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Old 29th April 2019, 07:49   #21
bigd
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ok thanks for the info
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Old 29th April 2019, 08:15   #22
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Quote:
Originally Posted by marinabrian View Post
Assuming a good battery which is fully charged......

96Ah battery, that will still start the car at a 50% charge level, and the car electronics that correctly go to sleep leaving a quiescent current of 45mA.

This level of discharge will allow for about 1080 hours, or 45 days......six weeks give or take


Brian
I've had zero (battery) trouble since the original post, but I did get around to checking under the battery cover yesterday. Applying similar logic to my 55Ah battery would give around 26 days/3-4 weeks before trouble starts.

I think I know where I stand.
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Old 29th April 2019, 16:13   #23
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Quote:
Originally Posted by COLVERT View Post
You won't charge a flat battery on a 450 mile run.

Lets say you averaged 50 mph on that run. This would mean that the best you could hope for is 9 hours on charge.
( 450 divided by 50. )

The regulator in the alternator would limit the charge going into your battery at about 5 amps. ( about the same as your average charger. )

You need at LEAST 24 hours on a charger to get your battery anywhere near fully charged. So your 9 hours running would just not cut it.



9h x 5A = 45Ah, A 'flat' car (typical, not stop start or deep discharge type) battery is typically at 70-80% of full charge when the voltage drops below 10.5v - too low to start the car. 45Ah is over half the capacity of the battery so should be sufficient. kicker here is that the alternator needs enough voltage from the battery for the field coils in the first place, which it won't get from a flat battery - which is the real reason it won't charge on a 450mile run (you won't get 5A spare charging out of it + running the car and it's accessories).


24hrs on a 2A charge, or 12 on a 4/5 Amp charge will be enough. Failing that charge well enough off of the car (i.e. 3-4 hours at 4/5A) to give it enough to work properly and the long run should do the rest.
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Old 29th April 2019, 19:11   #24
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Quote:
Originally Posted by Avulon View Post
9h x 5A = 45Ah, A 'flat' car (typical, not stop start or deep discharge type) battery is typically at 70-80% of full charge when the voltage drops below 10.5v - too low to start the car. 45Ah is over half the capacity of the battery so should be sufficient. kicker here is that the alternator needs enough voltage from the battery for the field coils in the first place, which it won't get from a flat battery - which is the real reason it won't charge on a 450mile run (you won't get 5A spare charging out of it + running the car and it's accessories).


24hrs on a 2A charge, or 12 on a 4/5 Amp charge will be enough. Failing that charge well enough off of the car (i.e. 3-4 hours at 4/5A) to give it enough to work properly and the long run should do the rest.
The battery does not supply voltage to the alternator field coils !!!!

You've got your percentages and voltage figures wrong as well.

The maximum amperage from the alternator well exceeds anything the car accessories can call for. ( possibly twice that required. )

The maximum draw by the alternator ARMATURE is six amps to give an output from the field coils of 110 amps.---
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Old 29th April 2019, 21:13   #25
HarryM1BYT
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Quote:
Originally Posted by COLVERT View Post
The battery does not supply voltage to the alternator field coils !!!!

You've got your percentages and voltage figures wrong as well.

The maximum amperage from the alternator well exceeds anything the car accessories can call for. ( possibly twice that required. )

The maximum draw by the alternator ARMATURE is six amps to give an output from the field coils of 110 amps.---

All correct!


In addition, putting 45Ah of charge into a battery, will not equal 45Ah of charge available for discharge. A battery is not 100% efficient, nowhere near that.



On reflection, I was wrong about the 450 mile / 9 hour run being able to fully charge a flat battery. Ability to put current into a battery, is limited by the battery and the charging voltage available. Charging voltage by the alternator is between 14.1 and 14.4v, no more than the output of most modern chargers rated 3 to 6amps. An alternator will initially put around 20-25 amps of charge into a battery, following starting the engine, but that rate will quite rapidly decline as the battery voltage increases once on charge.
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Old 29th April 2019, 22:50   #26
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Certainly about a week with an ageing battery, but another odd issue. -

As the available voltage from the battery starts to drop, the side and tail lights come on, even though the switch is off. They go out again as the voltage drops further. Ideas?
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Old 30th April 2019, 07:04   #27
HarryM1BYT
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Originally Posted by p2roverman View Post
Certainly about a week with an ageing battery, but another odd issue. -

As the available voltage from the battery starts to drop, the side and tail lights come on, even though the switch is off. They go out again as the voltage drops further. Ideas?

Is that not the usual way the 75 indicates to you that the battery is almost flat? I have never understood why it seems to be designed to do this, it just seems so such a strange thing to flash the lights outside, when the driver is in the driving seat. My Ford used to flick its dash dial pointers, which was much more visible to the driver and sensible.
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Fix a poor handbrake; DIY ABS diagnostic unit; Loan of the spanner needed to change the CDT belts; free OBD diagnostics +MAF; Correct Bosch MAF cheap; DVB-T install in an ex-hi-line system; DD install with a HK amp; FBH servicing.

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Old 30th April 2019, 09:23   #28
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Quote:
Originally Posted by COLVERT View Post
The battery does not supply voltage to the alternator field coils !!!!

You've got your percentages and voltage figures wrong as well.

The maximum amperage from the alternator well exceeds anything the car accessories can call for. ( possibly twice that required. )

The maximum draw by the alternator ARMATURE is six amps to give an output from the field coils of 110 amps.---

Sorry, got the armature and field coils confused there for a moment. You're still not going to get 6A (at 12v) from a flat battery though. So, no, I don't think you are going to get enough charge (ever) from an alternator running with a flat battery to start with. Who ever said that batteries were 100% efficient? 45Ah to top up 20% of a battery rated at 96Ah is not claiming 100% efficiency!



disclaimer: calculations not guaranteed to be accurate as can't be rst to get calculators out and whatnot. your mileage may vary. make your bed and lie in it, what goes around comes around... blah blah blah....

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Need a T4 ?: T4 Owners Map thanks to Stevestrat ( use at your own risk)

Where?:
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Mods/Retrofits:

PCV vortex 'filter'; bluetooth; inline thermostat; reversing sensors; plenum spyhole ; headlamp washers ; Diy mp3 player replacing CD multichanger; FBH with remote; Headlamp washers; black/chrome front grille, rear blind; Xenon projectors
To do:
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Old 30th April 2019, 17:35   #29
HarryM1BYT
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Quote:
Originally Posted by Avulon View Post
Sorry, got the armature and field coils confused there for a moment.

No need to shout!
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Fix a poor handbrake; DIY ABS diagnostic unit; Loan of the spanner needed to change the CDT belts; free OBD diagnostics +MAF; Correct Bosch MAF cheap; DVB-T install in an ex-hi-line system; DD install with a HK amp; FBH servicing.

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Old 2nd May 2019, 21:06   #30
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I needed that shouting Harry cos my hearing aid has got a flat battery.--


Perhaps a quick charge from my alternator might provide a cure.-----
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