No auto stop-start on 2013 Focus - bad battery?

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Originally Posted by watercooled

Some articles seem to imply Fords will do some sort of regenerative braking to assist in battery charging; I wonder if this just means the alternator is loaded more heavily during braking, as I don't see any additional components in the parts list that could relate to this?

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I was intrigued by this so I did a little digging. It seems that some Fords have a clutch on the alternator, the idea being that under hard acceleration, the alternator can be de-clutched so that more engine power goes to the wheels. Under braking the alternator is clutched back in to charge the battery - the assumption being that the battery will be partially discharged and able to load up the alternator.

That last bit is essential, because under a 'normal' set up where the alternator is permanently engaged, the battery will nearly always be permanently charged, so there is little load for the alternator to provide a braking effect.

It seems an odd arrangement for Ford when the stop/start needs a fairly full battery to cope with frequent stop starts in urban driving conditions, but perhaps the algorithm takes into account throttle demand before de-clutching the alternator?

You would think it would be better to limit and cut out charge electronically?
I can understand them using a clutch on my 20 year old 328's large air con compressor to cut out when you go WOT but 20 years on surely it isn't the best way to do it?

That can be tricky, generally the way to control the output is to alter the excitation voltage on the rotor - that is controlled by the regulator circuitry. But there is usually some residuals magnetism, and under no load conditions the voltage can rise to high levels, so you then have to run it open cut, and use diodes that can withstand those voltages. A simple magnetically operated clutch might be simpler and more reliable.

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Originally Posted by peterb

I was intrigued by this so I did a little digging. It seems that some Fords have a clutch on the alternator, the idea being that under hard acceleration, the alternator can be de-clutched so that more engine power goes to the wheels. Under braking the alternator is clutched back in to charge the battery - the assumption being that the battery will be partially discharged and able to load up the alternator.

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That is interesting, I saw that behaviour in my Alfa described as: The alternator charges the battery to 85% and then cuts out to save load and hence fuel. If the ECU sees that you are engine breaking, it turns the alternator back on to gain some charge until you are at 100%. So they push it as an economy feature, perhaps because you expect performance from an Alfa but my cars have often drawn comments like "that must drink fuel" and people look surprised when you point out that performance through efficiency means that no, it really doesn't :)

I bought my car as a three year old car so just out of warranty, and in the couple of months I have had it the stop-start has never kicked in but that just means I don't have to press the disable button so I have been OK with that. The forums say it is always the battery either on the way out or replaced with the wrong type.

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Originally Posted by kalniel

I'd have thought you could feel engine braking in most cars to be honest, regardless of regen . Extent just depends on how large the flywheel is and what the software tells the fuelling to do to smooth it off.

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Yeah I just thought it felt a bit stronger than other cars I've driven. I don't mean so much when you lift off the accelerator, more when you're braking and push down the clutch a bit early. I couldn't say for sure though, it's subtle if anything.

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Originally Posted by DanceswithUnix

That is interesting, I saw that behaviour in my Alfa described as: The alternator charges the battery to 85% and then cuts out to save load and hence fuel. If the ECU sees that you are engine breaking, it turns the alternator back on to gain some charge until you are at 100%. So they push it as an economy feature, perhaps because you expect performance from an Alfa but my cars have often drawn comments like "that must drink fuel" and people look surprised when you point out that performance through efficiency means that no, it really doesn't :)

I bought my car as a three year old car so just out of warranty, and in the couple of months I have had it the stop-start has never kicked in but that just means I don't have to press the disable button so I have been OK with that. The forums say it is always the battery either on the way out or replaced with the wrong type.

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Yeah that's more or less the way I see it at the moment - not having to press the disable button every time I get in! :P

It seems a bit strange to be holding a lead acid battery at less than a full charge though, doesn't that encourage sulphation? Or is 85% high enough to avoid that?

FWIW, I'm back from a ~200 mile round trip and it's changed nothing - no cut-out and battery still at about 12.3v.

Quick update for those interested:

Given winter is approaching and I noticed the starter making an unusual noise a few days ago on a particularly cold start, I ordered a Yuasa YBX7100 from mdsbattery and I've just finished installing it. It's fairly awkward to do (especially in the dark) due to both the awkward positioning of the battery and how it's boxed in, but I eventually managed to do it.

I got in the car and everything worked as normal; no need for a radio code, and even the clock didn't reset, so maybe I did it quickly enough that nothing fully discharged. After checking it started OK, I switched the ignition back off and went through the BMS reset sequence I found on another forum. I was initially worried as nothing happened for about 5-10 seconds but eventually the battery light on the dash blinked.

I went for a drive to warm the engine up (and to give the ECU a chance to re-learn as the manual suggests). Got to a set of lights, and lo and behold, the engine switched off and the A symbol lit on the dash!!!

FWIW, the new battery measured 12.80v fresh out of the box, and the one I've just removed currently measures 12.32v with no load attached.

Definitely not a fault alternator?

Unlikely; it's been the same for several months and is consistently around that voltage with the engine off, and >14v with the engine running (as mentioned in OP) i.e. nothing IMO to suggest a faulty alternator. The battery won't hold a full charge even after being left on a battery charger overnight, plus it's ~4 years old.

Fair enough I did read that in the original post but always worth checking

The radio will probably be like my last works van, serial of the radio is coded to the chassis number so the d can? (assuming d can) bus can make the checks without you having to find the code.

Amusingly my wife's Mini started to keep running and only stop starting once in a blue moon, I assumed it was the battery as it was the original 2011 battery from new, one day it started to work again and really confused me, I couldn't do a code scan as I hadn't got my new cable to check it. It was the air con condenser leaking, this backed up the crap aircon. Turns out the engine kept running to try and pull the internal temps down, of course the engine doesn't need to be on if you have the aircon off and I had turned it off for some reason. Still cost me about the same as a bloody battery and half a day removing the majority of the cars front end to get it out!

got the cable and the battery still charges to 89% so fingers crossed it last a bit longer!