Use TWO EXTRA car batteries to start a car?

Can I jump start a car by using an extra *TWO* car batteries in parallel to do the boosting?
Is this safe?
Is it likely to damage any of the batteries involved?

Thanks - William
--------------- BACKGROUND INFO ---------------------
My car battery tends to run down. The reasons are not all that important here.
I can't use a mains charger on the car's own battery unless I take the battrey out out. And I don't want to keep doing that.
Near to the car I keep a couple of old car batteries which I use to jump start the car.
QUESTION: If my spare battery is struggling to start then car then is it alright to put *BOTH* spare batteries in parallel and then use them to jump start the car?
In other words, there would be three batteries in parallel (if you count the battery in the car).
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posted to alt.engineering.electrical,
uk.rec.cars.misc,uk.rec.cars.maintenance
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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
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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?
--
Skipweasel
Si hoc legere scis nimium eruditionis habes.
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contains these words:

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

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On 22 Jan 2006, MG wrote:

I am not clear where you would wire the bulb.
Does "bad" mean bad battery in the car or the weaker boosting battery?
And are you connecting battery-1 positive to battery-2 negative to give 24 V before completing the circuit with the bulb in seris?
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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...
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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On 22 Jan 2006, Floyd L. Davidson wrote:

I would like to do this rotation as it makes a lot of sense but the two spare batteries are physically too large for my car.
They were not ones I used myself but ones I was given.
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wrote:

Forget your plan.
Buy one of these: http://www.maplin.co.uk/Module.aspx?ModuleNo7385 (or get one from ebay, there's loads - search for "Jump Start")
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On 22 Jan 2006, Ed Chilada wrote:

At only 17 Amp-hours it seems to me to be essentially a smaller than usual car battery.
It's kind of what I've got isn't it, except when I take my spare battery into the house to charge from the mains I do not have such a neat carrying handle.
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On Sun, 22 Jan 2006 20:49:15 GMT Message-ID:
wrote:

And you have to have a separate battery charger, this unit just plugs into the mains and charges itself.
And it's easy to put in the boot in case your battery goes flat outside Asda.
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WM wrote:

Yes they are, there is only three reasons for the battery going flat,
1. Battery not holding the charge.
2. Alternator not charging the battery enough.
3. 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.
--
ThePunisher
Latitude: 54.67N
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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:
http://www.batteryfaq.org

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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
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On 22 Jan 2006, bob mcree wrote:

My spare batteries might struggle (don't know for certain) for a combination of reasons:
They were getting a bit old when I got them and *may* have been thru deep discharges or getting weak cells or whatever.
Hard to tell battery condition from voltages alone. Need to use a big battery-load tester.
Cold weather (when I need them most) reduces their power output.
I do not need them so very often so I am liable to not charge them up just when they need it.
They will self-discharge.
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Gave us:

Don't put them on a concrete floor either.
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That rule no longer applies.
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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.

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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. : > : >
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On 23 Jan 2006, Pop wrote:

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

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WM wrote:

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
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