Adding second battery to car.

[brrapp]

I've got an old Ford pickup, it has a 2.5 diesel engine which can be a bit slow to start if it's been sat for a week or two as I don't use the pickup regularly. I'm going to add a second battery tray and wire in a second battery to give it a bit of extra cranking power. I just happen to have a brand new spare battery sitting (my son wrote his car off a few days after I'd fitted a new battery). The only problem may be that the main battery is a big one 940Amps and the spare one I have is just 570Amps Can anyone foresee any problems linking these two different sized batteries in series?

[battered]

Yes, you will f* everything. Wire them in parallel. Black to black, red to red. Alternatively, if you don't understand why series wiring is a very stupid and dangerous thing to do, leave it well alone.

[brrapp]

battered said:

Yes, you will f* everything. Wire them in parallel. Black to black, red to red. Alternatively, if you don't understand why series wiring is a very stupid and dangerous thing to do, leave it well alone.

Sorry, I meant in parallel, just got my words mixed. It was just the different cranking Amps I was concerned about. I know it wouldn't make a difference in the short term (would be just like getting a jump start from a smaller car), but don't know if it will mess up the charging system.

[anonymous-user]

only thing i can think of is possibly a bigger alternator?

[paintman]

If it won't upset lots of electronic things that will then need a reset just fit a battery isolator switch.
But if it's like that after a week I'd be looking at testing for what's causing the drain.
Be an idea to fully charge the battery with a battery charger as short runs aren't going to put much charge into the battery compared with what's taken out by starting.

Edited by paintman on Friday 15th July 08:31

[Mr2Mike]

An additional battery won't really help if the problem is down to high resistance connections (internally corroded crimps, rusty ground connections etc.) or a tired starter motor. Have you determined the actual cause of the problem e.g. checked for voltage drop at the battery and starter during cranking?

[SlimJim16v]

Batteries wired in parallel like this need to be exactly the same, otherwise the more powerful one will discharge into the other.

If your huge std one won't start it, you have a problem that needs sorting.

[Jazoli]

Why not just put a trickle charger on it rather then mess about with adding batteries?

[battered]

SlimJim16v said:

Batteries wired in parallel like this need to be exactly the same, otherwise the more powerful one will discharge into the other.

No they won't, you can wire a small 12V car battery in parallel with a big one. It is true that if one is charged and one not then the charged one will discharge into the uncharged one, but this is nothing to do with their size. If they are in use in parallel, then both will have an identical state of charge, regardless of their size. The alternator will see them as one big battery and charge it accordingly. No harm will come to the alternator by adding one battery, even if both are partly discharged then upon starting the engine the alternator will just deliver the higher charge current in the same way that it would if you started a car with a discharged battery and then turned on lights, heaters, etc.

What is true is that if your existing battery is failing after a week, you have a problem. It should cope with this easily and fixing the real problem is the better way to go. As others have said, check your terminals, earth straps, check for leakage with everything turned off, and check the battery isn't just dead.