Battery Capacity

Maybe your land rover has coincidentally developed an engine earth fault? High resistance connection somewhere?

Tim

I'd check the connections very carefully. You are putting your meter across the battery terminals and nowhere else?

Do the headlights dim when you try to start it?

less compression on smaller cylinders in the Jag.

jag battery around 600 cca

lr battery around a thousand

Your figures are spot-on, sir. I was kind of thinking maybe the voltage drop from the jump leads I was using could be the culprit, but that Jag battery might just not have enough grunt, even if I could attach it directly to the LR's clamp terminals (which I can't).

I had not understood that from your post.

Even lorry size jump leads would give a very significant drop, you always need a battery on the vehicle to act as a reservoir for the charge you add from the leads. Crocodile clips only have a tiny contact area.

Last year an old, old customer came to the door to say could I help, he has a Citroen something diesel van, having driven a hundred miles or so, he got home, turned it off in the back alleyway to open up his garage, and it would not re-start, no go from the battery, I went down with my super sized jump pack, expecting it to leap into life, it would barely turn over !! When I investigated I found that the (very recent) cheapo battery that someone had put on, despite being the correct massive size, had died in some major way internally and would not buffer the power from the jump pack, and the crocs on the end of my jump pack had started to burn away. I put on a nice new Bosch battery and it leapt into life.

The jump leads would be the problem. Very few indeed have crock clips that can handle anywhere near the maximum current a decent battery can deliver. And on many, not the cable either.

Most cars are jump started when the car battery still has something in it. So jump leads made with this in mind.

Surely this is obvious!

If the cable from the battery to the starter motor has a cross sectional area of (say) 25 square millimetres then the contact area between a jump lead and contacts at each end (the donor battery, and the recipient car) ought to be at least similar to 25 sq. mm. This could be achieved using a screw-on clamp of some sort, but there's no way a croc clip could achieve it.

I doubt that even a big croc clip could achieve more than about 1 sq. mm. when clipped onto the connectors on the recipient car. At the 400A required to crank the engine even 0.005 ohm resistance will lose a couple of volts giving very slow cranking.

If the recipient battery is flat but otherwise not faulty, connecting to a donor car with its engine running at reasonable revs will ensure that the donor battery has about 15v across it - so this will deliver a large charging current to the recipient battery - probably something near

100A. Leave it connected for a few minutes (expect the croc clips to get quite hot), and the recipient battery will be at least a little charged, so its current plus the donor current will usually be enough to crank the failed engine.

Maybe that is because such leads are not designed to carry starting current, they are supposed to be used to *charge* the battery from a donor vehicle.

I wasn't happy about that aspect, either. You have thumping great leads yet the contact area is tiny. It's highly unsatisfactory.

Well, the old battery has suffered a similar internal failure, so it's ability to act as a buffer is pretty well non-existent I'm sorry to say. I've never been in this situation before. It really had me scratching my head. You think you know it all and then some novel set of circumstances crop up and you're left floundering for an explanation. :( Thanks for clarifying everything.

Or just to supplement the flat battery. But it makes sense to use the donor vehicle to charge that low battery via the jump leads for a few minutes before attempting a start.

Im surprised you dont have one of these

12V-Car-Battery-Tester-Analyzer -

Actually, I have now got one that shows CCA (literally a couple of weeks ago). At the time I put on my older snap on one that loads the battery and shows voltage differences, but that showed very low volts in the first place and bugger all under load. As it might have been a charging fault leading to the flat battery, I put a petrol fiesta battery (I had lurking around) on the vehicle (connected properly) just to get it started, which it did without a problem (showing how useless jump leads crocodiles can be !). Then I was able to check the charging and whether there was any parasitic drain. Having done that I gave the cheapo battery a charge up and was then certain the problem was a duff battery. Such a dramatic failure is not common, especially with a battery less than three years old, he couldn't remember who or where the battery came from, but someone had bolted him up with it. The new Bosch weighed about 2kg more than the cheapy, yet the claimed CCA rating of the cheapy was nearly the same as the Bosch.

In my experience of watching others on many occasions, even the average mechanic does not know this, let alone the man on the Clapham Omnibus.

[snip]

Many years ago, went on holiday to the south of France in our Renault

14, which had been starting and running perfectly for several years.

Parked outside the acccommodation where we stayed, and left the car unused for probably 2 days. Then went to start it: not even the courtesy light came on as I opened the door. The radio did work. There wasn't enough power to operate the starter solenoid when I turned the key.

So walked into town to buy a replacement battery.

I can only suppose that driving all day down a motorway had overheated the battery and killed it - though it was probably several years old.

I have a camping caravan holiday campsite and it has happened here last year it identified two with dead short cells which saved the jump leads damage often get people asking if I could charge their flat car or caravan battery, the tester is great especially with so called calcium and AGM leisure batteries which seem to show a good off load voltage ie over 12.4 volt but zero CCA health

-

At least it went home to die. ;-)

Perhaps as it cooled it fractured a cell-to-cell connection. It wasn't unknown that long ago.

Chris

Vibration, heat, old age. Without dismantling no-one will know.

The BMW branded one on mine failed just as the car passed three years old. I'd bought the car at 2 years old - so I assume the original.

The car started normally. Drove to the shops (in daylight) and it was totally flat after about being parked an hour. Absolutely no warning before.

Myself I think this is by design - they fail a few weeks outside the warranty.