19 replies to this topic

[fenghuang]

Are there any reasons to not upgrade to an alternator with a higher output, and is there a point where there's limited value in having more amps?

[Ethel]

I suppose there's technically a limit in what current the wires from the alternator to the solenoid can carry. The downside is, as you get nothing for free, a higher output will take more engine power to make it. Any alternator is only going to produce sufficient current to keep the battery voltage from dropping due the amount of load you put on the system.

[KernowCooper]

Standard car no advantage as soon as the regulator sees the battery voltage rising then it will regulate the alternator output down exactly the same as a standard alternator, if you drive by night and have a brace of spotlights and the heater on and the HRW and the wipers on then the higher power alternator will supply the current.

 

There is the belief that a larger alternator will charge the battery faster, there is some truth in it, but under testing we did on the test bed the larger alternator only put in a few more amps for a very short period, it would not put in its maxium output amps of say 70amps, till the battery was fully charged, the regulator kicked in quickly and regulated it back.

[fenghuang]

So is a 45A alternator in a pretty standard 998 about right?

Edited by fenghuang, 28 September 2015 - 01:44 PM.

[Swift_General]

If electrical system is standard then that will be plenty.

[gazza82]

45A is probably a lot more than an old worn dynamo will produce anyway

[Cooperman]

Do it correctly and prepare a power loading chart.

List every item which could possibly be switched on at any one time and note the wattage of the items.

Add them all together and divide by 12. That is the amperage needed to drive all the items at the same time. Add 20% as a reserve factor and fit the next alternator size above that.

It's called 'electrical engineering' rather than 'guessing'.

[KernowCooper]

The circuits and amps used are in my recient post in the FAQs Electrical, I have done the work for you.

[cal844]

The circuits and amps used are in my recient post in the FAQs Electrical, I have done the work for you.

as always!! 

 

i love this forum, especially for the technical and electrical gods!!

[fenghuang]

... It's called 'electrical engineering' rather than 'guessing'.

The thing that getting a degree in electronics taught me, albeit 2 decades ago, was to always defer to those with practical experience before trusting your own theory.

Edited by fenghuang, 28 September 2015 - 08:48 PM.

[fenghuang]

The circuits and amps used are in my recient post in the FAQs Electrical, I have done the work for you.

 

Extremely useful. Thanks for putting that together.

 

I think winter driving at night could easily see a load that exceded 45A.
Add in a 150W audio amp (which I'm sure many would consider not excessive) and a 75A alternator starts to seem justified.

 

Suppose it's a cold, wet night, and it takes 4 attempts to start the engine, maybe 2 seconds each. (I'm guestimating here, and assuming the car is in a state that many Minis are. ie It's in need of a little TLC.) I think that's a draw by the starter motor equivalent to about 0.3Ah, compared to a typical 075 battery with a capacity of 60Ah. (Have I got my maths right?) Pleanty of juice in the battery for that!

How does replenishing that charge fit into the load on the alternator, on a cold, wet night?

 

How much difference to mpg does a 75A alternator make compared to 45A? (Presumably the mpg is porportionately effected by how much current the alternator is supplying, so mpg will be better on a warm sunny day.)

[KernowCooper]

The cranking amps is approx 150amps, once started the regulator will allow a charge of the battery but within a few mins the regulator will cut back on the amount of amps to prevent the battery getting hot, this changes though if the regulator see a heavy current draw, i.e headlights +spotlights and normal running amperage, the regulator will then up the amperage to run the additional current requirements and keep the battery above the discharging state. At no time is the battery providing amps to power the circuit once the engine revs are above 2500rpm, if it is  then you have more current draw then the alternator can provide, obviously this changes when you pull up at traffis lights for a short while, as amperage output from a alternat is proportonate to alternator revs.

 

So for example if you have a 45a alternator and you current draw is 55amps then the battery will start putting the 10amps back into the circuit and a 30a/h battery will be flat in 3hrs, but in reality it wont work like that because the voltage at the battery will drop off to the point where the lights get that dull you'll be forced to pull over long before then unless you bring the amps drawn back within the alternators maximum output.

 

The more load you put on the alternator the harder it has to work so yes it will require more power to drive it, but as a owner of a Classic Car, then I would not be worrying how much it effects the fuel consumption, as other elements of driving are effecting it more so.

 

If you do upgrade to a 75a alternator then what I do on the conversion is change the alternator output cable between alternator and solenoid, as the standard Brown cabling is not up to the extra amperage

Edited by KernowCooper, 29 September 2015 - 09:50 AM.

[fenghuang]

Thanks for the info.

[Brams96]

Don't spend your money on a new alternator, just swap the 45A regulator for a 75A version, the casing & wiring for both is the same. They can be had on ebay for around £20. If you're worried about your amp then you should do 'the big 3', there are plenty of topic on this. Bigger cabling will provide more current carrying capability. As for MPG, this will only be affected by the load on the engine, so more load = less mpg.

[fenghuang]

No amp fitted ... yet ... so not an immediate condern, but an ebay swap is certainly worth considering. Thanks.