Use TWO EXTRA car batteries to start a car?

you can connect up as many in parallel as you like to get the result you need.

personally I would get one good battery on the vehicle and a good spare if needed.

mrcheerful

The message from WM contains these words:

Yes, but it's not always wise. Batteries have a very low internal resistance - if one of them is for some reason (like a dying cell) at a lower voltage than the other, HUGE currents can flow with the potential for monsterscarythings happening. In most cases this is unlikely - just as ordinary jumpstarting is safe enough.

More to the point however why not fix the charging problems in the existing vehicle?

The right thing to do is to avoid the problem. If you really must place batteries in parallel, normally is not a problem but, as other poster explained, when connecting a fully charged battery to one that is discharged and possibly faulty a lot of current rushes in to charge the low battery.

If I had to do it, I would place a 12V light bulb in series between the good and bad battery and leave it for 5 minutes or so to equalize the voltages and then short the bulb out before cranking. Once I had no jumper cable, but had plenty of 12Gauge wire so I made improvised jumpers. You can not start the engine on 2 long 12 gage wires but after waiting 20 minutes the good battery transfer enough charge into the discharge battery to let me crank. In conclusion the high resistance wires limited the charging current but in time charged the battery at least partially.

This is all emergency jury rigging that should be avoided.

Mauro

Yes.

Relatively.

Probably not.

But, it won't necessarily accomplish what you want. You're probably best off just using one battery. Whatever it takes to keep that one battery from being too discharged to start the vehicle, is what you should do.

When you put two batteries in parallel, the charged battery will drain into the discharged battery. You end up with more of the good battery's charge going to recharge the discharged battery than going towards starting the vehicle. It is conceivable that a totally discharged battery would result in two batteries half changed... without enough umph to start the engine!

With a total of three batteries, it just spreads the total charge out among all three batteries.

You would be far better off to replace the discharged battery with a fully charged battery, start the vehicle, and then jumper to the discharged battery and let it recharge. Next time, you swap in the battery you took out this time. Or rotate all three batteries if you like.

But still, the best solution is to figure out how to keep the battery in the vehicle fully charge so that you don't have to jump start it. Whatever it is you are doing that discharges it, isn't being done right...

Forget your plan.

Buy one of these:

(or get one from ebay, there's loads - search for "Jump Start")

Yes they are, there is only three reasons for the battery going flat,

1. Battery not holding the charge.

1. Alternator not charging the battery enough.

2. Something draining the battery when the engine is switched off.

1 and 2 are easily fixed but 3 can be a pain in the arse, take your car to a competent auto electrician.

No problem as long as one doesn't have a shorted cell (rare problem). Diesel trucks come with two batteries.

Some good information on batteries and how to charge them etc:

if your spare batteries are "struggling" to start the car it is quite possible that one will develop a shorted cell and if you have it in parallel with another battery when that happens there could be disastrous consequences. batteries can and do explode. putting old batteries of dubious performance in parallel is NOT a good idea.

-bob

On Sun, 22 Jan 2006 19:27:22 GMT, bob mcree Gave us:

Don't put them on a concrete floor either.

That rule no longer applies.

true, the material from which cases for batteries have been made has been for quite a few years immune to the old problems of increased self-discharge if you put the battery on a concrete floor; or a copper floor for that matter.

After reading the thread, here's what I'd recommend:

1) Fix the problem. If the car craps out when you're not at home, your spare batteries are useless. 2)At a minimum, buy or build a trickle charger. Charge spare battery A for 24 hours in even numbered months, and spare B for 24 hours in odd numbered months. You don't want to leave the batteries on the trickle charger forever - that leads to mossing, nor do you want to leave them for extended periods without periodic charging - that leads to sulfation. 3) Can you charge the car battery in the car with a long extension cord? If it's simply a matter of getting a long cord, do it.

Ed

I have 4x 85aH leisure batteries together in parallel in a trolley for powering a 3kw inverter, and it doubles up as a massive car booster \ starting battery. Unsuprisingly, I've not yet found anything it won't turn over, no matter how dead it may have been!

As others have said, the only risk is if you hook up a battery with a dead cell in parallel with another - but if all are "know good", or new, you'll be fine.

Alan.

Sounds like you could start an aeroplane engine with that sort of setup!

Anybody notice the OP seems to be gone?

: >> Don't put them on a concrete floor either. : >

: >

: > That rule no longer applies. : : true, the material from which cases for batteries have been made has : been for quite a few years immune to the old problems of increased : self-discharge if you put the battery on a concrete floor; or a copper : floor for that matter. : >

: >

Yes

1) Not as safe as having one good one in the car.

2) Whatever is draining the charge is causing heat and corrosion at the cause of the draining. That is unsafe.

3) The starter motor is designed to run for only short periods of time before they overheat. (The inefficiencies in the motor create heat.) Even one battery has enough power to burn out a starter motor several times over. The glowing internal parts of the starter motor are unsafe around any gasoline.

If you inadvertantly hook any one of the three up wrong, there is a chance they will explode - either from heat boiling the fluid or from hydrogen gas, and then spray hot sulfuric acid on everything in the area. As far as damage to the person connecting them - as long as you have a rubber apron and an acid-resistant suit and hood/face shield , and do it outdoors or in a well-ventilated area, and be careful to hook it up only when alert and sober, it won't be too bad.

However, instead of stacking batteries -

I might suggest that you get one of those dual batteries, the kind that has a switch to throw in case the "usual" part of the dual battery goes dead (if they still make them.) With one of those, you only have to "switch over" on the battery itself to start the car.

Or just get rid of the battery drain.

You mean *almost* gone! :-) But I'm still here. Been very busy for a few days.