[336c]

I am after a bit of advice, im putting together a camper trailer setup with a fridge, radio and optional air compressor onboard. Over the last two months i bought a 75ah agm battery and a 160w solar panel. Last week i purchased a dometic cff45 fridge without knowing that it consumes 5.2a . The radio consumes about the same and the compressor would be 25a.

I have tested out the charge rate on the solar panel and it is more than sufficient to hold chage for both appliances but im unsure that at night the battery would last for 10 hours on just the fridge. I checked the voltage drop and at 12.7v the fridge kicks in drops to 12.3 if running longer drops .1v every 5 minutes.

The radio wouldnt be running constantly of course nor would the compressor but i just want to make sure that overnight the fridge doesnt cut out....

The cff45 has a battery monitor so if it senses a certain voltage it will shut off and turn on at a higher voltage. My other idea was a 240v inverter as on 240v it uses .4a but no battery protection (except from the inverter which i think is 10.5v)

i had a bit of advice from a workmate this arvo, he said wiring up a trickle charge setup via an anderson plug for when i am travelling and run the solar panel connected to the auxillary battery when stationary. would a dual battery system work that distance ( from the jimnys battery to about the front of the trailer)?

[bumstein]

Just my thoughts, regardless of dual batteries and charging systems etc. I focus on managing my fridge.

To manage the fridge I avoid keeping it open for long periods, get stuff out earlier then it’s got the opportunity to lower the temp again and I put the warm beer in the next morning when the batteries are going to get a charge. I also turn my fridge off overnight, it might run through the evening but it gets latched, lives in an insulated bag and kept in a ventilated area in shade and hopefully a light breeze. I travel some very hot areas but I still find the temp hold down is still pretty good.

[sideways]

I have a 40L engel in my Sierra and it runs off a commodore battery (not rated but probably around 80a/h) with a voltage sensitive relay as an isolator. I also run a camp light and some other minor stuff (chargers ect) off the second battery.

It's fine running overnight, it hardly takes anything from my battery. I pull the fuse out so there's no load on the battery and wait a minute or 2 before measuring the voltage. Naturally the voltage drops far more under load and we're really interested in the batteries settled voltage. Mine was normally 12.4-12.5v.

Running it through an inverter will bring greater inefficiency to the system. Your average cheap inverter is probably in the realm of 75-80% efficient and it will also draw power when it's not even running anything.

When looking at amps you've got to factor in voltage you're running at to get an idea of how much power you're using. 12v x 5.4 A = 64.8 watts. 240 x 0.4 = 96 watts. So even without factoring in the inefficiency of the inverter you're already using 50% more power, though one would think it wouldn't need to cycle for as long if it's got more power at its disposal.

In the case of something like a compressor fridge, peak amp draw doesn't really matter too much as they don't run constantly. If you draw say 5amps for 5 minutes per hour and 2.5 amps for 10 minutes per hour, you've drawn the same amount of power per hour and you've put the same amount of power into compressing the refrigerant. The only real caveat here is voltage drop (lost as heat energy) along the cable. A lower amp draw will have a lower voltage drop so will be slightly more efficient that way. We aren't talking about big amp draws here though so it's easy and affordable to just use oversized cable.

It depends on conditions and the temperature your fridge is set to, but IMO you can safely factor an overnight battery usage of 1Amp per hour. Mine seems to use much less than that overnight. Say for reasons of battery life you don't want to discharge below 50% of the batteries capacity, assuming the battery is in good health that leaves you with 37.5 A/H to play with. Straight off the bat, as we're only drawing 1 amp per hour we can say that will run the fridge alone for 37.5 hours. If your radio takes 5 amps and the fridge is running that's 6 amps per hours total and 37.5 divided by 6 is 6.25 hours running both. You might run the radio for 3 hours in the arvo that's 3x5=15 amp hours drawn from the battery, leaving you with 22.5 amp hours in the battery which will run the fridge for 22.5 hours. Of course, this doesn't factor in what the solar panel does but they don't work real well at night, when it's cloudy or when you've parked your camper in the shade so I think it's not really fair to have to rely on the solar panel unless you know you're going to be sitting on a sunny beach or similar.

I don't know if you've looked into dual battery systems but they basically come in 3 types.
1. Manually selected. Boat switches and so on. When you want to isolate your second battery from the starting one you turn a big mechanical switch. When you want to hook them back together you also manually select it. Given your camper set up, you could achieve the same thing by just unplugging the anderson plug. These are simple and cheap but I don't really like them because I'm only human and I know at some stage I'll forget to unhook it and end up with both batteries flat and not be able to start the car or I'll forget to hook it back up and end up with a totally dead second battery.

2. Voltage sensitive relays. These automatically connect and disconnect the 2 batteries when certain voltages are achieved. For example mine disconnects the batteries when the voltage drops below 12.7 (when the engine is off and any current is being drawn) and hooks them back together at 13.2v, when the engine is running and the alternator is charging. They normally have an LED which indicates what they're doing and you can wire in a switch for manually linking the 2 batteries together, handy for jump starting off your second battery if you flatten your starting battery. I run these in my cars and I think they're great. Brand dependent they cost around $50 upwards.

These above 2 options have a down side of potentially shortening battery life by mixing different sizes/types of battery. Personally I haven't found this a problem but I've always run conventional lead acid batteries for both.

3. DC to DC smart chargers. As their name implies these are a DC powered smart charger. They modify the voltage/current out put to achieve maximum second battery life and get maximum charge into your second battery. They can also be used to manage the output of your solar panel. The down sides are cost (hundreds) and they have quite a small charging capacity, 10 amp is a pretty normal size and if you've discharged your battery by 37.5 amps then you will have to drive for at least 3.75 hours to recharge it. The other options should charge faster, depending on the battery. You can buy bigger smart chargers but they get golly expensive. One cool advantage is they often have a 240v input so when your trailer isn't being used you can have the charger plugged in keeping the battery in tip top condition, you could also have it plugged in at a caravan park.

I'd run the compressor off the Jimny's main battery myself. I'd do this for a number of reasons.

Less voltage drop from those long cables. My compressor was a whole minute faster pumping up 33s from my jimny than my sierra because the alternator could keep up on the jimny and it was getting about 3 more volts.

You're probably going to leave the trailer behind and do day trips at some stage. At which point, you'l need the compressor with the car.

You're normally pumping tyres up at the end of the day, taking the charge out the second battery just before it needs it.

You can mount it where most people stick the second battery in the engine bay, which is pretty neat. I hate digging out the tangled mess of cords and crap, then it also keeps it out of the cargo areas.

To see how much power your fridge really uses under camping conditions I'd fill it up with the appropriate amount of stuff, have it run on 240v until it stops cycling and then leave it hooked up to the battery and check the voltage periodically when it hasn't cycled for a couple of minutes. It can take a while to initially cool everything down and that's not really a fair load to place on the battery because it wont see that sort of thing when you're camping.