W203 Battery change

[Tomkinsm]

My C180K is amazingly still running on it's original Merc battery (15yrs old). I've ordered a new battery and was going to change this myself, until I read several threads warning about blowing SAMs when changing batteries, giving instructions on keeping power to the vehicle with a slave.

I recently changed by PCV pipe and as part of that job I disconnected the negative lead (which was the first step in recommended in the Haynes manual). I had no issues when I reconnected the lead.

So I'm curious, what's the difference between a battery change and removing a lead from the battery when doing any electrical work on the vehicle? I can't believe you have to keep power to the vehicle at all times - what if you were changing an airbag, for example?

Thanks for any answers. Car is going to my local indie on Friday for other things, so I'll probably ask them to change the battery now, but would like to know peoples thoughts on this.

 

[LostKiwi]

No difference whatsoever.
If you want to play safe changing batteries turn on the headlights before connecting the new one.

 

[Tomkinsm]

Thanks Lostkiwi

Physically there didn't seem to be any difference to me either, which was why I was confused that people were advocating a battery swap with a continuous live slave keeping the car powered. Any idea of the root behind these proposals?

Incidentally (you commented on my other thread), I wonder if this is a potential cause of my starting issues. Although the car cranks fine, the voltage when not running is only just above 12V (12.1 - 12.3) and dips to 11V when cranking. Running voltage is over 13 so I think the alternator is ok. But clearly at 15 years old it's due a new battery...

Mark

 

[umblecumbuz]

Or do as I have done in the past with an early (sensitive) 203.

Sit a slave battery in the drivers footwell and connect it into the obd2 port (it's always live). Then when you disconnect the normal battery you still have uninterrupted power.
No dangers, no spikes and no loss of data.

You can get the connecting leads and plug from the Bay - a few quid well spent.

 

[Blobcat]

Or do as I have done in the past with an early (sensitive) 203.

Sit a slave battery in the drivers footwell and connect it into the obd2 port (it's always live). Then when you disconnect the normal battery you still have uninterrupted power.
No dangers, no spikes and no loss of data.

You can get the connecting leads and plug from the Bay - a few quid well spent.

Just remember that the battery positive lead will still be live once you take it off the old battery so make sure you insulate it when you move it out of the way to get the new battery in.

 

[Chrishazle]

When I had my 203 battery changed at a local tyre fitter place (no, not KF!) I had them connect a slave battery to the "jump start" +ve terminal under the bonnet and the top of the suspension, so no interruption of power supply, no voltage spike to upset a SAM, just have to be careful with the battery leads when you disconnect then reconnect the new battery as they are also live!

IIRC (this was about 8 years ago) this was the method recommended by a certain gentleman known on here as TV!

 

[Tomkinsm]

Seems like there is some poor electronics design if interupting the supply blows the SAM.

And as I wrote originally, what do you do when you actually want power off, for example changing an airbag or some sensitive electronics?

 

[umblecumbuz]

Depends on the iteration of your 203 a bit.

Post facelift 203's had a more robust Sam unit, which would take connections/disconnections without hassle. Facelift was 3rd quarter of 2004, but whether or not the tougher Sam was introduced earlier, I do not know.

So total disconnection for any maintenance work should not be a problem.

 

[Tomkinsm]

Thanks for all the feedback

Mine is a prefacelift W203 (53 plate). So what I'm hearing from many of you is that disconnecting/connecting a battery is a lottery and risks blowing your SAM? For those that recommend the slave battery trick, how do you go about other maintenance activities which first require you to disconnect the battery?

I wonder what a Merc garage would do?