Jumping a car battery: that thing about not connecting the negative leads? All Crock!

[radhak]

I have always been nervous about jump starting a car battery - mine or anybody else'. And the reason has been that the instructions always said, connect the positive leads of the drained and the running batteries, but the negative lead on the drained battery end should not be connected to the battery itself, but to some other metal part. Not knowing the science behind that, I'd worry about choosing a wrong 'metal part' and something blowing up. (Yeah, I got good imagination !)

The other problem is, it has not always worked for me. Before, I used to think it was operator error. But now I am wondering.

Two months ago, my RAV4 needed a jump start. I tried using my wife's Corolla, but the jump did not work - the RAV4 coughed, spluttered and knocked, but that's it. I was about to move the negative lead to the battery itself (instead of the metal frame) when my neighbor drove up with his truck and jumped it for me from his battery. Huh, I thought - maybe the Corolla's battery is not strong enough? (It's new enough - the car itself is just a year old).

Yesterday I come out of a bank and a young lady calls out to me from her car to ask if I had jump cables. I do, so I offer to hook it up. (For whatever reason she's already barely holding back tears; maybe because her large dog in the back is panting away to glory, maybe she's having a bad day, but that's not the story).

Anyway - the jumping does not work. When she tries to start, the same thing happens - it coughs, splutters and knocks, but nothing further. I tell her that I suspect my batteries themselves are weak, or maybe need replacing, who knows? I apologize, pack up my cables and say bye. By now she's on the phone pleading with her boyfriend to come over and he's presumably refusing because she actually starts crying and all.

So I now think, what the heck, lemme try out my theory - and I reconnect the cables, this time negative to negative on the batteries, and ask her to try again. And voila! It starts rightaway!

She's relieved and even happy; though maybe she could have spent a tad more than that fraction of a second to thank me, but she was still on the phone yelling/upset at her guy, and maybe she grew up on the 'you are special' diet that fills one up with a sense of entitlement. But again, that's not the story.

Anyway, I am relieved, and sorta triumphant that I did not blow up anything and the jump worked. Is this because my battery is not strong enough to cause damage? Remember, I was using the RAV4 this time, different from the Corolla battery that also did not work when not connected to the negative lead.

I feel I'm justified to straightaway connect the positive and negative leads next time. Any comments on if that'd be inadvisable?

[LCHIEN]

First of all its important in the order of connection, positive leads should be connected first. If you connect the negative leads first then the whole car chassis is groound and when connecting the second positive lead, a small slip will allow the positive lead to contact the chassis which is everywhere and you'll get a huge spark.

By connecting the positive lead first, the negative leads can be attached without any danger of accidentally shorting to anything but a 1 square inch post, a small target to accidentally hit in the expanse of a whole engine compartment.


Now that's off the chest, connecting to the negative post vs. the chassis:
the current to the car being jumped really comes off the alternator of the running car and not the jumpee so the bracket near the alternator is a much shorter electrical path.

When you jump a car with a low battery the thing you are really doing is not running the starter motor from the running car but putting enough charge into the jumpees battery to start the car from his location - the jumper cables are a bit too small to start the motor directly.
Thus the battery and the alternator of the running car are providing a surge of current to the jumpees battery to get it charged a bit.
The most likely trouble you have was that you didn't get enough charge to the jumpees battery. Wait about five minutes if you can before attempting to start the jumpee.

By the time you tried again or the other vehicle came the jumpees battery got more charge into it.
As for the choice of terminal vs frame, I usually use the frame but the battery should work. The frame is a shorter electrical path (resistance wise) between all the points. (one alternators, two batteries, starter relay) than the battery post but ot won't make a critical difference if you wait like I suggested.

[mineengineer]

I think you want a little resistance in the circuit by connecting to the block instead of terminal, especially if it was misswired. I think doing it your way might have the potential to cause one or both of the batteries to explode.

[cwsmith]

I agree with Loring's explanation which is quite thorough. Even if you use an AC supply to jump-start a car (a 'charger/starter' is a key tool in my garage) the instructions say that you must give some time to charging the dead battery before you switch over to the starter mode. So if your battery is the least bit weak, and/or they have completely drained their battery, it may not be able to start the dead car until some charge is brought to their battery.

But, the biggest concern with just hooking both batteries directly together (positive-to-positive & negative-to-negative) is the chance that a spark may occur especially at disconnection, if connected directly at the battery where hydrogen gas is produced during the charging cycle. Such a spark, directly at the battery could very well ignite (read that as 'explode') any hydrogen gas venting from that battery. By having the negative connection made to some other area, even slightly remote from the battery, lessens the chance of that.

Perhaps this isn't the case for newer batteries, but with older lead acid batteries with vented caps, it was a real possibility, with the battery posts in immediate proximity.

Have I ever seen or directly heard of such an explosion? NO. But I don't think I want to either, so I still follow the rules... connect the "Red First" and always connect the "Black to body" (Red first, black to body). In any case, that's what Dad told me when I was 14 and I'm now 70, so it's worked so far for me.

CWS

[tfischer]

The reason you're advised to connect to the chassis ground and not the negative post on the battery has nothing to do with jumping effectiveness, and everything to do with the fact that a large spark (which will happen when you connect that last cable) could set off a battery explosion under sub-prime conditions.

So yes you CAN connect negative to negative, and you can usually get away with it. But in some cases it can be catastrophic.

It's possible you didn't have a very good ground on the chassis location you chose at first, which can indeed affect jumping performance. Otherwise it should work just fine.

-Tim

P.S. No offense but it rubs me the wrong way when people describe something they don't fully understand as "all crock".

[jking]

I appreciate the thorough explanations about the "why" behind connecting ground to chassis. The first time I heard that recommendation was when the newer cars started having a lot of electronics. The explanation I read at the time was that connecting ground to the battery would risk frying the circuitry. Only many years later did I hear someone mention the risk of sparking causing the battery to explode. I suspect with sealed batteries the risk is reduced, but, by how much I don't know.

Speaking from experience, the biggest issue I have with connecting the ground to the chassis is finding a good location on the newer cars. I have seldom had luck finding a location on the chassis that provides a good ground.

[JimD]

I've jumped my share of cars over the years and I usually connect battery to battery. I know about the spark risk but it's hard for me to imagine an explosive level of gas around a battery with the hood up. If you wanted to be extra careful you could fan air over the battery before making the connection. Hooking up the positive first is a good idea, however.

The other point I would make is there is a significant difference between good and inexpensive battery cables. Good ones have bigger wire (less resistance) and better clamps (also less resistance).

If you want to look for a good connection other than the battery you need to find a spot without paint. That isn't always obviously available. Lots of plastic and painted surfaces.

I agree completely with waiting a few minutes. I would also add that I haven't had any luck jumping completely flat batteries. Like if you leave your headlights on for a week or something. If the car being jumped cannot do any of the work, you just can't get enough current through even good cables and it takes too long to charge a battery through jumping cables (like hours). What starts the car is it's battery plus a little over the cables. If it doesn't quite get there when you first try it, waiting for additional charge to the weak battery is a good idea. If it won't crank after a few minutes, I think it's best to pull the battery, charge it (or replace it) and come back.

[leehljp]

Yes, I have heard over the years (since about the '80s) that in some cases a battery could explode but I haven't seen one. I do not work at a service station or auto maintenance store, so I don't see them regularly. My dad, in addition to being a farmer was a bulk gas distributor for this farming area. We kept lots of batteries in the warehouse and sold them to service stations and farmers. I have boosted cars, gas & diesel tractors and trucks as well as catapillars of old and other diesel/gas farm equipment. I boosted an occasional car/van off when in Japan and have also boosted vehicles off since being back.

I have tried the "grounding" to a metal to metal and experienced the same short coming as you did most of the time. If something didn't boost that way, and (it is usually 50/50 in the past 20 years with me,) then I go post to post and usually have little problems. I will say that sometimes I had to wait about 30 seconds to a minute and then had no problem (that indicated a very weak, dead or bad cell). I also keep heavy cables with me and recommend my girls keep a fairly heavy set. I have a set that has been around for about 30 years and are still in good shape. You can't boost a diesel engine off with a pair of cheap cables!

I think the "grounding" metal to metal instead of the post is a "safety" issue and a cover your bu** recommendation to prevent the shorting out and "potential" explosion. For me, it is like the spinning blade on a table saw. I watch what I touch and don't touch. I am not afraid to go post to post. My wife was sick for almost a month last winter and I forgot to crank her Prius during that time. The "starting" battery got very weak but not dead. Never the less, it would not start. I tried the metal to metal suggestion and it didn't work. (LOML was reading the book so I was being a good husband.) After it didn't work, I went post to post and it started as soon as I pushed the button and pressed on the accelerator.

I had a red-neck moment back when I was a teen and the only time I cursed in front of my dad. He asked me to check a battery to see if it had a bad cell. Instead of looking for the tester, I got a coat hanger and with pliers cut a piece of wire about 15 inches long. I jabbed one end well into one post. Then grabbing the other end well, jabbed down on the other post. The battery was NOT dead and did NOT have a dead cell! Before I could let go, the wire was red and sparks were flying as well as a few untimely chosen words in front of my dad! That was a learning experience! The wire left a mark on both hands for several months!

On the battery, the metal to metal is a safety suggestion. However, with experience, I will quickly go post to post.

OH, and If I have a can of WD 40 or other corrosion cleaning spray, I will spray before clamping onto the posts. On a few occasions, I have found a layer of corrosion between the cable & post can cause a vehicle to not start. If that is the case, boosting will be difficult until the corrosion is cleaned off or sprayed. DON"T use any flammable spray!

[billwmeyer]

I have had a battery explode. I hooked up the good battery first then when I went to hook up the weak battery...POW!!! I was on the ground with my ears ringing. I grew up on a farm and have jumped batteries since I was 10. I knew better, but I had never had a problem. This was a car battery with the vented caps, back in the late 70's. I was lucky as the side away from me blew so I was unhurt, but I was wondering why I was on the ground. I still hook up to the battery if I can get a good hold, but if not I hook up to the motor not the chassis. But I always hook up the weak battery first!

[capncarl]

All the right way/ wrong way discussion finally got around to the right way to do it wrong. Hook the positive wires up, then hook the negative wire to the dead battery, then hook the negative to the donor battery negative post. The donor battery most likely will have been in a moving vehicle and the radiator fan would have dispersed any explosive gas. Then crank the car and go get another battery.
capncarl

[radhak]

Wow - very interesting read. Thanks y'all!

I always wondered what the best sequence of the hookup would be. I do connect the positives first, but Cap's suggestion seems a good refinement.

I never thought to wait after making the connections. That should be on my next attempt. But I still maintain that connecting negative to negative proved to be far more effective than connecting to the chassis, even if risky.

ps : Tim, calling it 'crock', I was just being facetious for some humor...

[gerti]

I think one of the issues may simply be that not everything that looks like a good ground really is. For example a lot of the block may be aluminum, and aluminum oxide is an insulator. Or the materials may look metallic but are coated.

[mpc]

Many cars today have special under-hood connection points for jump starting. I've seen these on cars with the battery in the trunk, cars with batteries using side-mount screw posts, etc. Usually these connection points are in a plastic box - gotta open the cover first. (why? So that the hood can't get crushed down onto the posts in an accident leading to fire-igniting sparks. That's why the positive post of most batteries has a rubber protective hood on it too.)

Typically, I would:
1: start the "good battery" car and let it idle for a minute or two so that its alternator can replenish its own battery after starting the engine. Holding the RPMs a bit above idle helps a lot; some alternator+voltage regulator modules won't actually engage at idle RPM unless the battery is well below nominal voltage. Typically 1500 to 2000 RPM is more than sufficient. In theory, a tiny bit of hydrogen gas could be collecting around this battery now. Ideally all other electrical loads are OFF: headlights off, air conditioner/heater off (you'd be surprised at how many amps are drawn by the interior blower fan), etc. Just to give the charging alternator a break... Extra engine RPMs helps the alternator work less hard to develop the same output energy and it also means the little fan on the front of the alternator is turning faster and thus moving more cooling air too.

2: While the "good battery" car tops off its own battery charge, look around the engine compartment of the dead-battery car. I look for obvious safety issues like degraded wiring, brown stains on the intake manifold (indicative of fuel leaks/seepage - another area you want to keep sparks away from), etc. I also look at the battery itself: if the case is crack or bulging the battery has physical damage and that can be dangerous as well. It likely isn't going to explode when you try to jump it... but the owner needs to be informed that they are on borrowed time and the battery is going to strand them again very soon. If the top of the battery is dirty with a wet/greasy mess then that could be why it died in the first place. White residue around the caps or pressure relief vents? The battery chemicals were doing something bad - usually because the alternator or voltage regulator went bad and the battery was hit with excessive voltage and thus overcharged. That is fairly bad/dangerous situation and can't be ignored ==> leads to battery overheating. "Wow, the headlights are working great tonight" is a sure sign the battery is getting hit with 16 or more volts. 16 seems to be a fairly common "voltage regulator is shorted out" charging voltage on several brands of vehicles I've experienced. If the battery has removable caps, pop them off one at a time - carefully - with a rag covering them as you remove them to catch any splashes. Verify the water level covers the tops of the plates and that the plates are parallel. Low water means a leak or over-charging (boiled the water away) and the battery will have less than normal storage capacity... next time the owner wants to start they may need another jump. Use distilled water (like anybody carries this around in their car!) to refill the older lead-acid batteries but not the newer AGM batteries. Plates that appear damaged (not parallel to each other, chunks missing, etc) meant the battery internals are physically damaged and continued use of that battery might be dangerous... depends on the damage. I wouldn't jump a damaged battery personally.

Connecting the cables... what I think is a safe way:
3: positive battery post to positive battery post while holding the negative cable ends well away from the positive (red) clamps. Or connect using those special under-hood jumpering points if your car or the dead-battery car has such terminals.

4: Negative battery post or special under-hood negative jumpering terminal (if present) on the running car.

5: engine ground or chassis ground point or special underhood jumpering terminal on the dead car. This way the sparks happen away from both batteries. Avoid the intake manifold too if possible. The case of the dead battery car's alternator is a good ground in theory.. unless the mounting bolts are sloppy. (That'd be why that car had a dead battery too.) The engine front face, valve cover, and other "cap" like engine pieces are usually lousy ground points as they are aluminum or plastic parts typically with a coating and the mounting bolts likely don't have a solid electrical connection to them. Intake manifolds are often plastic on modern engines... aluminum on many other engines. Again with less than optimal grounding. In fact, on fuel injected cars you'll often see a small wire screwed to the intake manifold to ground it to the chassis or engine - so the various sensors used by the engine computer have a good ground. This scrawny wire though is way too small for jumper cable grounding - it'll melt like a fuse if you attach the jumper cable black clamp to something grounded by this little wire. Finding a way to clamp to the engine block or something non-aluminum bolted to the engine block is the ideal option. Other good ground possibilities are the big bolts/nuts poking upwards through the body sheet metal where the suspension mounts on strut-style suspensions (many front-drive cars and compacts). Sometimes you can see the beefy ground wire from the battery's negative post and follow it to where it is bolted to the engine or bodywork - that's a great ground point.

5: let the "good battery" car continue to run a bit, putting some charge into the weak battery. This is where having headlights and other stuff OFF helps - bringing a dead battery back to life is stressful for many modern alternators so give them all the help you can by removing other electrical loads. Holding the RPMs up a bit helps the charging alternator too - more input energy to work with so it can charge quicker AND it's own belt-driven fan will be turning faster, blowing more cooling air at the alternator. Heat is the alternator's enemy. The more AMPS it has to produce, the harder it'll work and the more internal heat it develops. That's why turning off headlights and other electrical loads is worthwhile.

6: after a couple minutes try starting the other engine. When it starts, quickly remove the negative jumper cable from the engine/chassis/ground terminal - again so the sparks happen away from the batteries. Hold the RPMs a little above idle on that engine for a few minutes to recharge the dead battery a bit before driving.

7: Disconnect the jumper negative cable from the good battery car. Then disconnect the positives in any order.

It's not uncommon for a car needing a jump start to end up needing a new alternator soon after. Charging a near-dead battery is a big job for an alternator especially if headlights or other big-amp items get turned ON at the same time. I've seen several cars overheat their alternators this way. Typical passenger car alternators are sized to handle the electrical loads of that car (headlights, fans, etc) with just enough extra capacity to "top off" the battery after a typical engine startup. Recharging a fully discharged battery may over-tax the alternator... that's why you need/use plug-in battery chargers instead. Work vehicles often have over-sized alternators so they tend to survive these situations.

Lead-acid style car batteries are not designed to be deep-discharged either. Getting such batteries to the point where the vehicle needs a jump start will have damaged the battery. It may seem to work once re-charged... starting the engine "normally" for a while. But it'll have less than 100% capacity after this damage... and likely will crap out again several weeks to several months later. "Deep cycle" batteries (used in boats, RVs, etc) are designed to survive 90% discharges but few passenger cars/pickups have such batteries - they're more expensive than normal lead-acid batteries. I don't know about AGM batteries - how they handle deep discharges. The "Optima" brand batteries include a few deep cycle models too.

Last thing: car batteries are sold with a "XX month" warranty. Typically 12, 24, 36, ... 72 months. My experience is that any lead-acid car battery older than its warranty will suddenly give up without much warning. Battery makers are pretty good at knowing just how long a battery is going to last it seems. I joke "you can calibrate your calendar with the battery warranty." If that battery was ever discharged to the point of needing a jump-start, or just discharged to the point where it barely started the engine, you can bet it won't last more than a couple months past that warranty date. So if you look at the dead-battery's sticker and see that it's near or past its warranty date, tell the other driver they need a new battery even if this one seems to be working now. It wont be soon... likely at a really inopportune time: dark outside, need to be somewhere NOW, kids are in the car and hungry, and it's raining too...

my over-worded opinion. I'm much more a car guy than woodworker obviously... and well known on a couple car forums.

mpc

edit: does the new forum software have a "time out" feature? By the time I finished typing that dissertation it said I wasn't logged in... I was when I started typing!

[woodturner]

Last thing: car batteries are sold with a "XX month" warranty. Typically 12, 24, 36, ... 72 months. My experience is that any lead-acid car battery older than its warranty will suddenly give up without much warning.

LIL here, and in my experience that is pretty true for aftermarket batteries, not so much for OEM batteries. I replaced the "3 year warranty" truck battery after 12 years and it turned out it was not really bad - it was the alternator (tested fine, but really was not). Might have gone another 12 years lol. I think part of the reason newer batteries seem to fail sooner is that most are "maintenance free" - so you cannot top off the water as it evaporated, which eventually causes the battery to fail. When we could top off the water once a month, batteries seemed to last much longer. Could also be other differences in technology and materials.

[radhak]

mpc, that's very very good info - both for background and practical.