Mains battery charging procedure

Dull Spark

Member
Joined
31 Aug 2017
Messages
103
Visit site
My car's manual is emphatic: if I wish to charge the battery in situ, I must not connect the negative charging lead to the negative battery terminal but must instead connect it to the engine block. (For a simple chap, the two are electrically the same aren't they?)

Is there a good reason for this requirement and if so, does it apply to charging the batteries I have on board?
 

VicS

Well-known member
Joined
13 Jul 2002
Messages
48,350
Visit site
My car's manual is emphatic: if I wish to charge the battery in situ, I must not connect the negative charging lead to the negative battery terminal but must instead connect it to the engine block. (For a simple chap, the two are electrically the same aren't they?)

Is there a good reason for this requirement and if so, does it apply to charging the batteries I have on board?

I imagine if you read the connecting / disconnecting procedure in full you will find that you connect the positive first then the negative, ( Disconnect in the reverse , ie negative first)

If so this is to avoid a spark at the battery terminal

Modern chargers sense the battery and even if not plugged in/ switched on a significant current flows

(Note similar procedure when using jump leads ... last connection made/ first broken is a negative away from the battery)

A spark which ignites any hydrogen and explodes the battery in your face will spoil your day.
 
Last edited:

William_H

Well-known member
Joined
28 Jul 2003
Messages
13,785
Location
West Australia
Visit site
Yes I agree with Vic it is all about moving any sparking away from the battery where hydrogen may lurk. Yes the same principle should be applied to boat batteries. ie onnect the last connection away (physical distance) from the battery.
ol'will
 

Plevier

Active member
Joined
22 Aug 2008
Messages
3,594
Location
Brighton
Visit site
I think there is more to it than that, at least with new cars.
We have a new SEAT car with stop-start, efficient battery charging (which I don't trust, shortens battery life I'm sure!) etc
Instructions are emphatic that using a charger, you must put the neg on a contact provided for the purpose, not direct onto the battery terminal. This contact is only an inch or two from the battery terminal, on the bulkhead, so the spark argument above doesn't hold.
There is some gadget between the battery post and the cable terminal, I can't work out what it is.
So I don't understand what's behind this. Maybe the OP's car is similar.
It's not going to apply to a boat system anyway!
 

knuterikt

Active member
Joined
11 Sep 2006
Messages
1,624
Location
Oslo, Norway
Visit site
When jump starting a car it’s good practice to attach the last connection (negative) to the chassis to prevent spark igniting gas from battery.
A charger should not be connected to ac supply when you connect the battery posts to charger. I have never seen sparks when doing the connections for charger in correct order.
 

lw395

Well-known member
Joined
16 May 2007
Messages
41,951
Visit site
Some cars have a current measuring shunt in the battery wiring. I'm pretty sure than my car has it in the positive side, but some might have it in the negative side.
The ECU may be measuring battery current and/or voltage when the car is 'switched off'. If you charge some cars direct to the battery terminals, you will get error codes or worse.

My car has a jumpstart/charge +ve terminal so that jumpstart current is not measured by the shunt sensor. It's also closer to the starter than the battery is, as the battery is in the boot. But any current the jump leads put into the battery will be measured.

As it seems to have got ten years out of the original battery, it must have some merit?
 

Dull Spark

Member
Joined
31 Aug 2017
Messages
103
Visit site
Having now read the rest of the car's manual.......

The issue may well be associated with the prevention of sparks, but there is a clear reference (in another part of the manual!) to possible damage to the entertainment or navigation (GPS) system in the car if the negative lead is attached to the battery.

I think the matter is probably only relevant to the motor industry and not to us.

Many thanks for your advice.
 
Top