Dual battery system - opinions?

Submitted: Monday, May 19, 2008 at 22:10

ThreadID: 57784 Views:3965 Replies:3 FollowUps:4

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wortgames

Hi, I know this is a common recurring thread, but I've done a fair bit of reading and I still have enough unanswered questions that I'm keen to chew the fat with anyone who cares to offer an opinion...

I have an older model Range Rover (whose single battery needs replacing), and I do a bit of camping. No compressor fridge (I use a 3-way on gas) and I don't have a winch but may get one down the track. I often stay put for a few days or more and like to run a bit of music, the odd gadget battery charger (via a small inverter) and a bit of LED / fluoro lighting. Nothing heavy (although I may end up putting some deep cycle capacity in the camping trailer if and when we go solar). I don't want to spend a huge amount, so we can rule out Optimas and the like.

I'm thinking along the lines of a matching pair of Exide N70EX, and hopefully a controller that will isolate the 'main' battery when the ignition is off, but still run the car's normal accessories (stereo, UHF, interior lights) etc off the aux batt. The reason I'm thinking matching starters rather than starter + deep cycle is because I'd kind of like to run both batteries in parallel for day-to-day use as it seems a shame not to make the most of all that potential cranking power - the Rover V8 needs all the help it can get to fire up!

So my questions, in kind of a big mess of interrelated issues, are:

Do I need a fully automatic controller or a simple isolator? How much will I need to spend?

Will a controller protect me in daily use against inadvertently leaving headlights on - or will they just drain the aux? Do all controllers generally have a low-volt cutout to protect the aux battery? Should I just rewire the stereo, UHF, cig lighter etc to the aux battery then use an isolator instead?

If I run both batteries in parallel for starting, what happens when the aux battery has been used? Will connecting full & 'flat' batteries together cause major problems? If so, presumably I would need to start on a single battery after camping? Could I use the low-volt cutout on the aux battery and a relay to automate this (ie, if aux battery volts>cutout, then both batts in parallel for starting; if aux batt volts<cutout then start on main battery only?)

Any other tips or suggestions?!

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Reply By: GerryP - Monday, May 19, 2008 at 22:33

Monday, May 19, 2008 at 22:33

How about something real simple like this... join the two batteries with a heavy cable and a heavy duty relay, which in turn is energised by your ignition circuit. This means that while the car is off, the batteries are separated. However, as soon as you turn the key, the relay comes in and connectes both batteries in parallel.

By installing an isolating toggle switch in the ignition wire to the relay, you can stop the relay from energising if you want to start on one battery only.

Very simple, very cheap and very effective for what you wish to do.

Hope that helps mate.
Cheers
Gerry

AnswerID: 304772

Follow Up By: wortgames - Tuesday, May 20, 2008 at 10:39

Tuesday, May 20, 2008 at 10:39

Hi Gerry, a simple isolating relay was kind of my initial plan, although I hadn't considered the toggle switch - which is a pretty neat (and cheap) solution I have to admit!

I'd really like to get some undervoltage protection in there too though - it is easy to leave some accessory or other (or even headlights) turned on inadvertently. It would also be good to know that on a longer stay I don't need to worry about the battery, just use the juice until it stops.

Maybe I need 2 relays? One on the main battery, triggered by the ignition circuit, and one on the aux battery, triggered by a voltage sensor? That way the aux battery would cut out when the volts dropped, and it might even keep the aux disconnected while starting.

I'm guessing though that there would probably be something on the market that already does all this?!

FollowupID: 570873

Reply By: Sand Man (SA) - Tuesday, May 20, 2008 at 10:42

Tuesday, May 20, 2008 at 10:42

I would leave all "normal" accessories (stereo, UHF, cig lighter, etc.) connected to the standard battery. These are (or should be) controlled by the ignition key and you would be unlikely to flatten the battery, unless the stereo system includes an a high watt amplifier.

Even using an inverter/gadget charger, or LED lighting will not draw much power from the battery.

If you are on a low budget, I would suggest you spend your money on a new starting battery and a jump start power pack in case of a flat main battery. The power pack can be recharged via the vehicle alternator when driving and could also be used to power your LED light and fluro, providing it is a versalite, or similar low drain device.

If you have a higher budget, I would recommend you look at a higher capacity power pack such as the Sidewinder (ABR) unit (advertised on this site), or a Blue Apple Thumper.

Only if you need to run a fridge or other higher current draining device, would you need to install a dual battery system and this should include a deep cycle auxiliary battery and be controlled by a "smart" isolator.

Bill.

Bill

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AnswerID: 304822

Follow Up By: wortgames - Tuesday, May 20, 2008 at 11:09

Tuesday, May 20, 2008 at 11:09

Hi Sand Man, I reckon that just about any device can drain a battery if you run it long enough! A few days camping with the CD player going and a few lights at night can really take the edge off a fully charged battery. Add to that 20w of interior lights left on all day when somebody leaves a door ajar and a bit of inverter / air compressor use and it's not hard to foresee a problem getting started!

I have a cheapo GMC jump starter / portable power pack which I use mainly for lighting away from the car but I really can't see one of these getting the Rangie going, unless it essentially contains a full size starting battery anyway - in which case price, storage and handling become an inconvenience and I might as well mount the thing under the bonnet anyway. It would also be good to have the two batteries in parallel for daily starting and future winch use etc.

I do own a gel cell which will probably wind up in the camping trailer along with a black panel or two for longer trips, but right now I need to replace the main starter in the car and I'm tempted to get a matching pair for all the above reasons.

FollowupID: 570876

Reply By: Mainey (wa) - Tuesday, May 20, 2008 at 17:00

Tuesday, May 20, 2008 at 17:00

Use CRANKING batteries as CRANKING batteries....

Read any battery manufacturers own web site and see exactly what they recommend their battery is capable of being used for.

Then read their oppositions web site to get a comparison !!

Mainey . . .

AnswerID: 304888

Follow Up By: wortgames - Wednesday, May 21, 2008 at 11:33

Wednesday, May 21, 2008 at 11:33

Hi Mainey, yep I am aware of your apparent dislike for the Extremes from numerous other archived threads on the subject!

I would have thought though that for an application where 99% of the time it is being used in parallel with the main cranker, and therefore being used for the same purposes as the cranker (ie, cranking and a bit of winching) that it would make far more sense to get a matching pair of crankers.

That said, and knowing that OCCASIONALLY the battery will be called upon to run a few accessories between charges, I would have thought that the Extremes would probably do a better job of handling the occasional camping trip than most other cranking batteries. Do you disagree with this?

FollowupID: 571064

Follow Up By: Mainey (wa) - Wednesday, May 21, 2008 at 18:18

Wednesday, May 21, 2008 at 18:18

I assure you it's not a personal thing with me at all.

As I said just look at (any) manufacturers web sites, I did not nominate Exide, but as you did, I will say on their battery site the extreme IS rated as a "passenger car CRCANKING battery" and not as a Deep Cycle STORAGE battery.
I don't dislike Extremes, I just believe they are over rated and over priced too, but that is only my 'opinion' and I don't sell them anyway!!

As you state you use yours in "an application where * 99% * of the time it is being used in parallel with the main cranker, and therefore being used for the same purposes as the cranker (ie, cranking and a bit of WINCHING) that it would make far more sense to get a matching pair of crankers ...... and knowing that *OCCASIONALLY* the battery will be called upon to run a *FEW* accessories *between charges*, I would have thought that the Extremes would probably do a better job of handling the occasional camping trip than *most* other CRANKING batteries."

Yes, this is 100% true, so I don't disagree with you !!

However, remember the Extreme has only a 6 Month warranty when used other than a Cranking battery.

Mainey . . .

FollowupID: 571124