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Dual Battery System

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  • Leighton De Wet
    Leighton De WetMember

    Hi Guys

    I’m planning on doing a dual battery system in my Ford Ranger to run a fridge, inverter and a few LED lights.

    What will be the best battery size and what else do I need to do a proper system that won’t fail over a weekend?

  • Melt de Kock
    Melt de Kock
    Melt de Kock on

    To start with one needs to determine what the amp/hour draw will be when looking at the size of a suitable auxiliary battery. The most widely used battery in the Leisure market has been the standard Deep Cycle 105A/h lead acid which gives you about 50 A/h to operate with taking into account that the battery will get hurt if drawn below 50% on a regular basis. The voltage cut out level on most fridges is below 12v which ensures that the battery takes a pounding on a camping two day week end.

    A standard 40l fridge draws around 3-4 amps per hour on average in an environment up to 25 degrees. Doing the maths this gives you 12.5 amps per hour before the battery starts to take strain.

    Luckily the fridge will switch off when it reaches the right temperature so this gives you some extra hours to play with. On average with 50 amp hours you can stand a day and night running the fridge at setting of zero to two degrees.

    Should the temperature go over 30 degrees or the fridge setting is at -10 degrees you easily double the amps drawn from the battery. That leaves you with around 6 hours. Adding an inverter will again increase the load when in use.

    A good option is to invest in a dry cell battery like the latest AGM Discover distributed by ENERTEC Batteries. For a variety of additional features the 100A/h AGM will give you double the run time as a standard lead acid battery as you can  bring  the capacity down to 20% before you start hurting the battery.

    To give your dual battery system a sporting chance here are some points to consider:

    When reaching your destination the Auxiliary battery must be fully charged. This can be achieved with the latest intelligent DC to DC chargers. (a solenoid unfortunately don’t qualify here)

    Should you make use of an inverter or have a bigger fridge than a 40l and the setting is around -5 to -10 degrees you need to supplement the battery with the likes of a solar panel. This must be done as soon as you arrive at your camp site.

    Make use of a device to give you proper feedback on how the battery is doing. Guessing wont get you anywhere. The CTEK Battery Sense gives you a graph telling you how the battery is doing. By having the info on your smartphone you can now make an informed decision.

    Leighton De Wet
    Leighton De Wet
    Leighton De Wet on

    Thank you so much for the assistance, the inverter I’m currently using is 1000w and the fridge I’m looking at buying is the Dometic CF 50 Fridge/Freezer

    I have a 100w foldable solar panel, will invest in a proper DC to DC charger and battery

    Johan Nel
    Johan Nel
    Johan Nel on

    In my opinion, dual battery systems should be designed around the specific needs of the owner and not generalised as only one way to go and that is with a deep cycle battery setup.  I have a Ford Ranger that I use as general adventure and tow vehicle. I have fitted a normal lead cell battery, 180Ah Truck battery, connected via a NL relay system.

    This setup works 100% for me in a off-road, camping without power situation, as I cash in on the ability of the second battery to be recharged within a 30 min game drive.  This supplies power for two fridges (80 l NL in trailer + 40 l Engel in my truck) set 3 and -3 degrees. The one used as a fridge is switched off at night. An inverter is also used at night to power my C-pap machine while I sleep. I have determined that my power needs would not exceed 30Amps per night and rationed myself to just that.

    During a recent 5 week trip, circling the Okavango delta and at one stage camping for 8 consecutive nights in Botswana Parks, wihout electricity,  I have never depleted the second battery below 80% of its capacity.  The freezer in the trailer was disconnected during game drives and only the one fridge was powered. When not travelling, I would run both freezers for at least 6 hours before going on a game drive again.

    My point is, deep cycle batteries cannot be charged in a short period of time, but large normal starting batteries can. Even if need to be, quickly be recharged by solar panel, in a fraction of the time a deep cycle battery.

    This works for a situation where one can take a daily game drive and are on the roll most of the time and certainly works for me. ( The cost of such a large Truck/Tractor battery is approx R2500)

    My 2c

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