Caravan Battery Charging On The Move

Submitted: Sunday, Feb 03, 2013 at 17:00

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Battery Power

In this article we explain why you might need an additional power source independent of the starting battery installed into your vehicle and we look in detail at types of batteries,

Member - G.T.

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

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

Sunday, Feb 03, 2013 at 23:16

GT
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

GT

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.

Cheers

John

J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

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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

Ross,

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.