Batteries

[les]

My battery died the other day, even the alarm warning lamp was out. I’m hoping something was left on to cause this but can’t think what. The Battery has now been fully charged and connected and I’m waiting to see if it discharges again after a couple of days just standing on the drive. I’m wondering if a battery can discharge itself without a drain on it, by perhaps an internal short and interested to know what sort of current should be drawn from a battery when all is off except an armed alarm? Can anyone advise please?

[Dogsdad]

Do not know what the discharge will be with your alarm armed.
My modern draws 0.25 amps when standing with alarm armed ( plus radio I think ).
A 35 amp hour battery will discharge in 6 to 7 days if fully charged to begin with at this rate of discharge.
Your rate of discharge will probably be less but will still give a flat battery eventually. Faster if not fully charged or if the battery is not in first rate condition
You should remove a battery lead connect an ammeter between battery post and lead switch on your alarm and see what the discharge is. This may give a clue to the problem.

R.M.

[les]

Thanks for your reply, it gives me something to go on. This has happened on my daily driver and the alarm system has always been a mystery to me, so I’m treading carefully!

[Dogsdad]

Let us know how you get on with this.

R.M.

[StillGotMy1stCar]

I have measured the current drawn on both my modern cars and one was 20mA (.020A) with a pulse of 30mA every second or so and the other was 20mA with a pulse of 38mA every second, the pulse being caused by the flashing LEDs. On both the current drawn is the same if the alarm is armed or not. To add to the confusion my most modern has a Body Control Module that draws 300mA for 15 minutes whenever you open and close a door or arm disarm the alarm.
I charge them up if not used for a couple weeks before they go below 12.2V that’s about 50% of full charge.
Batteries can self-discharge, I had this on my mower at the beginning of last year a 24Ah battery that would go flat in 48 hours. Just to get my moneys worth I emptied the acid, cleaned the cells out with bicarbonate of soda and water then refilled with the acid I had taken out and it has been fine since.
Regards John

[les]

Thanks for the further information, I’m pretty sure the battery is good but need to consider this point. My first instinct is to leave all untouched for a couple of days and see if the discharging repeats itself. The battery is now at 12.8 v.
There was a slim possibility that the tailgate release motor was running but I’m starting to doubt this now. If the problem occurs again I will be checking readings.

[Dogsdad]

Seems we are not talking about a Morris Minor here.
No idea then.
R.M.

[les]

No not a Minor, I considered the problem was a general electrical issue that could occur in any vehicle.

[JOWETTJAVELIN]

Does it have interior/boot lights which dim before going off, and was the radio switched off before the ignition?

[les]

Thanks for replying, the answer is no to both questions but I have an update: I tried to check on the current being drawn from the battery with all ancillarys off, unfortunately my meter gave an unsatisfactory result, being limited on the ranges available. However, out of the blue I thought of a guy that could possibly help, so with the battery fully charged, went to see him. He had a suitable meter and ran a test, to my surprise there was the smallest draw possible showing on the scale. Because of this he ran a further test with some fancy gadget, this time on the battery itself. It proved to be faulty. Although holding a good state of charge the state of health was only 66% and the print out read ‘Bad & Replace’ It was thought that the recent couple of frosty mornings was too much and the battery gave up. I didn’t realise what I thought was a reasonable battery, could lose all its charge so dramatically.

[JOWETTJAVELIN]

No problem mate, I don't know much about modern vehicles but do know a lot of parasitic drains are caused by the electronics which do not simply switch on/off but have a time delay or constant feed.

[les]

Yes, on my journey to sort this issue out, I came across plenty of mention of time delay with circuits on modern vehicles which have to be allowed for, waiting for time to lapse etc before coming to a conclusion! Fortunately my car is not that modern, just more so than the Minor ! Thanks.

[Chipper]

Do not know what the discharge will be with your alarm armed.
My modern draws 0.25 amps when standing with alarm armed ( plus radio I think ).

R.M.

Sounds rather excessive! I would get that checked out if that's the case; it should only be a fairly meagre drain of 20-50mA or so at most with everything switched off or on standby. 0.25A is about ten times what one would normally expect; perhaps you have a courtesy/glovebox/boot light staying on for instance...