Flat Battery - Invite To Car Experts For Help

[Prudie]

Sorry this is a long post and I have asked this before but would appreciate any help from you who have above average car electrics expertise (my husband has put these words together below):

Why does my car battery keep going flat? 2000 reg Mercedes SLK 230– the story so far:
The car had a new battery about 10 months ago as every time it was left for a long time (2 weeks or more) it would be flat. The car spent a number of days in a local garage run by an ex Mercedes mechanic who could not find any obvious issues causing the battery to drain and concluded the battery was knackered so sold me a new one.

September after a week's holiday came back and the car wouldn’t start. Jump started and seemed fine although it did seem ‘sluggish’ turning over.

Situation has gotten progressively worse until leaving the car for more than a day now sees it fail to start.

If the battery is given a proper slow charge this seems to help and the car will start OK for 3 to 4 weeks although again worsens over the weeks.

Occasionally (2 or 3 times in Sept/Oct) the car alarm has gone off for no apparent reason. My husband is wondering if the car alarm horn built in battery is flat/expired and is trying to charge when the car is turned off at random times hence draining the battery but doesn’t know how to test this.

Nearly all of the driving I do is very short distance (12 miles a day) with most ancillaries on.

What has been done to try and identify the issue:
 Firstly it was mentioned the interior lights sometimes drains the battery. Interior light disabled.
 The boot interior light (apparently another potential source of battery drain) has been checked with no fault found
 The battery has been repeatedly checked with professional battery drop test equipment and has been sentenced good each time
 The electric seat relay was replaced in September (this was due to the seat not working and failing the new MOT rules – not related to the battery drain)
 The charging system has been checked by 3 different vehicle electricians all of whom agree it charging correctly
 The car has been turned off and the current draw monitored using a clamp on Ammeter – the car draws around 250 milliamps at rest (i.e. after the car has been turned off and left to go to ‘sleep’)
 An in line ammeter has been fitted (again a draw of approx. 250 milliamps was seen) and each fuse removed in turn. The alarm/immobiliser and the radio are responsible for the majority of this draw on the battery. This ‘test’ was carried out with both the car turned off and repeated after about an hour of turning the car off and leaving it to go to ‘sleep’

Again sorry this is long but we would be interested to hear any thoughts on above from the petrol heads :-)

[davebro]

You say the car "fails to start" - does the engine turn over & if so is it normal or slow?

Starting takes a large current from battery to starter to earth - if any of those connections are fouled up - loose/dirty/whatever - then it might not start.

[davebro]

when the car won't start is there enough juice in the battery to light the dash, outside lights, radio etc?

[bhg481]

250 mA is quite a lot of current - quarter of an amp, to put it another way. It means you use 1amp-hour every 4 hours or 6AH/day; after 2 weeks that's 6x14 = 84 AH ie, can be enough drain to mean the starter won't work. I think you need to find out exactly where all those milliamps are used.
My last couple of cars have shut down a lot of circuits after about 20 minutes (radio goes off, interior lights go off); I suspect your car does something similar. Is that feature working?

[bhg481]

Another question - petrol or diesel? Diesels take a lot more current to turn the starter because they have a much higher compression ratio on the engine.

[Prudie]

Just to answer dave - I use the car daily so it doesn't go flat enough for everything to lose power (except after a long holiday) but each day after a full charge as mentioned I can hear the turnover gets more and more sluggish until the day it's not enough to turn the engine. With jump leads it fires up instantly. I don't mean to be rude and value anyone's input but all the people who have been doing the above are qualified mechanics and it has stumped them so it's not an obvious or standard fault.

[Prudie]

Petrol

[ToraToraTora]

if you only do short distances then the battery will be getting hammered by the starting. It takes approx 20 mins running to put the charge back in that it took to start. Diesel longer. Also I assume you don't have one of those cars that turns itself off when it stops in traffic etc, if so disable that. Over time the 250ma drain will also lower the charge.

[shoota]

I'm not an expert Prudie but I would suspect your daily usage with everything going is insufficient to keep the battery fully charged. A trip out of town for an hour or two during daylight with minimal accessory usage may be all that is needed.

[shoota]

Have you considered one of these (not necessarily from these people)?
https://www.selectsolar.co.uk/cat/48/solar-chargers-for-cars

[bhg481]

Has the alternator output been measured with lots of high-current devices on?
Can I suggest you buy something like this?

Amazon.co.uk User Recommendation

Just plug it into your auxiliary socket and check the voltage. When you turn on the ignition it should b around 12V; whilst you're driving it should be close to 14V. If it drops to around 12V whilst you're driving, whatever the electrical load, you might have an alternator problem; if it stays on permanently when you turn off the ignition it means that circuit is still live - should it be?

[ToraToraTora]

good idea just bought one of those, cheaper on Ebay:
https://www.ebay.co.uk/itm/Voltmeter-Voltage-Battery-Monitor-LED-Display-Indicator-12V-24V-Car-Boat-UK/293251567665?_trkparms=aid%3D888008%26algo%3DDISC.CARDS%26ao%3D1%26asc%3D20140109134348%26meid%3D7be4d9ad647642ec985b603db8f1b4ef%26pid%3D100042%26rk%3D1%26rkt%3D1%26mehot%3Dag%26sd%3D392432420502%26itm%3D293251567665%26pmt%3D1%26noa%3D1%26pg%3D2508447&_trksid=p2508447.c100042.m2062

[KARL]

I regret that I cannot explain your particular case but instead offer my own experience of a Golf. The seller made a mention of the battery being a good one but I thought he was being untruthful because it would be flat if the car was left more than 3 or 4 days unused (and everything switched off). that being my assumption I bought a new battery but the same happened straight away. I did what you have done: I pulled fuses out one by one and also relays. I ended up testing when everything was pulled out. The same result, the battery still went flat and I felt stumped.

I took the car to a garage, the guy asked me to bring both batteries in - fortunately I had not dumped the old one which turned out to be fine. He kept the car for five days and when I returned he pointed to both batteries and said they were duds, he could provide me with a new one. I said thanks but no thanks. Something was draining the battery/batteries. I resorted to disconnecting the battery once stopped for anything more than overnight and kept pondering and prodding.

There was a wire connected directly to the battery at the clamp and feeling I had nothing to lose I disconnected it, it looked almost as if it was a post-production fit. Since then the battery has not lost charge when the car has been out of use for a few days - in fact the original battery lasted me seven years, not long replaced. I have no idea what that wire was for and everything seems to work normally, although I do not use the factory fitted alarm which I think never worked anyway.

There are those who point to two things in particular that are likely to cause problems like the one you have. One is a radio with pre-set circuitry, etc. which needs a fair amount of electricity to stay set, The other is an alarm. Finally, some suggest that mobile phone add-ons can drain batteries. I am not qualified to comment on this speculation/knowledge. I hope you succeed in solving the mystery - these are very frustrating.

[Ryzen]

You should carry out a diode test on the alternator..diodes are used within the alternator to convert ac to dc voltage.If they start to fail you may still see a good dc voltage output but at rest, a bad diode will allow current to flow back from the battery to chassis negative.This parasitic drain will be undetected by general parasitic drain tests.
You say that a 250mA drain is evident..have you withdrawn fuses to determine the source of the drain?
If all else fails then a intelligent trickle charger will have to be used between usage.

[Hymie]

Bhg481 is correct; 250mA is an excessive discharge rate – only a very healthy battery will be able to start the car after being left for a week at this discharge rate.

Anything over 100mA should be a cause for concern – in my experience garage mechanics know very little about vehicle electrics.

Since the radio/immobiliser are responsible for the majority of the current drain, I would consider placing a switch in the supply to these (after all, who would nick a 20 year old Merc) – but you might have to re-code the radio each time you switch them off.

Admittedly driving only 12 miles a day, with multiple start/stops during those miles won’t help the situation.

One final piece of advice, turn your mobile phone on in timer camera mode with the flash off – set it to take a photo and quickly place it within the glove box (facing upward). If for some reason the glove box light is causing the issue, the camera photo will confirm this.

[bhg481]

Ryzen's comments about the alternator remind me of something I heard many years ago, when alternators were new to the car industry. If the bulb failed which showed that the alternator wasn't charging, then it could somehow cause the battery to discharge. Very vague, I know, as it was many (30?) years ago and may not apply to more modern alternators, but it might be worth checking that the warning light does come on when you turn-on the ignition and goes off as soon as you start the engine.