Cheap way to isolate 2nd battery

Submitted: Thursday, Jul 01, 2004 at 21:09
ThreadID: 14297 Views:1610 Replies:10 FollowUps:16
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Is there anything wrong with just using one of those red toggle switches to do this job? I know the new smart solinoid's are the way to go, but if you remember to manually isolate the battery it must be a cheaper option????????
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Reply By: Rosco - Bris. - Thursday, Jul 01, 2004 at 21:13

Thursday, Jul 01, 2004 at 21:13
A perfect solution, as long as the vehicle doesn't have an ECU.

Otherwise spikes can be a real problem ... same as on your home 'puter.
AnswerID: 66054

Follow Up By: Brew69 - Thursday, Jul 01, 2004 at 21:22

Thursday, Jul 01, 2004 at 21:22
ECU? Is that a chip? Have an carby GQ Patrol ...should be ok?
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Follow Up By: Rosco - Bris. - Thursday, Jul 01, 2004 at 21:26

Thursday, Jul 01, 2004 at 21:26
Cob

Sounds like it's an earlier model, sans computer. In that case go for your life. But use a quality switch, like those marine isolating jobs.

That's all I used on an earlier Landie ... worked a treat and cheap as chips.

Cheers
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Follow Up By: desray - Friday, Jul 02, 2004 at 19:14

Friday, Jul 02, 2004 at 19:14
Red toggle switch is fine use the same size cables as the normal battery cable , just dont turn it on or off with the engine running. No spikes or charging problems. I have used this system for 20 years now no problems so far. Turn one battery off at night and back on in the morning BEFORE starting the engine. ( Some yellow breakdown vans have used this system for years )
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Reply By: Member - Pesty (SA) - Thursday, Jul 01, 2004 at 21:28

Thursday, Jul 01, 2004 at 21:28
Hi Brew if your 4by is reasonably modern it will have an 80 - 100- amp alternator on it and if you run your 2nd batt down a bit, when you start up your alternator is going to punch 60-80amps down that line so your switch and wiring has to be rated high enough to take it otherwise it might get too hot and melt and burn, with not so good results. Dont take short cuts here.
AnswerID: 66058

Follow Up By: Brew69 - Thursday, Jul 01, 2004 at 21:34

Thursday, Jul 01, 2004 at 21:34
Thanks Steve, i think i might get an auto sparky to do the job. Just been spending heaps lately on bits and pieces and was hoping for a cheapy. I have 2 batteries in the Patrol, just not isolated.

The Brewman
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Follow Up By: Rosco - Bris. - Thursday, Jul 01, 2004 at 21:49

Thursday, Jul 01, 2004 at 21:49
Are you sure of that?

But anyway, no need for stress. Just use battery cables with the batts hooked up in parallel and a "Hella" type isolating switch to disconnect the aux when required. Simple, and no need for a sparky.
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Follow Up By: Member - Pesty (SA) - Thursday, Jul 01, 2004 at 22:39

Thursday, Jul 01, 2004 at 22:39
Yes a battery isolator would work fine, make sure it is a marine one, the normal ones are crap, but the original question was to use a toggle switch, different ball game.
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Reply By: Member - Camper (SA) - Thursday, Jul 01, 2004 at 21:54

Thursday, Jul 01, 2004 at 21:54
Hi Brew69
My auto sparky had a cheap way to do the job using a solenoid switch, but I can't remember the details.
Point is you don't necessarily have to throw money at the job there are cheaper solution apparently.

Cheers, Camper
AnswerID: 66065

Reply By: Eric Experience. - Thursday, Jul 01, 2004 at 22:06

Thursday, Jul 01, 2004 at 22:06
Brew 69.
Your switch will be ok, dont take any notice of the post about high currants. The way a battery works prevents this from hapening. The flatter a battery is the smaller the charging current, the max current any battery can take from a regulated alternator is about 1 tenth of its capacity IE if its a 70 ah battery it will take 7 amps. The only time you could get a high current is if you forget to turn the switch of when you are starting the motor. To prevent this from being a problem it is best to use relatively thin wire so the current is limited to about 20 amps which wont cause any problems to your wireing or you switch. Thick wires and high currants are a very high fire risk.
Eric.
AnswerID: 66067

Follow Up By: Member - Captain (WA) - Thursday, Jul 01, 2004 at 22:34

Thursday, Jul 01, 2004 at 22:34
Hi Brew69,

Be very wary about some of the advice here!!!

While some batterys (i.e wet cell deep cycle) have high internal resistance, other types of batteries have low internal resistance and as you haven't stated what type of battery you have (deep cycle, lead acid, gel cell etc...) do you know what you batteries max. current draw is?

Take Pestys advice and make sure you have a big cable, but to make doubly sure, put an appropriate fuse in the line as well. Using a small wire without a fuse means you are using the line as the fuse. This has so many fire risks, as I'm sure you could imagine. Its also why car makers fit fusible links (big amp fuses) to their batteries.

I can well understand you wanting to cut costs, but please take the time to find out the implications if you take shortcuts. Not wanting to sound all doom and gloom here, but don't want to see you with a fire hazard.

Cheers

Captain
Its not what you drive, but how you drive it!
LC 200 + AOR Quantum

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Follow Up By: Brew69 - Thursday, Jul 01, 2004 at 22:42

Thursday, Jul 01, 2004 at 22:42
Its a deep cycle ED4
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Follow Up By: Member - Captain (WA) - Thursday, Jul 01, 2004 at 23:02

Thursday, Jul 01, 2004 at 23:02
Hi Brew69,

While a deep cycle will have limited capacity to accept a charge, it will still readily supply heaps of amps when starting if you forget to switch it off. Also, you (or the next owner!!!) may change the battery type in the future, forgetting about this limitation.

There are many budget ways of isolating a 2nd battery. One way (NOT the only way) to do it, is simply wire the two battery +ve terminals together using proper (thick) battery cable with a fusible link. But, use a manual isolation terminal lug on the battery, these are cheap as chips.

This way, you can easily isolate your battery with a relatively "high amp" switch. Also, if you do leave it switched on when starting the vehicle, it can handle the high amps. Plus, you can change the type of batery without worrying about the type of isolation system you have used.

This is only one of many budget solutions. I'm sure many will post of other solutions too.

Cheers

Captain
Its not what you drive, but how you drive it!
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Follow Up By: V8troopie - Friday, Jul 02, 2004 at 01:07

Friday, Jul 02, 2004 at 01:07
Some very 'strange' advise from Eric here. Thick wires and heavy currents a fire risk?? Thin wires and heavy currents, yes, the wire gets hot - so, its safer to use the right size wire for the maximum current that "could" flow.
It has been my experience that a battery takes whatever charging current it requires at the present stage of charge.
This is limited to what's available from the source (alternator
It is also limited by the resistance in the circuit, eg. wire size, wire terminations, switch or relay contacts etc.

So, if the battery is old and perhaps has a high internal resistance, then it would indeed take less current and a longer time to charge. But, how would you know? You can't by just looking at a battery.
Klaus
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Follow Up By: Eric Experience. - Friday, Jul 02, 2004 at 21:49

Friday, Jul 02, 2004 at 21:49
Klaus + Captian.
The reason for using thinner wire is to reduce the fire risk. If you were to take the trouble to measure the currents involved instead of just guesing you would find that when a motor is started the cranking battery drops to approx 10 volts, so if you have .1 ohm in the wire the maximum current from the second battery would be 20 amps. [40 watts for 5 seconds]this will not heat up the wire or burn out the switch, if you use a thick wire and a fuse each time the switch is left on during the start the fuse will blow, the driver wont know and his beer will be warm. As for the charging current my figures are correct, the simplest way to find out is to measure it. Eric.
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Follow Up By: Member - Captain (WA) - Saturday, Jul 03, 2004 at 14:32

Saturday, Jul 03, 2004 at 14:32
Hi Eric,

I can see where you are coming from, but a slight error in your resistance calculation.

Assuming 20 amp rated wire (say 26 strand, 0.32 dia copper wire rated for 20 amps, resistance is 0.0091 ohms/m) on a length of 5 meters, total resistance is only ~0.05 ohms. Even with only 10 volts, this results in a potential current draw of 200 amps (I=V/R). Clearly this will be dangerous on 20 amp wire.

A 20 amp wire does not have an internal resistance to reduce the current draw, its what you connect (say a spottie) that has the high internal resistance to reduce the current draw. The 20 amp wire simply is capable of handling 20 amps.

In practice, the total resistance is marginally higher due to termination reisitance, but not by a significant degree. This is a brief explanation and there are more factors at work, but I hope it explains it enough to point out the dangerous practise of using small wires on battery isolators.

Cheers

Captain
Its not what you drive, but how you drive it!
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Follow Up By: Eric Experience. - Saturday, Jul 03, 2004 at 22:15

Saturday, Jul 03, 2004 at 22:15
Captian.
Sorry I dont know where you got the 20 amp wire from ,I stated a resistance not a wire size. Given the resistance of .1 ohm the diference between the cranking battery [10volt] and the deep cycle[12volts] is 2 volts the current is 20 amps. Even if we use your figure of .05 ohms we still get 40 amps, so I glad you dont do my books. Eric.
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Follow Up By: Member - Captain (WA) - Saturday, Jul 03, 2004 at 23:07

Saturday, Jul 03, 2004 at 23:07
Hi Eric,

Look, I don’t want to make a big deal of this, but you do not use the voltage difference between the two batteries to calculate the current draw of a wire between them. Measure the voltage in the joining wire, its not 2V.

As for using a small wire to connect two batteries together without a fuse or circuit breaker, that is simply dangerous. Take a look at an AG fuse, it is simply a very small wire in a glass vial. When too much current passes, it overheats and melts. A small wire coated in plastic is not that dissimilar, except the plastic catches fire when the wire melts!!!

If you want me to go into detail about the calculations involved I will, but bottom line is I am an Engineer and happen to know a little bit about this subject (OK, only a Chemical Engineer, not an Electrical Engineer, but have still studied DC electrics in some detail). I am not trying to “show off” here, simply do not want to see someone do something that is unsafe.

Cheers

Captain
Its not what you drive, but how you drive it!
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Reply By: Tim - Thursday, Jul 01, 2004 at 22:41

Thursday, Jul 01, 2004 at 22:41
A big marine switch will set you back about $35 which is much cheaper then the solenoids, just dont forget to isolate it and they work great.
Tim
AnswerID: 66077

Follow Up By: Boeing - Friday, Jul 02, 2004 at 18:40

Friday, Jul 02, 2004 at 18:40
Hi Tim, Do you mean a Colhesse Switch?
This would appear a safe and cheapish solution.........

Regards

Mark
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FollowupID: 327008

Follow Up By: Tim - Friday, Jul 02, 2004 at 21:52

Friday, Jul 02, 2004 at 21:52
Not sure what you mean by colhesse switch. The marine switch I am talking about is available at bias or supercheap autos, approx 150x150mm and mayby 75mm high(?) all black except for a small red button and you can turn it to Battery 1, Battery 2, Both or Off.
They are rated at 12V for 300A continuous load or can switch up to 175A which is more then enough for any 4by.
There are 3 terminals, 1, 2 and common, put everything that is currently connected to your battery positive on the common, and connect the battery positives to 1 and 2 and thats about it (there are a few other things like earth connections etc) and I don't think it can get much simpler then that.
Tim
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Reply By: Flash - Friday, Jul 02, 2004 at 02:30

Friday, Jul 02, 2004 at 02:30
Go with the switch, I've been doing it for twenty plus years. Works a treat.

Just put a circuit breaker in the circuit. I have a 30 amp breaker which works fine and protects you from shorts. Better than a fuse, because if it trips out it cools off and then resets automatically.
Cheers
AnswerID: 66094

Reply By: ianmc - Friday, Jul 02, 2004 at 11:19

Friday, Jul 02, 2004 at 11:19
I used TWO 50 amp fuses on the advice of my auto elec.
Battery is in tray of my ute & switch under dashboard so it is easy to use anytime I want.
One fuse is line where 2nd battery joins starter battery + terminal & other is
near + terminal on 2nd battery to ensure that wherever a short may occur
both batteries are fully protected and a fire cannot occur.
The dash location of the switch also provides a handy take off point for accessories.
Cabling is fairly heavy as the starter may draw on it & recharge current may be high.
AnswerID: 66128

Reply By: Baz (NSW) - Friday, Jul 02, 2004 at 12:49

Friday, Jul 02, 2004 at 12:49
Brew69 i have a Sure power Multi-battery isolator for sale in the trader section have a look if your interested make me an offer close to the price.

Baz.
AnswerID: 66136

Reply By: Mick - Monday, Jul 05, 2004 at 08:57

Monday, Jul 05, 2004 at 08:57
Get a better quality one, not from super cheap-been down that road.
Get HELLA or NARVA, or other heavy duty steel keyed switches, as the plastic red keys vibrate and lose contact.
AnswerID: 66486

Reply By: -OzyGuy- - Tuesday, Jul 06, 2004 at 09:39

Tuesday, Jul 06, 2004 at 09:39
Concider this: Your 4x4 is worth lotsa muny!

If after reading the posts above you are not confused, and are not worried about burning it to the ground then, Ok.

However as is stated, you have to always remember to actually do the work and do the switching of the actual "manual switch" yourself....

For the price of a decent restraunt meal you can get an Isolator that is specifically designed to do the job automatically and without any human intervention.

I know my choice!

AnswerID: 66638

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