoided at all costs. Trying to start a motorhome with a flat battery is no fun.
  3. We must never (accidently) try to start the engine using our house batteries.


Here are five ways of using your vehicle engine and alternator to charge your motor-home house batteries. I’ll start at the cheapest option and work my way up to the most expensive (and perhaps best) option.

Option 1 – Switch on the dash

With this option you only need a very heavy duty switch (perhaps 100amp rating or better) that simply connects the positive terminal of the house batteries to the positive terminal of the crank batteries. When the switch is on, current from the alternator will charge both the house batteries and the crank batteries. When the switch is off the two battery banks are not connected (thus preventing you from flattening the crank batteries at the same time as the house batteries).


  • Simple to install
  • Cheap (approx $100)


  • Manual – you have to remember to switch the connection on and off
  • Forgetting to turn the switch off could result in both sets of batteries being flat, leaving you stranded
  • The house batteries can only be boosted – not fully charged.
  • Does not prioritise charging of the batteries (ie can not charge the crank battery first – then switch to house battery).


Option 2 – Ignition controlled relay

This is really just a refinement of option 1. Here the manual switch is replaced by a relay (suitably rated) and this relay is activated (ie powered) when the ignition switch is turned on. This removes the manual switching disadvantages of option 1. Note – if you are considering using this option, be very sure that you use an accessory feed from the key switch. Make sure that this deactivates the relay BEFORE the starter motor engages (most accessory feeds do this). This is important to avoid drawing power from the house batteries while starting the vehicle.    


  • Fairly simple to install
  • Cheap (approx $150)


  • The house batteries can only be boosted – not fully charged.
  • Does not prioritise charging of the batteries (ie can not charge the crank battery first – then switch to house battery).


Option 3 – Voltage controlled relay


Another refinement of option 1 – a bit smarter. A special relay is fitted to interconnect the two battery systems. This relay is able to monitor the voltage of the crank battery and join the two battery banks together ONLY once a sufficiently high voltage has been reached. This effectively prioritises the charging – making sure the crank battery is charged before joining the two batteries together.   Unfortunately, if the house batteries are very flat, some power will immediately flow out of the crank batteries and into the house batteries.    

When the engine is turned off and the alternator stops producing power, the voltage of the crank battery will quickly fall and the relay will seprate the battery systems.


  • Fairly simple to install
  • Cheap (approx $200)
  • Does prioritise charging of the batteries (ie charges the crank battery first – then switches to the house battery).


  • The house batteries can only be boosted – not fully charged.
  • Current can flow out of the crank batteries and into the house batteries (and visa-versa).



Option 4 – Diode isolation


To avoid the issue where one battery bank can feed the other (flatter) battery bank, it is possible to fit diode isolation. This is simply a box with 3 terminals on it. A diode allows current to flow in just one direction. You can then be sure that power can flow into the battery (via the diode isolator) but not back out and into the other battery.

The big disadvantage of this system is that diode isolators introduce a 0.6 volt drop in the system. Now this does not sound like enough to worry about, but it is quite significant. 0.6 volts could mean the difference between charging your batteries to 65% or 85%.  My advice is to avoid diode based isolators unless you know what you are doing and are able to overcome the 0.6 volt issues (and this can be done – write to me if you want to know how).    


  • Fairly simple to install
  • Cheap (approx $100 – $200)
  • Stops discharge from one battery into another


  • The house batteries can only be boosted – not fully charged.
  • Both sets of batteries will receive less charge (due to the voltage drop across the diodes).



Option 5 – Two alternators


With this method you simply fit a second alternator to the engine. The two battery systems remain completely isolated from one another. The original alternator charges the crank batteries as it always did. The additional alternator is responsible for charging the house batteries. This is actually my preferred option. Electrically speaking it is simple (mechanically it can be difficult to find room and fit the second alternator). This option avoids most of the pitfalls of the previous methods by keeping the two batteries isolated from each other. It is also possible to replace the regulator on the house alternator with one that more closely matches the preferred charging characteristics of house batteries.        


  • Electrically simple
  • Can be setup to more fully charge the house batteries
  • Avoids discharge from one battery into another
  • No switching or manual intervention required


  • Mechanically tricky – it is not easy to fit a second alternator
  • More expensive (say $300 – $600)



There are other ways of using smart regulators or even inverters powering 3 stage chargers to your house batteries. Presented here are just 5 of the simpler and cheaper options.

You should also be aware that charging from your alternator is not free (the bloke that first said “there are no free lunches” was a physicist for sure). When you use your vehicle engine to charge your batteries your vehicle will have reduced power and/or consume more fuel. However, this increase in fuel consumption compares favourably when compared to the fuel consumed by the typical small generator.  So the lunch is not free – but it is quite cheap.

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10 Responses to “Charging from vehicle alternators”

  1. Dave Says:

    Awesome website Gavin, and we always click on all the adverts too.
    My Queen Jenny and I have been following ‘Hobohome’ since we discovered you last year. We were hoping to meet you on our recent ‘Oz 360 Quickie’ in our Toyota Coaster, but we somehow lost you between Katherine and Geralton. We are currently parked up at a fantastic place in Clifton QLD, where Vaughan the proprietor, has a massive shed that he rents out to folk like us, who need decent facilities to work on our motorhomes. Can give you more on that, if you need such a place when you are in QLD again. In the mean time, our best regards to you and Tracey and thanks again for such a wonderfully informative website.

  2. Hobo Says:

    Hi guys,

    Thanks for the comments about the site – it is nice to know that people enjoy it and find it useful. We plan to be in the west for another year or so, keep in touch and we will catch you on the other side!

    G & T

  3. frosties Says:

    hello hobo we have just finished our motor home it took 2 years to do, its a 40ft intanational 82 v8 petrol 4 speed auto the new rules that has come out about gas fit out and seat belts and so on drives ya crazy i had to get a seat belt engineer to give a writen report and sign off that cost $400.oo and that i had to follow the specs supplied ,also gas had to be checked and fitted by gas fitter that was $1,000 that was just to conect the gas pipes i had to do the rest the 240volt had to checked by a fully qualified sparky with stickers,but the pitts dont care about the 240 volt , we live in perth i bought the bus from SA $6,000. and up to date it cost me $35,000 we have i double bed 2 single beds ,shower ,tiolet plenty of room and it is licened to carry 5 cos one can sleep on the lounge or we can carry 1 child in booster seat

  4. Iain Says:

    Hi guys.
    Just found out about you today and was browsing the site and stumbled across this article.
    You may not be aware of another option for charging deep cycle batteries.
    I am just in the process of fitting a dual battery system to my 4×4, as well as 2 deep cycles on my camper trailer and have found what I believe is the best way to maintain the batteries.
    CTEK have just released a new charger that is fed by you main battery/charging system.
    Rather than just switching over as with a voltage controlled relay, the CTEK unit produces charge and discharge cycles.
    The 8 cycles follow the routine shown below:
    Desulphation, soft start, bulk charge, absorption, analysis, recondition, float, pulse.
    This maintains the battery and not only fully charges the battery but extends it’s life.
    The model number is D250S, and a PDF can be found at
    I hope you find this useful.
    Cheers and happy travelling.

  5. Baz & Deb Says:

    Hi Guys,
    A great lot of helpfull info in these pages, keep up the good work.
    Have you ever considered the costs of charging the batteries from the alternator due to the load placed on the engine and reduced fuel economy. I’m no electrician or much of a mechanic but am assuming that an alternator under load would be similar to an a/c compressor or are my assumptions incorrect.

  6. Hobo Says:

    You are 100% correct – there is no such thing as a free lunch when it comes to energy. It has to come from somewhere – so yes it will add a load to your vehicle engine. However – it is probably more economical than running a generator when you get to the camp.

  7. Ken MacAulay Says:

    Howdy Folks,

    I love cruising through your website and it’s many informative articles.I bought an =’88 Mazda T3500 conversion when I retired in March and have been getting around the country in it. I’m solar powered and can take advantage of the many great beaches along the Murray. Regarding charging from the vehicle alternator, the previous owner had fitted a 12v – 12v 3 stage battery charger that tops up my house batteries if I get too many days of cloudy weather. It does a great job and I can shut off my solar panels if I wish when I’m on the road. Does it hurt the solar panels to be shut off (before the controller)?

  8. Hobo Says:

    Hi Ken,

    I see no issues with opening the circuit between the panels and the controller – however, I would ask why you would want to do this? The charge coming from the panels is completely free – the charge coming from the vehicle alternator is not. Charging from an alternator places additional load on the engine and it burns more fuel accordingly.

    I have a similar setup to you (slightly more manual), and I control the charge from the alternator and leave the solar panels doing their job all of the time. The solar regulator prevents the battery bank from becoming overcharged. I have a 120amp 24v alternator and when I ask that to charge my house batteries (early in the morning when they are at their lowest), I can distinctly hear the additional load coming onto the engine. I don’t have the data to back this up, but I am sure I get better fuel economy in the summer when I almost never charge the house battery from the vehicle alternator.


  9. Fesmech Says:

    Hi Guys
    Thank you for your awesome site and all the information you provide within it. I noticed that you top up your house batteries via Hobohomes alternator when on the move, which of the above 5 options do you use?

  10. Hobo Says:

    We use an ignition controlled system with a manual boost. This means that I can control the charge while driving (big boost if we have a short drive ahead and the batteries are low – or none at all if I think the solar will do the job today). I am a bit of a fanatic when it comes to looking after my batteries (Tracey would express it a little more strongly than this, she says that I spend more time looking after them than I spend with her :-)). For someone who wants to spend less time looking at digital read-outs etc, the fully auto method would be better.

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