Electrical woes

[BRG75]

So, do you want to diagnose your car with an oscilloscope, or a poor man's AVO.

Do you want to set your ignition with a laptop, or a RIZLA paper.

I'd prefer the later in both cases, but a 75 is still worth the effort of modernising.

Sorry for jumping in, but have we really progressed; you breakdown and get conveyed back home, or open up your 5 draw cantilever tool box, from the boot.

[SD1too]

Originally Posted by SteveThackery

My point was that if the alternator is working OK ... then it should never be necessary to use a charger because the car will keep it topped up in normal use.

When the car is used for very short distances there is insufficient time for the alternator to replace the charge used to start the engine. The battery's level of charge therefore decreases and the effect is cumulative. Use of high current consuming accessories such as the heated rear window accelerates the process, as does the rather high quiescent current drawn 24 hours per day by the 75/ZT. Eventually the battery declines to turn the starter motor.

In this scenario the alternator is working correctly and the battery is not intrinsically faulty. It's a situation that has cropped up many times on the forum and the only way to resolve it is by external charging.

Originally Posted by BRG75

Sorry for jumping in, but have we really progressed ...

Kev has thanked us for our advice and says that he's taking his car to an auto electrician on Monday. We await further news.

Simon

[SteveThackery]

Originally Posted by SD1too

When the car is used for very short distances there is insufficient time for the alternator to replace the charge used to start the engine. The battery's level of charge therefore decreases and the effect is cumulative.
Simon

Obviously that is theoretically possible, but is it a realistic scenario? It must be decades since I've heard or experienced someone getting regular flat batteries because their journeys were too short. It has certainly never happened to me.

Be honest: have you ever come across it? And how long ago was that? Back in the days of dynamos?

Edit: sorry, I see you reckon it's happened "many times" on the forum. I remain sceptical; open-minded, but sceptical.

[MSS]

Originally Posted by SteveThackery

Obviously that is theoretically possible, but is it a realistic scenario? It must be decades since I've heard or experienced someone getting regular flat batteries because their journeys were too short. It has certainly never happened to me.

Be honest: have you ever come across it? And how long ago was that? Back in the days of dynamos?

Edit: sorry, I see you reckon it's happened "many times" on the forum. I remain sceptical; open-minded, but sceptical.

I don't normally post these days and am currently serving a self-imposed ban for matters relating to gardening, specifically manure and nuts, but felt the urge to chip in on this important topic.

The scenario that Simon describes is a very real one. A car battery will charge at a maximum rate equal to around 10% of its capacity i.e. no more than 7A to 8A for an 096 battery type as fitted to the 75 MG/CDT. This means that most cars will have to be driven for 12-15 hours in order to fully charge a battery from the depleted state allowing for the inefficiency of the charging process. Check any car's battery by applying a charger and you would be surprised at how much energy (voltage*charge current*time) is needed to bring the battery up to its full capacity.

The reason one does not hear of many people not being unable to start their car due lack of stored energy in the battery is that despite a very high instaneous current, starting a modern car actually consumes very little energy. Therefore cars keep going in terms of being able to start but the battery life is gradually reduced due remaining at less than 100% charge.

I posted here on this matter with actual number and quite a lot of detail couple of years ago.

[SteveThackery]

Originally Posted by MSS

The scenario that Simon describes is a very real one.

Thank you, and in which case I apologise sincerely to Simon.

[SteveThackery]

Originally Posted by SteveThackery

Thank you, and in which case I apologise sincerely to Simon.

Nevertheless, we are talking about a proposed scenario where the battery repeatedly gets too flat to start the car because its journeys are too short to charge it sufficiently. As you said, it takes very little energy to start the car, so these journeys would have to be very short.

Let's assume the starter draws 150 amps. My car takes about one second to start, but let's double that to two seconds to give ourselves headroom. So a cold start takes 150 x 2 = 300 amp-seconds.

You suggest a battery might charge at seven or eight amps, and so let's go for seven amps to be on the safe side. At seven amps, it takes 300 / 7 = 43 seconds to put back the energy consumed during the cold start. Obviously the charging process isn't 100% efficient, so let's assume 70% efficiency. That pushes the run time out to one minute.

One minute is a heck of a short journey. If you think my estimate of the starter current is too low, then double it to 300 amps. Now it'll take a two minute journey. Reduce the charge current to 5 amps if you wish - were now at 171 seconds. That's still less than three minutes, and we've incorporated a lot of "on the safe side" figures.

I hope you can see that my scepticism isn't just because I am a habitual disbeliever. It's based on sound reasoning.

Are you going to suggest that the Rover alternator is inadequately specified, such that there is zero charge current when the lights and HRW are on? Because that seems to be the only remaining scenario that supports your argument about cumulative discharging of the battery.

[torque2me]

Originally Posted by SD1too

When the car is used for very short distances there is insufficient time for the alternator to replace the charge used to start the engine. The battery's level of charge therefore decreases and the effect is cumulative. Use of high current consuming accessories such as the heated rear window accelerates the process, as does the rather high quiescent current drawn 24 hours per day by the 75/ZT. Eventually the battery declines to turn the starter motor.

In this scenario the alternator is working correctly and the battery is not intrinsically faulty. It's a situation that has cropped up many times on the forum and the only way to resolve it is by external charging.


Kev has thanked us for our advice and says that he's taking his car to an auto electrician on Monday. We await further news.

Simon

Booked in for Wednesday (13th Jan). They want it all day. They mentioned it could be an alternator problem (rectifier pack) besides a parasitic drain so I'll have to dig out my spare unit.

I'll post more when things clear!
Kev

[torque2me]

Originally Posted by BRG75

I'd prefer the later in both cases, but a 75 is still worth the effort of modernising.

What modernising options on a 75 do we have BRG75?

Kev

[torque2me]

Originally Posted by SteveThackery

One minute is a heck of a short journey. If you think my estimate of the starter current is too low, then double it to 300 amps. Now it'll take a two minute journey. Reduce the charge current to 5 amps if you wish - were now at 171 seconds. That's still less than three minutes, and we've incorporated a lot of "on the safe side" figures.

I hope you can see that my scepticism isn't just because I am a habitual disbeliever. It's based on sound reasoning.

Are you going to suggest that the Rover alternator is inadequately specified, such that there is zero charge current when the lights and HRW are on? Because that seems to be the only remaining scenario that supports your argument about cumulative discharging of the battery.

I'm basically with you on this Steve though it does depend on RPM as most alternators will not charge on tickover or lowish revs. So, for any journey at 10 miles or under the battery is unlikely to have received adequate charge for the time taken to do 10 miles.

I've been doing 30 to 40 mile journey's and always ensure revs are 3000 in the lower gears and kept there longer. As the auto-electrician said to me, one cannot rule out the alternator without testing it as it may continue to draw current even when the engine is switched off. Hope they can track down the issue.
Kev

[SteveThackery]

Originally Posted by torque2me

I'm basically with you on this Steve though it does depend on RPM as most alternators will not charge on tickover or lowish revs.
Kev

I think all we can say about that is "it depends". It depends on the electrical load, the idle rpm, the state of charge of the battery, and the specification of the alternator. So I don't think we can be definitive on this.

One of the reasons for moving to alternators from dynamos is that alternators would, indeed, charge even down to idle speeds. Obviously, though, the output from the alternator does reduce at low speeds, but how much depends on the circumstances.

My post above was just trying to demonstrate that it really takes very little running time indeed to replenish the charge used during starting. Clearly a cumulative flattening of the battery due to ultra-short journeys is possible in theory, but in practice? - it's never happened to me, nor to anyone I know. Has it happened to you? Honestly?

I remain of the view that very few owners have a usage pattern that consistently flattens the battery. If it happens to someone, I'd bet good money it being one of three possibilities:

1/ faulty alternator
2/ faulty battery
3/ something discharging the battery