Disconnecting battery to charge?

[Cap2018]

Hi all, very confused here. I’ve been gifted a 12V 8A intelligent car charger that I would like to charge my AGM battery with. The resting voltage is low and the ISG hasn’t been working as a result. My owners manual recommends a slow (10 hour) charge so the charger should be suitable. The owners manual states that you must never charge the battery without disconnecting it but I am also aware that disconnecting the battery can stuff up the ECU. How do you charge your batteries? And which chargers do you use? ANY advice or experience would be hugely appreciated. Thanks

[ruggedscotty]

disconnect the battery - saves any issues with the charger affecting the car electronics.

your car electronics should not be affected, however your radio may lose its preset stations,.,

[Cap2018]

ruggedscotty said:

disconnect the battery - saves any issues with the charger affecting the car electronics.

your car electronics should not be affected, however your radio may lose its preset stations,.,

Thanks for the response. What about the issues with the ECU needing to ‘relearn’ everything? overblown internet rubbish?

[Pothole]

Cap2018 said:

ruggedscotty said:

disconnect the battery - saves any issues with the charger affecting the car electronics.

your car electronics should not be affected, however your radio may lose its preset stations,.,

Thanks for the response. What about the issues with the ECU needing to ‘relearn’ everything? overblown internet rubbish?

I think so, yes. IANAFTM.

[ruggedscotty]

gumf…

u will be ok. unless something is drastically wrong with the ecu it should be able to cope with that.

[Cap2018]

Pothole said:

Cap2018 said:

ruggedscotty said:

disconnect the battery - saves any issues with the charger affecting the car electronics.

your car electronics should not be affected, however your radio may lose its preset stations,.,

Thanks for the response. What about the issues with the ECU needing to ‘relearn’ everything? overblown internet rubbish?

I think so, yes. IANAFTM.

IANAFTM?

Thanks for the response

[Pothole]

Cap2018 said:

IANAFTM?

Thanks for the response

Sorry, should be F-TM

[scarble]

it'll be fine, if your stereo is one that needs a code make sure you have the code, probably inside the front page of the manual. Would be amazed if you even lose your stations.

[Cap2018]

scarble said:

it'll be fine, if your stereo is one that needs a code make sure you have the code, probably inside the front page of the manual. Would be amazed if you even lose your stations.

The code is scratched into the radio! luckily it isn’t too obvious

[Monkeylegend]

I don't disconnect mine to charge and have never had an issue.

[Cap2018]

Monkeylegend said:

I don't disconnect mine to charge and have never had an issue.

Interesting. What car/ battery? and how many amps does the charger supply?

[Monkeylegend]

Cap2018 said:

Monkeylegend said:

I don't disconnect mine to charge and have never had an issue.

Interesting. What car/ battery? and how many amps does the charger supply?

I keep my SL on a 12amp trickle charger all year round, attached to the 70amp auxiliary battery in the boot, and have used a normal fast charger on the starter battery which from memory is 35 amp, both without disconnecting.

The trickle charger on the auxiliary battery keeps the starter battery charged as well.

When I did disconnect the battery I got all sorts of alarming messages when I reconnected but most disappeared after a run and restart.

I had to get one message cleared, can't remember what it was now, but the Indy garage I use told me to charge in situ without disconnecting.

Also when I renewed both batteries I used an old battery to maintain the connection which avoids issues with messages and resetting.

[DuraAce]

Never disconnected it (20+ cars). Never had a problem.

Battery charges quite nicely while still connected with engine running and Alternator working!

[Cap2018]

Monkeylegend said:

Cap2018 said:

Monkeylegend said:

I don't disconnect mine to charge and have never had an issue.

Interesting. What car/ battery? and how many amps does the charger supply?

I keep my SL on a 12amp trickle charger all year round, attached to the 70amp auxiliary battery in the boot, and have used a normal fast charger on the starter battery which from memory is 35 amp, both without disconnecting.

The trickle charger on the auxiliary battery keeps the starter battery charged as well.

When I did disconnect the battery I got all sorts of alarming messages when I reconnected but most disappeared after a run and restart.

I had to get one message cleared, can't remember what it was now, but the Indy garage I use told me to charge in situ without disconnecting.

Also when I renewed both batteries I used an old battery to maintain the connection which avoids issues with messages and resetting.

I am leaning towards leaving it connected, I’m sure a lot of the alarms/ error codes would clear after a short drive but it doesn’t seem necessary to have to put up with that anyway

[Cap2018]

DuraAce said:

Never disconnected it (20+ cars). Never had a problem.

Battery charges quite nicely while still connected with engine running and Alternator working!

Good to know! That was my logic when asking for the charger and then I checked the manual. Not sure if that was a good idea!

[journeymanpro]

do not disconnect it to charge if you have stop/start. if you do this then the battery management system will not know the battery state of charge and potentially cause other issues. charge on the car and connect the negative of charger to cars metalwork not direct to battery.

you will likely find a device connected to battery negative that monitors current in and out.

[Olas]

journeymanpro said:

do not disconnect it to charge if you have stop/start. if you do this then the battery management system will not know the battery state of charge and potentially cause other issues. charge on the car and connect the negative of charger to cars metalwork not direct to battery.

you will likely find a device connected to battery negative that monitors current in and out.

All cars in produced in the last 30 years have been *NEGATIVE EARTH*
This means that the Negative terminal of the battery is connected directly to the body of the car.

It makes absolutely no difference if you connect the charger to the battery negative terminal or the bodywork because they are already directly connected to one another - from the standpoint of reading a circuit diagram this means that the entire monocoque IS the battery negative terminal.

Alternator output is typically 65+amps and 14 volts which far exceeds the capacity of a commercially available battery charger so there is no need to disconnect anything - the circuits within the car were built to withstand alternator output so charger output isnt even worth considering.
Further - ECUs are protected by diodes. Diodes only allow current to flow in one direction.

In summary - just charge the damn thing!

[Cap2018]

Olas said:

In summary - just charge the damn thing!

That's what I needed to hear!! All excellent advice, I just needed the reassurance!

Outdoor extension cable arriving today so will get it charged this weekend.

Thank you

[thebraketester]

How long are you leaving that car for without driving it?

[Monkeylegend]

Cap2018 said:

Olas said:

In summary - just charge the damn thing!

That's what I needed to hear!! All excellent advice, I just needed the reassurance!

Outdoor extension cable arriving today so will get it charged this weekend.

Thank you

Being PH we need to know what car?