Thread: Dual batteries, question for the sparkies

Shifter I forgot to ask you, is your motor home used all the time or just for now and again trips. The reason I asked is that our Smart Chargers were designed for heavy duty use, although they will fit any monitoring situation and when we started the Smart Charger range years ago, one of the things we came across was the better makes of 12v fridges that had automatic low voltage cut-outs, were all set to 10.5 volts which is way to low for a cranking battery if you want to start something afterwards, but continually discharging a deep cycle battery down to this level is not a problem, whether it is to run lights or as tdi130dualcab puts it “to keep the beer cold”.
Our smallest Smart Charger, the SC40 has a programmable cut-out voltage which is set ex the factory at 12 volts but the customer or installer could change it to 10.5 for use with a fridge that does not have auto low voltage cut-out and we know that many of our customers have used cranking batteries as there auxiliary battery and we never received any feedback that indicates anybody had had any problems.
So it it depends on how you use and maintain the batteries as to how they will last and Shifter what ever you have been doing seems to work so keep it up.
One point I would like to raise about the charge voltage. When the voltage applied to a lead acid battery goes over 14.2 volts, a chemical reaction starts and this reaction has a type of cleaning affect on the battery’s plates ( this is a crude by simple way of putting it ). It is a good practise to do this to a lead acid battery for about 2 to 4 hours every 30 days but to do it continually will have an adverse affect on the battery.
By the way, if you go to some of the major battery manufacture’s web sites, you can get a wealth of info and also try the VW web site. They don’t make batteries but there web site used to have some great data relating to car batteries.
regards.
PS. Smart Charger is the trade name we you for one of out range of products that's why is used in the text above. 8)

Thanks for all the useful and discriptive discussion.
A bloke who lives in the NT - Broome, has written a couple of books about Motorhome / Caravan / 4x4 electrics, 12 & 24 volts stuff, which are excellent. I think his name is Collyn Rivers. He goes in to great discriptive length about "smart" chargers, regularors, batteries of different types, the whys and wherefores. If you plug in Collyn Rivers to Google he will appear as if by electricary! Well worth the read and purchase. some good Solar stuff in there too.

Shifter.

Hey drivesafe,

I was intending on running 6 B&S cable (next size up from 8 I'm told) to a deep cycle in the back of the disco, but then run from the negative terminal through the floor to the chasis. Are you saying you should run both cables from the front?

Simon.

Hi Simonit, yes and although it sounds like a good way to get a good earth, it's surprising how many vehicles have poor returns through there chassis because of paint or insulation or any number of other things that can reduce the conductivity. Most vehicles use the body work of the vehicle for a earth return for lights and other low current accessories and you may be ok using the chassis, but to guaranty that you will get the best possible control over the power supply, both when charging and when using the auxiliary battery,what ever size cable you use for the positive, use the same size for the negative. If you use the chassis you may find that although you have used 6 B & S, which has an 80 amp rating, for the positive cable, because there is the slightest bit of resistance through the chassis, you may only have a 10 amp return so your 80 amp wire will only be able to supply 10 amps. And another point to consider is that as the current requirements go up so does the resistance.
You can do a rough test to see if there is a resistance problem.
Take the leads off your auxiliary battery and connect a large load to the leads, say a few driving light globes. The measure the voltage between the main battery positive terminal and the end of the positive wire where you have the globes connected. Note the voltage. Next repeat the teat this time from the main battery’s negative terminal to the point where you have connected the earth to the globes.
If it is a good earth then the second reading should be lower or the same. If it is higher then the earth is no good.
If you do this test when there is both a positive and a negative cable then the readings should be the same.
Sorry I waffle on a bit but I find it is best to start how to do something and the REASON why.
Regards. 8)
One more point when running battery cable through a vehicle or to a caravan or trailor, put circuit breakers at both bettery positive terminals to prevent a fire in case of an accident.

Did I count right, Tdi130dualcab?
4 batteries? and solar!


what, no wind generator?
:wink:

and to keep this "technical"

what size batts for which purpose?

With batteries, unlike a woman's view, the size does NOT matter.
When you are planning to add an auxiliary battery ( or 2 ) to your vehicle and/or caravan or trailer first decide where you are going to fit the battery and more importantly, how much space you have at the location where you want to fit it. The space alone my rule the size of battery.
If you have ample space then the next question is what type of battery.
Using a cranking type battery as an auxiliary battery under the bonnet when a winch is fitted the the vehicle is probably better as you can parallel the two batteries to use when wincing.
If you fit the auxiliary battery somewhere else and /or you will only be using it for house keeping operations, fridge, lights etc., one 12 volt deep cycle battery is better suited to this type of use, and if you have the room and need to run off the auxiliary battery for long periods of time you are better off fitting two 6 volt batteries wired in series.
Another consideration is what power will be available from your alternator.
That is not how BIG is your alternator?
If you know that you will be able to keep revs up above 1500 RPM ( this figure will very slightly from one type of vehicle to another ) for most of your driving time and most of the travelling is going to be in daylight hours then the good old 45 amp alt will work fine.
If you are going to be travelling at night with a few driving lights running and a fridge but you are still likely to be travelling at a good speed then it would be better to have a larger alternator.
If you are in a special situation where you know you are going to be travelling through rough country requiring low speeds for long periods and worst still doing it at night then it will not matter how big your alternator is, it will not be turning fast enough to cover the power that you are using let alone trying to charge either of the batteries.
A common situation that occurs in the Golf country in the wet, is for a vehicle to travel for hours and when it stops, the battery is flat.
At low revs and under load, an alternator will not be producing a high enough voltage as well as low current.
Good alternator voltage is 13.8 to 14.2. Lower voltage will not charge the battery ( s) properly and continuously high voltage will cook the battery ( s ).
You should work out these and other factors before doing any thing.
Regards. 8)

Your right on the money mate. Excellent advice. When my motor home is not in use I remove the house battery - deep cycle, and trickle charge with a smart charger. I made up an attachemet for a standard 12/24 volt battery charger. it keeps it fresh and in good nik. I need to add a higher charging alternator and a "smart" regulator and all my electricary problems are over. Same same for a 4x4 witha dual battery system.

Shifter.

Originally posted by Pedro_The_Swift
Did I count right, Tdi130dualcab?
4 batteries? and solar!


what, no wind generator?
:wink:

and to keep this "technical"

what size batts for which purpose?

O.K. to give a brief on the "4 batteries"

battery 1: stantard battery for the landrover.

battery 2: 28 Ah SLA (sealed lead acid) fits in standard battery compartment next to main battery. Used for light in tray of vehicle, running fans and charging torches and battery showers whilst at extented camps.

battery 3: 75 Ah deep cycle, backed up by a 60W solar pannel. The sole purpose in life of this battery (and pannel) is to keep the beer (I mean fridge running). This battery lives in a box in the tray of the landy....next to the fridge.

battery 4: 65 Ah deep cycle. Lives in the camper trailer and only gets charged while driving (or at home in the garage). This battery purpose in life is for lights, fans and (hevan forbid) portabile DVD in the camper.

Thats my little setup, gets well used....

Cheers