Alternator Capacity

[EggsBenedict]

Hi,

Not sure what capacity the standard alternator is, but I was wondering if there is any reason why it has to be more than 50A. Looking at the load, I think it breaks down something like this:

Item	Watts	Number	TotalWatts	Volts	Amps
Lights	55	2	110	13.5	8.15
Dashboard Lights	2	6	12	13.5	0.89
Stereo	50	1	50	13.5	3.70
Wipers	30	1	30	13.5	2.22
Indicator	12	4	48	13.5	3.56
Brake Lights	21	2	42	13.5	3.11
Tail	5	2	10	13.5	0.74
ECU	10	1	10	13.5	0.74
Fuel Pump	30	1	30	13.5	2.22
Engine Fans	50	2	100	13.5	7.41
Heater Fan	30	1	30	13.5	2.22

Total Watts: 472 Total Amps: 34.96 Amps x 1.5: 52.44


I've added the 1.5 factor as it's what this guy has as his upper boundary (1.25 -> 1.5): http://www.racemettleltd.co.uk/alt.html.

I'm just thinking of possible ways to gain a bit of underbonnet space.

Cheers,

Eggs

Edited 10000 times to fix the table

Edited by EggsBenedict on Monday 29th July 18:07

[QBee]

Standard alternator on the Serpentine Chimaera is a 100 amp Land Rover part.

[EggsBenedict]

Good to know. And therefore would power a/c, heated seats, heated windows, auxiliary lighs etc. that I don't have on the Chim.

[QBee]

Have a look at the fuse chart in the Bible for those answers....

....but don't forget as well as running all electricals (assume a female passenger once in a while), the alternator has to charge the battery too.

The eclectic windows are a 30 amp fuse, and the cooling fans are 25 amp fuses EACH. 20 amp fuse for the ECU. Ecelctic mirrors are also a 20 amp fuse, and main beams are a 25 amp on some cars

[QBee]

Heated seats are a 25 amp fuse too.....motors and heaters are your main sources of power drain, and head lamps count as a heat source in that respect

[Bassfiend229hp]

QBee said:

Heated seats are a 25 amp fuse too.....motors and heaters are your main sources of power drain, and head lamps count as a heat source in that respect

+1 :yup:

[EggsBenedict]

Yep. Don't have heated seats, won't miss them in this car! Electric windows are an intermittent load, so I don't need to consider them as something the alternator would need to power constantly, likewise electric mirrors.

QBEE, don't know what your female passengers do to bump up electrical load, but I haven't noticed my wife or daughter contributing to it much...

It's a good hint to look through the fuses, though I think there's a fair amount of 'starting load' on most electrical items, which means that these have to be set higher than the 'running load' so as to avoid popping fuses as soon as you flip a switch. Certainly true of cooling fans - this is why there are circuits available to stagger the start up of cars with twin fans.

[chris1972]

What are you saving the space for and how much space do you intend to save by using a lower capacity alternator?

[EggsBenedict]

I want to think about where the alternator goes, because I'm working on a plan for an engine swap. I don't have an exact %age of space i want to save, more about where it's positioned that will facilitate a much greater space saving at the top of the engine. because it's a different engine I'm considering, I don't have to worry so much about belt routing.

[jamienshelly]

I know Sardonicus has fitted a smaller alternator and possibly Phazed, I think tat a 70 amp unit has been used.
I would think a 50 amp unit would be to small, as you have to think of every eventuality.
What would happen if you are stuck in traffic on a hot rainy night, you have lights, wipers, cooling fans, heater blower either cooling you down or keeping the screen clear, plus trying to keep the battery topped up. The space saving will be minimal and you will have to add or modify brackets etc I believe.

[phillpot]

jamienshelly said:

plus trying to keep the battery topped up.

But surely if the alternator is "doing its job" nothing is being drawn from the battery?


ETA.... Once the engine is running and power used to crank engine has been replaced.

Edited by phillpot on Wednesday 31st July 06:30

[jamienshelly]

phillpot said:

But surely if the alternator is "doing its job" nothing is being drawn from the battery?

Other than starting the car, providing power for the alarm the interior light, and the clock.
I know there is not a lot of draw on the clock but it adds up, and if the alternator is at its limit or near its limit for long periods it will fail sooner. plus the battery size used in our cars are usually a 70ah unit so if the battery is ever low on charge it will be drawing more than the alternator can supply.

[QBee]

You might well get away with it in summer with regular use. The headlights won't often be on, the heated seas ditto, the windows will be permanently be down and the battery won't get much discharged.

If you intend to use it only to go to PH meets once a month in winter, after dark, cold and wet, pub four miles away, your battery will be dead before March.

[phillpot]

jamienshelly said:

the battery size used in our cars are usually a 70ah unit so if the battery is ever low on charge it will be drawing more than the alternator can supply.

Nonense imho !


Yes, battery can supply up to 70ah (for starting) but will only take what an alternator (or any type of charger) can give it. It will obviously take longer to re-charge with a smaller output alternator.



Bring back the Dynamo

Edited by phillpot on Wednesday 31st July 06:33

[jamienshelly]

phillpot said:

Nonense imho !


Yes, battery can supply up to 70ah (for starting) but will only take what an alternator (or any type of charger) can give it. It will obviously take longer to re-charge with a smaller output alternator.



Bring back the Dynamo
The 70ah is the minimum and it will probably be around 500 to 550 cold cranking amps.
if the battery is flat it will draw far more amps than if charged. When you hear a belt squealing due to a flat battery it is the alternator stalling that causes the slip and that is usually with an alternator that is factory fitted.
A 70 amp alternator will more than likely do the job and I think that is what others have fitted but for the size and weight saving a 50 will more than likely be a false economy.

Edited by phillpot on Wednesday 31st July 06:33