current output

You have to understand that amps are pulled and not pushed.
The BCDC may well be a 40a or a 140 amp unit but if the target device only wants 1 amp then that's all it will take.
Think Warragamba dam as the BCDC and a garden hose as the cable. Just because you have a gazillion liters of water available does not mean you will ever need it or indeed be able to stuff it down the hose.

How much current might flow through your Anderson depends totally on what gear you have connected and how much current it wants plus the size cable delivering the current.
Can't see how you could ever pull 40 amps unless the batteries in the trailer were flat.
Perhaps write down all the trailer devices and what power they consume and that will get you close to what might flow.

Mine is a Redarc 12/20 charger.

I presume that means that the max it will ever put out to the anderson plug is 20A. I have a 150w 12v three way fridge in the van so presumably in theory there should be suffcient power. I just wanted to check that this was the case. Hopefully I won't stuff up the 50W wattmeter.

Correct
The 20 amps is the maximum amount of current that your charger can supply.
Even if you 5000 amps feeding the Redarc, unless it goes faulty, it can only give out 20 amps.
However you are in teh pooh. 20 amps at 14 volts = 280 watts (20 x 12 = 240w)
50 watts at 14v is a little under 4 amps. (50 @ 12 v is slightly more than 4 amps)
Have a look at this - calculator

William, the meter in the link is 50 amps not 50 watts so you'll be ok.

Blue M I have had an LV1240 bcdc charger in my patrol for approx 1 yr I put a 130 amp/volt meter in line a couple of months ago and it was pulling 33 amps off the alternator to charge 2 x 105 ah agm's every things been working well so far. Now the redarc charger requires 60 amp fuses I had 50 amp fuses in line before installing the charger when I only had 1 battery to charge straight of the alternator via a dual batt system.so I just left them in place and put a couple of 60 amp fuses in my tool box just in case. So a couple of weeks ago we had an extremely hot day and for some reason the 50 amp fuse near the starter battery blew so I have now put in the correct 60 amp fuses so I would expect they have factored in start up surges so it can draw more than 40 amps sorry about the long story.

-
Batts,

Fuses operate by err, fusing. That is, they melt. To melt they require an elevated temperature which is created by the current flow adding to the ambient temperature.
So maybe your 50A fuse was subjected to a significant temperature in the engine bay (it can be as high as 80 degrees or more) adding to a current flow of less than 50 amperes. With luck the 60A replacement fuse will be OK.

Batts
Current out is not the same as current in.
A 40 amp unit supplying 40 amps to the batteries will draw more than 40 amps.

Yes that would be it Allan I had a similar thing happen once with a 70 amp trip switch mounted on the fire wall it worked well until we got those couple of extra hot days that we all experience each yr then it tripped out.

Yes Mal it will draw more of the batt especially when it boost the out put that's what I was trying to say in a round about way because they have a 60 amp fuse fitted on either side of the charger so one would expect that it can also surge up well over the 40 amps on the out put side I probably should've have mentioned that.

Yes. Thas the one I have. I just didnt want to put too much current through it. Hopefully all OK.

William, if you have a look at those specifications you will see that it will handle 100 A for a little while (at least while you have time to switch the motor off or disconnect the meter.)

The charger is mounted in the rear of the ute and is 12V20A. The Anderson plug cnnects to the auxillary battery. The BCDC charger charges the auxillary batery.

I dont understand the difference between charging the battery in the caravan via the Anderson Plug compared with charging the auxillary battery when its running my Waeco in the rear of the Ute. What am I missing????

William you wrote:

"I have an Isuzu DMax with a 90A Alternator which also charges a second battery in the vehicle through a BCDC charger. I also have an Anderson Plug at the rear of the vehicle for charging the caravan battery."

Then:
"Mine is a Redarc 12/20 charger.

I presume that means that the max it will ever put out to the anderson plug is 20A. I have a 150w 12v three way fridge in the van so presumably in theory there should be sufficient power."

So from the above your charging a battery in the car from the charger mounted in the car. You also have another connection via the Anderson plug form the aux in the car to the battery in the van which has a three way fridge connected to it?

If this is the case there are a few issues with this setup, one the charger in car charging an in car battery and then another in the van is not an ideal setup as the charger won't get the true picture as to the charge state of the van battery.

If you also have a three way fridge connected to the van battery then it will draw 10 amps or more, this again will confuse the charger, not to mention only leave 10A to charge your two auxiliary batteries.

Hi

I started this thread with a question on the anticipated output at my utes rear anderson plug as I didnt want to damage my 50A wattmeter if I connected it up. I am interested in understanding the way my ute and van are wired up. I have received some good advice from several forumites regarding the wisdom of my 20A BCDC charger being conncted with both my auxillary battery and the caravan battery which I hadn't considered. I therefore hooked the van up to my anderson plug via the wattmeter with my 3 way fridge in the van connected on DC and a small fridge in the rear of my ute running off the auxillary battery. This is the result of my test.

No motor running
12.6V at the ute battery
12.7V in the van battery
Minimal watts going through the watt meter.

Ute motor running
13.2V at the ute battery
12.7V at the van battery
160W going through the watt meter approx 12.9V

Turn off the fridge in the van with motor running
13.2 V at the auxillary battery
13.2V at the van battery
80W going through the wattmeter

I therefore conclude the rear anderson plug is serviced through the BCDC charger as I would expect to get more volts if the lead were from the alternator. The anderson plug charges the battery in the van which in turn services the 3 way fridge. This I think because the volts in the van battery rises when I turn off the fridge with the motor running.

If this is the case then I agree that the system is incorrect. Unfortunately due to the spaghetti wiring its hard to physically see the route of the cables in the van and ute.

I would welcome any comments as to whether or not my logic is correct. Having said that the last forum comment appears to have the same set up as me and experiences no problems.

The mind boggles.

Bill mate lots of people have poorly to badly set up systems of whatever and "don't experience problems".

Just because the system does not fall over in a screaming heap, does not mean it has "no Problems"

your figures show that without the 3 way fridge, you have some chance of charging the van battery in reasonable time ...... but dropped to 12.7 volts, you will charge the van battery but incredibly slowly.

If you are trying to charge both batteries and run the 3 way fridge all off the output of your dc to dc charger ...... none of it will work well.

it might work ... but not well.

cheers