Club 80-90 Forums

Anyone got any idea how long I would need to run the engine, to replace the juice I drained using an 18v (65W) laptop for 2 hours ?

More curiosity than anything else, but has a certain relevance for me

Well...if its taking 65 watts @ 18 volts that's 3.6 amps. (W=V x A) For 2 hours that's 7.2 Amp hours.
But @ 12 volts it's 5.4 amps , so it's 10.8 amp hours.
What's your alternator's output at a given engine speed? You'll need and ammeter and a volunteer to hold engine speed while you check the output, unless you just want a value at tickover.
Unfortunately that's where the alt's output is at it's lowest...

I hope this helps (hope i'm correct, too!)

Would be at tickover.

Altho willing to go for a quick cruise if required

So if you've got an 80 amp alternator giving full output that will take 80/10.8 hours = about 8 minutes.
Doesn't sound like enough but that's what the numbers say.
You'd have to go for a quick drive. Plus some extra to put back the cranking amps...

TBH I think that sounds about right in my head. Cheers

(Its a lesiure battery charging surplus to the main)

Assuming that the battery will except and take on that amount of charge in that amount of time.....theory certainly not born out in the real world. Batteries are designed to give out high current and discharge far quicker than they charge.
Going back to basics....charge rate is controlled by the intensity of the magnetism in the rotor windings. The rotor is an electro magnet spinning inside copper windings (the stator) the strength of the rotor magnet is controlled by the regulator which determines just how powerful the rotor magnet (alternator output) is by monitoring battery charge state.
IN OTHER WORDS.....the alternator DOES NOT SIMPLY kick out full charge as soon as the battery drops from full charge, it infinitely varies output depending on battery charge state.
The only time (in theory) that the alternator kicks out it's rated output would be when charging a from flat...
So 8 minutes at 80amps on a partially discharged battery just isn't going to happen.
And besides....charging at those rates for anything more than a few minutes will seriously heat the battery/warp plates and gas like german shot putter after eating 20 cans of beans.
Martin

I suppose the next question would be.....why then do modern vehicles have 100amp/hour plus alternators...simple answer: with a near flat battery and on the coldest, darkest and frosty mornin,g the alternator is directly powering (through the battery), the heated rear window, the blower motor, the lights, the heated mirrors, the heated washers, the radio, the ignition AS WELL AS trying to re-charge the battery.
Martin

You're absolutely right Red - I should have put "in principle", with hindsight!

I didn't want to overcharge Tatty's grey matter

Ok, say I have a 2 fully charged batteries, then I drain 18v @ 65w for 2 hours, how long with a std alternator, would I "theoretically" need to go for an "average" run (say 30mph @ 2500 Rpm)
Oh and its in the daylight, and its dry (so no lights or widscreen wipers etc)

A good 30 minutes. Trouble is, none of these calculations hold much weight as they don't take into account the capabilities of the battery/batteries but to my knowledge you need at least a 9 mile drive to get your battery up to a good state of charge again - longer if possible.

Red Westie wrote:I suppose the next question would be.....why then do modern vehicles have 100amp/hour plus alternators...simple answer: with a near flat battery and on the coldest, darkest and frosty mornin,g the alternator is directly powering (through the battery), the heated rear window, the blower motor, the lights, the heated mirrors, the heated washers, the radio, the ignition AS WELL AS trying to re-charge the battery.
Martin

Not to mention the heated *ss support on some models!

Alternator rating is very colored... a 45amp alternator will only put out 45amp at max alternator rpm (not same as engine rpm) for a short time because as alternator heats up output drops. At idle it's probably putting out about 5-10amp. That's why newer vehicles use 80-90-105amp alternators... so they can actually get 20-40amps out of them at normal rpm

Link to some alternator specs:
http://www.amplepower.com/products/alt/index.html" onclick="window.open(this.href);return false;