Dual Battery Alternator question

Hi Pomgonewalkabout

You have not described much about your dual battery setup, so I'll assume it is not a parallel connection but is the most common voltage senstitive relay type system.

As such it will suffer from the same issues as a normal car battery charging situation in that it will take several hours to charge the depleted battery as typical car system effectivily limits current to prevent over charging on an all day drive.

A voltage stepup system works faster.

Robin Miller

Sorry,
I use a Redarc

cheers

About 2 hours driving around

Depends largley on the battery itself -

AGMs will recharge more quickly than wet cells
Starting batteries recharge quicker than deep cycle
New batteries recharge quicker than older batteries

Run your vehicle for an hour a day and see how you get on. If stationary, check the voltage with a multimeter while recharging - if its around 14 volts, thats fine, if it less than that, turn up the hand throttle and see if the voltage comes up - the higher the voltage, the quicker the recharging.

Hi Pomgonewalkabout

As mentioned by the others it will depend on the type of battery but your alternator will do the job no problem.

The Redarc joins the main and aux batteries at a set voltage and in turn the alternator chargers the aux.

AGM's will accept around 27 amps per hour. Some deep cycle batteries will only absorb 15 amps per hour and will take a very long time to recover.

Generally 2 hours a day drive time will keep your fridge going. Fit a battery protector to the fridge so that the fridge cuts out before damaging the battery. There are also low voltage alarms that are great as they beep when the battery gets down to a selected voltage.





I would also test the battery when you get a chance and charge it up on a 240V Smart Charger.

Regards

Derek.

If your 100A/h battery only lasted 48 hours on your 40 litre Engel, a couple of things may have happened.

1. Your battery wasn't fully charged to start with.
2. You were running the fridge as a freezer.

Assuming the fridge is not of the vintage variety, the maximun consumption is 2.7 amps.

Also, if the battery is a "wet cell" type, rather than an AGM type, it will never get a full charge from the Alternator. Best thing is to whack the battery on a 240 volt 3 stage charger every now and again to ensure 100% charge.

Now, if the auxilary battery is in a camper trailer, of caravan and coupled to the towing vehicle's charging sytem, you will need the addition of an Arrid "twin charge" or similar, to overcome voltage drop and ensure adequate voltage at the battery end.
The reason I say this is because a 100A/h anything is a big battery to have under the bonnet.

Hi Pomgonewalkabout, unless you are running heaps of other accessories other than the fridge, the size of your alternator is not going to make too much difference.

If you have the standard Defender set-up with both batteries under the seat, the maximum current you are likely to get while driving is about 20 amps with the battery in a low state of charge, no matter what type of battery you are using.

Unless you have up the size of the original cable, the long run of cable from the alternator to the batteries will limit the charging current because of the voltage drop.

“ IF “ the battery is that low then to charge the battery up while driving will probably take at least 5 or 6 hours just to get the bulk of the charge back into the battery. To fully charge it will probably take 8 to 10 hours.

If this is your everyday drive vehicle then you battery should have been pretty well fully charged before you started out on your trip.

If on the other hand, the defender is only used infrequently then you need to get the battery charged and maintained by a 240 battery charger.

Also, I wouldn’t take too much notice of the volt gauge, none of these entry level gauges are very accurate and are only good for giving the most basic idea of the battery’s state of charge and can be out as much as 20% or more in either direction.

To get a accurate reading that gives you a better that 5% accuracy you need to spend upwards of $500 and this is usually just not worth it.

There are a number of good landy sites that can give you assistance as well.

Cheers.

Many thanks for taking the time to reply

The Fridge is an Engel 40th Anniversary 40 litre model.
Powered by a 100A/H SLA via a Redarc isolator.
Before that the dual battery was a 80A/H and we used to be able to run the fridge for about 3 days without starting up the truck.

After leaving the campsite on Tuesday we had a 7 hour drive home and after emtying the fridge I put a few ice blocks in it and left it switched on (number 1)

The battery is now showing 10% charge and the fridge ran for 40 hours.

I know that you shouldn't run down the battery that low, but this is the first time.

I am a little surprised that we get less time out of the 100A/H battery than we did with the 80A/H

In the last few weeks though I have made a few modifications.

1. I have ran a length of 6BS twin battery cable to the rear of the truck next to the trailor socket and terminated with a 50amp Anderson ready for when we pickup our Adventure off road trailer. This is protected by a 100amp fuse.

Maybe there is a voltage drop, I will have to measure the voltage at the rear Anderson when the battery is charged again. I did measure this once and it was 12.7V from memory

2. I have added a opposite lock dual battery monitor with alarms.

The 100A/H battery sits next to my starting battery under the passenger seat.
IT is a SLA battery that I purchased from Jaycar, maybe tthat is the problem!
It doesn't like the Birdsville track! And can't handle the rough stuff?

The Trailor is supplied with a 100A/H battery and when I know what type and brand maybe I will purchase another one so that I have two the same and get rid of the Jaycar one.

The idea is that whilst driving with the trailor my dual battery will be in parrallel with the trailor battery both charged via the Redarc.

I guess having now got rid of the rooftop tent we will no longer be in one place and will be able to drive around, explore and charge the battery backup

cheers and thanks for the replies