Page 1 of 1

Alternator Efficiency

PostPosted: Tue Nov 08, 2011 5:33 am
by CIrons
I have read the chapter on Alternators in 'Living on 12 Volts'. I found it most useful, especially the section on alternator sizing as I am looking at fitting one to the Volvo Penta 2030 (27HP max) diesel engine in my 33ft Alden designed timber cutter.

The question of alternator efficiency is mentioned on page 37 in connection with sizing, but only in the doubling of the horse power figure which also allows for 'engine ageing'. Coulomb's recent reply to someone wanting to build a generator powered by a 2.5 hp motor suggested that the efficiency of small frame Ample alternators is about 30% and the large frame ones 40%.

One of Ample's competitors talks about sizing HO alternators on the basis of one HP per 25 Amps. They do not say whether the Amps are the rated ones or the ones that get as far as the battery terminal, i.e. allowing for the 5%-10% loss in the wiring as well as any drop due to the alternator's 'efficiency'. In addition, I assume the efficiency of the battery also comes into play. So how does their sizing figure stand up to Ample Power's specs.

Tests with a Vetus 25Amp rated 3 step mains battery charger show that the actual current at the battery terminal amounts to an average 18.8 amps each hour during a full charge from 11.0 Volts 'full' on a good 150Ahr AGM battery of which 15.5 amps are actually stored there. This means that battery efficiency is about 82% overall from start to finish. When the battery is shown to be between 50% and 72% 'full', however, the current remains above 20 amps per hour at the battery terminal. About 40 amps are stored during this two hour period when battery storage efficiency appears to be about 100%, an improvement over the average of about 82%, if my battery monitor is accurate (see attached chart showing manual recording of the cycle from the battery monitor display).

Assuming you can choose when to use the engine driven alternator in the charging cycle, the best time to do so appears to be during this period. The charger can increase the state of charge from 50% to 72% when storage efficiency (above 50% full) is at its highest. I see this as the 'optimal charging period' but its parameters seem to vary from one battery to another, depending on their condition. I would complete the cycle with energy from contemporary solar sources because AGM's like to be topped up every cycle.

The question then arises as to how many amps arrive at the terminal from an Ample alternator, specifically the 4023, so that I can compare it with the measured performance of the mains charger. I note that at 14.6 Volts, the upper set point, my AGM battery accepts less than 15 amps from the charger, that is 219 watts, whereas we are sizing the alternator system on the amps produced cold at, say, the maximum engine speed of 3500rpm in this case, that is 14.6 x 121 amps which is 1767 watts, about 8 times more power than I can use. I can't help thinking I've missed something somewhere.

The spreadsheet attached is an attempt to figure out the sizing issues by using and extending the method described in 'Living on 12 Volts'. It suggests that about 20 amps rated output per horse power is the figure to use. The Volvo Penta has no more than 5HP available at the PTO with an effective maximum rpm of 3000. I would have to make do with one AP 4023 belt driven by the PTO to augment the 150 or so amps sometimes produced by the contemporary solar energy sources for charging the 450Ahr House bank. Not really enough, I think, even with the standard Volvo Penta alternator charging the start batteries. It looks like I may have to use the engine for about 4hrs per day when sailing which may not always be acceptable.

From a user's point of view, my figures show that DC power is very costly to produce with an alternator belt driven by a low powered engine when compared with contemporary solar derived power from wind/water generators and solar panels. According to these figures, the 4023 example produces only 25 hot Ahrs per hour stored in the battery at full alternator RPM. At that rate, we should be thinking in terms of only 20% overall charging efficiency from a HO alternator producing 5 useable amps per engine horse power. Even during the brief 'optimal charging period', the more efficient storage process in the battery increases the overall efficiency by only 5%, from 20% to nearly 25% overall charge efficiency. If these figures are correct, they make for a fairer means of comparison with non-fossil fuel powered alternatives than the 30% figure.

In view of the above figures, I am thinking about coupling an AP4059 directly to the PTO to maximise the HP available from the motor. The 4059 offers 25.5 rated amps per HP consistently across the range of speeds that my engine runs. At a charging speed of about 2500 RPM, the 'optimal charge time' allows 33 amps per hour to be stored which equates to a usable 6.8Amps per horsepower – an improvement of 1.6 stored amps per horsepower over the belt driven 4023 at nearly twice the purchase price, a 30% improvement in performance for a 40% increase in cost.

Have I got these numbers right? Constructive comments on my arithmetic and logic would be appreciated. I need input to select the better alternative.


PS Please state file types suitable for uploading. None of the usual ones are accepted so promised data is not attached.

Re: Alternator Efficiency

PostPosted: Thu Nov 10, 2011 12:20 pm
by Coulomb
Alternator efficiency depends on a few factors. A tin-can alternator
from the auto parts store may be in the 30-40% range. Ample alternators
are in the mid 40% for small frames and mid 50% for large units.

Power is the product of Volts times Amps, so one horsepower per 25 Amps
is meaningless with knowing the voltage. Fourteen volts at 25 Amps is
350 Watts, about 47% of one horsepower.

Engine torque also comes into play. Small gasoline engines don't have
the flywheel mass or torque to work through the power pulses that the
alternators produces -- they stall even though their rated power is

You don't loose Amps through the wiring. Volts yes, but the regulator
makes up for that. Power is lost, of course. Whatever current leaves
the alternator goes somewhere. If the batteries won't take it, then the
alternator won't produce it.

You logic is flawed in several places. If a large frame alternator will
fit, you'll get more Amps per fuel consumed than a small frame. It
doesn't take long to pay the higher alternator price.

If you can't make the large one fit -- you're in the same position most
boat owners are.

Re: Alternator Efficiency

PostPosted: Thu Nov 10, 2011 6:54 pm
by CIrons
'Power is the product of Volts times Amps, so one horsepower per 25 Amps
is meaningless with knowing the voltage. Fourteen volts at 25 Amps is
350 Watts, about 47% of one horsepower.'

Are the amps you're talking about above the rated ones or the delivered ones?

Re: Alternator Efficiency

PostPosted: Sat Nov 12, 2011 4:03 pm
by Coulomb
You don't have it yet.

An Amp produced is an Amp delivered.