How many batteries can I charge?

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi KA, and a DC/DC device, even a small 6 amp unit, will increase the voltage drop at the fridge, potentially causing problems for the fridge.

A good few years back, I carried out an extensive number of tests, with different set ups and even when increasing the cable size to 25mm2, there was still far too much voltage drop being caused by a 15 amp 3 way fridge for the house batteries to get a decent charge while driving, and there was no way a 25 amp 3 way fridge could be connected to the house batteries without discharging the batteries while driving.

All the testing I did showed most efficient set up ( and one that Dometic also recommends ) is to run the two separate power supplies, as covered earlier.

Early last year I developed a new set up using a 1,000w Sterling inverter equipped with a new Australian patented RVD, which makes the inverter safe to use while driving.

The inverter is then used to power either a 60 amp Sterling or Promariner battery charger.

The reason for the Sterling or Promariner charger is both have up to 60 amp capacity but more importantly, these chargers come with three separate isolated outputs.

This allows the fridge to be powered off one output and the house batteries to be charged by one of the remaining outputs.

So even with a 25 amp 3 way fridge, you still have 35 amps available to charge the house batteries. All powered down a single 25mm2 twin cable, run from the cranking battery.

When the ignition is turned off, the inverter is turned off and the charger turns off, leaving the house batteries and 3 way fridge automatically isolated from each other.

There is a bit more to the set up, including a specially programmed isolator in the tow vehicle, but this gives you an idea of what can be done and while this system is specifically designed to work with any voltage output, high or low, from the alternator, but I still recommend using one of LeighW’s booster fuses to make this set up more efficient.

[Steve H Junior Member Find all posts View Profile Close Steve H]

Hi again Steve, and how much power are you contemplating to need.

Are you looking at stopping for night, using a bit of battery power and then driving to another stopover?

Or are you likely to be free camping for a few days between drives?

Our future plans will be mixed. Almost always unpowered sites. There will be the odd time like Easter when we will set up unpowered for a week or so. There the Solar panels will be connected to help keep batteries up and van fridge running on gas most likely. Then maybe once a year or so, we would be hoping for a 4 week trip where we would be on the move most days with the odd 2 or 3 day stop in between. I was originally just contemplating how my alternator would go charging all 3 of my batteries (while running the van fridge from the van battery) while we are travelling. Once stopped, van fridge would be switched over to gas and van battery just powering small appliances, lights etc... Car doing its own thing, disconnected from the van. I have little to no voltage drop as the cables I have used are more than adequate. Not sure what it is classified as but the copper diameter is 8mm.

[KA_WA_11 Junior Member Find all posts View Profile Close KA_WA_11]

Drivesafe,

Thanks for the information, I will need it change my set up to the van so it has a separate supply to the fridge.

[Steve H Junior Member Find all posts View Profile Close Steve H]

Drivesafe, further to what I wrote above, I have taken your advice and will not even connect the van fridge to 12v. I've decided it will be much easier to have it as gas or 240 only. I will however run a cable from the van battery to the auxiliary battery in the car for keeping van battery charged while driving. If I was to run a separate set of cables from the cranking battery to the van fridge for running the fridge on 12v while driving, I would probably need those cables to pass through a second dual battery isolator or some kind of shut off solenoid to prevent the cranking battery from running down if I forgot to disconnect it when I stopped. Correct?

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi KA and happy to be of some assistance.

Hi Steve and a simple fix is to get a “Fridge Switch”.

This device monitors motion and if the van is still for a given time, some minutes, the switch disconnects the fridge from it’s supply.

As soon as you get going again, it connects the fridge again.

And all done automatically.

[Steve H Junior Member Find all posts View Profile Close Steve H]

What size circuit breaker would you suggest on the cable from cranking battery to van fridge if I were to do it?

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi Steve and standard convention is decide on the size of the cable/wire you need.

This must be large enough to handle the fridge current plus decrease the voltage drop.

My suggestion is, because of the distance, to use 8B&S twin.

8B&S twin ( 7.9mm2 x 2 ) has a continuos safe current rating of 65 amps, and while this high current is not required, the large size cable will reduce the voltage drop at the fridge.

As 8B&S has a constant safe maximum current rate of 65 amps ( 75 amp in open air ), the safety device should not be rated at more than 60% of a cable’s maximum.

So it’s a 40 amp fuse or circuit breaker.

Sorry for the long drivel but it makes it easier for you and anyone else following this thread to sort out the correct and SAFE protection for any DC circuit.

[Steve H Junior Member Find all posts View Profile Close Steve H]

Hi Steve and standard convention is decide on the size of the cable/wire you need.

This must be large enough to handle the fridge current plus decrease the voltage drop.

My suggestion is, because of the distance, to use 8B&S twin.

8B&S twin ( 7.9mm2 x 2 ) has a continuos safe current rating of 65 amps, and while this high current is not required, the large size cable will reduce the voltage drop at the fridge.

As 8B&S has a constant safe maximum current rate of 65 amps ( 75 amp in open air ), the safety device should not be rated at more than 60% of a cable’s maximum.

So it’s a 40 amp fuse or circuit breaker.

Sorry for the long drivel but it makes it easier for you and anyone else following this thread to sort out the correct and SAFE protection for any DC circuit.

No Spot on. I'd rather you explain it so I know why I am using a 40a breaker rather than just use it and have to ask next time. Thanks heaps. I have decided on using the separate cable from the cranking battery to power the fridge while travelling and will incorporate a fridge motion switch in this circuit also to prevent flattening my start battery. Also I will connect the van "house" battery to the aux battery in the back of the car so it will be charged while driving also. Van Fridge will not be connected to the van battery. Hopefully it will all work as planned and be a perfect setup. Thank you all for your suggestions and comments. Steve.

[Steve H Junior Member Find all posts View Profile Close Steve H]

What is your charge voltage at the cranking battery terminals with the motor at 1500RPM and with all batteries fully charged?

Charge voltage at the cranking battery was 14.09. Inserted the voltage booster diode today and what do you know? 14.55volts!!!. The extra half a volt should help it all out. What an awesome little bit of gear that thing is.