This is not specific to our cars but I thought it interesting. The battery on my wife's car suddenly gave out yesterday and the car wouldn't start (hate it how you never get any warnings these days when a battery is about to go). So I called the AAA and they came out and did a quick check (measured cranking amps were 150 versus 550) and I opted to get one of their batteries. I figured he would be installing it with the car off but he hooked up his power pack and told me to first start the car, which I did. He then proceeded to disconnect the old battery--the car continued to run since it was on alternator power--and he installed the new battery.

The idea behind doing it this way was to retain all of the various radio settings, etc. without needing to reprogram.

I thought this was a pretty slick idea but was wondering if it could be safely done with our cars when replacing the battery?

Doug

 

Interesting - I was under the impression that disconnecting the battery when the car was running was something you absolutely should not do.

Plus, what's the big deal about removing the battery on an XK8 anyway - reset the windows, enter the radio code (if it has one) and you're done. Not worth risking a power spike or shorting the positive lead for.

 

I tend to agree and my choice would be to go with one of those "memory minder" gizmos with the 9 volt battery to retain settings. Intuitively having the car running seems like a chancy thing to do but googling around it looks like there are quite a few opinions that say it is okay (but there is a risk for alternator damage).

Doug

 

His power pack was taking the load from the alternator, acting as a substitute for the car's own battery.

There's the danger that the positive terminal might touch a metal surface during the swap which could do serious damage to the electronics. Not worth the effort in my book.

 

As far as I can recall once we started the car off his power pack he DIS-connected it so that he could remove the battery and the car was completely running on alternator.

Doug

 

Ewww, bad idea. Did the guy say- 'hold ma beer an' watch this'?

 

Yeah--the whole episode gave me pause but the guy was so confident with what he was doing it was obvious he had done this numerous times before and I figured it must be okay.

Doug

 

That 'diagnostic procedure' is FINE with a DC GENERATOR (AKA car from the 50s) but an ALTERNATOR needs a battery reference for the voltage regulator to function properly.
Some cars need the ECM to have a reference pin to the ALT to monitor load and charging.

There are several ways to damage 'alts', REVERSE POLARITY, and disconnecting the battery while under load are a few that I can think of.

Just thinking out loud.

bob

 

What he could/should have done was to attach the leads of the power pack
over the the battery leads. Disconnected the old battery, connect the new
battery while the power pack acted as a memory minder the whole time.

The power pack then acts as a huge memory minder gizmo.

Then again, there is another thread where it is claimed somehow that
a lithium power pack risks over voltage. I don't see how if the power
pack is designed to output 15V or less, but that is the claim.

 

I don't see any advantage of having the car running or started while this is being done. Perhaps using his jumper pack to start it first to ensure 'the battery is the problem' but then turn it off and put the new one in. Funny too since there was a recent post of AAA changing batteries and not tightening the cables properly, would have been a hoot is you got home and tried starting and nothing cause of a loose cable.

 

At Halfords, they plug a power source into the cigarette charger to retain settings while they're replacing the battery.

 

Gents, it's never necessary ... never good or even acceptable practice ... to make or break a battery connection with the ignition turned on.

This is not to say we won't get away with it in the vast majority of cases. But those other cases can get pretty ugly.

 

There was a similar sky-is-falling story from AAA about never, ever jump starting a Jag supposedly because it will fry all the electronics. I wish they would get their stories straight.

 

Come to think of it, AAA vehicles do a lot of jump starts.

If the driver wants to use a memory minder, then why is
he not carrying one in the truck?

Just like carrying a 10mm wrench for the battery clamp.

Or did he use water pump pliers?