Non starter.

[GrahamF]

So the car has sat in the garage for 3-4 months. It went in as a runner, with a new battery and hi-torque new starter fitted last June, which performed well all summer and autumn. Charged the battery the other day until it said it was fully charged, with the puddle lamps shining brightly, but as soon as I turn the key the dash cluster dims, there's a couple of clicks and the battery dies. No puddle or interior lights at all. Put the charger back on and within half an hour the battery shows fully charged again.
Where do you start? I am no mechanic and certainly no auto electrician.
In the dark of Rugby.

[Seaking]

Start with the battery change it or jump start

seems like your battery has failed

cheers Glenn

[mole42]

Check the battery connectors, if they are even slightly loose you'll get that symptom.

If you have one of those battery disconnectors, check it's tight (guess how I know that?)

[GDPR]

I'm with Glenn here - start with a jumpstart, and if that works suggest a new battery - mine failed a couple of months ago, and I got a cracking battery for £45 delivered via fleabay

[mole42]

If Graham has a duff battery it'll be under warranty so he should get a free replacement.

[Jonat127]

When you say battery dies, does it come back to life when you turn the key off?
If it is just dimming right down when you turn the key to crank the starter then it could be the starter is stuck and just pulling all the power which dims all the other lights.
My starter got stuck after an 8 month rest.
In the end I had to leave ignition on then run a wire directly from the power input on the solenoid to the battery. I touched this wire onto the positive terminal repeatedly. Each time it clicked I took the wire off. Eventually after a dozen or so shocks the starter sprang back into life.

[GrahamF]

I will try the jump lead option at the weekend.
The battery looks charged until I turn the key, it then drains as soon as I turn the ignition on, even before I attempt to turn the engine over.

[Mark Serj]

I have had 2 duff new batteries in a row recently so that would be where I would look first Graham. The supplier said they never get duff batteries blah blah blah, but when I returned it and they tested it, it was shot.

[GrahamF]

Swapped the cars around tonight and with a decent donor car the Stag fired up and ran sweetly. It wouldn't then restart as soon as I switched it off after running for 10 minutes. Maybe my battery charger is knackered, anyone recommend a new battery charger to purchase. What sort of amps do you need for charging a Stag battery reliably?

[LAMBRETTLAD]

I would have left it running at high tickover for at least 30 mins, or a good ten mile run (if day time ).Your battery may be deeply discharged .

Better than a charger is an optimiser to first charge then to keep it in good charge after the recharge ,if the car is left standing.

I also bought a Clark portable jump battery pack for around £80 .

[marshman]

Swapped the cars around tonight and with a decent donor car the Stag fired up and ran sweetly. It wouldn't then restart as soon as I switched it off after running for 10 minutes. Maybe my battery charger is knackered, anyone recommend a new battery charger to purchase. What sort of amps do you need for charging a Stag battery reliably?

Battery capacity or "size" is specified in AH, Amp Hours. So in very simple terms a 45 AH battery will supply 45 amps for 1 hour or 1 amp for 45 hours or 10 amps for 4.5 hours. In reality the actual times are diffrent due to battery chemistry and internal construction but you get the idea.

Now the charging process is the reverse. So in theory if the battery is totally discharged it would take 45 hours at 1 amp to fully charge, or 1 hour at 45 amps. In practice the battery should never be totally discharged, but you get the idea. So with a flat battery charging using the alternator you would have to run the car at a good speed for at least an hour to geg anywhere near fully charged. Most half ddcdnt chargers manage around 4 amps so need in theory around 10 hours to charge a flat battery. In practice it is a lot longer for two reasons. First ghe charging process is not 100% efficient, you have to put more in than you get out. Secondly as the battery charges the charge current tends to fall. So you quickly get to 70 or 80% of a full charge, but the last 20% can take as long as the first 80%.

To be kind to the battery it should have a nice long slow charge when it has been heavily discharged. This gives all 6 cells within the battery chance to fully charge and equalise. In the old days you could get trickle chargers which did a great job of nursing batteries back to health. Companies likd Ctek make modern electronic chargers which take care of everything for you, conndct it and wait until it tells you the battery is charged. They do not always like starting to charge a full discharged, totally flat battery. With chargers for car batteries bigger is not always better unless you want a starter/charger which will do the job but is not very good at preserving the battery.

Modern batteries seem to last many years if looked after and not left in a discharged state, my record is 16 years and counting in my wifes mx5.

Roger

[GrahamF]

Thanks Roger, very helpful.

[GrahamF]

New battery fitted, the 8 month old one only registering 5 volts, so replaced under warranty. Great to have a new one on with plenty of power. The car ran a dream.

[rare1]

Just put a new battery on mine after 10 years. Had 4 duff battery's in a row on my wife's SLK all changed under warranty. Had car checked at Mercedes garage for electrical problems and all clear.

[Mark Serj]

2 duffers in a row for me