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Trailer battery charging

Discussion in 'Camping Trailers' started by Fittster, Nov 12, 2017.

  1. Fittster

    Fittster Member

    Hi everyone, I have a question about charging my camper trailer battery while driving.

    In my vehicle I have an aux battery all set up with a DC-DC charger and everything works fine.

    I have run cable from the AUX battery under the bonnet to the rear of the car (Anderson plug at the end).

    This plugs into my trailer.

    I recently did a trip to Alice Springs and it seems that the trailer battery doesn’t fully charge, it only gets a bit over 50%.

    I was told by an electric guy that this setup will work but like I said, it doesn’t seem to be. I would expect that after hours and hours of driving it would get full. Granted, I did have a large fridge running in the trailer at the same time but surely it should still get to 100%.

    I have a theory but I’m not a professional on this stuff. The battery in the car is much smaller than the one in the trailer. Could it be that the trailer battery is only filling to what the AUX in the car is at 100%? The redarc is between the cranking battery and AUX in the car. Could it be switching off when the AUX is 100% and therefore the trailer one stops being charged? I was told it would work because you’re just making the battery bank bigger.

    I know I could put another red arc in the trailer and run the cable to the rear of the car from the cranking battery but I was hoping I could save the money.

    Anyone have any ideas?

  2. Tink

    Tink Well-Known Member

    When you say Redarc what model are you talking about?
    If it is a VSR like an SBI12 then all it will see is one big battery (aux plus CT battery combined) and charge both to about 95 odd percent given time. Voltage drop is your enemy. What size cable to back of vehicle and then to CT battery?
    Or it is a BCDC something or other, it needs to be installed as close as possible to the battery it is charging so you need two of them, one at the aux under bonnet and one at the CT battery
  3. Fittster

    Fittster Member

    It's a BCDC1220 so it looks like I'll need another one and move the cabling to the cranking battery.
    Thanks for your help Tink.

  4. dabbler

    dabbler New Member

    Adding to Tink...
    Connecting batteries with different capacities is not recommended. In most scenarios the larger capacity battery will never charge fully and the smaller battery will discharge faster.

    Depending on how you charge and discharge, this can shorten the smaller battery's functional life.

    Battery chemistry and charging rate may be an issue too.
  5. Fittster

    Fittster Member

    One last question... The setup under the bonnet is connected to my ignition. I keep reading this thing about smart alternators and I'm not sure what charger I should get for the trailer (since I won't be able to run a wire from the trailer to the car ignition - well I don't want to).

    I have a 2015 Pajero so I'm assuming this has smart alternator technology? I don't fully understand this system but looking at the manual for the Redarc BCDC1220 it really does matter with where I put one of the wires. Is there a charger that will just "work" without me having to worry about this smart alternator stuff?
  6. Batts88

    Batts88 Well-Known Member

    Looking at the maual you can wire it up the way you want the red power wire will be connected too the starter battery and the blue wire connected to the red somewhere near the charger there is a diagram as well

    Section 2.2 blue wire trigger the 2 paragraphs explain what to do for the BCDC1220

    Then scroll down to the first Installation figure 2.7a for a pic of how it looks
    Last edited: Nov 13, 2017 at 6:48 AM
  7. Batts88

    Batts88 Well-Known Member

    Before you spend more money on another unit is the wiring running to the trailer large enough it needs to be reasonable large over that distance I have learnt from others on this site Redarc have a basic size guide in section 2.6 and I think they have a better one in the hand manual. Also you may need another eleco or someone who can check how much power is getting to the aux and trailer battery there may be something not wired up correctly or a wire put on or left off by mistake. Also check for a fuse near the battery to see it's not blown and is the correct size according to the instructions.

    I got my charger from these people last yr no problems and a lot cheaper than from Redarc themselves.
    Last edited: Nov 13, 2017 at 6:50 AM
  8. Tink

    Tink Well-Known Member

    Battery chemistry and charging rate is important. Should only connect similar chemistry batteries and charge accordingly.
    BUT connecting two similar chemistry batteries, even if different capacities is NOT an issue. The charger or alternator won’t care, just one big battery. The fridge drawing the power won’t care, just run for longer. If the batteries are not of a similar age, then maybe, just maybe, their internal resistance may be different but eventually they will equalise and both will FULLY charge.
    Albynsw likes this.
  9. Albynsw

    Albynsw Well-Known Member

    I am guessing that your cabling is undersized and the voltage drop is stopping you getting sufficient charge
    Aaron Schubert and Tink like this.
  10. Tink

    Tink Well-Known Member

  11. Fittster

    Fittster Member

    I got all the wiring from an auto elec who works at battery world in Canberra (Phillip). I think you're right though, I bet he only gave me 6mm wire where I've read that you should use up to 10mm?
    If I go for another redarc I have figured out the wiring. The blue goes with the red and I have an ignition relay under the bonnet. But yes, redarc are so expensive. I think if I double check the wiring size (I hope it's at least 8mm, such a pain to get it fed through some of the grommets!) and look at another type of charger I'll be right to go.
    To test the current can't I just turn on the car and use a multimeter on the anderson plug at the back? What numbers should I be seeing while the car is running?
    FYI - The second battery in the bonnet runs my driving lights, UHF and a couple of acc sockets and USB sockets. Putting the Anderson plug at the back on to the cranking battery isn't a problem, I think I'd prefer to separate them anyway if there's going to be problems trying to run the camper battery off the AUX.
  12. Tink

    Tink Well-Known Member

    Just be carefull with cable sizing. There is 6mm “auto” cable which is only 4.59mm square, there is 6mm square and then there is 6B&S which is 13.5mm square. Running to a camper battery should be at leat 6B&S although I prefer to use 16mm square tinned solar cable. See comparison charts which also show ratings, etc.
  13. Fittster

    Fittster Member

    I've done a bit of testing and with the car running I'm getting 14.5V when testing at the anderson plug at the back. That sounds pretty good to me and is what it should be to charge the battery in the trailer. The cabling is coming off my AUX in the bonnet.
    I have also plugged up my little solar panel and getting a bit over 14V from that. Could it actually be a problem with the battery itself? I'm using one of these:

    I'm going to leave the solar on all day and see what the battery gets up to. Nothing is running so it should all be going towards charging.
  14. Batts88

    Batts88 Well-Known Member

    If your 2nd battery is a just a deep cycle it's not a good idea to run driving lights off them. Aslo the driving lights should usually run off the main starter battery unless there is a reason you can't.
    Albynsw likes this.
  15. Fittster

    Fittster Member

    Ok I'll double check that one!
  16. Fittster

    Fittster Member

    So, it looks like I might just have a dud battery. I charged it up to 93% then the sun went down. A few hours later it was down to 88% (nothing using the battery). This morning down to 80%, then a couple of hours later down to 75%. It's just losing charge very quickly by just sitting there.

    The battery isn't very old and the run to Alice Springs was really it's first time in the trailer. We did take a very corrugated road though so maybe it got damaged then?

    I'll take it in to get checked but maybe I should just go and buy an AGM deep cycle 100a/h and just replace it?
  17. Batts88

    Batts88 Well-Known Member

    If you have a large fridge try and get a battery with more storage capacity if you have room in the camper. Getting a couple of days at least out of it so you don't have to keep checking it also have a look at Aussie batteries they have great specials like their 130ah is on sale for $269 delivered in the link below. I have one of their 105ah it's fine there are lots brands others might suggest they found good as well. Also I'm not sure if you already know but batteries come in left, right configuration so the terminals are in the right spot for your set up.

    Also fitting a small voltmeter in the camper would be handy they use hardly any power and you can keep an eye on the battery level so you get a longer life out of it. I also printed a battery voltage chart for agm and lead acid and keep them in the glove box eg: below.

    Attached Files:

    Last edited: Nov 15, 2017 at 2:10 AM
  18. Fittster

    Fittster Member

    Alrighty, after lots of testing I have found that the load drop is in the camper trailer wiring - I think. I cabled the car myself and the auto elec gave me the wire which is pretty solid, I just can't remember the size he told me (yes, I went on the word of someone who does this as a job). I get over 14V continuous output at the back of the car while it's running. I'm assuming that's good.

    But when I plug the trailer in and test at the battery in the trailer (so testing on the positive cable at the furthest end which is about an extra 4m) I see it go up over 14 but then drop to 11, then 13's/14/s then drop. It just cycles through this so I am assuming that the trailer cable is only standard 6mm rather than the 13.xmm or whatever 6B&S is at.

    I can see where the existing wire runs, just not sure how I'm going to feed the new wire through. I have a feeling this is going to be a pain to upgrade!
  19. Fittster

    Fittster Member

    Hmmmmm I forgot to add I have put a Ridge Ryder 20A DC-DC charger in the camper and ran the test on the input wire. I'll try breaking the connection to the charger and testing direct off the wire. If it works, maybe the charger is drawing too much power to start up and then senses low power so shuts down, then does it over again and again.

    I did however go to an auto elec to buy some cabling (after I called them) and he tried to sell me 6mm. I stated I wanted 6B&S and he came back with 8mm... I said it again and he basically refused to sell me the cable until I brought my trailer in to him! WTF?!?!

    I can't seem to find much on the charger itself, maybe there's an issue with it running on a smart ignition? Either way, this trial and error stuff will end up costing me a fortune!
  20. Tink

    Tink Well-Known Member

    Where do you live? May be anle to help with cable supplier.
    Voltage should not be going up and down at trailer if it is stable at back of vehicle.

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