Trickle charging battery why cant put negative on terminal?

ks789 · Feb 25, 2020

One for spruce if hes about, though ive no doubt some others of us know too ? Im charging my van battery, left in the van, using a trickle charger, which I usually charge my leisure battery with. Im doing it as I havent done much mileage lately. On one site online, they say dont put the negative on the battery terminal, but to put it on some bare metal instead, otherwise it could cause an explosion!!!

This is an article by cars direct - when and how to use a trickle charger.

Ive done this before, no problem so im thinking their advise is wrong? Ive heard not to put the negative on the battery terminal before, but I think this was in relation to doing a jump start maybe?

Cheers all for advise ?

Davy G · Feb 25, 2020

ks789 said:
One for spruce if hes about, though ive no doubt some others of us know too ? Im charging my van battery, left in the van, using a trickle charger, which I usually charge my leisure battery with. Im doing it as I havent done much mileage lately. On one site online, they say dont put the negative on the battery terminal, but to put it on some bare metal instead, otherwise it could cause an explosion!!!

This is an article by cars direct - when and how to use a trickle charger.

Ive done this before, no problem so im thinking their advise is wrong? Ive heard not to put the negative on the battery terminal before, but I think this was in relation to doing a jump start maybe?

Cheers all for advise ?

I think it might be because of the chance of a spark when you make the second crocodile clip connection igniting gasses from the battery. If you connect one clip to the battery first and then the second one a little distance away, I'd think the second one would be the one that might spark as it completes the circuit but would hopefully be a safe enough distance from the battery to not be a serious hazard. A spark would occur when connecting two sources of electricity of differing value, on the second connection .

I think I'm right but I'm sure Spruce will enlighten us.

As I think back to my youth in the 1970s I remember charging ropey old car batteries for our familie's hot dog van. No sealed batteries in them days so the vent caps had to be loosened before charging. Van was parked thirty yards away in a council estate so all the batteries had to be carried to the house and charged on the kitchen worktop. Loads of horrible gasses as they were charged at up to 6 amps. Never realised that there was a risk of explosion. Jeez! When I think about all the risks I've taken down the years, it amazes me I'm still here. ?

ks789 · Feb 25, 2020

Davy G said:
I think it might be because of the chance of a spark when you make the second crocodile clip connection igniting gasses from the battery. If you connect one clip to the battery first and then the second one a little distance away, I'd think the second one would be the one that might spark as it completes the circuit but would hopefully be a safe enough distance from the battery to be a serious hazard. A spark would occur when connecting two sources of electricity of differing value, on the second connection .

I think I'm right but I'm sure Spruce will enlighten us.

I read further from the article and there were some reviews/comments after the article and they reckoned it was written by an idiot. I had heard before not to put it on the negative but I think only for the purpose of jump starting. I think any trouble is so unlikely, but they're covering themselves from any potential actions.

Davy G · Feb 25, 2020

ks789 said:
I read further from the article and there were some reviews/comments after the article and they reckoned it was written by an idiot. I had heard before not to put it on the negative but I think only for the purpose of jump starting. I think any trouble is so unlikely, but they're covering themselves from any potential actions.

I think you might be right. The way things have gone I think they look at what might be possible to go wrong and advise on that basis, no matter how remote the risk.

When I worked on road works we started out with traffic cones marking out holes and orange lights at night. After close to twenty years it had become so tight that sometimes on small jobs it took longer to set up signs, barriers and walkway-diversions than it did to do the job. I used to say "We have to cater for a blind man on a bicycle".

Pjj · Feb 25, 2020

I was in the motor trade for 16 years , and this advise has been around for a long time as has been said I think it was aimed at not causing a spark with the fumes that can come from charging a battery causing an explosion , I have always charged batteries all my life by putting the charger on the battery terminals and never had a problem , I think with modern cars it might also have something to do with the electronics , I know the battery is supposed to be disconnected from the vehicle wiring before charging to stop the potential current damaging things like the ecu and other sensitive electronic components , Ime sure @spruce will give a detailed explanation .

Tango · Feb 25, 2020

One issue charging a battery in situ while connected is that trickle chargers tend to have thin wires. If you connect the charger to a battery that is still hooked up to equipment and there's something turned on (that you don't realise) then the sudden connection / turning on can cause excess current to be drawn down those thin wires to power that device.

On another forum i used to frequent their advice was to always turn on the headlights when jump starting (the idea was that it would prevent a voltage spike - cars are designed to run from an alternator at 14.5V anyway so this is nuts). Bonkers, headlights draw a LOT of current and takes that away from the starter motor. Thankfully jump leads are incredibly hard to burn out but if you're using a trickle charger then those wires are gonna be smokin within a few seconds. Oh and yes i did try the headlights (i was young and gullable) and er.. yeah the car behaved rather weirdly.

ks789 · Feb 25, 2020

Pjj said:
I was in the motor trade for 16 years , and this advise has been around for a long time as has been said I think it was aimed at not causing a spark with the fumes that can come from charging a battery causing an explosion , I have always charged batteries all my life by putting the charger on the battery terminals and never had a problem , I think with modern cars it might also have something to do with the electronics , I know the battery is supposed to be disconnected from the vehicle wiring before charging to stop the potential current damaging things like the ecu and other sensitive electronic components , Ime sure @spruce will give a detailed explanation .

I hope I havent damaged mine then, as I havent disconnected anything from the battery. have done it before and seems ok. Its strapped in place from the previous owner so a bit of a faff to remove it indoors. Fingers crossed.

Davy G · Feb 25, 2020

ks789 said:
I hope I havent damaged mine then, as I havent disconnected anything from the battery. have done it before and seems ok. Its strapped in place from the previous owner so a bit of a faff to remove it indoors. Fingers crossed.

I wouldn't think you've done any damage. I'd think you'd be safe enough with a slow, gentle charger. I'd say the bigger risk might be if you were using a high ampage booster charger like the ones they have in car repair garages. Especially with the modern, electronic systems being so sensitive and probably delicate. I've charged batteries in situ in the past without any problems. Gently does it I'd think.

ks789 · Feb 25, 2020

Davy G said:
I wouldn't think you've done any damage. I'd think you'd be safe enough with a slow, gentle charger. I'd say the bigger risk might be if you were using a high ampage booster charger like the ones they have in car repair garages. Especially with the modern, electronic systems being so sensitive and probably delicate. I've charged batteries in situ in the past without any problems. Gently does it I'd think.

Yes its got 3 I think settings, I just leave it on the 2 amp slowest setting. leave it till morning and jobs a goodun (we hope)

Pjj · Feb 25, 2020

ks789 said:
I hope I havent damaged mine then, as I havent disconnected anything from the battery. have done it before and seems ok. Its strapped in place from the previous owner so a bit of a faff to remove it indoors. Fingers crossed.

Hopefully it will be fine I think the manufacturers say disconnect everything to cover themselves should anything happen so that they don’t have to pay , bit of a get out clause

spruce · Feb 25, 2020

Pjj said:
I was in the motor trade for 16 years , and this advise has been around for a long time as has been said I think it was aimed at not causing a spark with the fumes that can come from charging a battery causing an explosion , I have always charged batteries all my life by putting the charger on the battery terminals and never had a problem , I think with modern cars it might also have something to do with the electronics , I know the battery is supposed to be disconnected from the vehicle wiring before charging to stop the potential current damaging things like the ecu and other sensitive electronic components , Ime sure @spruce will give a detailed explanation .

Here's my take.

The issue that this revolves around is jump starting a vehicle with a flat battery with a vehicle with a good battery.

There is a reason why that battery went flat. It could have been a drain caused by an interior light left on. That battery doesn't pose a risk of explosion as the cells haven't gassed.

But a battery that has a faulty cell could have produced gas in an effort to start the car. So the procedure when using jumper cables is to start by connecting the first red cable to the positive terminal of the vehicle with the flat battery. The other red battery clamp is then connected to the other vehicle. The black clamp is then connected to the negative terminal of the flat battery and the last clamp is connected to a good earth on the vehicle being jumped from. Its usually in the engine bay well away from the battery. In the old days the chrome front bumper was a good place.

When connecting this last negative clamp there will be a spark. Hence the reason for the procedure. What is the chance of the vehicle's battery being jumped from having a build up of gas? Hardly any, but we follow the procedure just to be safe.

When the vehicle has been started its always best to let both engines run for a while. Then disconnect in reverse order. Here is where there is more chance of a spark. The biggest concern to me when connecting and disconnecting jumper cables is to ensure that the second positive clamp doesn't accidentally touch the chassis of the vehicle as that will cause a direct short.

Personally I prefer to let the vehicles engine run for a while before trying to start the car with the flat battery. Then I will start the vehicle with the flat battery and let that run for a while as well. I then switch both engines off and disconnect the jumper cables in that reverse fitting order. Once done I will try to start the car with the flat battery. If it starts then great, I have reduced the risk of any damage being done to the electronics side of things. But I have left both engines running and removed jumper leads adhering to the procedure.

What about a battery charger? When you connect both clamps to the terminals hopefully you will have the charger switched off. With the charger not powered you aren't going to get a spark. You then ensure that both clamps are secure and then switch on the charger. I have never heard of anyone advising to connect the positive of the charger to the positive terminal and the negative to the body. But there is no reason why you can't do that if you wish.

Again, when you have finished charging your battery you switch the charger off and then remove the negative clamp from where ever you have placed it.

Pjj · Feb 25, 2020

spruce said:
Here's my take.

The issue that this revolves around is jump starting a vehicle with a flat battery with a vehicle with a good battery.

There is a reason why that battery went flat. It could have been a drain caused by an interior light left on. That battery doesn't pose a risk of explosion as the cells haven't gassed.

But a battery that has a faulty cell could have produced gas in an effort to start the car. So the procedure when using jumper cables is to start by connecting the first red cable to the positive terminal of the vehicle with the flat battery. The other red battery clamp is then connected to the other vehicle. The black clamp is then connected to the negative terminal of the flat battery and the last clamp is connected to a good earth on the vehicle being jumped from. Its usually in the engine bay well away from the battery. In the old days the chrome front bumper was a good place.

When connecting this last negative clamp there will be a spark. Hence the reason for the procedure. What is the chance of the vehicle's battery being jumped from having a build up of gas? Hardly any, but we follow the procedure just to be safe.

When the vehicle has been started its always best to let both engines run for a while. Then disconnect in reverse order. Here is where there is more chance of a spark. The biggest concern to me when connecting and disconnecting jumper cables is to ensure that the second positive clamp doesn't accidentally touch the chassis of the vehicle as that will cause a direct short.

Personally I prefer to let the vehicles engine run for a while before trying to start the car with the flat battery. Then I will start the vehicle with the flat battery and let that run for a while as well. I then switch both engines off and disconnect the jumper cables in that reverse fitting order. Once done I will try to start the car with the flat battery. If it starts then great, I have reduced the risk of any damage being done to the electronics side of things. But I have left both engines running and removed jumper leads adhering to the procedure.

What about a battery charger? When you connect both clamps to the terminals hopefully you will have the charger switched off. With the charger not powered you aren't going to get a spark. You then ensure that both clamps are secure and then switch on the charger. I have never heard of anyone advising to connect the positive of the charger to the positive terminal and the negative to the body. But there is no reason why you can't do that if you wish.

Again, when you have finished charging your battery you switch the charger off and then remove the negative clamp from where ever you have placed it.

The hand book with my 2003 Bmw 5 series says to jump start it use the connections on the engine not to connect jump leads to the battery in the boot it says that this way it will protect delicate electronics . It also says to remove the battery from the car if charging it to prevent potential fumes from going inside the car . Obviously Bmw don’t remove the battery very often as they would know what a nightmare of a job it is to get it out !,,,, when I have charged it I leave it in situ ???

Tango · Feb 26, 2020

Davy G said:
Especially with the modern, electronic systems being so sensitive and probably delicate.

Some are far more resilient than the makers let on.

spruce said:
The biggest concern to me when connecting and disconnecting jumper cables is to ensure that the second positive clamp doesn't accidentally touch the chassis of the vehicle as that will cause a direct short

I jump started a woman years ago with a car that is known for delicate electronics. She was trying to hit on me afterwards telling me how i'd saved her day (repeatedly) and as i'm standing there talking to her i accidentally touched both clips from her end of the jump leads together - with my car still running and the leads attached. You should have seen the sparks flying from those croc clips!!! - engine pitch changed dramatically too. I threw them on the ground and had to kick them apart as i couldn't do it by hand - they were sticking together so foot on one and a good swift kick with the other foot. Car survived that with no noticeable signs of problems. Battery was dead within 6 months though :~

Like i said in my previous reply the biggest issue that i can see is that if the battery is connected, you might overload the charger and either burn out the wires or cause the charger to go up in flames (only if you start using stuff in the car like the headlights). For that one single reason it's best to disconnect it so you can't screw up.

Iron Giant · Feb 26, 2020

I charge my battery in situ with my Numax charger, if you disconnect a battery fully it can reset the ecu and you can potentially lose the mileage reading and have to recode the radio, when I had new battery fitted last year they connected a little machine direct to the van I think it was somewhere under the dashboard to keep all electrics live whilst changing the battery, it kept the total mileage on the van but it did reset the trip meter to 000 rather than showing the miles I had done since I last filled the van with diesel.

spruce · Feb 26, 2020

Pjj said:
The hand book with my 2003 Bmw 5 series says to jump start it use the connections on the engine not to connect jump leads to the battery in the boot it says that this way it will protect delicate electronics . It also says to remove the battery from the car if charging it to prevent potential fumes from going inside the car . Obviously Bmw don’t remove the battery very often as they would know what a nightmare of a job it is to get it out !,,,, when I have charged it I leave it in situ ???

This current van I have as jump points/charging points under the bonnet as the battery is under a panel in the floor of the driver's cabin.

My van's instruction manual also says to remove the negative terminal when charging the battery. It also says that charging should be done in a well ventilated place after following the manufacturers instructions. (This battery also has a gassing tube running from the battery 'outside' the floor of the van.) When reading it I'm getting the feeling that I'm reading an instruction manual on how to disarm a landmine and still remain alive.

Electronics will only be damaged if there is an electrical 'feedback somewhere. We generally know that if we reverse polarity on our leisure battery terminals we will 'cook' our controllers. I guessing the instruct manual has to cover all scenarios.

If I have to recharge a battery with an ordinary battery charger I will use both battery terminals and not disconnect the negative lead. I still have to remove stuff out the back end of my van going to scrap. It has a new battery inside it so I put that battery on charge from time to time. I will connect it up to both battery terminal in the same way as I always do. Removing a battery lead can cause you more headaches than its worth such as losing a radio if you haven't the codes and/or trip/fuel consumption/average speed data.

Flat batteries and jump starting a vehicle is obviously a potentially dangerous thing. A customer had the AA out to start his Ford Ranger with a flat battery. (He later discovered his dash cam didn't switch off with the ignition.) The AA guy was wearing leather gloves with long 'sleeves', he wore a face shield and a protective apron. During the process his customer was standing right next to him. So this guy had all the PPE but didn't understand the reason behind the equipment. I thought it was funny tbh. When an old world war 2 bomb is found the first thing they do is clear the area of people.

ks789 · Feb 26, 2020

spruce said:
Here's my take.

The issue that this revolves around is jump starting a vehicle with a flat battery with a vehicle with a good battery.

There is a reason why that battery went flat. It could have been a drain caused by an interior light left on. That battery doesn't pose a risk of explosion as the cells haven't gassed.

But a battery that has a faulty cell could have produced gas in an effort to start the car. So the procedure when using jumper cables is to start by connecting the first red cable to the positive terminal of the vehicle with the flat battery. The other red battery clamp is then connected to the other vehicle. The black clamp is then connected to the negative terminal of the flat battery and the last clamp is connected to a good earth on the vehicle being jumped from. Its usually in the engine bay well away from the battery. In the old days the chrome front bumper was a good place.

When connecting this last negative clamp there will be a spark. Hence the reason for the procedure. What is the chance of the vehicle's battery being jumped from having a build up of gas? Hardly any, but we follow the procedure just to be safe.

When the vehicle has been started its always best to let both engines run for a while. Then disconnect in reverse order. Here is where there is more chance of a spark. The biggest concern to me when connecting and disconnecting jumper cables is to ensure that the second positive clamp doesn't accidentally touch the chassis of the vehicle as that will cause a direct short.

Personally I prefer to let the vehicles engine run for a while before trying to start the car with the flat battery. Then I will start the vehicle with the flat battery and let that run for a while as well. I then switch both engines off and disconnect the jumper cables in that reverse fitting order. Once done I will try to start the car with the flat battery. If it starts then great, I have reduced the risk of any damage being done to the electronics side of things. But I have left both engines running and removed jumper leads adhering to the procedure.

What about a battery charger? When you connect both clamps to the terminals hopefully you will have the charger switched off. With the charger not powered you aren't going to get a spark. You then ensure that both clamps are secure and then switch on the charger. I have never heard of anyone advising to connect the positive of the charger to the positive terminal and the negative to the body. But there is no reason why you can't do that if you wish.

Again, when you have finished charging your battery you switch the charger off and then remove the negative clamp from where ever you have placed it.

Much obliged for reply spruce. Should be fine then, I always have them connected before switching on. yes I thought it odd in the article about using the trickle charger, but if just jumping the battery I will remember to put negative to bare metal. cheers again.

Tango · Feb 26, 2020

spruce said:
When connecting this last negative clamp there will be a spark.

Not always, I have used real expensive jump leads in the past with an anti spark device fitted (probably a large capacitor). Jumped loads of cars with them and not a single spark.

The moment our company permanently got the contract (and those leads) they vanished and got replaced with a really nasty set that makes Halfords look expensive.

Tango · Feb 26, 2020

Tango said:
Not always, I have used real expensive jump leads in the past with an anti spark device fitted (probably a large capacitor). Jumped loads of cars with them and not a single spark.

These don't spark.. but look at the price tag - over £200!

https://www.mad4tools.com/sealey-proj-12-24-booster-cables-7m-450a-25mm2-12-24v-electronics-protection

AGlassAct · Feb 26, 2020

I used to run the battery test bay at flyBe. Charging them was never the issue - problems arose when reconnecting and disconnecting them on the aircraft, usually for reasons covered above (idiots leaving equipment on in the flight deck). You’ll have no issue charging a battery on its ‘terminals’, why would expensive chargers have reverse polarity protection otherwise. The other thing that hasn’t been mentioned is how would a battery charge if the negative terminal on your leisure battery isn’t connected to the chassis? A lot of DIY installs I’ve dealt with don’t have VSR’s and all the wires go directly to to the battery. If you connected the negative terminal to the vehicle chassis in these instances the battery wouldn’t charge as it doesn’t have a ‘return’ path. In the OP’s scenario it would charge but you’d then be putting an imbalance of current in to one battery (if you have more than one leisure battery) as the guidelines are that all main battery leads are the same length, so it makes sense that the charging leads follow the same rules.

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Trickle charging battery why cant put negative on terminal?

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