Can you charge your car battery while still connected

Tafia

Original Poster

2,658 posts

231 months

Monday 20th July 2009

Hi folks,

This morning after three weeks of not being used I tried to start my Honda Accord diesel and found a flat battery. I checked the voltage at 12.1 volts.

I called the main dealer to ask if it was safe to charge the battery on the car without disconnecting the car leads as I didn't want to lose my codes.

At first he said yes then a techie beside him asked if I had the sat nav model. When I said yes he then said it might not be safe to charge it with the leads connected! "We have had cars in with the ECU blown due to charging on the car"

I decided to jump start it from our Toyota. Alternator voltage then was 14.5. I ran the car for a few miles then checked the battery volts again. 12.7V

Odd that a drop of just 0.6 V could cause the battery to fail to start the car!

A chum with a Mercedes has his on trickle charge with car cables connected when he parks it for the winter and his dealer says that will cause no harm. Anyone know who is correct? On the car or with car cables disconnected?

Monday 20th July 2009

There are battery chargers and there are battery conditioners . . . . . . all pretty much the same, the conditioners arent as quick to charge, but contain a controller that cuts the amount of power fed into the battery once its at or near optimum.

Monday 20th July 2009

Tafia said:

Hi folks,

This morning after three weeks of not being used I tried to start my Honda Accord diesel and found a flat battery. I checked the voltage at 12.1 volts.

I called the main dealer to ask if it was safe to charge the battery on the car without disconnecting the car leads as I didn't want to lose my codes.

At first he said yes then a techie beside him asked if I had the sat nav model. When I said yes he then said it might not be safe to charge it with the leads connected! "We have had cars in with the ECU blown due to charging on the car"

I decided to jump start it from our Toyota. Alternator voltage then was 14.5. I ran the car for a few miles then checked the battery volts again. 12.7V

Odd that a drop of just 0.6 V could cause the battery to fail to start the car!

A chum with a Mercedes has his on trickle charge with car cables connected when he parks it for the winter and his dealer says that will cause no harm. Anyone know who is correct? On the car or with car cables disconnected?

Is the ECU "live" without the ignition switched on? If not then how would a proper charger hurt it? Genuine question...

Monday 20th July 2009

Charging batteries while leaving the car connected will be absolutely fine.

What you shouldn't do with a lot of modern cars (Mercedes are a good example) is jump start them with a booster pack, although normal jump starting is fine.

OP - your voltage is only part of the problem - it's the current that the battery can supply that's going to be the issue. Also have a look what voltage the battery is putting out when you crank the engine. Notalot I suspect.

*Al*

3,830 posts

205 months

Monday 20th July 2009

I'd personally never charge a car battery insitu with the leads connected.Electronic 'spikes' can damage electrical circuits.

Monday 20th July 2009

*Al* said:

I'd personally never charge a car battery insitu with the leads connected.Electronic 'spikes' can damage electrical circuits.

But wouldn't the circuits actually have to be connected to the battery for this to happen? If the ignition switch is off how many of these will there be?I don't see an issue with a good charger/conditioner like "AccuMate" etc...?

Monday 20th July 2009

esselte said:

*Al* said:

I'd personally never charge a car battery insitu with the leads connected.Electronic 'spikes' can damage electrical circuits.

But wouldn't the circuits actually have to be connected to the battery for this to happen? If the ignition switch is off how many of these will there be?I don't see an issue with a good charger/conditioner like "AccuMate" etc...?

I'd bet good money that on modern cars the alarm, immobiliser, ECU, clock, ICE, etc are connected to the battery full time, regardless of whether the ignition is on or not.

Old cars aren't a problem, but anything remotely modern is probably a nightmare of potential 'spike' damage by using chargers or 'Booster packs' to jump start them.

Monday 20th July 2009

Orb the Impaler said:

What you shouldn't do with a lot of modern cars (Mercedes are a good example) is jump start them with a booster pack,

Why, please...?

Monday 20th July 2009

esselte said:

*Al* said:

I'd personally never charge a car battery insitu with the leads connected.Electronic 'spikes' can damage electrical circuits.

But wouldn't the circuits actually have to be connected to the battery for this to happen? If the ignition switch is off how many of these will there be?I don't see an issue with a good charger/conditioner like "AccuMate" etc...?

All ECU's fitted to modern vehicles are required have protection against these type of situations, spikes should affect them. Unless you're near a university thats doing EMP Pinch testing.

dugt

1,657 posts

190 months

Monday 20th July 2009

surely the battery is charged by the alternator when the car is running? so it should be ok to charge when the car is not running

ive never disconected cables to charge batteries

and sometime modern cars dont start, even with a small voltage drop, because they need a certain level of volts to fire, but they will let you turn the engine over and run your battery down even more, really annoying

doug

well done doug a man with a brain when engine is running a voltage of 14.0+/- v olts will be applied to a battery.when starting voltage will drop to maybe 10v theres a change of 4v minimum so voltage spike?Remember suck squeeze bang blow it aint rocket science!Back to you original posting if you battery was showing 12v it should have started all day.I would look for loose connection which wouldnt cope with current when starter was applied.Again suck,squeeze,bang,blow its simple physics really!

andrewmorpeth said:

if you battery was showing 12v it should have started all day.

When my diesel BM's drop under about 12.4v they don't turn over fast enough to fire, over 12.5v they will fire up ok. Need quite a healty battery to start a 6 cylinder diesel from cold in the winter.

andrewmorpeth said:

Remember suck squeeze bang blow it aint rocket science!Back to you original posting if you battery was showing 12v it should have started all day.I would look for loose connection which wouldnt cope with current when starter was applied.Again suck,squeeze,bang,blow its simple physics really!

Good grief. Pubs kicked out already?

andrewmorpeth said:

......if you battery was showing 12v it should have started all day.....

Not so I'm sorry to say.

The starting performance of a car battery is limited by the current it can pass. As a battery deteriorates, the voltage will fall fairly slowly. However the ability to provide 12 volts across a large load is what is required to drive the starter.

A battery tester will test the voltage drop at the same time as applying a load, impossible to replicate with a normal voltmeter.

This thread from Honest John Forum gives some good tips about battery testing
http://ww.honestjohn.co.uk/forum/post/index.htm?t=...

Tuesday 21st July 2009

mybrainhurts said:

Orb the Impaler said:

What you shouldn't do with a lot of modern cars (Mercedes are a good example) is jump start them with a booster pack,

Why, please...?

Because he read it some where on the internet....

Tuesday 21st July 2009

iirc the merc chargers are designed by a company called ctek - very cool very clever chargers - http://www.youtube.com/watch?v=qHSI_qHOI4U

that is why you can hook them up while the battery is still connected to the car as some chargers are more than a bit dumb and can damaged stuff

Tuesday 21st July 2009

Surely, if you connect up a maintainer (with the battery still connected) and with the maintainer plugged in via a spike protecter, you'll be fine.

Tafia

Original Poster

2,658 posts

231 months

Tuesday 21st July 2009

samuelellis said:

iirc the merc chargers are designed by a company called ctek - very cool very clever chargers - http://www.youtube.com/watch?v=qHSI_qHOI4U

that is why you can hook them up while the battery is still connected to the car as some chargers are more than a bit dumb and can damaged stuff

I asked the guy with the Mercedes if he was using an expensive charger with special protection and was surprised to learn he uses a cheapo one from Aldi!!

Tafia

Original Poster

2,658 posts

231 months

Tuesday 21st July 2009

*Al* said:

I'd personally never charge a car battery insitu with the leads connected.Electronic 'spikes' can damage electrical circuits.

Do you have any problem with losing the codes for the radio, locking etc?

Tafia

Original Poster

2,658 posts

231 months

Tuesday 21st July 2009

esselte said:

*Al* said:

I'd personally never charge a car battery insitu with the leads connected.Electronic 'spikes' can damage electrical circuits.

But wouldn't the circuits actually have to be connected to the battery for this to happen? If the ignition switch is off how many of these will there be?I don't see an issue with a good charger/conditioner like "AccuMate" etc...?

Dealer tells me there are several items powered up when the car is parked. One I hadn't thought of is the module for the door locks which sits there waiting to be contacted by the owners key.

I mentioned that years ago folks would leave their side lights on all night when parked in the street yet a mechanic told me that nowadays if they leave a car door open in the workshop overnight , it can flatten the battery.