Wiring 2 12v batteries in parallel advice.

[stupid head 10]

4-5 times a year i live out of my van, for between 3-7 days at a time, and i could really do with an extra battery and it's power.

I have to charge my phone and gps via the cigarette lighter socket and use the interior lights for extended periods due to reading and studying maps to plan the next days walking etc.

 

So, i'm wondering if it's ok to connect the spare battery i have to the one currently in use, in parallel, ie +to+ -to-.

 

Obviously, with the van not being run for several days the one battery can become flat, this happened recently, in the middle of nowhere, and i was lucky enough to get a jump start from a group of travellers who were passing by.

 

So, for extra capacity, is it ok just to join two in parallel . ?

I'm not really wanting to go down the route of having a dedicated leisure battery, as the second battery could help with starting the van, plus it means installing an isolator etc.

 

Any advice. ?

[Sid Umpley 10]

the problem is. you car battery is designed for a single function, to provide enough umph to turn over a starter motor. its just not designed for long term usage such as lights etc.

 

a leisure battery on the other hand is.

 

leisure battery and a zigg unit is the answer

[corsaman132 10]

I think you can only connect two 'identical' batteries(as in same ampage etc...).

[stupid head 10]

the problem is. you car battery is designed for a single function, to provide enough umph to turn over a starter motor. its just not designed for long term usage such as lights etc.

 

a leisure battery on the other hand is.

 

leisure battery and a zigg unit is the answer

 

Thanks for that.

 

Thing is, i already have a spare battery, albeit a "standard" 12v, and i'd rather not shell out any money as i'll be heading out again next couple of weeks for 7-8 days, and money is already tight.

 

I suppose i'm asking -

A, is it worth doing. ?

B, is it safe. ?

 

Both batteries will be under the bonnet btw.

[stupid head 10]

I think you can only connect two 'identical' batteries(as in same ampage etc...).

 

Yeah, one website i read said that, if one has a lower ampage it could draw power from the more powerful battery, over heat and explode. !

Though someone else said that was total, well, you know.

[Mikes10 10]

The main problem with connecting 2 sets of batteries in parallel

each set of batteries will be slightly different from the other

even from the same manufacturer, therefore you get the first set

discharging into the second set, the current flow will be small

but none the less discharge will occur.

 

Now, if a cell fails in the first set, then the first set

will be at 10V and the second set will be at 12V, the second

set will discharge quite rapidly into the first.

 

Connecting a high current diode in series with each set will

overcome the problem of self discharge and failure discharge

however, this changes the circuit characteristics for charging

the batteries, and the diodes themselves would be physically

quite large and expensive.

[Demonic69 10]

Running them in parallel would be fine from a draw point of view. Charging may be a little more difficult as the alternator only puts out a standard amperage, though a lot of vans have a bracket for a second alternator. it depends how much your alternator puts out.

You'd be better off with 2, new, identical batteries as stated to help with any mis-matched current drain, or one really beefy battery.

You could even just have a 12v high-amp switch to switch from one to the other when one starts getting low

[big500 10]

You could but the battery with the largest capacity as the main battery on the van, mount the second battery under the bonnet, ground the -ve on the second battery. Then run a substantial baterry cable (I used the earth wire off a mig welder) between the 2 positive terminals. Fit a battery Isolator swith to this cable and leave it in the OFF position. Fit a split charge relay under the bonnet. Connext the 2 battery +ve terminals to the split charge relay using heavy guage wire on the "Contact Side" connect 1 side of the relay coil to earth and the other side of the coil to the WL wire on the alternator. You can then run your gear off the main battery and if it goes flat just turn the isolator - hey presto instant additional charged battery to start the van. The second battery is isolated unless a) the engine is running, and it is charging or b) you have turned the isolator to the ON position. If you forget to turn the isolator back OFF after using the second baterry you will be in the same position you are now however. Alternatively you could rewire the leisure side of the van to the second battery and preserve the main battery