I'm trickle-charging a car using direct connections to the battery. I thought this couldn't be simpler: ventilate, unhook the battery, connect the positive first, then the negative, then turn on the power. Then I saw this article...

Locate a Suitable Ground Location

Finding a suitable location to attach the negative terminal of your charger is the key to safe and effective completion of your trickle charger circuit. Ideally a spot of bare metal on the frame, or a large bolt attached to the chassis or engine block should be used, but if one cannot be located, a section of the frame that is free of excessive dirt, grime and oil will suffice. The negative terminal on the battery should never be used, as it can cause an explosion or fire.

Attach Cables

Connect the alligator clips. There should be a black clip and a red clip. Take the red or positive cable and attach it to the positive terminal of the battery to be charged. Once you have ensured you have a solid connection, connect the black, negative wire to your ground location located in step 2. It is important to ensure that this negative wire is solidly attached before plugging in your charger. Do not touch the wires while the charger is plugged in and turned on, as serious shock or injury may occur.

Connect the black clip to the negative batter terminal and the red clip to the positive one. Once everything is in place, connect the trickle charger to a power socket.

...which is confusing, not only because it seems to contradict itself (first it advocates never using the negative battery terminal, then at the end it says to use it after all), but also because there's a mess of disagreements in the comments, for example...

I am a professional, factory Toyota trained technician. I can tell you right now, that you, and the author of the original post don't know what you're talking about. ... Secondly, you do NOT hook a battery charger up to charge a battery by applying an alternate ground source, i.e. an engine block. The ground connection for a battery charger, used at a 2-4 amp rate (trickle charging) should ALWAYS be applied direct to the ground side post of the battery. Hence why, you remove the battery from the vehicle.

Now I'm not sure who to believe...

Various other sites seem to say what I'd always thought, which is to use the two battery terminals, but some people here seem convinced that this risks an explosion, even if you don't turn on any power until both croc clips are connected:

[to prevent] making a rather dramatic mistake. BLACK goes to GROUND [usually a metal bracket or large bolt head that's attached to the engine well sheet metal], and RED, to the positive post on the battery

What do I do with the negative (black) croc clip on my trickle charger, to ensure safe, stable charging?

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    Well, as I explained, that's what I thought at first and what I've always done, but a whole lot of people are saying that there are dangers to doing this, so I was hoping for an answer that explains if the concerns are legitimate or not. Commented Jan 9, 2016 at 18:47
  • Here is a video showing what can happen if you just do red to red and black to black youtube.com/watch?v=F3bu0TN_0eE
    – rviertel
    Commented Dec 12, 2016 at 3:46
  • Do note that trickle chargers by definition have low amperage, and thus the risk of spark is lower than for high-power chargers. Also, if you let the battery charge fully before taking the leads off, the possible hydrogen generated has long ago dissipated. Furthermore, some intelligent microprocessor controlled charges can help to eliminate the spark. I'm not claiming that you should connect the negative to the battery terminal, just saying that the risk of hydrogen explosion is lower than what people think.
    – juhist
    Commented Jan 27, 2017 at 20:35
  • I generally charge my batteries in a well ventilated place (outside, still in the car). I usually connect the clips straight to the battery without disconnecting the car wiring loom from the battery, and then I plug the charger into the wall outlet to power it in. Never seen a spark anywhere near the battery in all the time I've been doing it, because of the order of connection..
    – Caius Jard
    Commented Aug 31, 2017 at 23:34

7 Answers 7


Yes, as everyone is stating, a spark igniting hydrogen gas could cause an explosion, causing injury from small parts, sulfuric acid or both. Very nasty. Hydrogen gas is a byproduct of electrical energy created from a chemical reaction of lead plates submerged into the acid and water. Sparks can happen internally, too. Like when the lead plates warp from water evaporating out. This is why some are cynical about 'sealed' cells on an unmaintainable lead acid battery. Spark wouldn't necessary come from jumper cables or charger cables in this case, though, but rather simply attempting to start the vehicle.

It seemed like there were a couple of specific questions you had that I will attempt to answer.

Don't attach negative clip to the neg terminal on the dead battery unless you want the car's wiring removed from the battery. Attach to clean, unpainted part of frame or engine block bolt. This will keep any possible spark or spark arcing away from the potential gas. The contradictory part of whatever website you wrote from in your original question post was obviously either a mistake or the part where it spoke of attaching to the negative terminal was describing the process of jump-starting from a charged battery to a dead one. In that case you always attach dead-side first, with negative clamped to frame or engine, but the positive and negative attached to their perspective terminals on the charged battery (on the other side of cables). Again, the exception is if you remove the car's wiring.

The reason you go from dead to charged is to reduce the risk of spark. You can't take the risk of spark away completely, which is why it's better to attach the negative clip away from neg battery terminal. Same principal with a charger. Don't plug into power until clips are clipped. Just like going from dead to charged when charging from another battery, don't plug in charger until clips are on.

Another question I think you had was if there are other types of automotive batteries that don't have this risk. The answer is that unless it is lithium ion (or similar) batteries found in hybrid/electric cars or sometimes found in an emergency kit, they will ALL be some kind of lead acid type, even though there are a few different kinds of lead-acid batteries in use. They all hold the same risk.


Here is a link to a credible source regarding a study that took place with lead-acid batteries and explosions. It also includes all the other information I stated above about safety measures like hooking the negative to frame or block instead of terminal and the reasons for these safety measures:


Edit: additional info for an additional question. Should car wiring be removed from terminals before charging the battery?

This is a double-edged sword so you'll have to ultimately decide for yourself.

When working on the electrical system for repairs, you should remove the negative cable off of the terminal, anyway, so you don't accidentally short a circuit and damage an electrical component. The problem with cutting the car's circuit is that many ECU/PCU units have adaptive learning to help it control everything and disconnecting the circuit will force it to 'relearn' timing, shifting, etc.** This is due to Keep Alive Memory (KAM), a chip that stores information, which will erase when disconnecting a car's circuit from it's electrical source. The problems can vary from simply waiting for the 'relearn' process to even having to perform manual resets with a scan tool in order to get certain functions working again. Worst-case scenarios can require replacement of ECU altogether in order to get certain functions working again. Worst-case scenarios are usually only possible with early 2000's and later. (90's won't have these problems but still may need to relearn engine cycles, tranny shifting, etc.). There are pocket lists that specify specific problems with specific models out there, but you probably won't find one big, conclusive list for all problems with all models, so research on your specific model is always a good idea. There are KAM savers that plug into a cigarette lighter and use a 9-volt battery to Keep the KAM chip from erasing info while the battery is disconnected, but this will only be helpful if you can complete the repairs before the 9-volt battery is drained, maybe 30 minutes, +/-. Obviously, this won't help while charging the battery. So that is the downside to removing the cables from the battery to charge it. That and the risk of spark increases. Also, if you remove the cables, you have to connect the charger's ground clamp directly to the negative terminal on the battery instead of the frame or block. My previous edit stated otherwise and, after rereading it, I realized the error in that statement.

The benefit of removing the cables is that it will be impossible for a power surge to damage anything electrical-related in your car. Since the battery charger is plugged into A/C current, surges can obviously cause electrical damage. That's why I say the answer is a double-edged sword, because there are risks either way you go. If it were me, I'd keep the cables connected while charging, but use a GOOD surge protector for the charging device. By good I don't mean a $10 cheapie. If you keep the cables connected, follow this process: Plug the charger's positive clip onto the positive battery terminal. Then plug the negative clip onto the frame or engine block as far away from the battery as possible. Then plug the charger into a good surge protector, followed by plugging the surge protector to the wall outlet. Then turn the charger's switch on. Lastly, turn the surge protector's switch on. If it doesn't have one, buy a good one that does have a switch. A few final points: It's safer to use a lower amperage charger, between 2-6 amps, than to use a faster charger. It's bad for your charging system if you don't charge the battery completely, so be sure the charging process is complete. Lastly, I believe it IS necessary to have the car's ground cable connected to the negative battery terminal for the charger to charge it IF you clip the charger's ground to the frame or block. If you want the cable removed, you'll have to connect The clip directly to the battery. I'm sorry if reading my previous edit was misleading. If you decide to remove the cables, the negative cable ALWAYS goes first to prevent accidentally creating a complete circuit with a tool, jewelry, etc. Always use eye protection while working with the battery. I hope this answered your questions to your satisfaction.

**This adaptive learning can throw some people off after improving their vehicle performance. For example, say you complete a post-coil(s) ignition system overhaul. This will improve spark efficiency, which will subsequently advance timing. The ECU is used to the more retarded timing and will have to learn the improved cycling. This can fool some people into thinking they did something wrong but, if you just drive it around for a while, it will eventually relearn the more efficient timing. This process can vary widely, from 25 to 100+ miles. It may even take a few days of separate start-ups. During the process, you may experience erratic idling and acceleration, alarming exhaust, etc. Unless it's extremely obvious there's a problem with the work that was done or the problems don't go away, it should clear up on its own.


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    Great answer, thanks for the explanations. One last thing I'm unclear on - to use a ground location, the neg battery terminal needs to be connected, right? So the positive should be connected too - since you should never have only negative connected? So therefore this should only be done with the battery connected to the car? I'm asking because some sources say to charge with the battery unplugged to protect the car's electronics. Commented Jan 14, 2016 at 15:06
  • The negative terminal will at least need to be connected to the chassis, yes. The only way to charge the battery when the negative cable isn't connected to the terminal is to attach the negative clip directly to the terminal. If you don't want the cables connected then you'll be forced to clip directly to the terminal. Commented Jan 15, 2016 at 1:12
  • Not in my wildest dreams did I imagine such a complete answer to my own question. Cheers @Mustangguy809! Commented Dec 2, 2016 at 3:22
  • Does this explanation (gases from lead + water) still hold valid if the auto battery is Li-Ion?
    – atconway
    Commented Jun 15, 2020 at 18:13

The "best" thing you can do is not use the crock clips on the battery while in the vehicle in the first place. Most trickle charges come with a permanently mounted set of leads, like you can see in this Black & Decker version:

enter image description here

In the picture you can see three different types of leads (as looked at from the standpoint of the common attachment lead):

  • Top: Crock clips
  • Middle: Permanently mounted leads
  • Bottom: Cigarette lighter

Using the permanently mounted leads provide you protection from several aspects. First, you can attach them to any point in the electrical system which provides a good ground (negative lead) and hot (positive lead). When attached to the charger, this will provide for the charging needs. Secondly, since the common attachment lead completely encloses when connected, there's no worry of an open spark occurring. If placed in an accessible place, you wouldn't even need to open the hood of a vehicle to get this attached.

You can use the crock clips directly on a battery which is not located in the vehicle. Use the cigarette lighter version for a quick attachment source. Use the permanently mounted ones for a quick disconnect under the hood without the need for fumbling with either of the other two.

  • Nice. My charger doesn't have those, but it uses the same type of cable so I'm sure I could buy them. Can you give an example of a typical place you might mount the positive lead? (the negative lead could go to any exposed metalwork in the engine, right?) Commented Jan 13, 2016 at 11:59
  • Correct on the negative. Just needs to be a clean metal bolt which provides a good ground. The same with the positive, though there's going to be fewer places to mount this. I used a small power distribution point which had a common "lug" on it to mount mine. You can also connect it directly at the battery terminal (if there's something available there) or there may be something at the under hood fuse panel. It just has to be somewhere in the positive side, close enough to a ground point attachment for the other. Commented Jan 13, 2016 at 12:10
  • For the permanently mounted leads option I have the option of a chassis screw in the vicinity of the battery (motorcycle) for negative mounting, but the chassis metal where the bolt goes through is painted. Does that disqualify it as a proper ground?
    – atconway
    Commented Jun 15, 2020 at 18:17
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    @atconway - No it doesn't. You could just take it down to bare metal at the contact point using some sandpaper or some other abrasive. You can always check between the would be ground and the negative side of the battery with an ohm meter (digital multimeter) to check for continuity if you're worried about it. As long as you have good conductivity between the two, you should be golden. Commented Jun 15, 2020 at 18:33

Ignition of gas by sparks is clearly what everyone is talking about here. The argument about connecting the terminals first before plugging in the charger is not really a protection from sparks. If the charger is not plugged in, there will be a brief flow of current from the battery to the charger which could in theory cause a spark. This could be due to a capacitance inside the charger, or even capacitance between the charger cables. Likewise when it comes to disconnecting the charger cables, depending on the design of the charger, there could still be a small current flowing into the charger if the charger was unplugged, which could in theory cause a spark.

Having the negative connection made away from the battery would remove any possible spark from the location of any ignitable gas and so would remove the chance of an explosion.

Since the charging current is relatively small, there should be no problem connecting the negative terminal to a clean remote location, since there will be minimal voltage drop between the remote location and the battery terminal.


1) From an electrical perspective, you always connect negative of charger to negative of battery. The closer the better (i.e. put the clips directly on the battery) to avoid losses and heating in the wiring of the car.

To check for this put your voltmeter across the battery when you are charging, then put it across the output of the charger. Any difference is lost in the wiring. A voltage under about 13.2V at the battery means either your battery is just about dead, or very little charging is taking place. Reading the amps going out the charger will also reveal much.

i.e. watts in = volts @ chargeramps at the charger watts into battery = volts@batteryamps @ battery

 difference = lost power == heat
 losses are either in the wiring, or are being used by some electrics in the car. 

2) Always do this in a well ventilated place, preferably away from the car and other flammables. Hydrogen, Oxygen and combustables (e.g. your house) are not your friend.

3) Observe basic safety precautions - open battery breathers (if present), fire extinguisher, circuit breakers, equipment fuses, gloves, eye protection etc.

4) Hydrogen build up is mainly due to electrolysis that takes place at the electrodes as part of the charging process. It doesn't happen when you are driving because the vibration loosens the hydrogen and oxygen bubbles from the plates all the time. In the static 'charging' situation this doesn't happen - so you can do it by lifting a corner of the battery about 10mm (1/2 inch) and dropping it about every 1/2 hour. You will hear the bubbles coming to the surface. If you are in a well ventilated area, all is well. If you are smoking you may singe your eyelashes.

5) You are more likely to have a spark ignite hydrogen at the end of the charge process than at the beginning (see above). Switch the charger off at the wall and wait for everything to settle before removing the leads.

6) Attaching the leads is more tricky since you will get a spark either way round - either the residual battery charge feeds the charger(if the charger is off), or the charger feeds the battery and sparks (the charger is on, battery voltage is low). Either is sufficient. There is no substitute for doing what you can to get rid of these naughty gasses - ventilation, vibration, time.

A car I had in the 60's - I think it was an original mini or Beetle actually had positive battery terminal to chassis. Blindly connecting charger negative to chassis and positive to either pole of the battery would have caused problems (read fire, smoke, brokenness) in the charger, battery or car - especially in the old equipment we used back then.

If the battery to chassis connection is poor, the car will not start anyway since much of its power will go into the bad connection rather than the starter motor. This place will get hot and burn you or the car or both.

We old timers chuckle at the safety precautions that are so often taken to ridiculous lengths - until someone loses a house, car and perhaps life.

Only then do we understand.

  • Interesting... and completely different to the other answers. Your point 1) matches my experience: after I moved my negative clip from the battery terminal to a position on the chassis that is connected based on the other answers, the charging rate appears to have reduced by about 80% (though that could be because it's non-linear in volts). Point 5 makes a lot of sense - I never understood why everyone seemed so worried about hydrogen build-up before charging. To be clear, are you advocating physically removing the battery from the car? I'd like to see what the others make of this... Commented Jan 19, 2016 at 10:56
  • p.s. regarding point 6, what about if my croc clips, similar to Paulster's image above, connect to the charger via a cable with an SAE plug, and I clip the cable to the battery before plugging the cable into the charger, and unplug the croc clips before unclipping them? That would mean any spark happened safely down the cable, at the SAE connector, right? Commented Jan 19, 2016 at 11:03
  • @user568458 - your ps is on the money. When the car is in an unventilated basement I would probably remove it if I want belt and braces safety, otherwise not, the carbon monoxide is more likely to kill me when I start it. I'll edit my response to answer your first query
    – ChrisR
    Commented Jan 19, 2016 at 14:25
  • Everyone should be mindful of the potential for gases at all times. Remember that the alternator charges the battery and, if the battery died, there could still be trace amount of gases. The gases can can also get stuck into pockets. I think if there were no cause for safety concerns until after charging, there wouldn't be so many documented cases of battery explosions when hooking up cables before the charge. Commented Dec 2, 2018 at 21:06
  • Also, increased resistance will increase heat and time during a charge. If one decides to clip away from the negative battery terminal, resistance will naturally be increased. That is simply another trade-off for safety. But it all comes down to choice. Commented Dec 2, 2018 at 21:06

As far as I understand it, the risk is that, in the course of normal operation, the battery releases hydrogen gas, and if there is a spark close to the battery, this gas could ignite and cause an explosion.

When you connect a set of jump leads to the car, there is a chance of a spark as the second lead is connected and the circuit completed - and as you should always connect positive first, this risk can be avoided by connecting the second, negative, lead elsewhere in the vehicle.

  • Is this still a risk if, as mentioned in the question, I connect both clips before activating the charger? Also I remember reading somewhere that only certain types of car battery release hydrogen gas (I've lost the link but it seemed to imply that most modern ones don't?) Commented Jan 13, 2016 at 10:12
  • I was just trying to decide that - In theory as long as no current is going to flow, then there shouldn't be a spark, and so there shouldn't be a risk. Most modern batteries are sealed ('maintenance free'), so, again in theory, shouldn't release any gas - but then do you trust them...
    – Nick C
    Commented Jan 13, 2016 at 10:16
  • That makes sense One last thing I'm unclear on - to use a ground location, the negative terminal needs to be connected between battery and car, right? So the positive should be connected too - since you should never have only the negative connected? So therefore this should only be done while the battery is connected to the car? I'm asking because some sources said to charge with the battery unplugged, to protect the car's electronics. Commented Jan 14, 2016 at 15:08
  • Yes, this is talking about connecting it in the car - obviously if the battery is out of the car, you can't use a remote connection...
    – Nick C
    Commented Jan 14, 2016 at 15:24
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    The other reason for doing it this way is that the dangling lead is going to be connected to chassis anyway. If it inadvertently brushes anything on the car it will most likely be chassis and not do harm. If on the other hand, the dangling cable is positive, touching it to chassis will put a full 12V into a short circuit, 100s of amps if it attached to a battery at the other end. Lots of heat and probably a fire as it will weld itself to the chassis. No need for hydrogen in this equation
    – ChrisR
    Commented Jan 19, 2016 at 14:41

The danger of explosion derives not from the ignition of hydrogen but of the 2:1 mixture of hydrogen and oxygen. 55 years ago I experienced this and was lucky that the sulfuric acid shower missed my eyes. Yet it didn't occur to me that this was the reason for connecting the negative cable to the frame well away from the battery ! Whoops !

  • Welcome to the site. The main site is intended for questions and answers, not discussions. You're welcome to swing by the chat room and air your views and experiences there :)
    – Zaid
    Commented Dec 19, 2016 at 18:16
  • @Zaid, Richard Martin is explaining, through his experience, that the negative croc clip should be connected last - well away from the battery on a good earth point to the chassis/ engine and HE is correct... I have had to help a colleague who had a battery explode on him and 3 cars covered in acid... So, my point is that he has give a correct answer, not just views...
    – Solar Mike
    Commented Dec 19, 2016 at 20:32

Is the majority of people seriously stupid enough not to understand that the car inherently isn't grounded? The car stands on rubber. There's no ground. Hence why when a power line falls you are supposed to stay in the car. The car battery is attached to the car that's why you are supposed to put it on the negative.

If you put the red and the black on to the car and touch the frame of the car, for example the A, b or c pillar you'd die. It would also heat up. If the power coming through the wires would be strong enough the metal would burst and the car explode. The energy can't go anywhere but stay in the frame since the car isn't grounded. Good night, I wish you all some common sense.

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    I have touched the chassis of a car many times while its battery was being charged. I aint'nt dead.
    – Chenmunka
    Commented Jan 9, 2019 at 18:38
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    This also has nothing to do with the question asked Commented Jan 10, 2019 at 9:51

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