General: car electronics

[Simmy]

the daddy of all volvo electrical faults is when the battery gets a low voltage situation even though the car will start and run without problem the low voltage triggers an srs hddd code and cannot be reset even with vida or a dealer software the only way to fix this is to remove the crash module and have a specialist download the info off the eprom chip remove the marker left by the low voltage and then re load the eprom chip with the original info without the code marker this involves striping out a lot of the car to acess the module . ask me how i know. so not all that reliable . the specialist had at least 20 volvo modules in for this service when i was there . so it must be a very common problem..

[Tannaton]

The problem is electronics on cars are very precise, reliable and specific. Car batteries are not, they start to deteriorate from day 1, the voltage can vary, as does their voltage drop under load, and the further dimension of it ageing.

Expensive cars now have lithium batteries which are much more predicatable.... and circa 20x the price. Car manufacturers do not do it deliberately, that is a conspiracy theory.

As someone else said, the systems on Volvo cars are the best around, there's redundancy on major controls (for example throttle position sensors with digital and analogue outputs). A warning lamp on the dash of a Volvo would likely have been a breakdown in anything Ford, Fiat or French....

[Rob76]

Thanks to all who replied.
I enjoyed the discussion. Having been in electronics for 60+ years fixing items from valve radios to S.A.M Defence systems my conclusion is car electronics could do with a major rethink.

[StanC]

A comment about stop-start systems. It would be good to know just how effective they are in reducing actual emissions. Every time the engine cuts out and needs to be restarted the energy required to restart it has to be put back into the battery by the alternator in the minutes immediately following the restart. This bit of extra work done by the engine powering the alternator uses additional fuel that would not otherwise be used.

Some university study done a while back concluded that the energy required to start a hot engine is roughly equal to the energy needed to simply keep it idling for about 20-25 seconds. So, if during a stop-start cycle the engine is stopped for less than 20 seconds (usually the case for my motoring) the whole process actually results in more energy - and therefore more fuel - being used than if the engine had just been left idling.

Stop-start is the biggest single drain on batteries. By simply entering/leaving a car park (stopped at the barrier for ~2 sec) or being in a slow-moving queue (repeatedly stopping for ~5-10 Sec), can result in the battery becoming depleted, AND the work done by the alternator/engine in attempting to put charge back into the battery results in very little, if any, fuel being saved. I put my XC40 B4 on a smart-charger (Ctek) a couple of times a week just to ensure the battery is maintained and the indications are that it needs it.

Stan.

[pinballdave]

Originally Posted by StanC

A comment about stop-start systems. It would be good to know just how effective they are in reducing actual emissions. Every time the engine cuts out and needs to be restarted the energy required to restart it has to be put back into the battery by the alternator in the minutes immediately following the restart. This bit of extra work done by the engine powering the alternator uses additional fuel that would not otherwise be used.

Most modern stop-start cars also have the alternator controlled by the ECU. So during idling and normal driving, the battery is charged up to around 80% and maintained at that level. During braking (both engine braking and friction braking) the alternator output is turned up to maximum to provide a 'regenerative braking' load and harvest the remaining 20% charge without using any additional fuel. So if you mostly coast to a halt rather than sudden heavy braking, you've got the extra energy needed to restart the engine for free, and the stop-start cycle will still save you fuel no matter how briefly it's stopped.

Some university study done a while back concluded that the energy required to start a hot engine is roughly equal to the energy needed to simply keep it idling for about 20-25 seconds.

This is often quoted by those who dislike stop/start, but by my reckoning the figures are out by at least one order of magnitude (maybe two!). It is most likely that the study was measuring the amount of idling needed to replenish the charge in the battery, but this is limited by the amount of power that can be extracted by the alternator.

The energy required to spin a hot engine will always be the same, whether it comes from the battery, or from the ignition of the fuel in the cylinder. So nominally the energy needed from the battery to start the car will be exactly the same as the energy extracted from the fuel used for the same number of revolutions. I admit that there are some significant efficiency issues with this statement, as there will be electrical losses when using the starter motor, but there are also similar thermal losses when running on fuel, so the two amounts are still comparable.

The big difference is that the alternator is likely to be rated at 120-180 Amps, whereas the starter motor draw will be several times that, so it will take significantly longer for the alternator to generate enough power to replenish the battery (and also power all the other electrical loads in the car at the same time).

I put my XC40 B4 on a smart-charger (Ctek) a couple of times a week just to ensure the battery is maintained and the indications are that it needs it.

If the battery is the correct stop-start type, then it won't really need it (but it won't do any harm), all you are doing is topping up the battery from the 80% level (that the car normally maintains it at) to 100% full. Unlike a standard lead acid battery, an AGM battery doesn't suffer much degradation from not being stored fully charged, and is easily able to cope with this type of use.

This is the reason you shouldn't use a standard lead acid battery in a stop start car, even if the stop-start function is disabled, as the car only maintains the battery at an 80% level, to allow some capacity for the alternator to perform 'regenerative' charging during braking.
A standard battery will suffer from sulphation unless it is fully charged regularly, and is likely to fail in months unless you give it a proper top up charge every few weeks.

[StanC]

I don't know about other cars, but my XC40 B4 R-Design does not do engine braking regeneration. It only regenerates upon using the main brakes, and this is to recharge the small 48v battery for the electric motor, not the main starter battery. So, the main (starter) battery in my car gets no support from regeneration, it is recharged only by the (petrol-driven) alternator.

You say that: 'The energy required to spin a hot engine will always be the same, whether it comes from the battery, or from the ignition of the fuel in the cylinder. So nominally the energy needed from the battery to start the car will be exactly the same as the energy extracted from the fuel used for the same number of revolutions'. This is where we differ. For the same number of revolutions, the starting of a 'stopped' engine takes far more energy than simply maintaining an idling engine. The crankshaft and flywheel of a stopped engine need to be accelerated up to and held at cranking speed until ignition. This creates a lot of angular momentum and is where the bulk of the energy goes in starting an engine - accelerating it from zero and holding it at cranking speed - and, according to the study, this equates to ~20-25 sec of simple idling, depending upon the size of the engine.

Incidentally, the battery specified for my car is EFB, but I guess that AGM would be the better bet.

Stan.