Earth/negative 'tag' in engine bay for charging

[Wibbly]

Why do Audi do this to avoid folk connecting a charger directly to the battery terminal. I.e. what's the point?

What would happen if I chose to use

a. the battery negative terminal, or
b. another earth point such as the battery tray bolt

when wiring in a permanent 'charger connector' of the type supplied as an acessory by CTEK

 

[Evil Derboy]

Because it easier to connect to two points under the bonnet then strip half the boot apart to get to the battery on some cars.

 

[Wibbly]

Because it easier to connect to two points under the bonnet then strip half the boot apart to get to the battery on some cars.

So this is an 1.4 A3, and the negative 'tag' is a few cm from the negative battery terminal. But the manual makes it clear you should use the tag and NOT the battery terminal for a charger connection (positive goes direct to battery as normal)

Question is why do they go to all this trouble, and what happens if you ignore it?

I wondering if this is to do with the battery management system electronics, or to do with keeping the connection away from the phyical battery to reduce spark/explosion risks (as the negative terminal should be connected last and disconnected first, so this would make sense, I guess)

 

[Phazer]

It's because of the types of battery (AGM etc) that are fitted. They need to be charged in a "smart" controlled way. Directly connecting them to another power source or charger can damage them pretty quickly and also damage the car's monitoring circuits (you'll see the small gauge wire connected to the battery).

So do as the manual says as it could get expensive otherwise.

 

[Wibbly]

It's because of the types of battery (AGM etc) that are fitted. They need to be charged in a "smart" controlled way.

So you are saying the car controls the charging from the charger?

 

[DJAlix]

Modern vehicle electrics and related control modules are very sensitive. Connecting directly to the battery for charging and especially "jumping" can cause massive failures, and as mentioned above the points provide a much simpler way of connecting when / if a battery is located in the boot or in the case of say a UK Q7 under the drivers floor.

 

[Wibbly]

Modern vehicle electrics and related control modules are very sensitive. Connecting directly to the battery for charging and especially "jumping" can cause massive failures

Ok I get it. But do you know why that would be the case - isn't the battery connected direct to the earth point via a big fat cable anyway? And a short one at that, when the battery is in the engine bay anyway...

The only explanation I can find from an on-line search is around avoiding battery gassing exposions as I hypothesised above.

(I'm an electonics engineer by training, so happy with a technical anwser if you have one)

 

[Phazer]

So you are saying the car controls the charging from the charger?

No I'm not and you know it

 

[Phazer]

Here's the notes from the workshop manual regarding charging the A3 battery. You'll note that the car is actively monitoring battery capacity. The terminals provided protect this monitoring circuit, I've not dug into the wiring diagrams to see if there is any active protection on there but manufacturers generally don't fit things if not needed for cost reasons.

"Wherever possible, the battery should be installed in the vehicle and connected while it is being charged. Otherwise, the charging current will not be taken into account in the capacity calculation of battery monitor control unit -J367-"

 

[desertstorm]

The reason you don't connect directly to the negative terminal is because there is a current shunt on the negative terminal that the car uses to measure charge going to or from the battery. Just been out and took a picture on my 2008 A4 3.0 TDI.
This is the J367 battery monitor control unit. It sends a signal back to the alternator and in my case the diagnostic interface which calculates the battery state of charge and system voltage. If you connect directly to the battery lug the car doesn't see that the battery is being charged.

 

[Wibbly]

Thanks!

 

[Phazer]

To add to the above there is a further warning in the workshop manuals:

"For vehicles with start/stop function and battery monitor control unit - J367- , the black charger clamp (-) must be connected to the vehicle body earth. Connecting it to the negative terminal of the battery - A- will cause the start/stop system to malfunction."

Presume this is due to the discrepancy between battery state and the charge level the car is expecting, which makes sense against the post above.

 

[Wibbly]

All makes sense now. Thanks.