 | Home > Maintenance & Modifications > Doing a dual battery system properly. |
 
|
|
|
| Pilgrimmick Member Since: 16 Nov 2015 Location: Highlands Posts: 582  
|
If just running a fridge, would not solar be a better option, run for weeks when parked up, no expensive second battery to buy and maintain.
There is a thread somewhere about what solar panel to use for this very job! 80" 1948 Lightweight V8 Bowler Tomcat 130 Station wagon 90 300tdi (Santana PS10 pick up) Range Rover L322 (Ful fat) |
||
15th May 2018 9:47am |
|
| Bluest Member Since: 23 Apr 2016 Location: Lancashire Posts: 4206 
|
I've not given solar a lot of thought as I don't have a roof rack for a panel. With a portable panel it would only be able to be used when not driving, so evenings and early morning, not in the best sun. Plus, if it's sunny I'll probably try to camp in the shade. If we stay put for more than a day or two I'd choose a site with electric hook up. 2007 110 TDCi Station Wagon XS
|
||
|
15th May 2018 1:26pm |
|
| Cupboard Member Since: 21 Mar 2014 Location: Suffolk Posts: 2971 
|
If you want to go solar than I think a portable panel would be best so you can park in the shade and put the panel in the sun.
However, back to the original question. A normal lead acid battery can have a few hundred cycles down to 50% DOD, a fancy AGM (Odyssey) down to 30% DOD. Power consumption of a fridge depends on the fridge brand, the insulation and the ambient temperatures but an average of 2A (that is 2ah/h) would be about right using figures from National Luna. If you have an Engel set to 10C and it's only 15C outside then obviously it will use less than that. If you want to camp up at 4PM and leave again at 9AM the next day that means you need 34Ah capacity using this example. People might claim wildly less power consumption from their fridge but in all the tests I've seen using real fridges in real conditions with actual electrical testing kit, 2A seems in the right area. You're correct they cycle on and off, 2A is the average. If you're using a standard flooded battery and need 34Ah out of it regularly then you need at least a 68Ah battery. Add a bit more for contigencies, charging phones, running lights etc. FWIW you can fit a pair of 019 batteries in the standard Puma/TD5 battery box and they're up to about 100Ah each. A 019 is what Land Rover supplied with Defenders from the factory. Then you need to get power out of the battery. A short, protected run of heavy cable to a fuse board, then a longer run of cable to the fridge. Using me as an example, I have a Snomaster fridge by the back door. I have a 90Ah battery under the passenger seat, that goes pretty much straight to a 100A fuse and then using 16mm2 cable run in conduit in to the driver's seat box. There it goes on to the input stud of a 4 way fuse board, also connected to the input stud (so therefore bypassing the fuse board and being protected just by the 100A fuse upstream) is my ARB twin compressor. Running from a 20A fuse on the board is my fridge. I've had to extend the cabling that was originally supplied and have used 8mm2 cable to minimise the voltage drop. The stock fridge cable uses ~4mm2 and over the total cable run I get a drop of about 0.4V when the fridge is running. For the cabling, make runs as short and direct as possible, make sure that the cable is protected phycially with a sheath, conduit and grommets as needed. All cables need to be fused as close to the source of power as possible. The fuse rating depends on the size of the cable, some useful information here: https://www.12voltplanet.co.uk/cable-sizing-selection.html https://www.12voltplanet.co.uk/fuses-guide-uses.html How to get energy in to your battery. On a Defender that's got the aux battery next door to the main battery (so minimal voltage drop along the cables), isn't running solar and is using the same battery chemistry for both batteries there is no need to use a DC/DC charger. A split charge relay will do the job fine, costs less, is simpler and giver you the advantage of being able to jump start yourself should the need arise (and you've sized things appropriately). However, if you do decide you want to run a DC/DC converter for whatever reason, your question was how do you size it. Look at the maximum current that you want to draw from the battery, in this case a fridge which will take 5-10A when running. Subtract that from the rating of your charger and you're left with the maximum available current to charge the battery. 1/10th the capacity would be about the minimum, 1/5th capacity would be nice and give you a bit of headroom. Too small and nothing bad will happen but it will take longer to charge. Too large and nothing bad will happen but you've spent more money than you need and the battery won't charge any quicker (than a correctly sized one, it will charge quicker than one that's too small). Something in the 20-30A range would be plenty. CTEK makes a nice 20A one that has a solar input for future expansion. When fusing your batteries, bear in mind that power can come from both ends. If you have, for example, your main battery under the seat and an aux battery in the back of a pickup bed then you need a fuse at each battery to protect the cable from both of them. In my set up, I have 16mm2 cable and a Victron 230A split charge relay. Both batteries are sat in the box next to each other so the chances of a fault occurring are minimal so I haven't fused them. As soon as you're running a cable out of the battery box then you need a fuse, and make sure that any pre-fuse cabling is well protected from rubbing against any metalwork. If you want to buy a Snomaster BD/C-40 SINGLE 40l fridge/freezer give me a shout  It's in perfect condition. 18 months old complete with case and remote etc. and 3.5 years remaining on the warranty. The problems with it are a) that it's only a fridge or a freezer and I want something with a freezer compartment that's a fridge and a freezer. b) it's a bit rattly. This is annoying when it's right behind your head, you wouldn't notice it if it was in the back of a station wagon with a load of stuff between you and it, or in a pickup, or in a 300TDi. |
||
|
16th May 2018 1:04pm |
|
| Bluest Member Since: 23 Apr 2016 Location: Lancashire Posts: 4206
|
Thanks Cupboard, most useful. It's given me a lot food for thought. Solar isn't for me at the minute I don't think. One of the reason I fancied a B2B charger was the simplicity of adding solar later if I fancy giving it a go. The other reason I liked the idea of B2B is that I understand you get a "fuller" charge into the aux battery than you would with a relay. Ring do a 30 amp one and I also saw the 20 amp Ctek you mentioned. Ctek is a bit more expensive but has more charge stages, such as a special mode for recovering deeply discharged batteries that the Ring doesn't. The Ctek even has a fancy add on module that enables the accessories to run off the starter battery while the engine is on so that the Ctek doesn't have to fight the accessory load while charging the aux battery, but that is a pricey set up. I may still just go with a split charge relay to set everything up and then save for the B2B later as it would be a realtively simple upgrade.
I have a little used 120ah AGM battery left over from my previous installation, but I've a feeling it is bigger than the LR starter battery, so might need to sell it and buy something that will fit in the passenger seat box. Intention is to have B2B/relay under the drivers seat along with a fuse board to run two hella sockets (fridge and CB) and 4 usb ports. In terms of battery charge, assuming there is some load on the aux battery, can I use a volt meter to tell me when it is discharged? At what voltage would would an AGM be discharged down to 50%? Finally, thanks for th offer of the fridge. Probably wont be buying 6 months. Don't even know what size I want yet. 2007 110 TDCi Station Wagon XS |
||
|
16th May 2018 8:42pm |
|
| MadTom Member Since: 10 Sep 2013 Location: Olomouc Posts: 616  
|
In Discovery 2 TD5 I have 2 AGM batteries - main is 95 Ah - good foe everything, also for winch. Second is 75 Ah - as big as possible for available space. It can power the compressor fridge for 3 days in hot summer. No fancy voltage sensing relays or chargers, just one big 150 A relay, thick cables and fuses between batteries. And a small switch to block the main relay. Relay is controlled from ignition switch - without ignition, the relay is disconnected. Both batteries are charged together from alternator. It works for 6 years now. "Drobek" = The Small One - Discovery 2, "Blufínek" = The Blue Thing - Defender 130, and for me at least Ford Mondeo
 |
||
|
17th May 2018 7:21am |
|
| Mo Murphy Member Since: 01 Jun 2008 Location: Letchworth Garden City, Herts Posts: 2223 
|
Bluest,
In my experience and perhaps for you as a long term aim, to run your fridge you will best be served by being able to run your fridge off the split charge for when you are travelling, solar for when parked on a site with no mains hook up and a 230v camping mains hook up for when available. Make no mistake, staying a couple of days on a campsite in southern Europe in the summer with the fridge running will run your auxiliary battery down to the point of no longer providing enough current so having a supplimentary supply is important if you want to keep your beer cold and your ham pink 😊. Mo The Land Rover 90 - Many are called, few are chosen. 50 Shades of Pennine Grey |
||
|
17th May 2018 8:03am |
|
| Cupboard Member Since: 21 Mar 2014 Location: Suffolk Posts: 2971
|
Maybe sometimes. The Defender alternator is a little bit smart, but not as smart as a modern "smart" alternator as fitted to things like the current Ford Ranger. The Defender is kicking out ~14.4V all the time which means that it will quite happilly fully charge your battery as quick as a battery will take charge. Where a battery to battery charger really shines is when you've got a long cable run to your aux battery. If you're trying to charge a battery in a trailer or at the back of a canopy then having something to boost the voltage after some has been dropped along the cabling makes sense. If you're putting a thick cable run over a short distance then you'll gain nothing in terms of charge speed using a smart charger.
That's a very difficult question to answer. You can't accurately measure how much charge is left in a battery whilst it has a load on it, you can have a vague guess but that's about it. More information here: http://batteryuniversity.com/learn/article..._of_charge |
||
|
17th May 2018 9:22am |
|
| MadTom Member Since: 10 Sep 2013 Location: Olomouc Posts: 616
|
Yes, for longer trips in summer or winter I use some extension cord and 10A 14,2V power supply as a charger. I just connect the charger to the aux battery and continue using fridge or webasto heater I winter. I was thinking of solar cell - but I need at least 100W, and not fixed to the car, because I'm trying to park in a shade. And this type of solar is a bit large. In our Discovery 2 we are travelling in 4, so not enough space. Solar is on the list for planned Def 130 camper.
Or I can just start the engine and let it idle for 2-3 hours, just like a diesel power generator, to charge the battery. For my use probably cheaper, than solar of bigger battery. Important is, I can calculate with 3 day of fridge run just from aux battery, and in case of real need, I can switch to main battery for at least one day. Just remove one fuse and add a bit o wire with 2 connectors and fuse inside. Frigde has some low voltage cut off, to protect the battery. "Drobek" = The Small One - Discovery 2, "Blufínek" = The Blue Thing - Defender 130, and for me at least Ford Mondeo |
||
|
17th May 2018 10:43am |
|
|
|
| All times are GMT |
< Previous Topic | Next Topic > |
Posting Rules
|
Site Copyright © 2006-2024 Futuranet Ltd & Martin Lewis
