Caravan Battery Charging On The Move

Can I get my vehicle wired directly from the alternator to the caravan batteries via Anderson plugs? Regards G.T.
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Reply By: olcoolone - Sunday, Feb 03, 2013 at 17:22

Sunday, Feb 03, 2013 at 17:22
Yes but it's more advisable to go from the starter battery or starter motor.
AnswerID: 503991

Follow Up By: Sand Man (SA) - Sunday, Feb 03, 2013 at 18:04

Sunday, Feb 03, 2013 at 18:04
Yep, I agree, especially from the starter (primary) battery.

The best method to achieve this is to run a reasonably heavy duty cable from the primary battery with an inline fuse or circuit breaker to protect the circuit and terminating at the rear of the vehicle in a 50 amp Anderson connector.

Depending on how the caravan (or Camper) is wired you then connect the cable to the vehicle Anderson connector which completes the circuit to the remote battery.
In an ideal situation you would have a dc-dc charger mounted in the caravan, close to the battery which would apply a smart charging regime with a multi-stage process.
Some of these dc-dc chargers (I use a Ctek D250S dual) have a built-in isolator in them which keeps the primary vehicle battery and the remote battery system electrically separated.
Without this feature, it would be possible to drain the vehicle battery if the caravan battery is discharged and the vehicle circuit left connected.

This is the reason why you connect to the primary battery as a smart isolator can determine the voltage level of the primary battery, before it activates the alternate circuit.


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Reply By: Ross M - Sunday, Feb 03, 2013 at 23:16

Sunday, Feb 03, 2013 at 23:16
As mentioned, yes you can, but I do not agree with Olcoolone on this.
Because the alternator is the highest charging voltage point in the vehicle, if you want the highest voltage possible to be applied to long, suitably sized wiring via Andersn plugs to a caravan, then the charge terminal on the back of the alternator IS definitely the best place to have the positive cable attached to.

All the negative wires in the charge line must also have the same integrity as the positive wires, so the neg of the big charge wire is best bolted directly to the alternator case or it's mounting bracket. It seems no one does this, they unwittingly connect it to the battery negative.

Because of loads and length of wiring from alternator to main batteries, to connect the charging supply to the +ve terminal of the main battery may deprive you of a very important 0.5v drop or even more. If you are dragging power off the battery the weak link in the charge/flow circuit is the size of the wiring from alternator to battery.
This does develop more and more voltage drop across it while alt is delivering heavy charge loads.
Unfortunately, most people never think of this alt charge wire at all. Funny that, they think the power comes from the battery. How wrong they are.

After all, we are trying to maximize the charge to the caravan, not provide it with ho hum, possibly sort of ok, she'll be right mate type wiring configurations.

Some people do use the DC/DC multi stage switch mode type chargers, they do a good job.

Although you still require good wiring and anderson connections, you can use a 12v to 240v inverter and have it run a multi stage switchmode battery charger to charge the van batteries. This is not as efficient but does work and you may have these components anyway. Many people have all sorts of reasons for not doing it this way though.

Best of luck sorting out the wheat from the chaff on this. I don't go along with a lot of the normal firmly held beliefs people seem to have and stick to, without due consideration to the way charging circuits actually work

Ross M
AnswerID: 504030

Follow Up By: Member - John and Val - Monday, Feb 04, 2013 at 08:18

Monday, Feb 04, 2013 at 08:18

I agree with Ross here. As he says, from an electrical standpoint it is far better to connect both positive and negative right at the alternator in order to avoid voltage losses. In practice it is usually much easier to connect to the starter battery terminals. Connecting here has an added bonus - there is a fusible link (extra-high current fuse) between the alternator and the starting battery. I actually use a double fusible link fed by the alternator - one feeding the battery in the usual way, one leading to the auxilliary ( house) batteries etc.

Something that hasn't been mentioned is that it is essential to have a suitable switch in the engine bay to connect and disconnect the positive line to the aux battery. Usually this will be a voltage sensitive relay (VSR) which connects the aux circuitry only while the alternator is providing charge. Also, there should be a suitable fuse or circuit breaker in the engine bay to supply the aux wiring.

Ross has referred to dc-dc chargers, and I have found mine an excellent investment.

You may find Electricity for Camping a useful read.


J and V
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Follow Up By: Allan B (Member, SunCoast) - Monday, Feb 04, 2013 at 09:26

Monday, Feb 04, 2013 at 09:26

Whilst there may well be 0.5v drop in the alternator/battery cable at high charge current, most if not all, late model cars take the alternator regulator voltage-reference from a connection at the battery rather than a reference from within the alternator. This then has the result of maintaining the appropriate charging voltage at the battery terminals and the cable loss is of no consequence.

Certainly there will be a higher voltage available at the alternator terminal to use for auxiliary charging which may be useful to overcome losses in the cabling to the auxiliary battery. However, and this is a very fine point, in the circumstance of where the auxiliary battery is fully charged and there is a high load on the alternator from crank battery charge plus head & driving lights etc. the alternator output voltage may be raised to a level that could cause overcharge to the auxiliary battery. But I repeat, this is a fine hypothetical proposition.

I would prefer to take the auxiliary connection from the cranking battery terminals and ensure that there was an adequately sized cable from the alternator.

By far I consider the best arrangement is to use a dc-dc charger at the auxiliary battery to overcome all cable losses, act as an isolator, and provide multi-stage charging. This is especially so where the auxiliary battery is located in a caravan with long cabling as in G.T.'s case. Then also, by nature of dc-dc chargers, it does not matter if the charger is connected to the alternator, cranking battery, or anywhere else provided that adequate current is available.

I have two auxiliary batteries, each with it's own 20A dc-dc charger and a relay which places these batteries in parallel when the motor is off. It is the best setup I have ever used.


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Reply By: Member - G.T. - Monday, Feb 04, 2013 at 17:15

Monday, Feb 04, 2013 at 17:15
Thanks to all those who replied. The reason for my inquiry was that I was contemplating using an Ark Pak aux battery set up. However it `s out put is only 6amps, not enough to charge the caravan batteries as well as the aux battery, so I thought that as an alternative I could charge the van batts thru the alternator.
As it turns out my Waeco 80l needs 7.5 amps, so this idea bites the dust!
Thanks again, Regards G.T.
AnswerID: 504076

Follow Up By: Gronk - Monday, Feb 04, 2013 at 18:43

Monday, Feb 04, 2013 at 18:43
You certainly can charge the batts from the alternator....and unless you have a modern 4wd that has a lowish charge voltage ( but even this can be fixed with the diode fuse ) there is no need for a dc/dc device ..
Now...if you want to spend the money on one....their is nothing really wrong with them....but you will charge your batts quicker...90% of the time...just by using the alternator !!
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