Electrical Charging Aux Battery through Merit Plug

[rjstokes]

Hi all,

Another techie question. We've got a Waeco Fridge charging unit (RAPS 12R-U2) in the tub of our ute, with a Cigarette adaper (10 Amp) and a Merit plug (20 Amp). It connects power to the fridge when the voltage increases (when alternator's running) depending on a switch on the front.

I've also put together a battery box (100 amp/hr AGM battery) that will live in the tub for running the fridge free camping or down the beach for the afternoon. I'm tossing up getting a CTEK C250S Dual DC-DC charger in the tub, replacing the Waeco unit, to charge the aux battery (and then connecting the van to this battery via an Anderson plug eventually).

However, I'm wondering if I could simply connect the Aux Battery to the Waeco unit using a Merit-Merit style plug (20 Amp) and charging it via the Alternator (I've measured the voltage at the Merit plug as 14.4 volts with all car services on). I'm just not sure if the Merit is up the the task or if Alternator charging will kill the AGM battery. I guess if the Aux battery is dead flat the current to exceed 20 Amps? A local expert advised I need a $400 DC-DC charger (insisting the Ctek and Redarc are no good) and 10mm cable run from the Car Battery to the charger. Surely not?

Any advise welcome.

Cheers.

 

[achjimmy]

People charge there batteries directly through a 7 pin plug with don't be rude ant wire so it's possible and it won't kill the alternator. But the larger the cable the less voltage drop you will get and the more efficiently it will charge. I can't see why a merit wouldn't work well. Most popular plug after the trailer plug is an 50amp Anderson . In my system my tug auxiliary and van batteries are linked. I use an Anderson and 13mm cable but fuse it at 30amps which is not that far away from a merit rating. I don't see any need for a DC to DC but they are popular. One disadvantage for me if you connect through a DC to DC charger is it will isolate the batteries which I don't want. The biggest drawer of power when we free camp is the engle fridge. So should we choose to stay of power and not drive for say more two days then I need the van coupled with the tug to send the power back.

Lately I have found the vans solar more than adequate for recharging the vans batteries with need to be conected?

 

[rjstokes]

How about using a Merit to keep the aux battery topped up in the car (that would be 6mm cable from the car battery to the Waeco unit, then a Merit cable (20 Amp).

Then putting a Ctek 250 in the van. Hopefully that would work around the voltage drop issues, allow for solar in the future and be a safe interface to plug an external battery into the van should the van battery fail. The only issue I see there is the wire gauge from the car to the Ctek in the van (I think it's only 6mm at the moment).

If not a full re-wire might be in order!

 

[rjstokes]

Out of interest, I measured the voltage drops across the unit. At the Car battery - 14.7V, at the Waeco unit - 14.4V, tailgate 14.4V, Van battery switch 13.7V. I'm guessing from that a Ctek C250S Dual would work fine given they cut in at ~13.2V.

 

[jvp]

Hi All
A Merit plug is good for low to medium loads of fridges etc where when disconnected the male plug is not live. If you connect two batteries with Merit male / female the male centre pin will be live & exposed & easily short circuit to earth or vehicle body. There not designed for the job intended. Anderson plugs can't go wrong. Hope this helps.
jvp

 

[rjstokes]

Thanks JVP, thats the sort of thing I wouldn't have thought of until the fuse kept tripping! I've got a Projecta isolator under the bonnet that provides power to the 12pin plug (for the van). I'm thinking of tapping into that and putting an Anderson in the plug in the Tub for charging the Aux battery. That will mean the Alternator will charge the Aux battery, van battery and power the fridge in theory. Presumably that's ok? Whats to stop the Aux/Van battery's overcharging?