Charging for work not required or quoted

System

rafhelp wrote: »

NO youre wrong.

IF the battery was completely dead then how would the remote central locking, radio and other electrics work!!

seriously ive asked several other mechs and car owners who all say the same thing.

The purpose of alternator is to charge the battery. No lights on dashboard when you start the car says electrical issue. (as other electrics work). Sorry a mechanic should know this.

OK I'll bite.

I am a time served qualified mechanic with City & Guilds 383 Pt3 Repair and Servicing of Road Vehicles.

I am also a bit conversant with electronics unlike most mechanics having been an electronics engineer qualified to BTEC HND which is one step below degree.

So now we've got that out of the way....

Remote central locking, the radio and mirrors only require a small amount of current to operate, typically no more than 1 amp, two at the most which is typically around 1/60th to 1/120th of the total ampere hour capacity of the battery.

A starter motor can pull anything from 60 amps to over 200 amps depending on the size of the engine. That is typically around 75% of the total ampere hour capacity of the battery.

You can therefore have a battery which has enough charge in it to power the ancilliaries but not enough to turn over the starter motor. Usually what will happen when you try to do this on an almost flat battery is the dashboard lights will go out.

You seem to want diagnosis for free. Sorry but nobody does that any more due to the number of people like you who would expect a mechanic to put in an hour fault finding because you don't know how to do it yourself then fix it yourself or go somewhere cheaper once you got the answer. So now they charge.

As for your comment about the level of service you're getting there's a saying "you get what you pay for". I don't know a single place anywhere within 30 miles of me who is as cheap as you are saying other than people who do it on their drive ways using axle stands and I'm in the north east.

debtdebt

When I had a Range Rover, the alternator was on the way out. The early symptom was a whining sound at certain revs. Something to do with the bushes or spindle going or something like that. Anyway, when the alternator finally went, the car wouldn't start even with a jump start. I had a mass of lights and warning lights illuminate on the dashboard and that was it. The alternator on a 3.6tdv8 is a !!!!! to get too as well.

OP, when the garage advised you that they'd change the alternator for £25 was that including the part as well?

Strider590

rafhelp wrote: »

Wrong. Some electrics like radio and mirrors did work so the battery can not have been totally shot.

Right so the battery has enough charge to run minor low current devices, but not enough to shell out 100 amps to turn the engine over?

So the battery is shot......

As for your dash not working, my Vauxhall did this when the battery was shot. The voltage drop caused by starting the car, was enough to mess up the dash.



As for the "professionalism", I suggest you watch this....
https://www.youtube.com/watch?v=wlQeavkp5ow

BeenThroughItAll

Tarambor wrote: »

I am also a bit conversant with electronics unlike most mechanics having been an electronics engineer qualified to BTEC HND which is one step below degree.

Remote central locking, the radio and mirrors only require a small amount of current to operate, typically no more than 1 amp, two at the most which is typically around 1/60th to 1/120th of the total ampere hour capacity of the battery.

A starter motor can pull anything from 60 amps to over 200 amps depending on the size of the engine. That is typically around 75% of the total ampere hour capacity of the battery.

You can't say '1A' is 1/120 of the capacity of a 120Ah battery. It isn't. 1A drawn for 1 hour would consume 1/120 of the (theoretical) capacity of the the battery.

An amp is a measure of current draw, an Ah a measure of capacity.

Your example is meaningless. A 1AH battery with appropriate terminals and cabling could happily theoretically provide 200A to a drain for 20s or so. How else would those tiny booster packs work?

I do not have a BTEC in anything related.

rafhelp

debtdebt wrote: »

When I had a Range Rover, the alternator was on the way out. The early symptom was a whining sound at certain revs. Something to do with the bushes or spindle going or something like that. Anyway, when the alternator finally went, the car wouldn't start even with a jump start. I had a mass of lights and warning lights illuminate on the dashboard and that was it. The alternator on a 3.6tdv8 is a !!!!! to get too as well.

OP, when the garage advised you that they'd change the alternator for £25 was that including the part as well?

no part would cost £60

rafhelp

All cars ARE different. When the battery on my car is flat I know from experience that NONE of the electrics will work end of.

I have read you can jump start a car without an alternator:
https://www.reference.com/vehicles/recharge-dead-car-battery-bad-alternator-47990004188ad9cb

System

BeenThroughItAll wrote: »

Your example is meaningless.

I was trying to think of a simple way to explain it at the end of a very long day at work.

BeenThroughItAll

Tarambor wrote: »

I was trying to think of a simple way to explain it at the end of a very long day at work.

Unfortunately, you've not explained anything, because the detail is nothing like as straightforward as your post makes it out.

Joe_Horner

BeenThroughItAll wrote: »

You can't say '1A' is 1/120 of the capacity of a 120Ah battery. It isn't. 1A drawn for 1 hour would consume 1/120 of the (theoretical) capacity of the the battery.

An amp is a measure of current draw, an Ah a measure of capacity.

Your example is meaningless. A 1AH battery with appropriate terminals and cabling could happily theoretically provide 200A to a drain for 20s or so. How else would those tiny booster packs work?

I do not have a BTEC in anything related.

He can say that if he's trying to avoid providing a full thesis on automotive electrics.

Charge and discharge rates for battery technologies are generally rated as percentages of their capacity in amp-hours. So, for example, with a traditional flooded lead acid, normal charging should be at around 10% of the capacity rating (so 5A for a 50Ah battery).

Importantly, the capacity also varies greatly with the discharge rate:

Capacities are quoted for the "20 hour rate" or 0.05C - the current at which it will go flat in 20 hours. So, for that 50Ah battery, the 50Ah only applies if the load is 2.5A (2.5 A x 20 hours = 50 Ah). That's roughly the load that the locks, radio etc might give.

As the discharge rate increases the effective capacity decreases - quite drastically at high loads. It varies from battery to battery but, typically, the capacity at 2C discharge (a 100A starter motor on a 50A battery) will be about 20% of the .05C rating that's quoted on the case.

So your 50Ah rated battery in good condition will run your 2.5A stereo for 20 hours (2.5 A x 20 hours = 50Ah) but only run your 100A starter motor for 6 minutes (100A x 0.1 hours = 5Ah).

In flat, or nearly flat, condition the situation for high loads gets rapidly worse, which is why a car that's cranking so slowly it won't start will still run the hazard lights for an hour or more while you wait for recovery,