Moderator
When planning on running very thick (almost welding) cable from my cranking battery to charge an auxiliary battery about 4m away, this is a response I got from someone who has always seemed knowledgable on the subject:
"... it is a technical impossibility ... to charge a caravan battery from the vehicle alternator - UNLESS you are using a DC/DC charger. The cable from the vehicle should be used to direct power the fridge only when travelling.
Anybody that tells you they can direct charge a second battery at six metres plus distance from a modern vehicle alternator is dreaming. An alternator regulator can only sense the battery which is closest - which is the main starter battery. The only charge you can get into the caravan battery is the potential voltage difference between the starter battery and caravan battery - 2A generally and 4A if you are lucky. You may even find that lead from the vehicle on the caravan has a diode in the line which drops the voltage .6V."
I really hate that I can't get my head around why DC charger's are either essential or redundant!
Master
SubscriberIt has always been my understanding that the problem is the vehicle alternators regulator - it wants to hold the start battery at 13.8v, while the deep cycle battery to get 100% charge needs 14.0 to 14.5v, so the frig battery is never fully charged.
The exception is AGM batteries, which are happy at 13.8v.
That's my take on it, experts on here will soon tell us if I am wrong.....
Collyn Rivers tells the theory of battery charging a lot better than me though.
Terry
80 109" 2.6 P ex Army GS, saved from the scrappie.
95 300tdi 130 Single cab tray.
2010 Guzzi 750
Swaggie
Ranga, he does have a point. If there is so much volt drop in the cable the second battery may not charge. In that case, you need something to bring the voltage up.
With the cable you are running, though, I don't think you will be getting much volt drop in the cable.
Are you running a dual battery system such as one of those trackside units?
I thought they managed charging voltages.
Actually, thinking about it further, I might not be quite right.
As the voltage drops at the second battery, the current flow will drop, reducing the voltage drop in the cable, increasing the voltage at the battery. Something like that. Just thinking here.
The net result is the battery will charge. Just at a slower rate than the battery on the shorter cable.
But, I wouldn't worry about it if your using thick cable.
Oh, and give DC/DC chargers a miss.
Moderator
Yep, a Traxide SC80Originally Posted by Mick_Marsh
AT REST
Sounds reasonable, except people have been charging caravan batteries off the alternator for 40 + years
Swaggie
Use that then.
Doesn't that have small charging cables for the second battery?
You only need small cables to trickle charge a battery. The reason we have thick cables is for the cranking amps. What comes out of the alternator is, what, 2.5 sq mm, 4 sq mm, 6 sq mm cable.
Moderator
That's what I thought - why do we need DC to DC chargers all of a sudden? Perhaps modern alternators and/or auxiliary batteries are different these days?Sounds reasonable, except people have been charging caravan batteries off the alternator for 40 + years
Swaggie
Yes, because there is not much volt drop.Sounds reasonable, except people have been charging caravan batteries off the alternator for 40 + years
In my line of work, we deal with longer lengths of cable. If you were trying to charge the battery 15km away with 1.5 sq mm cable, I'd say you'd have a bit of a problem.
Swaggie
The ones I have seen have all sort of electronics in. They pulse charge at varying rates. Much like some of the 240vac chargers I have. Probably un-neccessary for your application.
AT REST
Some things he says dont make sense I had a piranha db setup in my d1 no diode I have a supercheap db Setup in the s111 no diode and a sidwinder db140 in the d1 now still no diode if you have an isolator why would you have a diode that drops the voltage? All 3 setups have charged the batteries to full charge on 6 plus hrs of driving no problems. The way i see it the average dc dc charger at 20 amps will put a lot less power back into the batteries than an alternator at up to 140 amps for the sidwinder or 80 for the sc80 over the same time
The amps to the battery are only limited by the wire and alt output provided the voltage is over about 12.8 the batteries will charge
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