Charging with battery connected.

Sounds like the type of charger I have that monitors the battery state. If it is it can be left connected indefinitely. Keeps the battery topped up, without fear of overcharging or causing any damage to the cars electrics. Mike.

Thanks for that. I was assuming it would be kicking out a lot less than the alternator would so I'd probably be OK. I'll go and stick it on now and I'll know where to send the bill for the new ECU. :-)

I've not had any trouble with the battery apart from a couple of occasions where I'd left the heated rear window on the previous time I'd used the car, sat in the car for a few minutes with the ignition on setting up the Sat Nav to find it wouldn't start. Almost as if the heated rear window was draining the battery somehow while it was parked up.

Could just be an old, dying battery.

Maybe but only a coupleof years old and it has only let me down those two occasions that I've had the heated rear window on and sat for a few minutes with the ignition on. You could well be right and those few minutes were enough to kill it.

IME the average life of a new battery is about 5 years. But one thing that batteries do not like, is being deep discharged, which could easily occur if a heated rear window were left on. They take quite a lot of current, which is why many cars fit a timer to them. I think a couple of deep discharges would certainly have an efect on a batteries longevity, but to reduce it to 2 years seems a bit too much IMO. Mike.

My BMW E34 has been off the road for nearly 2 months. All the time connected to my charger, switched to it's maintenance setting. Started it a couple of times with no problem, so I know my advice is OK. Mike.

I'm sure my heated rear screen is only on with the ignition unless I have a strange fault. There is only a small, dim warning light in the switch so it is easy to leave on. The voltage was very low on those two occasions so I probably haven't done it any good. A local motor spares place is advertising an intelligent battery so next time I'm passing I'll make a note of the manufacturer and google to find out what it is supposed to do. Nigel

Only joking about the bill. Mines an E36 with that bloody infamous BMW rough tickover problem. Nigel

Of course it's ok. It's an urban myth that car electronics can be damaged by a domestic battery charger.

Anything much more than a couple of weeks can be a problem these days.

Usually sticking valves on an early 24 valve.

It probably has been that. I'm lucky enough to have off road parking under a large carport with an external mains socket so leaving it on charge is no problem. I do wonder if the extra current taken by the Sat Nav and alarm might confuse the 'intelligent' charger a bit.

No - they're looking at voltage. Unless the current drawn by those was greater than the capabilities of the charger. Which it won't be.

I note that some 98-01 fiat puntos do the same; you can leave the light switch or rear demister switch 'on', and although neither are visibly on, they drain the battery. Good design.

I also wondered when I seen it in action why the lights don't come on at all when the switch is on; all other brands of car I've seen enable the rear and front sidelights when the ignition is off, but not headlights.

I would ignore the 'intelligent battery'; it'll be marketing nonsense. OEM batteries last for up to about 6 years, so I wouldn't bother with an expensive battery, unless you're going to be switching the ignition on and off a very large number of times every day, coupled with many short journies.

That should be fine.

You only need to flatten a battery once and leave it in that state for a while to turn it in to an old dying battery.