Alternator wires

Ok I'm being really lazy and asking questions before I get the multimeter out but multimeters only get you so far with 1m leads chasing loomed cables in the rain.

I'm coming around to the charge the batteries from the alternator school of thought. So I bought a dual battery charger from maplins this allegedy discriminately charges the lowest battery that it's linked to.
http://www.maplin.co.uk/lead-acid-battery-split-charge-module-37767
However I need to run the isolated charging lead from the alternator to this box. This is quite a head scratcher because I've 3 wires coming from the alternator ganged into a loom a meter later.

The alternator is a 55amp bosch in a merc 308. Tailing from it are 2 red wires what look to be 32amp and a thinner blue that looks like a 16 amp.

Does anyone know what these wires are for/going to and where to pick them up on route to the battery without bypassing the ignition? There's only one 600amp core attached to the +ve of the starter battery.

Also the van is spec-ed for an 85ah battery is it harmful for the alternator to exceed this (by say 165ah)?

Aidan_M_M

the 2 big wires are usually the same , a loop , and the small one is the D+ ,or switching wire , that becomes live when the engine runs .

thanks aidan

why does it need to loop? are they both going to the battery direct?

Aidan_M_M

one of tyhem goes to the battery , the other is often used as a take off for a strong12v supply.

Is there anything else between the larger cables and the batteries do you know? I'm guessing the voltage regulator is en route for one of them. This circuit won't work unless I isolate all the charge headed to the battery...I'm highly tempted to put the relay back in the box.

Aidan_M_M

Most modern vehicles have the regulator built into the Alternator , since oh , early 90's?

Any factory built Merc I see normally takes the leisure battery poer from under the LHS seat at the fusebox , or from the battery + pole direct , through a strong fuse(30-40A ish) , and into a heavy duty relay , switched by the D+ wire . nothing fancy at all .

Thanks again Aidan,
What I have isn't a relay it's more similar to a split charge diode block, rated 20A max.
There's nothing suitable coming from the fuse box last time I looked.
Also it's self switching so I don't need the D+.
The problem I'm having is that it needs to be in series with the alternator not parrallel.
A relay would be much easier to install in hindsight.
Back in the box it goes.

Aidan_M_M

I think them diode blocks are for a different application really , I like the relay idea , it's simple , cheap , and reliable . And it's a sure fire way of separating the leisure battery from the engine one , when the engine is off.

I hear you knocking

They did specify in the vague instructions that it was ideal for campervans/solar etc. and it is filed under a relay in maplin. A single pole in series with 3 possible circuits is a tall order though especially when most of the wiring runs through the chasis rails.