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Alternator needed for electric fan?

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John-B
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Alternator needed for electric fan?

Post by John-B »

July's DM has a comment that an alternator is required for an electric fan. My Dart has a dynamo and I have noticed that the charge rate goes up a lot when the fan goes on, but it doesn't seem to drain the battery. However, I haven't needed to idle for a long time in hot conditions.
Has anyone found that a dynamo with an electric fan is a severe drain on the battery?

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Re: Alternator needed for electric fan?

Post by Vortex O'Plinth »

An electric fan certainly adds a significant load to the electrical system but if the cooling system is adequate the duty cycle should be infrequent. The most arduous condition could be something like driving at night, in the rain and with the fan running, when the load could take the full output of the dynamo. This is probably an infrequent scenario.

I have an electric fan fitted and have retained the original (positive earth) dynamo. Originally the fan would run for at least 50% of the time, but even so there was never any problem with a discharged battery. I've since fitted an aluminium radiator and the fan now rarely comes on at all.
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Re: Alternator needed for electric fan?

Post by Kbeal »

It's only a problem if as suggested above in extreme situations, eg. idling for ages in hot conditions. I remember a few years ago queueing for hours to get in to Le Mans classic. I was running an alternator - all fine, but the chap in front of me, also in an SP with an electric fan but running a dynamo ended up with a flat battery.

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Chris_R
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Re: Alternator needed for electric fan?

Post by Chris_R »

If you think about it, it's down to supply and demand. The dynamo can supply a certain amount of amps, the fan demands a different amount of amps. If the fan demands more than the dynamo can supply the extra will come from the battery. If the dynamo can supply enough amps then it won't.
I don't know how much power the dynamo can deliver at different engine speeds so if you can get that information you should be able to work out if it can power your fan, and if not, what is the capacity of your battery to supply the power that the dynamo cannot. How many Amp/Hours is it? Using the reserve of electricity from the battery is not a problem unless you drain it below the point where it can supply enough for the car. If you have a fan drawing 10amps and a 70amp/hour battery you can run the fan for 7 hours before the battery is exhausted (in theory). Of course if you are at night with headlights on as well, each of those draws 6 amps so that's another 12 amps, you're now up to around 22amps and your battery life is now about 3 hours. I think wiper motors draw another 8 or 9 amps. If you're queuing for something like Le Mans classic for hours you are probably also turning the engine off and restarting it, using significant power every time you use the starter. It all adds up.
Alternators, but not all alternators are able to deliver 50 or 60 amps, and in some cases more, at engine idle speed. Hence the preference for fitting an alternator if you place a higher demand on the electrical system.

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Re: Alternator needed for electric fan?

Post by Brian-H »

When a battery is discharging due to load, you also have to take into account Peukert's Law , which basically says that the higher the discharge current, the lower the Ampere-hour available

https://batteryuniversity.com/learn/art ... ry_runtime

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Re: Alternator needed for electric fan?

Post by Ian Slade »

An alternator charges at idle, a dynamo requires higher rpm, so when stuck in traffic you will discharge the battery unless you set the rpm to about 2000rpm, the more electrical devices running the higher the rpm, on hot dark night and raining it may be impossible to balance the load, there again it is unlikely that you will find those positions, but not impossible, be prepared to load shed or shut the engine off, which every one did upto the seventies when alternators became more a standard fit. Driving today requires less thought, therefore thought is now redundant.
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Re: Alternator needed for electric fan?

Post by Sydsmith »

Like many SP250 owners I have an electric fan and a dynamo. I have had the car 10 years and it still has the same Bosch 95 amp hour battery fitted. I use the car winter and summer long and short runs, town and country, stop start and open road and never had a problem.

I don't know the current consumption of the fan, but a simple test to check if you need to change things is to check the amp meter reading at tick over, switch on the fan and look to see how far into the negative the meter moves, then raise the revs until the meter is equal, to check how many revs you need to supply the current needed to support the fan consumption, if the revs required are within the limits of your normal driving style you are fine.

That having been said, there are many types of fan, those that use more current tend to be more efficient and use more current for a shorter period than the smaller fans, 10 mins at 10 amps against 20 mins at 5 amps for instance, so its swings and roundabouts and of course the bigger fans will need more revs to equalise. Syd

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Re: Alternator needed for electric fan?

Post by Chris_R »

Ian Slade wrote: Tue Jul 07, 2020 2:25 pm Driving today requires less thought, therefore thought is now redundant.
I never thought of that!

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Re: Alternator needed for electric fan?

Post by Petelang »

Your Dynamo brushes and probably the armature will not last long if it's delivering it's maximum most of the time and it will consume lots of power to turn it compared with an alternator, plus as mentioned above, you get more output at lower engine speeds and alternator regulation tends to be better for the battery so I would opt to have an alternator any day.
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John-B
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Re: Alternator needed for electric fan?

Post by John-B »

Well, I'm not going to change to an alternator. My dynamo was reconditioned about five years ago and I have a 70 A battery and positive earth.

The DM article seemed to imply that it was essential to have an alternator and I wondered what people thought about it.

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