two batteries in the camper trailer

G'day Spade,
Only my personal opinion, but I think you're buying more trouble than you need. I'd be inclined to fit a 2nd battery the same as the one you already have (space permitting of course) and wire them in parrallel too. I would imagine the fridge is the biggest/most constant draw you would have on the battery/ies and the other things would mainly be intermittent type uses such as flouro lights and maybe a water pump, possibly a radio/stereo system.
I have done this in my camper trailer and it works well.....no need for a separate battery for lights, radio, pump.....the 2 SuperCharge Gold 105 a/h batteries handle everything just nicely. I have a similar set-up on the Patrol to what you've described, with a simple solenoid (which I control via a switch). My 2nd battery now resides in the cargo area of the Patrol (but that is of no consequence for what we're discussing here).
If you MUST fit a battery of a different capacity in the trailer as a back-up, I would venture to say (just my opinion) that you would be best advised to have a separate solenoid mounted in the vehicle, with separate cabling from truck to trailer; keeping the whole extra trailer battery totally isolated from the fridge battery.
Cheers
Roachie

G'day Spade. I think Roachie's hit the nail on the head. As I understand it, (and I'm no expert) different types/sizes of batteries will cause problems. There's a wealth of info out there on this topic if you check through the archives and also on google. The general consensus is, if possible put one big battery in that will handle the whole job, and if that's not possible two or more matched batteries, wired in parrallel. For camp trailers agm batteries are IMHO the only way to go, even though they cost a lot more. They are totally sealed, can be mounted on their sides, or in any position, wont leak or gas and accept charge a lot quicker than a conventional lead acid battery. As for a solenoid, there are plenty of different types out there. Have a look at the Redarc voltage sensitve one. It doesn't connect until the car is producing 13.6 volts, which means your car battery will get first go at the charge, and then it will charge the CT battery. It disconnects when the voltage drops back to 12.6.

Whatever battery you end up going for, make sure you get sufficient capacity, so you are not discharging below 50% to ensure good battery life. Also if you go with the agm's, don't hook them up to a conventional $50 battery charger - a good way to kill the battery. The agm's are far more voltage sensitive than a lead acid battery and need a good three stage charger. That's my two bobs worth. Ferris.

Spade,

The most important thing with a remote battery is to make sure that it actually gets a full charge. Usually the voltage drop in even large cables means that the charge current is quite limited meaning that the battery takes a very long time to charge from the vehicle. Long term undercharging is very detrimental to your expensive batteries. Putting 2 batteries in just makes it worse.
One solution is to use something like the Arrid Twin Charge
http://www.allabout12volt.net.au/indexcat.html
which provides up to 20 amp charge, correctly regulated, even if there is voltage drop in the supply or a load on the battery. Not cheap, but neither are good batteries.

Phil I

Some good advice here already Spade. In my view keep it simple. Don't think you need a separate battery for the fridge. I agree with Ferris. AGMs are ideal for the CT. I've got two 120AH AGMs in parrallel in the CT and they are brilliant. They charge via a standard solenoid and very thick double insulated welding cable. Measured voltage drop (without load) is 0.1V. They charge quickly and hold their charge well (ie don't need to be on trickle charge). The two identical batteries act like one big one. I also have a 105 AH DC in the 4B.

I carry a 25 Amp 3 stage charger to use if I have access to 240V or with my 1KVA gennie, but rarely use either. I get 5 or 6 days at least, running fridge (70 litre dual temp Reefer), heaps of lights, radio etc without driving or charging. With a bit of daily running around (which tops up the vehicle DC) I can get a week or more without suplementary charging.

For cheaper AGMs some of the Chinese ones are pretty good. I've got Fullriver and can not fault them.

There is no point putting in 240 amphours of battery if you can't recharge within a day - if you don't recharge them promptly, they will Sulphate and not last long. If you are just going away for weekends and will charge them with a 25 amp charger when you get home - fine.

If you plan to charge them from the car you will need to run a MINIMUM of two 4 gauge wires from the main battery to the Trailer batteries to keep down voltage drop. An ARRID Twincharge will help - at $300. You will need to drive for for up to 8 hours to charge up the batteries (dpending on Alternator Capacity and other loads).

Paralleling batteries of different size does not cause a problem.

Paralleling batteries of different chemistry does cause problems - but nomore than putting in one battery of the wrong chemistry e.g. if you put in a Calcium Calcium Maintenance free battery but your Alternator is designed for Calcium-Lead batteries then the Calcium Calcium will charge slowly - whether it is the only battery or in parallel with another battery.

Mike
- Licenced Electrical Contractor
- Degree in Electronics Engineering

Thanks all for some good responses here. If I could indulge further I would appreciate some further replies. I have searched archives however could not find specific answers to what I want to do.

1. I have a fullriver 120ah and am looking for another the same, as the existing 120ah will not last a full week in many conditions.
2. I have a good 3 stage charger (CTEK Multi XS7000)
3. The 75ah auxillary battery in the vehicle is a flooded deep cycle. There is no danger of flattening the starter battery as it is isolated with a simple solenoid (nothing fancy). There is little room in the Pajero and the battery was the best fit.
4. I have a parrallel cable to the trailer (will check the guage) connected straight to the existing c/t battery.

I am convinced that I should simply parrallel the two c/t batteries that will be the same size and type (thanks comments above). So what I am really after now is advice on what is the most efficient way of getting the vehicles alternator to charge the c/t batteries given what I have.

1. Is it necessary/wise to isolate the vehicle auxillary from the c/t batteries.
2. If I use an isolator/solenoid does it need to be mounted near the vehicle auxillary or can it be mounted in the c/t. (If I mount an isolator in the c/t the connecting socket on the vehicle can be used as a power source)

I have also been working (rightly or wrongly) on the theory that if the c/t battery is at the correct distance from the vehicle alternator (spitting out current at say 14.5volts) the voltage drop of approx 1 volt will see an ideal voltage to charge the AGM battery in the c/t. I am sure I read this on this forum somewhere.

Thanks all for your assistance here.

Spade Newsom