This week I redid my dual-battery system as it was few years old and in the early days as I really didn’t understand the technicalities of AGM batteries and I managed to shorten their lives to the point of being useless.So I thought I'd post up a write up of what I did and the hows and whys.
My original set up was a hybrid thumper deal put out by ABR Sidewinder it consisted of 8 x 15 Ah 12 V DC AGM batteries connected in parallel of which there were 4 under each front seat. These were connected to a 120 A isolator (ABR Sidewinder) mounted just in front of the Main battery. Circuit protection is provided by 2 auto reset 50A circuit breakers, one before the isolator and one before the batteries. This gave me 120 Ah of battery life in theory. The truth is this style of charging will only ever get your dual battery to charge to about 80% as the alternator just doesn’t have the spare capacity.
Now my wife likes her food fresh (no canned peas for that girl!) and the only way to really get her out camping in the early times of our relationship was to have fresh food, you know the stuff you need to displace drinking material for in the fridge. As a result to keep us both happy we ended up with an 80 L Waeco. Now this Fridge/ Freezer is a monster and used to fit in my old Terrano no problems but it is a bit of a squeeze in the JK Unlimited but we make it work. The size of this fridge comes at a power cost though, it draws a peak current of 7.5 A and an average of about 2.5 A over 24 hours that's about 60 Ah.
To get the best life out of AGM Deep Cycle batteries you should really only run them down to 50% although is won’t hurt them to go down to about 20%. So to only run my system down to 50% I'd need a 120 Ah battery system. The reality though is those current draws are at STC (Standard Test Conditions) or 25 deg C and here in Queensland local temp is pretty much higher than that even in winter so the fridge will draw more current over a day.
So the greater draw and lack of full charge led me to overdraw the battery system on a couple of occasions thus shortening its life. It would still show a phantom charge but when a load was connected it just died.
Our camping style is more relaxed touring with lots of days where the Jeep will just sit there while we go off for a walk, swim, etc. So the replacement system had to enable us to be self sufficient for a couple of days without driving.
So with our requirements in mind and a target of only running the system down to only 50%, I did an energy budget and multiplied it by two to get the capacity we were shooting for. In our case it was 70 Ah a day so 140 Ah was the minimum size battery we were looking at. As I have said STC is unlikely to be encountered so we have built in a little fat to cope with higher temperatures and any future needs. In this case it is only a touch more than 1 A per hour over the day.
The new system:
CTEK D250S Dual DC-DC Charger- located against the firewall under the factory Optima
http://www.batteriesdirect.com.au/fi...82612906_1.pdf
2 x REMCO RM12-100DC (100 Ah Deep Cycle AGM) Batteries – 1 under each seat, yes they fit
http://www.federalbatteries.com.au/RM12-100DC/
130 W 18 V 7.22 A Solar Panel, - attached to the Congo Cage
14 m of 6mm^2 solar cable
2 MC4 connectors
2 50 A Anderson plugs
1 Trailer Vision Anderson plug dust protector
Your battery choice also dictates which charging option you should go with. AGM/Gel batteries need multi-phase temperature controlled chargers or they will not have a long life.
A cool new to car level chargers is MPPT. This technology has been around for years but it has only just made its way into DC-DC for car use. MTTP chargers are the go because they give you the peak charge possible. For example the panel above is a 130 W, 18 Vdc, 7.22 A panel (P=IV, so l=P/V or 130/18=7.22 A,). Without MPPT I wouldn’t actually get 130 Wpk of charge out of the panel I’d effectively only get 12 x 7.22= 86.6 Wpk. Where an MPPT charger comes in is it is actually a DC-AC converter with a transformer that steps the panel's output voltage from the level it is putting out to the charge voltage nominally ~14 V this have the effect of increasing the current (in this case to about 9 A). The charger then converts this back to DC for the battery.
Another factor to consider is temperature compensation; in hot conditions a battery will actually take more charge, so a hot battery can lead to overcharging which will reduce a battery's life as well, just like taking it down too low.
So how much to do it all:
I paid $279 for the D250S Dual through an E-bay store in Melbourne (have seen them for up to $470) as it can take the charge from a panel and/or an alternator and has a built in isolator and has a temperature sensor for the battery.
The panel was an E-Bay cheapie at $300
And the batteries came in at $350 a pop after some serious haggling with Battery World.
And about $50 for the rest of the gear.
Now this was not the cheapest option but last time I was out the cheapest option let me down. Now I have a system that should keep me running for up to 4 days to 50% battery without having to start the Jeep or up to a week if I let it get down to 20%. Odds are I’d probably have to drive a short distance in that time period, but my need to do so to charge the battery has been eliminated.
I know this was a long post but I felt the need to share.
OTTO
Rebuilding my dual battery system
