I'm thinking about getting hold of a small stationary diesel engine
like a little Lister or Petter cement mixer engine, to drive a 12v car
alternator, this will be a low cost battery charger for a boat .......
now , how can i match the right size engine to the right amps
alternator ? What will run what in practice ? Has anyone tried this? I
might close couple the alt to avoid belt drive power losses.
Modern alternators develop peak output at about 15,000 rpm and don't charge
much below about 3000 rpm. Unless you have a particularly high speed engine,
you can't close couple them.
The calculated output wattage has to be divided by the efficiency to get the
input power. When I did this exercise using a 2 1/2 HP engine, the thing
stalled dead as soon as I tried charging a battery. Assume an efficiency of
about 0.25 for a start. There are various ways of reducing the output of an
alternator but the easiest is to run it slower.
Don't forget that alternators don't self excite. That means that they will
not charge a completely flat battery as they need to draw a small current to
provide initial field excitation.
If you get a modern alternator, you will also have to decide where to
connect the sense terminal. Again the easiest is to connect it to the output
at the alternator so converting it to machine sensing. It's not as good on a
car but will do if you're charging a battery off load.
Finally, a lot of alternators don't appreciate running open circuit, i.e.
without being connected to a battery. It is a good idea to leave a small gel
cell across the output to protect the electronics. A large capacitor may
Sure, i'd have to put 3:1 gears on it, a car alternator cuts in at
over 3000rpm, i just have a thing about v-belts at the moment after
getting my overalls caught in a 30hp deisel,and loosing my trouser leg
! I'll want to drive a standard car alt 50/60 amps, because these are
10 quid from the breakers, or no quid if you are quick on a dumped
On 2 Feb 2005 09:11:16 -0800, firstname.lastname@example.org (Rich.) wrote:
I built a little set 20 years ago with an 11AC alternator on a Petter
AA1, even with a faily high rpm engine like that you still need a belt
drive to get proper charging speed for a vehicle alternator. It was
noisy. I'm sure it would have run a bigger alternator than that
without bother. If you want to close couple it, maybe an old
commercial or marine dynamo would be worth looking out for.
Traditional & Modern canal craft repairs
Vintage diesel engine service
The Petter A series (AA1, AC1. AA1 is smaller in power output and cylinder
bore to the AC. The AC1 has had some modifications over time. Earlier
engines have the injector in the side of the head, hidden inside the air
ducting. Later ones have the injector in the top of the head and the ducting
is cut out around it. The AC1 is current production by Lister Petter!) are
small engines, mainly made of aluminium, and are more portable than the
Lister singles (LD1, SL1, LR1, SR1, ST1. All the Listers look identical on
the outside (Except for the rocker covers on the ST1!). The LD1 is the
smallest in power, the ST is the larger. The cylinder bore and RPM increase
down the range.). These Listers are quite(!) heavy, and much taller than the
Petters. All will drive an alternator, but watch out for the RPM on the
fixed speed engines. These are not all the same, and some mixers run at
quite low RPM, which means putting a larger pulley on the engine, leading to
a longer belt being required.
We have a Lister LR1 (Ex Mixer) which is about 3 HP at 1000 RPM (If I
remember right!). It has a pulley directly attatched to the flywheel of
about 12" or so diameter. It drives a standard Lucas alternator OK. It is
not too noisy, just the same as a Mixer in fact, a bit clanky.
The charge light circuit is powered from the positive feed to/from the
battery being charged, the light and a swich being mounted on the engine. We
used welding cable to take the charge to the boat battery to minimise
We have now come up with a new system. The alternator charges a battery
mounted on/by the engine, this battery powers an invertor (600 Watt in this
case). This feeds into a mains extension cable to the boat, where it runs
the 240V circuit, and a battery charger to charge the boats battery.
I hope the above helps....
If you use the battery sense configuration, voltage drop is compensated for
by the alternator. Basically there are 3 terminals on a Lucas alternator.
I'm assuming one of the ACR series since that's what I have messed with. The
battery (+) and indicator terminals are obvious as is the case to earth (-).
The S terminal is a sense connection. The electronics try to maintain this
at 14.2 volts, i.e. charging voltage. If you connect this terminal directly
to the battery, you eliminate the voltage drop in the (+) lead. With a bit
of fiddling, you should be able to make the (-) side remote sensing as well.
Our Alternators are all "used".
One is a 90 Amp from a Volvo 940.
One is unknown, probably about the same.
We did use a Lucas one, probably around 35-60 Amp.
Never had an alternator with more than two terminals...+ feed and field
I do however understand the reference to "Battery sensing". Some of them
"Charge Management" thingys are meant to convert the Alternator to Battery
I used to have an AB1 on the boat diving an alternator just for
Sold it to Kim and replaced it with a 5kva 230v genny and battery
I had to get the engine started and warm before I connected the field
wire on the alternator and even then it struggled for a few
I was going to say that too .......
Still works just fine, but you are right, it does struggle to start with & I
wonder if that isn't the governor struggling to deal with a sudden load, the
compression failing slowly or the design of the engine being more designed
for a steady-applied load.
Noisy too. It's the only engine my neighbours complain about!
"Spring in the air?"
"Spring in the air yerself !"
Sorry, Rich, did you mean me? Petter AA - or AB - output? Not sure, really,
I just fire it up and connect it to the battery! Should I see if I can find
a tag on it? - though Julian will probably know ........
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