Stop/Start not working, battery charge level required?

A Jaguar XE but I will guess most cars the same, if the battery is low, it disables the stop/start, we went around 75 miles yesterday and no stop/start, battery about 1 year old, but car not used much, so stuck on charge today.

I have an energy meter so seemed good idea to use it.

This shows quite a short time at 3.8 amp and 3 amp, then dropped to 0.8 amp which will clearly at that output take a long time to charge to a point where it drops to 0.1 amp and charger will then show as fully charged, but started charge 9:26 am and dropped to 0.8 amp by 9:38 am that's around 0.6 Ah, it's a 90 Ah battery, so that is nothing really, so why no stop/start?

That's odd

I wonder how the car measures battery charge and makes the decision?

There might be a Technical Document you can download from Jag or an owners site.

(It may be extremely large)

My neighbours XF has a big battery plus a little one which he thinks is for stop-start

I have wondered that myself, stop/start does not work with heated seats on or head lights, so maybe simply alternator output, if output of alternator over x amps no stop start?

I looked back at the monitor 2 watt = 0.1 amp it would take a long time to charge at that rate, the charger alternates between 0.1 amp and 0.8 amp with a battery over 12 Ah, so at 14.4 volts it drops to 0.1 amp, and at 12.8 volts it will switch back to 0.8 amp, so battery sits most of the time at 12.8 volt, each time it drops below the threshold it switches back on for a few seconds, at 0.8 amp then quickly hits 14.4 volt and drops again to 12.8 volt. Can only use smart charger as a AGM (VRLA) type battery.

I normally charge battery for at least 24 hours, but last time had a problem with another car, so charger said fully charged, but don't think it was, the car is not used much, it is wife's, I use an old banger, so have to charge it from time to time, at over £200 when this battery was fitted, don't want a repeat, changing battery involves telling the engine management it has been changed, because it is only fully charged every so often, normally sits around 80% and only chargers on over run.

I really do think it is going too far with fuel economy, but son also has an XE our tax £30 per year, his over £100 because the engine management on his gives him 189 BHP and wife has 164 BHP engine exactly the same, just the mapping changed, and he has 4 wheel drive.

He actually gets better MPG than my wife, although no surprising she scares me with her driving, hence why not used much, mainly use a nearly 20 year old Honda Jazz, or Kia Sorento that she does not like driving as they have manual gearboxes, only 5 speed, well if you include low ratio 10, but never used, her is 8 speed, glad not manual with that many gears.

There can be a problem with modern cars and charging the battery when it doesn't really require it. Like a mobile phone the battery is monitored and the battery charge is calculated by an equation that works out how much charge/how much use /how much standby time etc. If you charge the battery, especially if it is removed or disconnected, this calculation then becomes incorrect. The charge strategy is not to actually keep the battery fully charged as they actually last longer if allowed to run at something like 70% charge. If you charge the battery and refit it, the battery monitoring will still assume the battery has the amount of charge it calculated. The reason for this is to reduce parasitic load used by the alternator and for this reason, it only charges at a rate that it has calculated that it needs to.
Gone are the days when you could just bung a Lucas ACR alternator on and it gives full charge all the time!

Oh and disabling stop - start is very beneficial to your timing chain, it is one of the reasons why mechanics forums hypothesize the BMW N57 engine has either very high failure rate or very low depending on usage....

I realise that using a charger maybe not best idea, and normally I would not charge it, but wife had been stopped from driving because of cancer treatment, so it had sat in drive for around 8 weeks.

So from what you are saying she will need to see dealers and get computer reset, however another 4 weeks before she can stop self isolating so will have to wait until she can go to Chester 100 mile return trip so not some thing to be done in hurry, and it also entails driving through lock down areas.

My worry is when last battery failed there was no warning, it just stopped starting, so a little worried taking it for long runs.

ericmark said:

My worry is when last battery failed there was no warning, it just stopped starting, so a little worried taking it for long runs.

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Have you considered something like a CTEK intelligent battery charger and charging it with the battery still connected and in the car? Assuming the battery sensor circuit remains "awake" which I'm sure it does, it should keep tabs on the battery receiving a charge and not attempt any selective disabling of circuits to save power.

Our Kia's (definitely not a Jaguar) start and stop had the same issue. We took it to the dealership and they said the battery simply needs changing, as we do few and short trips only. So I drove 80 miles to no avail. Then we went on holiday and only as we had travelled 600 miles, it worked again.

Bodgedbuild said:

Have you considered something like a CTEK intelligent battery charger and charging it with the battery still connected and in the car?

Click to expand...

Battery has never been removed from the car, the IAN 102626 CAR BATTERY CHARGER ULGD 3.8 A1 from Lidi is very like the Ctek, all stages are constant current where one stage of the Ctek has constant voltage, it will not unlike the Ctek return to full output, once it has dropped out of 3 amp rate it will not return on its own, and it will not restart after a power cut, and minimum voltage is 7.3 for a 12 volt battery under that it assumes a 6 volt battery, and it has a built in volt meter, but it is a Smart charger which I know I must use with an AGM/VRLA battery.

As far as I am aware there is nothing the Ctek would do which is any different to Lidi as far as battery care goes, if I have it wrong then all ears.

Thank you @chris1982 reassuring to realise it can take that long.

After talking to my wife and queried her she says she may have used front screen heating, which would explain why stop start did not work, I was unaware of this when I started post, sorry had I known at the time of starting thread I would not have asked the question.

Bodgedbuild said:

Gone are the days when you could just bung a Lucas ACR alternator on and it gives full charge all the time!

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That was a blast from the past, I remember repairing them, there was a fault with regulators which first caused a non-regular charge patten and then failure, we started using an oscilloscope to monitor charge pattern to reject faulty regulators. Also battery sensed worked OK for years, but machine sensed would burn a hole in centre of the slip rings, and the rectifying pack was poor, the Durite replacement was a lot better with same diodes for field as main output, so we added an extra 3 small diodes and used the original to form a split charging alternator. They were so much better than the old 10 and 11 AC Lucas alternators.

I only found one alternator with current control the old CAV used mainly on Buses with a 440 regulator that had M1 and M2 terminals connected across a lump of steel with saw cuts, which acted as a resistor for regulator and ammeter. However I note now Sterling do a special regulator to stage charge batteries so I suppose that has all changed?

Worked with a few weird alternators including a Delco with 12 and 24 volt outputs used on a Grove crane, crane used 12 volt for all but starting, and the system did not really work, each service we would swap batteries as other wise one would go flat, the relay Bosch used worked better.

Also the French single phase alternators.

Today more interested in the Tonum regulator used on railway engines with the steam turbine generator, dynamos were still used when I started, but things like bucking coils were only found on Buses and like, so since mainly wagons I worked on never got into them.

You seem more up to speed on alternators than me tbh, Interesting history too...
I've always been tempted to fit a Lucas ACR in place of a modern "on demand" one and disconnect the IBS. On 5 & 7 Series BMW's they are a constant headache!
Back to your charging, can I confirm you are charging from the jump-lead point under the bonnet and not direct to the battery? The charge would then flow down all the correct circuits and also the negative lead IBS "shunt" would also register current flow in the correct direction. I believe a cigarette socket could be used but i'm not sure if yours become dead once switched off?

Yes charge from jump-lead point under the bonnet, mainly as having the bonnet cracked open not a problem, having boot left open is, as motor opened and closed so can't just put it down until touching cable it's all or nothing.

The Bosch alternators started to rectify the star point and the output was massive from such a small alternator. And easy to change brushes and regulator could do it with many cars without removing alternator just two screws. Early ACR was just 16 amp output, not really enough for modern car, seem to remember sons narrow boat had two 80 amp alternators. And some had a switch mode unit which combined to output so could charge the domestic battery at 160 amp, OK 400 Ah as 3 batteries in parallel, but my experiments with batteries have shown time no amps is important.

That little 3.8 amp Lidi charger even on the Kia Sorrento battery or caravan battery does not charge at 3.8 amp for long, it may sit at 0.8 amp for days, but the 3.8 and 3 amp stages are completed in 5 hours even with a rather discharged caravan battery. So 20 Ah pushed into the 90 Ah battery at 3.8 and 3 amp, but the remaining 70 Ah can take days to get into the batteries.

But at 0.1 amp and 12.8 volt I am not sure if leaving the charger connected is actually helping? I note with the jump start battery 12 Ah VRLA even after 3 months standing the battery takes minutes before charge rate drops, the AGM or VRLA seem to hold their charge well, but Jag has not been filled up since March, it has done a few trips, but not enough to keep it fully charged.

At first I only charged the Sorrento and Jazz, I left the Jag knowing the AGM battery does hold charge better, but no soon had lock down been lifted and wife went in for cancer treatment so using my car the really old Honda Jazz, her car just sat there. Did use Sorrento to visit her brother and to move stuff from old house towing trailer, and very poor roads to her brothers house, Jag would ground. So Jag really a white elephant and not used. She loves the Jag, mainly because automatic, but no room in it to move stuff, and not as easy to park in hospital car park as Jazz.

Yes it drives nice, but I have never been cab happy, I had a HGV but never really used it, let the medical lapse. Would not mind getting a train ticket and drive the local steam trains, but that's different.

I did say "consider" fitting an ACR
I was plagued with battery sensor problems on an E60 5 Series BMW and then on an E61 when the wife bought one. The initial fix was to disconnect the IBS, my car actually ran for over a year without it connected. I did play around with the sensor on the wifes car though because it kept disabling functions and was suffering gradual battery drain. Heated seats failed, heater fan only worked at low speeds etc. In the end, after a lot of playing around with knock of BMW factory diagnostic tools the problem was cured by telling the ECU it had a different battery to the one actually fitted, never had any low charge issues after that.
I wonder if disconnecting your IBS might be worth trying if the car is not used for long periods? I would still keep it on a trickle charge though....