12 replies to this topic

[JustSteve]

I need to buy a new alternator but not sure if this will do... http://www.minispare...px|Back to shop


Got to power lights, wipers, stereo (only 2 6x9's) and a cigarette lighter (phone charger)


Should I get http://www.minispare...x|Back to shop?

[MRA]

45 Amperes is plenty for your application, alternators and batteries should be "matched" to avoid overcharging your battery and damaging it.

[JustSteve]

45 Amperes is plenty for your application, alternators and batteries should be "matched" to avoid overcharging your battery and damaging it.

cheers MRA, just wanted to check

[MRA]

That's ok

[ajtom94]

i run a battery that should be in a pug 206 2litre diesal and the 70 amp alternator - never had an issue with both

[MRA]

45 Amperes is plenty for your application, alternators and batteries should be "matched" to avoid overcharging your battery and damaging it.

cheers MRA, just wanted to check

I made a mistake with what I said above, KernowCooper has kindly offered to add an ammendment to this topic to correct it.

[KernowCooper]

45 Amperes is plenty for your application, alternators and batteries should be "matched" to avoid overcharging your battery and damaging it.

Not strictly true The regulator in the alternator would cut the amps on a upgraded alternator back back long before that happened the alternator supplies only the amps to run whats the load thats required and to keep the battery voltage at the correct level, if the battery is charged then the only time the battery is supplying any amps is when the load exceeds the amperage of the alternator.

You can confirm this by putting a fluke clamp meter on a charging system where the battery voltage is around 12.45v (50%) and upon starting a 60amp alternator will not put 60amps into the battery, the fluke will indicate approx 30amps and very quiclkly drop back as there is no load on the electrical system, switch the lights on and the regulator will sence the load and increase the amps.
All you have with a upgraded alternetor is reserve capacity to deal with a increased load on the electrical system.

I get asked will a upgraded alternator charge my battery faster.

And the answer to that is not a straight forward Yes Or No, It depends on the load at the time the battery is being charged.

Example

The early mini had a lower output alternator then the later mk4s 45amp the 16ACR gave a max output of 34amps

So if the load on the electrical circuit of all the electrics turned on was say 30amps (unlikely without extra spots) you have in reserve from the alternator 4amps so the flat 45 amp/hr battery under these conditions would take from flat 11.25hrs to charge
With the same load but a 45amp alternator you have 14amps in reserve from the alternator so the 45amp/hr battery would take 3.21hours to charge. But the times given could not be achieved in real life as the alternators would always cut the amps back as the voltage of the battery rose, so its a linear curve of charge.

So the summary is that a alternator will only put out as much amperage as the load requires, it will maintain the battery between 13.7-14.2v during the charge cycle and when a flat battery draws current from the alternator the alternators regulator will sence the batteries increase in voltage and cut back the amps.

And I didnt copy and paste it from any yellow book or the web its from 30years experience of auto electrics before a gentleman asks

Edited by KernowCooper, 18 March 2013 - 12:47 AM.

[Spud_133]

And all at twenty to one on a Sunday night. You my friend, are keen!

[JustSteve]

thanks guys, and thanks ever so much KernowCooper for putting so much effort into that! At what point is an uprated alternator unnecessary?

[KernowCooper]

When the current alternator has the cabability of running all the load of the electrical system and no discharge is taken from the battery, if that makes sense, read the post I'm hoping will be pinned here http://www.theminifo...vehicle-earths/ will explain it indepth.

Edited by KernowCooper, 18 March 2013 - 11:53 PM.

[Cooperman]

You can do an 'electrical load calculation'. Add together the wattage of each electrical item on the car. So, for example, the headlights will be 55 watts each, so that's 110 watts to begin with. Then add sidelights, instrument lights, heater fan, radio/audio system, rear lights, number plate lights, brake lights, heated rear screen plus anything else. Divide the total by 12 and that is the total amps which could be drained with everything on at the same time. Add 15% to 20% as a reserve and that is the minimum amperage of the alternator you need. Well, that's how I do it, but Dave (KernowCooper) is the main man with electrical stuff.

[KernowCooper]

Thats the way Cooperman and usually it comes out around 30amps max with everything on, depending on the cars specification

[MRA]

Whilst doing some work for a large automotive wiring harness company we used to have to allow a factor for every bundle, as wires in bundles, trunking etc will heat up and increase the resistance. overheating in a harness is very bad, as the copper has a tendency to "cut" through the softened insulation and then short out against another wire, a similar issue was with the Metro main connector to the steering column. they had one large wire going to this connector which then split into 2 wires went through the connector and then on the other side joined back up to one large wire, what happened was one of these wires would get sufficiently hot to then melt and short out :-(