Wednesday, Jul 31, 2013 at 23:11
Will,
I think I can see how you've done your maths now. Your result of 110.7 is actually Amp-hours per day. That's fine, we can work with that.
However, since you have a 240V fridge running off an inverter you need to allow for the inefficiency of the inverter. I don't know what yours is, but lets work on 80% efficiency. So your fridge will need 100/80, = 1.25 times the placarded consumption, = 1.25 x 808, = 1010 Watt-hours.
So now your total watt hours per day = 1010 plus 300 plus 120 plus 100, = 1530 Watt-hours. Divide by 12 to give amp-hours, = 127.5 Amp-hours per day.
That's a lot, but you say you want to be conservative, so we'll run with it.
You've said you're considering 2 x 120AH Thumper batteries, = 240AH battery capacity. I'd say with your daily consumption that 240AH would be the minimum you should consider because the best balance of daily use vs total economic life of the batteries is to only take them down 50%. In round figures, ONE day without charge will get you to that point. Two days without charge will flatten your batteries.
Mate, you're going to have quite a job getting adequate charge into those batteries to both charge them and offset your use and also to maximise your unpowered camping.
I agree with outbackjoe - a 20 amp dc-dc charger is unlikely to be enough unless you plan to spend all your time driving or idling the vehicle.
There is merit in Joe's suggestion that you use the higher output of the alternator to bulk-charge your batteries to about 80%, then finish them off with a dc-dc smart charger. Others may disagree, but I don't think there's any way you can do this on the cheap with your projected useage and those batteries.
Sooo...think about this....
A Ctek D250S coupled with a Ctek Smartpass. As I understand it, this system will use the high output of the alternator (Smartpass maximum is 80 Amps) up to a certain point, then finish off the charge with the 20 Amp multi-stage D250S. The system is good for battery capacities up to 800 AH, so you could increase your battery capacity if you wanted to in the future.
Here is a link to the manual. Page 11 shows the configuration which I think you need, and a bit of text to support the idea.
To maximise the benefit of this system, you NEED BIG CABLE from alternator all the way to the D250S/Smartpass, and from there to the van battery. If it's 6 metres from car alternator to the Ctek installation in your van, you will need two runs (a positive and a negative) of 2 gauge (= 2 AWG, = 2B&S). That's big stuff and will be expensive. Assuming your Ctek gear is about a metre from the battery, you will need 6 gauge for both +ve and -ve. I used
this calculator to arrive at those figures. I used 13.8Volts, 80 amps, 5% loss between car and van, 6m x 2 for a cable run, = 40ft round figures. Between Ctek and battery, 1m cable x 2=6ft, 2% loss, 80 amps, 13.8 volts.
I think you will need a 40 to 50 amp multi stage mains charger for when you need to run a generator. A good quality one with a high power factor (0.9 or so) could possibly run off a 1kVa genny, though you might need bigger. More research needed. The large mains charger will reduce the running time of the genny and minimise annoyance to other campers. If you're not thinking of a genny, just hooking up to mains, then a smaller, cheaper, less efficient mains charger could suffice.
If you're thinking about solar to extend your time off-
grid without running your car or generator, then you're going to need a fair bit of that too, plus a decent regulator. The Ctek D250S can take a solar input which is nice and convenient, but I don't know what its max solar input is. I suspect it will not be enough in the above scenario, so a separate solar regulator may be needed. A subject for another post if you're interested.
Cheers
AnswerID:
515684
Follow Up By: Member - Frank P (NSW) - Wednesday, Jul 31, 2013 at 23:37
Wednesday, Jul 31, 2013 at 23:37
Will,
In the above I assumed the two thumpers were going to be house batteries in a caravan, but re-reading your original post I see they would both be under the bonnet in a self-contained van.
If that is the case and with your projected useage I think you're dancing with the devil, because if you split them conventionally into engine battery and house battery you won't have sufficient cacpacity in the house battery. If you then draw on the engine battery to increase your house capacity, which you seem to suggest in your post, you run the risk of two flat batteries and nothing left to get you going again.
Anyway, if you do the conventional split I would still recommend the above setup to charge the second battery, but obviously with smaller cable as the runs will be much shorter. Or put a third battery somewhere in the van, parallelled with the second battery under the bonnet and proceed as above.
Your time on-site with one house battery will be quite limited unless you use a generator or run your car engine for some hours on a daily basis, or plug into mains.
FollowupID:
794998
Follow Up By: Will T2 - Friday, Aug 02, 2013 at 12:28
Friday, Aug 02, 2013 at 12:28
Thank you Frank for this detailed answer.
I am now certain this fridge is not a viable solution and I am now thinking about buying a new one. I found some very low consumption 12v fridge rated at 0.15kWh/24h which is 5 times less than what i got now and no need to go through the inverter.
I am thinking of buying one of those and go back from the start with it.
I have been looking into the ctek range and am very interested on the Ctek D250S coupled with a Ctek Smartpass option.
And you did understand correctly the first time, the 2 batteries will be both leisure battery in the van and I will keep the actual 2 under bonnet cranking batteries.
I will keep you guys posted with my new plan just to see what you think.
Thanks,
Will
FollowupID:
795079
Follow Up By: slowflow - Friday, Aug 02, 2013 at 15:46
Friday, Aug 02, 2013 at 15:46
Have a read of this link hope it helps
http://www.fridge-and-solar.net/agm.htm
A purpose built fridge sounds like a great option.
Others have suggested the CAT brand of batteries but you would need to
check their specs.
East coast marine have battery monitoring systems BM-1, undoubtedly there are many others, they maybe worth looking into for your application.
Barry
FollowupID:
795090