Car battery

[EFLImpudence]

I read on t'internet that with the electronics in modern cars, there is a constant drain on the battery of up to 50mA.

I measured mine this afternoon at 150mA - hence the three times acceptable - so that would be 50Ah in a fortnight instead of only 16ish.

 

[JohnW2]

I read on t'internet that with the electronics in modern cars, there is a constant drain on the battery of up to 50mA.

Oh, I see.

I measured mine this afternoon at 150mA - hence the three times acceptable - so that would be 50Ah in a fortnight instead of only 16ish.

Right, I understand. If that 150mA is 'unavoidable', that's obviously a problem if you're likely to leave the car unused for appreciable periods - with those figures, it probably would not start after a couple of weeks or so of non-use.

Although I haven't done many measurements, I have the same problem. I'm using my car progressively less these days, and the battery (even if a newish and fully charged one) was going flat after 2 or 3 weeks of non-use. It seems that most of that was down to the alarm, so I confess that I've now had to disable the alarm - which has helped a lot! With the alarm off, one imagines that it needs hardly any power when 'idle' - on the face of it, all one seems to need is just a tiny amount to keep clock(s) going and for the electronics which respond to an 'electronic key' - but I dare say there is plenty of 'use of electricity' of which I am unaware!

Kind Regards, John

 

[securespark]

Plus, why, if you jump start it, is it alright for the rest of the day but not the next morning?

You mean once you have jump-started it, you can restart it all you want for the rest of the day but in the morning, after standing overnight, it has discharged?

 

[EFLImpudence]

Yes, and when jump-started, half a mile and it will start on its own and then rest of the day normal driving, it's ok.

I know that's what they do but am trying to understand why.

I can understand that they wear out and get 'weaker', as I called it (after all eight AA batteries are 12V but they won't start a car) but what happens in the first five minutes to make it alright again (temporarily) but leaving it overnight is back to square one (constant drain aside).

I will do what you said and disconnect it tomorrow evening and see.

 

[stillp]

One of the failure modes that can occur is self-discharging, when even if the battery is disconnected it will run down over time. I would guess that's what's happened to yours, so if you jump start the car it's OK until left for a while.

 

[JohnD]

IME, at this time of year, all the time and effort you spend testing and investigating is wasted. You will get a new battery in the end, so just get on and buy one.

If you want, you can verify the boot light and interior light are off, and put the battery on charge overnight, but if it fails again the following night, just buy a new one.

 

[EFLImpudence]

Thank you John - sympathetic and tolerant as ever.

 

[JohnD]

Thank you John

glad to help.

 

[EFLImpudence]

When did you help?

 

[securespark]

If you have, or can borrow, a CTEK charger, they have a reconditioning mode, where they can recondition an old clapped-out battery from 2V upwards.

I did it for my Skoda which was left languishing on the drive and it worked a treat. Worth a try, if you can get your paws on one.

 

[JohnW2]

One of the failure modes that can occur is self-discharging, when even if the battery is disconnected it will run down over time. I would guess that's what's happened to yours, so if you jump start the car it's OK until left for a while.

Indeed. In fact, is that not the 'usual' initial failure mode of virtually all rechargeable batteries - i.e. they will seemingly charge, but actually will not store much electricity and will then loose it spontaneously quite quickly after the charger is disconnected? That seem to be the usual change before the 'final' one when the won't even seem to charge

I suppose one way of looking at it is that the purpose of all rechargeable batteries is to 'store electricity' (in a chemical form), so when they start ageing/ deteriorating, their ability to store electricity will deteriorate (both the amount and 'how long it stays there')

Kind Regards, John

 

[securespark]

When we bought our last car, a 59 Honda, it took us a while to realise the battery was wearing out. It got to the point where just listening to the radio with the engine off for 20 minutes would kill it.

I fitted a Stop/Start AGM Bosch S6 096, then just under £100. It's been fantastic and 5 years & 100K later it is still going strong. I understand they are now around £125. Compared to other AGM's they seem a decent price and are the bog's dollocks.

In fact, yesterday we went to pick up a canoe and needed the headlights to unlock the container. They were on for a good 25 minutes with the engine off and it started without complaining.

 

[ban-all-sheds]

Plus, why, if you jump start it, is it alright for the rest of the day but not the next morning?

Because it's shagged.

 

[ban-all-sheds]

However, with the three times the acceptable drain, after, say, a fortnight (50Ah instead of 16) it would be considerable so should be investigated.

Don't leave your car at an airport car park while you go on your hols.

 

[ban-all-sheds]

This will vary consdierably according to the type (chemistry) of the battery - whilst the more modern types (NiCad NiMh etc.) tend to maintain their nominal voltage throughout most of their discharge life, and then 'suddenly plummet', the voltage of lead-acid batteries such as you're talking about tends to reduce progressively throughout discharge.

I've never wondered about this, as so far my numbers have not come up on a Wed or Sat evening (I'm considering a strongly worded letter of complaint), but if a chap were to go out and spend, literally literally, £millions on a brand new car, why would he be stuck with the same 150-year old battery technology as the chap who buys a Dacia? Not knocking Dacia - they just happen to make the cheapest new car on the market right now.

I know that lead-acid batteries are good at handling short duration monstrous loads, but even so, have we really not managed to design anything better in the last 158 years?