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recharging a dead battery with the car, jumping car with second battery.

fuzzybabybunny

fuzzybabybunny

Moderator<br>Digital & Video Cameras
Moderator
i had to get jumped today, which got me thinking of buying a portable battery jumper.

but i was thinking.... i've got two batteries in my car... one is dead and sitting in the trunk (i know because i tried unsuccessfully to jump my car with this battery). the other is in my car, obviously. can i connect the dead battery to my car and then hook it to the charged battery to jump my car, and then drive around to recharge the dead battery? that way i have two charged batteries and can jump myself anytime, instead of buying a seperate portable jumper. a guy at sears auto said this could destroy my alternator though, charging up a dead battery.
 
hmmm... I'm not sure what the difference is between a dead battery and a low battery (low enough that it can't start the car). Is it bad to recharge a battery via the car that I couldn't start the engine with (it was trying to turn)?
 
Originally posted by: SoulAssassin
If you had two matching batteries (cca/age/everything) you can wire them in parallel, that won't work here. Your other option is an isolator...it's not worth the hassle in your case.

http://www.piranhaoffroad.com....fo_isolator_DBE180.htm
Click to expand...

Why does one need exact matching batteries?, I've used 2 different batts. wired in parallel without any problems. OP previous poster was correct when he stated it's not a good idea to try to recharge a dead or near-dead batt with your car, the Alt. isn't designed for this and could likely burn out. Counter guy @ Autozone told me the #1 reason for returned alt's. is customer installing then getting a jump and trying to recharge dead/low battery, alt. get to hot and poof! it's shot..
 
If you plan on running dual batteries, you ALWAYS need an isolator.

Car electrical systems were designed to keep a battery full, not to charge it from a severely discharged or dead state.

 
Unless you're using the second battery to run a fridge or something, it's unlikely to be worth the hassle of getting a second battery setup...
 
How about fixing your car so it starts like it's supposed to?

What's with all this talk of having to jump start?
 
Originally posted by: fuzzybabybunny
hmmm... I'm not sure what the difference is between a dead battery and a low battery (low enough that it can't start the car). Is it bad to recharge a battery via the car that I couldn't start the engine with (it was trying to turn)?
Click to expand...

Dead battery won't hold the charge even if you set it on a battery charger for weeks.
Low battery can be recharged.
 
Originally posted by: Gillbot
If you plan on running dual batteries, you ALWAYS need an isolator.

Car electrical systems were designed to keep a battery full, not to charge it from a severely discharged or dead state.
Click to expand...

If it can top off a battery, it can charge it. You'll just need to leave the car running for 2-4 days straight. 😉

ZV
 
Originally posted by: BUTCH1
Originally posted by: SoulAssassin
If you had two matching batteries (cca/age/everything) you can wire them in parallel, that won't work here. Your other option is an isolator...it's not worth the hassle in your case.

http://www.piranhaoffroad.com....fo_isolator_DBE180.htm
Click to expand...

Why does one need exact matching batteries?, I've used 2 different batts. wired in parallel without any problems. OP previous poster was correct when he stated it's not a good idea to try to recharge a dead or near-dead batt with your car, the Alt. isn't designed for this and could likely burn out. Counter guy @ Autozone told me the #1 reason for returned alt's. is customer installing then getting a jump and trying to recharge dead/low battery, alt. get to hot and poof! it's shot..
Click to expand...

Wait, that doesn't make any sense. I've jumped and been jumped many times cars with completely dead batteries. Every time I removed the cables and let the alternator charge them back up. Not one dead alternator so far. Whats the point of jumper cables anyway if every time you used them you blew an alternator.
 
Originally posted by: Bryf50
Originally posted by: BUTCH1
Counter guy @ Autozone told me the #1 reason for returned alt's. is customer installing then getting a jump and trying to recharge dead/low battery, alt. get to hot and poof! it's shot..
Click to expand...

Wait, that doesn't make any sense. I've jumped and been jumped many times cars with completely dead batteries. Every time I removed the cables and let the alternator charge them back up. Not one dead alternator so far. Whats the point of jumper cables anyway if every time you used them you blew an alternator.
Click to expand...

There's nothing wrong with it provided the battery isn't actually "dead". If it's just a case of a good battery being left with the lights on overnight or left to sit for a few weeks without starting then it shouldn't be a problem. But a battery that is completely dead (that is, sulfated, has a shorted cell, has a reversed cell, etc) can cause excessive strain on the rectifier and voltage regulator which may lead to failure of those parts.

Since both the rectifier and the voltage regulator are built into the alternator assembly on most (all?) modern cars, it is generally required that the entire alternator be replaced if one of those fails.

ZV
 
Originally posted by: Zenmervolt
Originally posted by: Bryf50
Originally posted by: BUTCH1
Counter guy @ Autozone told me the #1 reason for returned alt's. is customer installing then getting a jump and trying to recharge dead/low battery, alt. get to hot and poof! it's shot..
Click to expand...

Wait, that doesn't make any sense. I've jumped and been jumped many times cars with completely dead batteries. Every time I removed the cables and let the alternator charge them back up. Not one dead alternator so far. Whats the point of jumper cables anyway if every time you used them you blew an alternator.
Click to expand...

There's nothing wrong with it provided the battery isn't actually "dead". If it's just a case of a good battery being left with the lights on overnight or left to sit for a few weeks without starting then it shouldn't be a problem. But a battery that is completely dead (that is, sulfated, has a shorted cell, has a reversed cell, etc) can cause excessive strain on the rectifier and voltage regulator which may lead to failure of those parts.

Since both the rectifier and the voltage regulator are built into the alternator assembly on most (all?) modern cars, it is generally required that the entire alternator be replaced if one of those fails.

ZV
Click to expand...

how do you tell if a battery is dead? My spare battery has been sitting in my trunk unused for... uhhh... a year. Should i just take it to a battery place for disposal?
 
Originally posted by: SoulAssassin
don't do it, just get one of the chargers...this is the one I have

http://www.sears.com/shc/s/p_1...71486000P?sid=BVReview

only buy it when it's on sale, if it's not on sale this week it probably will next...more or less alternates.
Click to expand...

Hehe, the person who jumped me had this same exact model and it jumped me right up in an instant. Then I went to Sears and I'll be damned if I spend $60... So yeah, hoping that it's on sale next week.
 
Originally posted by: Zenmervolt
Originally posted by: Gillbot
If you plan on running dual batteries, you ALWAYS need an isolator.

Car electrical systems were designed to keep a battery full, not to charge it from a severely discharged or dead state.
Click to expand...

If it can top off a battery, it can charge it. You'll just need to leave the car running for 2-4 days straight. 😉

ZV
Click to expand...

No, the electrical system was not designed for FULL output over the extended period of time it would take to charge a battery from dead or an extremely discharged state.

That's the reason many people with large stereo systems burn out alternators repeatedly. They were not designed for high constant current loads.
 
Originally posted by: Gillbot

That's the reason many people with large stereo systems burn out alternators repeatedly. They were not designed for high constant current loads.
Click to expand...

I would say it has more to do with poor designs on the alternators. Way too much heat and not designed to remove it quick enough.


On Delcos (and most others), the weakness has always been the ability to keep the rectifier cool. Design better cooling and you move to a different weak part.

Even splitting the load across dual rectifiers on a single alt(currently what I am doing) helps a ton, there are even external mount rectifiiers out there with cooling fans to aleviate the problem.

Before my dual rect's I replaced the rect, diod trio, and regulator every year just about. This is in a DD with intermittant high loads, no expensive stereo equip. Once the rect melts down it hoses nearly everything.

 
Originally posted by: fuzzybabybunny
Originally posted by: SoulAssassin
don't do it, just get one of the chargers...this is the one I have

http://www.sears.com/shc/s/p_1...71486000P?sid=BVReview

only buy it when it's on sale, if it's not on sale this week it probably will next...more or less alternates.
Click to expand...

Hehe, the person who jumped me had this same exact model and it jumped me right up in an instant. Then I went to Sears and I'll be damned if I spend $60... So yeah, hoping that it's on sale next week.
Click to expand...

I think I got mine for $45-50.
 
Originally posted by: BUTCH1
Originally posted by: SoulAssassin
If you had two matching batteries (cca/age/everything) you can wire them in parallel, that won't work here. Your other option is an isolator...it's not worth the hassle in your case.

http://www.piranhaoffroad.com....fo_isolator_DBE180.htm
Click to expand...

Why does one need exact matching batteries?, I've used 2 different batts. wired in parallel without any problems.
Click to expand...

The weaker tends to drain the stronger and cause a premature failure. Not saying it won't work but it's not a Good Fucking Idea (tm).

 
Originally posted by: mooseracing
Originally posted by: Gillbot

That's the reason many people with large stereo systems burn out alternators repeatedly. They were not designed for high constant current loads.
Click to expand...

I would say it has more to do with poor designs on the alternators. Way too much heat and not designed to remove it quick enough.


On Delcos (and most others), the weakness has always been the ability to keep the rectifier cool. Design better cooling and you move to a different weak part.

Even splitting the load across dual rectifiers on a single alt(currently what I am doing) helps a ton, there are even external mount rectifiiers out there with cooling fans to aleviate the problem.

Before my dual rect's I replaced the rect, diod trio, and regulator every year just about. This is in a DD with intermittant high loads, no expensive stereo equip. Once the rect melts down it hoses nearly everything.
Click to expand...

I disagree, to an extent. Yes, there is way too much heat but it is also inside an engine bay. Furthermore, when the electrical system is maintained properly and used as designed the failure rate is minimal.
 
Originally posted by: SoulAssassin
Originally posted by: BUTCH1
Originally posted by: SoulAssassin
If you had two matching batteries (cca/age/everything) you can wire them in parallel, that won't work here. Your other option is an isolator...it's not worth the hassle in your case.

http://www.piranhaoffroad.com....fo_isolator_DBE180.htm
Click to expand...

Why does one need exact matching batteries?, I've used 2 different batts. wired in parallel without any problems.
Click to expand...

The weaker tends to drain the stronger and cause a premature failure. Not saying it won't work but it's not a Good Fucking Idea (tm).
Click to expand...

So your saying that during a discharge/recharge event the larger Batt. will "give up" some it's capacity to quickly recharge the smaller one??
 
Originally posted by: Gillbot
Originally posted by: Zenmervolt
Originally posted by: Gillbot
If you plan on running dual batteries, you ALWAYS need an isolator.

Car electrical systems were designed to keep a battery full, not to charge it from a severely discharged or dead state.
Click to expand...

If it can top off a battery, it can charge it. You'll just need to leave the car running for 2-4 days straight. 😉

ZV
Click to expand...

No, the electrical system was not designed for FULL output over the extended period of time it would take to charge a battery from dead or an extremely discharged state.

That's the reason many people with large stereo systems burn out alternators repeatedly. They were not designed for high constant current loads.
Click to expand...

Point. I was really just aiming for flippant. 🙂

I'm surprised that a drained (not dead or shorted) battery would pull more than 10-12 amps which is about the same load as turning on the headlights. That shouldn't come close to maxing out an alternator.

People with big car stereos will pull 75-100 amps to power the stereo (a 1,000 watt sub at 12 volts is over 80 amps). Putting that kind of amperage through a 12 volt automotive battery for any length of time is going to boil off the electrolyte in short order.

ZV
 
Originally posted by: fuzzybabybunny
how do you tell if a battery is dead? My spare battery has been sitting in my trunk unused for... uhhh... a year. Should i just take it to a battery place for disposal?
Click to expand...

Unused for a year or more? Subjected to the temperature extremes of living in the trunk of a car?

It's pretty much certainly dead and sulfated.

ZV
 
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