Simple Battery Question

[Andy Mathers]

Evening all, My Stag hasn't been started for about three weeks now and being keen to turn it over and fire it up tried to start it - and it seems that the battery is flat. It's not completely dead as the door lights came on but there's nothing like enough juice to turn the engine twice. So. I've attached a trickle charger to add some zest into the battery - just how long should I keep it on ? I've just had a go with a jump starter but that's low on power too so that's being charged up as well ! The battery was new two years ago so I believe I have a bad earth somewhere, I don't want to overcharge the battery and cause damage but I've no idea how long to keep the charger on. There.....told you it was simple !

[marshman]

If it is that flat then it will need a "good long overnight charge" on a "trickle" charger. If you assume its a 40 Amp Hour battery then divide 40 by the output of the charger (say 2 amps for a small one!) this gives you around 20 hours. Substitute the correct figures for your battery and charger. Its not very accurate because a) your battery is not totally flat, b) the charger current will vary and reduce the more charge the battery gets in it and c) the charging process is not 100% efficient, but it gives you a ball park figure.

Once you get it charged you need to find what is discharging the battery - they shouldn't go flat in a couple of weeks! Check the boot light, glovebox light and your radio to start with, also any alarm/immobiliser you may have fitted.

good luck

Roger

[jleyton]

As long as your charger is half decent, it won't overcharge the battery.

I'd leave it on overnight.

Good luck.

Cheers

Julian

[V Mad]

Simple answer, a trickle charge is useless (milliamps) for a full recharge of the battery. It is designed just to keep the battery topped up.

Put a proper charger on there for at least 24 hours. Assuming you have about a 50 ampere hour battery, 2 amps x 24 hours (= 48 ampere hours) will do business, enough to get you going.

[Andy Mathers]

Thanks Roger - It seems to do this after a couple of weeks left without starting, it could be linked to the radio I guess as that takes a direct feed off the battery and doesn't go through the ignition circuit at all - I guess it must draw something even though it's not switched on. I've recently put a lamp in the glovebox and checked that that's not the cause and my boot light isn't wired up at all. I've a poor earth somewhere on the interior lights which will need checking out at some time...probably during the dark nights of winter. In the meantime I'll just disconnect the battery when it's all fully charged. Thanks for your help,

[Andy Mathers]

Thanks VMad - I'll do that. Once it's charged up and quite happily burbling away afterwards I'll disconnect until I get a little time to investigate...

[Andy Mathers]

Cheers Julian - Not often that you can say 'ah I'll sleep on it ' knowing that it'll come good tomorrow !

[Andy Rundell]

i can tell you from practical experience that a faulty boot lamp switch (which doesn't turn the lamp off) will drain the battery in around 4 days

....... Andy

[ChrisLD364]

My dad had an Austin Cambridge - that was permanently on charge. He never found out what was draining it. Turned out my father in law had the same on his Morris Oxford - of course we didn't know each other at the time.

Same happens on my Defender - there's an old alarm system that has a live feed to it. So that's got an isolator on the battery. I fitted the one to the Stag as soon as I got it. Makes a good theft preventer and I always know that the battery will be fine. They're only a coupe of quid on eBay.

Chris

[DJT]

Andy,

With me working away from home until recently, my a Stag was regularly left for up to 3 weeks with the battery connected and not on trickle charge. It still had plenty of power left to start the engine even though the clock, immobiliser and radio were connected and working throughout.

I would suggest that you have another drain, or the battery is on its way out.

[Guest]

Hi Andy,
I had a similar problem with the battery going flat when the car was'nt used for a couple of weeks.It could have been the radio left on ,glovebox light not switching off or boot light.I fixed it by having a battery isolator
switch fitted to the side of the inner guard near the battery which I turn off when the car is not being used.It also is a good security system as the bonnet needs to be open to get at it.I have not had another flat battery since fitting the switch.
Cheers. Anthony

[wilf]

Andy, I can recommend a simple thing called a "discarnect" to allow you to easily disconnect the battery when the car is not in use. Under a tenner.

Also a battery maintainer/conditioner (designed to be left connected all the time the car is garaged) is a great idea, will extend your battery life immensely, and keep it topped up.

[jleyton]

Yep, totally agree with Wilf.

I've got both and they're invaluable.

Cheers

Julian

[Guest]

I have one of those battery disconnect things fitted at the battery earth and I unscrew it every time I put the car away. Forgot to do that last week and found the battery flat. I put a 12 volt lamp across the open connection at the battery earth terminal and it glowed, so something was "on". In my case it was the boot lamp, but in this case it is something else (his boot lamp is not wired up), so a systematic disconnection of the suspects should reveal the culprit. Unless of course the battery has had it - I have had failures in less than 2 years.

[wilf]

The problem with batteries left unused for longish periods is "sulphation" - this is when the battery acid attacks the lead plates, and then the battery capacity is much reduced. A battery "conditioner" can help slow this effect right down, hence the longer life of the battery. My old Cobra battery was 11 years old and good as new!