How can i add a second battery?

Hi. I am wanting to add a second battery so when im at a show i can run my install without running the power down on my main battery. 

I have seen it before where the person who owned the car had a battery under the back of the car and just plugged it up somewhere using jump leads. I was wondering if there is an easy way of doing it without draining the main battery or spending a lot of time and money installing a second battery with a cut off switch?

It will only be used for a few hours so the second battery doesnt have to last too long and i can just plug it upto a charger in the garage when i get home.

Any help would be appreciated, Thanks i advance mark.

p.s - This might have been explained somewhere already but i could not find it, If so i apologise. 

first i recommend that the battries be identical we just finished a threed about this  https://www.the12volt.com/installbay/forum_posts.asp?tid=80238&PN=0&tpn=1

next check out this site there is a nav on the right of the page you want #118 this will help u deside which kind of set up to use http://www.bcae1.com/

Thanks a lot.

That makes it a bit easier to understand. I thought it would have been asked before but i couldnt find it.

I think i will go for the diode type isolator as it looks easiest to set up. The car is only used at 6 shows a year so it isnt a big deal if i lose .6 v on the set up.

Thaks for your help.     Mark.

so lemme get this right from what that thread said, you are supposed to use the SAME EXACT Batts.? And if so under that rule, you shouldnt use a Red Top under the hood and Yellows as secondary? if i read that right i would say i would have to disagree with that too. i have seen many setups that come through the shop that have done that setup and everything works fine, and have yet to see problems.

From what i have read, If im understanding this right, they mean you cannot use a battery that has been on the car 5 years and then throw a brand new one on as your second.

You can use different types but they have to be the same age.

and i have seen many cars with 4awg power wire,10awg ground, wire ive seen phone cable for speaker wire, radios ing wires hooked up so that the radio only turn on when the parking light are on, but thay work with no real negative consequeence. the question to ask is it the right way to do it? are these people getting the most out of there equiptment? should any self respecting installer tell a custome that its right just beacuse it works and the car is not on fire? the answer is no. so next question are any of us batt. experts? probly not ,do the batt. experts say  that your battires should be identical in type, CCA ,and age, yes so why do we argue that fact?   

Just for another data point, there is another application that uses a battery for 'engine off' use, and an 'engine start' battery.  RVs.. motor homes and campers.

Here's an application where the engine start battery is a standard starting battery.. probably something like 12v 100ah. The battery bank for 'engine off' use is often something like (4) 6v 220ah deep cycle golf cart batteries wired in series parallel. An isolator is used, and while battery matching is *strongly* recommended for the battery bank itself, the age/size/usage level comparison between the battery bank and engine start battery really isn't a concern.

Personally, if you're not using an isolator of some sort then match the batteries. If you're using an isolator (correctly), then I'm not too awfully concerned about battery matching. The RV industry has been doing it this way for decades.. on hundreds of thousands (if not millions) of vehicles. 

In short, it all depends on what you're trying to do, and what you want to accomplish.

Jim

nouseforaname wrote: great thread, huh josh?

ya it seams like we just did this.

good point. but the main diffrence in rvs are that the battries when in use are compleatly isolated under no circumstance will thay ever power the same load. in a car with most isolator set up the battries are only disconected when the car is off therefore thay power the same load and charge at the same time .that will not happen in a rv.

Using the link you posted ( http://www.bcae1.com/) and example #118..

Most RV circuits that use a solenoid look like the 'Dual batteries without an isolator' with the addition of a single solenoid connected near the left fuse (off the front battery). They aren't nearly as complicated as the 'solenoid type isolator' circuit with its two solenoids and switches. The RV solenoid is trigger by ign+12; when you turn on the ignition the front and rear batteries are hooked together in parallel.

We took our RV out over the July 4th weekend.. while driving, the alternator died. At that point we were running on battery power; both the engine battery and the house batteries.

You said;

"in a car with most isolator set up the battries are only disconected when the car is off therefore thay power the same load and charge at the same time .that will not happen in a rv.."

That is **exactly** how it works in an RV with a solenoid based isolator system. When the solenoid is energized all the batteries are hooked together and power the same load and charge at the same time.

Answer this for me.. how much power is coming _out_ of a battery when the system voltage is 13.6vdc?