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BMS - How is it meant to work?

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My 2017 Mk5 (bought two weeks ago) gave the 'shutting down' messages and the stop/start didn't work (although it did on the test drive). So had my usual garage look at it when they serviced it - and checked it over for me. They charged the battery a bit and tested it, and it reported 'good, but needs charging' The stop/start worked again and the 'shutting down' messages went away. Three days later, after not being used, again the same messages and lack of stop start. It sounds like if the battery drops to less than 80% (or SOC calc less than 72%?) then it disables stop/start. So one would assume that in three days the battery has lost over 20% of it's charge. We took the car for a 40 mile round trip at 40-60mph, and the stop/start still didn't work.

I don't really care about the stop/start, in fact I'd probably turn it off during each drive even if it did work - I'm just using it as an indication of the battery not holding it's charge/recharging properly.

However, on further reading, it seems that the charging systems are also 'smart' and may choose to stop charging the battery before it's fully charged for various reasons.

So, I'm very confused... Can anyone explain in simple(ish) terms how this is meant to work? Is the car meant to charge to 100%? I have the car booked in again next week, but it would be nice to have some ides of what's going on. The car is 4 years old and has done 18k miles, so has spent a lot of time sitting around,  particularly in the last year or so. I've received some contradictory views, so would appreciate any advice/comments.

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You will find that, especially in cold weather, the battery will hardly ever be at full charge or even over 80% as the BMS rarely allows full charging rate in the interests of engine economy and CO2 levels. This means that not only does the stop/start not work but also the heated screen does not work, which is ridiculous in cold weather, as the system thinks that the drain would be too much to use those. You can fully charge your battery and reset the BMS and everything will work again but eventually, it may drift back to not working. It is a flawed system. My wife has a 2014 Fiesta without BMS and the heated screen works whenever the button is pressed. In my 2018 S-Max with BMS it is a lottery in cold weather as to whether the heated screen will work. I suspect that as the weather warms up everything will start working again. There are plenty of threads on this issue if you search for Stop/start or heated screen.

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The question wasn't what problems it causes, we're all now familiar with that, but HOW exactly does it work? Then we can better learn to live with it, or better still, try and get it fixed.

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Thanks for your help. I went to the garage so that they could see exactly what I had fitted, so that we could order a replacement, but they were still convinced that things would improve once I started using the car more. As I haven't had another 'shutting down' message, I'll leave it for now and see how things pan out. Any more 'shutting down' messages or other adverse symptoms and I'll replace the battery.

I still don't understand why it simply doesn't charge to 100%, though.... It seems crazy to deliberately push things into failing...

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Putting the battery on charge for an hour or so with a modern Smart Charger will not fully charge it, the car's Smart Charging system is not a lot different.

Driving for an hour may only add 4 or 5% to the State of Charge and contrary to popular belief it does not mater how fast you drive or whether you have accessories on.

The problem now is that the electrical demands in a modern car are so high. The modern charging system has been designed to cut down on emissions (an Alternator needs a lot of engine power to produce electricity) and to protect the longevity of the battery because excess heat kills Batteries very quickly. I have had cars in the past that had a separate  air duct to keep the battery cool, but that would probably upset the emissions now so I don't think anyone uses them anymore.

If you look at modern High End Smart Phones, the Batteries are now massive because the demands are so high, but they do not give longer use. That industry does not have emissions targets so they can afford to have quick charging with a lot of heat and consequently most are a long way past their best within 2 years.

I don't know if the Smart Charging system on your car is the same as mine, but this is how mine works. It only charges until 80% and S/S only works above 68%. When you first start, the Alternator can put out close to 30Amps net for a short time, it then gradually reduces to around 1 or 2 Amps, sometimes pulsed. That is when it is charging on the Conventional mode, which is most of the time. It sometimes switches to either a Regeneration or Refresh mode. These give the battery a different current/voltage charge compared to conventional and must be there to preserve the battery life.

Modern cars also have a much higher Parasitic Current Draw than older cars so they do need either to be run or charged more often. Your one may have an excess parasitic draw if it does not hold its charge once the battery is fully charged, that is assuming that your battery is ok. 



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3 hours ago, Tizer said:

The problem now is..............

Many thanks for your explanation - it makes a bit more sense now... 

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