Flat battery
in [Car Maintenance]
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From: Dave Plowman (News) on 23 Dec 2008 09:18 Had an interesting one late yesterday afternoon - a neighbour knocked on the door and asked for some help. His '01 BMW 325T wouldn't start. Flat battery - just a relay or two clicking when you tried. He'd started it earlier - normally - and moved it a few yards to wash it. Got the meter out and it was certainly flat - 9.5v on the battery posts. Loaned him my little Lidl charger and agreed if it wouldn't start the next day I'd take him to get a new battery. His lady wasn't pleased - she wanted to go shopping that evening. About a couple of hours later I went out and noticed their car wasn't there. Sort of concluded she'd nagged him into getting the AA or whatever out. This morning he returned the charger and said on a whim - or more likely to stop the earhole gbh - he'd tried to start it after an hour or so and it did. With a charger who's peak output is less than 5 amps? He kept the engine running while she shopped then put it back on charge overnight. By the morning the charger had gone to float mode so had decided the battery was fully charged. What's the explanation? -- *Oh, what a tangled website we weave when first we practice * Dave Plowman dave(a)davenoise.co.uk London SW To e-mail, change noise into sound.
From: Alex Buell on 23 Dec 2008 09:49 On Tue, 23 Dec 2008 03:52:03 -0800 (PST), I waved a wand and this message magically appears in front of Ian: > Perhaps you have never had this happen to you - if you haven't, it can > be difficult to imagine (a) just how flat the battery appears to be > and (b) just how quickly it returns to full vigour. I have had this happen to me. Putting it on a 20Ah recharger for ~36 hours cured it. -- http://www.munted.org.uk Fearsome grindings.
From: Conor on 23 Dec 2008 10:01 In article <5012386bcddave(a)davenoise.co.uk>, Dave Plowman (News) says... > This morning he returned the charger and said on a whim - or more likely > to stop the earhole gbh - he'd tried to start it after an hour or so and > it did. With a charger who's peak output is less than 5 amps? > > He kept the engine running while she shopped then put it back on charge > overnight. By the morning the charger had gone to float mode so had > decided the battery was fully charged. > > What's the explanation? > Starter motor is possibly fucked - maybe brushes. Just changed the one on the Capri last week - similar symptoms. Turned over fine if the voltage was there but once the voltage dropped a bit, i,e when parked overnight, it didn't want to know. What was really wierd is it'd turn real quick for a few seconds then if it missed firing and you instantly tried again, it'd turn over like the battery was flat. All connections absolutely fine, battery in good nick. I put a new starter motor on and it's been fine since. -- Conor I only please one person per day. Today is not your day. Tomorrow isn't looking good either. - Scott Adams
From: Dave Baker on 23 Dec 2008 10:05 Dave Plowman (News) wrote: > Had an interesting one late yesterday afternoon - a neighbour knocked > on the door and asked for some help. His '01 BMW 325T wouldn't start. > Flat battery - just a relay or two clicking when you tried. He'd > started it earlier - normally - and moved it a few yards to wash it. > > Got the meter out and it was certainly flat - 9.5v on the battery > posts. Loaned him my little Lidl charger and agreed if it wouldn't > start the next day I'd take him to get a new battery. His lady wasn't > pleased - she wanted to go shopping that evening. > > About a couple of hours later I went out and noticed their car wasn't > there. Sort of concluded she'd nagged him into getting the AA or > whatever out. > > This morning he returned the charger and said on a whim - or more > likely to stop the earhole gbh - he'd tried to start it after an hour > or so and it did. With a charger who's peak output is less than 5 > amps? > > He kept the engine running while she shopped then put it back on > charge overnight. By the morning the charger had gone to float mode > so had decided the battery was fully charged. > > What's the explanation? Assuming there wasn't an unusual parasitic drain or bad connection after he'd moved the car to wash it then the battery is probably near as dammit dead with very little reserve capacity left. It may be partially shorted inside already. However the charger would have put enough surface charge into the cells after an hour to start the car easily enough. If the engine and ignition is in good nick you only need a fraction of an ampere hour to turn the starter over at say 200 amps for 1 second. The true test will be to check the voltage after the surface charge has been removed by putting the headlights on for a few minutes and then letting it sit for a few more - or just wait until next morning before he starts it next time. If it shows 12.65V then well and good but if it's really on the way out the voltage will be much lower than that and it probably won't hold a charge for very long. -- Dave Baker
From: Dave Baker on 23 Dec 2008 10:11 Conor wrote: > In article <5012386bcddave(a)davenoise.co.uk>, Dave Plowman (News) > says... > >> This morning he returned the charger and said on a whim - or more >> likely to stop the earhole gbh - he'd tried to start it after an >> hour or so and it did. With a charger who's peak output is less than >> 5 amps? >> >> He kept the engine running while she shopped then put it back on >> charge overnight. By the morning the charger had gone to float mode >> so had decided the battery was fully charged. >> >> What's the explanation? >> > Starter motor is possibly fucked - maybe brushes. The battery had a no-load voltage of 9.5V. That's nothing to do with the starter motor given that it wasn't the starter motor that had flattened it as it wouldn't even turn over. If the battery was good but the starter bad you'd still see a normal no-load voltage at the battery. -- Dave Baker
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