How to jump a car battery from another battery *not* in another car
December 24, 2023 7:41 PM Subscribe
It appears The Google cannot give me any information that doesn't involve a second car attached to a second battery. I have a second battery, tricklecharged, and no second car. I might know how to do this but asking the hive mind so I don't blow anything up.
Battery 1 is a standard 12V lead-acid batt and is in The Car (2021, excellent condition). Battery 1 is flat, not dead ; only halfway (or less) through its rated service lifetime. Flat likely due to too many recent short trips. Current temps between 50-60F; almost never below 40F. Not trying to troubleshoot the battery -- want to jump the car, using it and a second battery. I have jumper cables.
Battery 2 is a 12V 100Ah AGM battery, hardly used, tricklecharging for months from mains power and reporting 12.9V via voltmeter.
Something I dug up suggests one can wire the two batteries in parallel, making them one big battery, and the charge from the 2nd + whatever's left in the 1st should be enough for the engine to start. Which left me with questions....
1. Is this true - wiring the two batts in parallel, using the jumper cables?
2. If yes, what's the recommended order of applying the leads to the posts of each battery? And I'm *not* connecting either of the car-facing jumper clips to the chassis, right?
3. Is it correct to assume I should *not* have removed either of the car's own + or - cables from Battery 1's posts, prior to proceeding?
4. If the engine does start: how long should I keep the batteries connected? I'm guessing not long - I don't want to kill the AGM, I should just disconnect and take the car on a nice long drive -- right?
5. I should disconnect the jumper cables in the reverse of the order from #2?
Thank you!
Battery 1 is a standard 12V lead-acid batt and is in The Car (2021, excellent condition). Battery 1 is flat, not dead ; only halfway (or less) through its rated service lifetime. Flat likely due to too many recent short trips. Current temps between 50-60F; almost never below 40F. Not trying to troubleshoot the battery -- want to jump the car, using it and a second battery. I have jumper cables.
Battery 2 is a 12V 100Ah AGM battery, hardly used, tricklecharging for months from mains power and reporting 12.9V via voltmeter.
Something I dug up suggests one can wire the two batteries in parallel, making them one big battery, and the charge from the 2nd + whatever's left in the 1st should be enough for the engine to start. Which left me with questions....
1. Is this true - wiring the two batts in parallel, using the jumper cables?
2. If yes, what's the recommended order of applying the leads to the posts of each battery? And I'm *not* connecting either of the car-facing jumper clips to the chassis, right?
3. Is it correct to assume I should *not* have removed either of the car's own + or - cables from Battery 1's posts, prior to proceeding?
4. If the engine does start: how long should I keep the batteries connected? I'm guessing not long - I don't want to kill the AGM, I should just disconnect and take the car on a nice long drive -- right?
5. I should disconnect the jumper cables in the reverse of the order from #2?
Thank you!
to take your questions in order
1) yes any time you are jumping a car the batteries are in parallel because you put connect positive to positive which then feeds to starter
2) connect it as if battery 2 was in a car…positive on bat 1 to positive on bat 2, negative on bat 2 to engine block
3) yes leave the leads in place
4) assuming your alternator works you should be able to disconnect immediately
5) I always do not sure if it matters
posted by 12%juicepulp at 8:15 PM on December 24, 2023 [1 favorite]
1) yes any time you are jumping a car the batteries are in parallel because you put connect positive to positive which then feeds to starter
2) connect it as if battery 2 was in a car…positive on bat 1 to positive on bat 2, negative on bat 2 to engine block
3) yes leave the leads in place
4) assuming your alternator works you should be able to disconnect immediately
5) I always do not sure if it matters
posted by 12%juicepulp at 8:15 PM on December 24, 2023 [1 favorite]
Best answer: Flat likely due to too many recent short trips.
Short trips should not kill a car battery. Here's why not, and once you understand that, you'll also understand how to treat your AGM battery when jump starting from it.
Cranking a car engine makes the starter motor draw a lot of current from the battery while it's turning over the engine. Exactly how much varies a lot, but 200 amps is a good enough figure for this kind of rule-of-thumb analysis.
The amount of charge that a given starting episode pulls out of the battery is the product of the cranking current and the time taken. So if it takes ten seconds of cranking to make your engine start, at 200 amps, that costs your battery 2,000 amp-seconds (coulombs) of charge.
Car batteries are optimized for peak current delivery, not round-trip charging efficiency, so again sticking with rules of thumb, to get the battery back to the same charge level it had before you cranked, you need to push about twice the charge back into the battery as starting the car took out of it. So to recover from a ten-second crank at 200 amps, you'd need to supply say 4,000 amp-seconds of charge afterwards.
A car alternator in good condition will typically push maybe 20 amps of charging current into the battery for as long as the engine is running; less if the battery is close to full charge, but we're dealing with an iffy battery here so that won't be a consideration. 4,000 amp-seconds at 20 amps requires 200 seconds of charge time - a little over three minutes.
So if all is working as it should, any trip longer than three minutes should be enough to recharge what it took to start the engine, even if the start was balky enough to need a whole ten seconds. And three minutes is shorter than pretty much any trip it's worth starting a car for.
If you're working with a car battery that has chronic difficulty turning over your car - as in, it's been hard to start more than two or three times in a row - the most likely cause is battery damage, often from having been repeatedly flattened below 50% capacity by e.g. an interior light staying on overnight, or from alternator degradation. This kind of battery damage does two things: it reduces the amount of amps that the battery can supply and also reduces the battery's ability to hold any charge you pump back into it. And it's easy to kill a battery well before its rated service life this way.
If you just wire a fully charged AGM battery across such a failing car battery with jump leads, you're going to see some sparks as you make the final connection because doing that will immediately start making the AGM try to charge the car battery at several tens of amps. So you do not want those sparks to happen near either of the batteries involved, in case there's a bit of loose hydrogen around. This is the rationale for making the final connection with the negative (black) jump lead onto a bit of the car body rather than directly onto either battery's negative terminal.
But the other thing that you do not want to do is try to pull a couple hundred amps of cranking current from an AGM battery over jump leads, and that's for two reasons. First is that a typical deep-cycle AGM battery is not designed to provide peak currents that high, will make only a piss-poor attempt at doing so, and might even take some damage in the process. Second is that voltage drop across the four clipped-together jump lead connections will often be enough to reduce the available cranking current to the point where the car struggles to start.
So your best chance of success would involve wiring your AGM battery across your car battery, positive to positive, negative to negative, with the final connection on the car side made to a bit of bodywork or engine some distance from the battery, and then leaving that arrangement sit for a few minutes. What you're trying to achieve is to get enough charge into the car battery that it can supply most of what the starter motor asks for when you do eventually turn the engine over.
If it won't start, do not try over and over and over; just leave the batteries wired together for at least as long again as they already have been before the second attempt.
Once the car is running, make sure that the first jump lead connection you break as you remove them is the one to the bodywork, because you will surely get another spark from breaking that connection and a charging battery might be emitting hydrogen that you do not want to ignite, because it can literally explode the battery if it does. I saw that happen to my Dad, and it resulted in much swearing and a rapid run to the shower to wash off the acid splashes. He survived largely unscathed, but his work shirt got big holes eaten out of it.
The order you break the remaining connections in doesn't matter as long as you never allow a loose end on a jumper cable to come into contact with anything else until both ends are disconnected.
posted by flabdablet at 11:05 PM on December 24, 2023 [16 favorites]
Short trips should not kill a car battery. Here's why not, and once you understand that, you'll also understand how to treat your AGM battery when jump starting from it.
Cranking a car engine makes the starter motor draw a lot of current from the battery while it's turning over the engine. Exactly how much varies a lot, but 200 amps is a good enough figure for this kind of rule-of-thumb analysis.
The amount of charge that a given starting episode pulls out of the battery is the product of the cranking current and the time taken. So if it takes ten seconds of cranking to make your engine start, at 200 amps, that costs your battery 2,000 amp-seconds (coulombs) of charge.
Car batteries are optimized for peak current delivery, not round-trip charging efficiency, so again sticking with rules of thumb, to get the battery back to the same charge level it had before you cranked, you need to push about twice the charge back into the battery as starting the car took out of it. So to recover from a ten-second crank at 200 amps, you'd need to supply say 4,000 amp-seconds of charge afterwards.
A car alternator in good condition will typically push maybe 20 amps of charging current into the battery for as long as the engine is running; less if the battery is close to full charge, but we're dealing with an iffy battery here so that won't be a consideration. 4,000 amp-seconds at 20 amps requires 200 seconds of charge time - a little over three minutes.
So if all is working as it should, any trip longer than three minutes should be enough to recharge what it took to start the engine, even if the start was balky enough to need a whole ten seconds. And three minutes is shorter than pretty much any trip it's worth starting a car for.
If you're working with a car battery that has chronic difficulty turning over your car - as in, it's been hard to start more than two or three times in a row - the most likely cause is battery damage, often from having been repeatedly flattened below 50% capacity by e.g. an interior light staying on overnight, or from alternator degradation. This kind of battery damage does two things: it reduces the amount of amps that the battery can supply and also reduces the battery's ability to hold any charge you pump back into it. And it's easy to kill a battery well before its rated service life this way.
If you just wire a fully charged AGM battery across such a failing car battery with jump leads, you're going to see some sparks as you make the final connection because doing that will immediately start making the AGM try to charge the car battery at several tens of amps. So you do not want those sparks to happen near either of the batteries involved, in case there's a bit of loose hydrogen around. This is the rationale for making the final connection with the negative (black) jump lead onto a bit of the car body rather than directly onto either battery's negative terminal.
But the other thing that you do not want to do is try to pull a couple hundred amps of cranking current from an AGM battery over jump leads, and that's for two reasons. First is that a typical deep-cycle AGM battery is not designed to provide peak currents that high, will make only a piss-poor attempt at doing so, and might even take some damage in the process. Second is that voltage drop across the four clipped-together jump lead connections will often be enough to reduce the available cranking current to the point where the car struggles to start.
So your best chance of success would involve wiring your AGM battery across your car battery, positive to positive, negative to negative, with the final connection on the car side made to a bit of bodywork or engine some distance from the battery, and then leaving that arrangement sit for a few minutes. What you're trying to achieve is to get enough charge into the car battery that it can supply most of what the starter motor asks for when you do eventually turn the engine over.
If it won't start, do not try over and over and over; just leave the batteries wired together for at least as long again as they already have been before the second attempt.
Once the car is running, make sure that the first jump lead connection you break as you remove them is the one to the bodywork, because you will surely get another spark from breaking that connection and a charging battery might be emitting hydrogen that you do not want to ignite, because it can literally explode the battery if it does. I saw that happen to my Dad, and it resulted in much swearing and a rapid run to the shower to wash off the acid splashes. He survived largely unscathed, but his work shirt got big holes eaten out of it.
The order you break the remaining connections in doesn't matter as long as you never allow a loose end on a jumper cable to come into contact with anything else until both ends are disconnected.
posted by flabdablet at 11:05 PM on December 24, 2023 [16 favorites]
Regarding 2 & 5.
The order in which you connect + to + and - to - makes no electrical difference in terms of jumping the battery and starting your car.
The only reason why there's a recommended order is because it's safer if you do something clumsy and touch something you shouldn't with the jumper clips.
I'll repeat the advice of connecting Red (+) first and Black (-) second.
When done, disconnect in the reverse order, Black (-) first then Red(+).
Once you've connected the + cable, and you're in the process of connecting -, if you happened to brush bare metal (engine or chassis) with the - lead by accident, nothing bad will happen, The - terminal you're about to connect to is electrically connected to the chassis & engine anyway.
This is not the case if you do things in the opposite order, if you've already connected the - lead, and while connecting the + you happen to touch the clamp to the engine or chassis, you'll make a bang and a flash which at best it will give you a fright, and at worst could start a fire or damage your car's electronics.
posted by HiroProtagonist at 6:59 PM on December 27, 2023 [1 favorite]
The order in which you connect + to + and - to - makes no electrical difference in terms of jumping the battery and starting your car.
The only reason why there's a recommended order is because it's safer if you do something clumsy and touch something you shouldn't with the jumper clips.
I'll repeat the advice of connecting Red (+) first and Black (-) second.
When done, disconnect in the reverse order, Black (-) first then Red(+).
Once you've connected the + cable, and you're in the process of connecting -, if you happened to brush bare metal (engine or chassis) with the - lead by accident, nothing bad will happen, The - terminal you're about to connect to is electrically connected to the chassis & engine anyway.
This is not the case if you do things in the opposite order, if you've already connected the - lead, and while connecting the + you happen to touch the clamp to the engine or chassis, you'll make a bang and a flash which at best it will give you a fright, and at worst could start a fire or damage your car's electronics.
posted by HiroProtagonist at 6:59 PM on December 27, 2023 [1 favorite]
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posted by chasles at 8:13 PM on December 24, 2023 [1 favorite]