12v petrol genny

[Neil Smith]

Hi all,

 

I am building a 12v genny out of a surplus Honda gx160 5.5 hp engine from an old mains genny so its got variable speed on demand according to load, and a new 120 amp alternator made by iskra that was bought for a machine in error, I have got a couple of questions please, the alternator has a 2.5" pulley and I have got 3 pulleys for engine a 3.5" a 5" and an 8". the engine max is 3000rpm I think, but I don't want to run it flat out so what one would be best.

Also can I just connect plus terminal to battery + and casing to battery - and that's it or do I have to connect the warning light terminal D+ to somewhere, there is also a W terminal but I think that's for tacho, will it self energise.

 

Neil

[bottle]

The alternator will need to run at 3,000 rpm. minimum and probably at 10,000+ rpm for maximum output, check on-line for the actual specification.

[Nickhlx]

I would have said that the alternator would need to run at a minimum of 6000 rpm at max engine revs to achieve the majority of its rated output, and also to get adequate cooling at high loads, but Bottle is the expert...

 

Nick

[Neil Smith]

I just spoke to lucas technical and they said it starts charging at 1600rpm as it is from a new Holland tractor, but he did not know maximum rpm, but he did say with 5.5 hp you will never be able to gear it up enough to over rev it, and that my 5" pulley might be a good start point.

 

Neil

[bottle]

I would have said that the alternator would need to run at a minimum of 6000 rpm at max engine revs to achieve the majority of its rated output, and also to get adequate cooling at high loads, but Bottle is the expert...

 

Nick

 

That is definitely a No, No

 

Have done a little searching and found this PDF for Iskra. http://www.iskra-agv.cz/us/pdf/alternators.pdf

 

Nick

 

You are correct that to get the majority of an alternators rated output it needs to run at 6,000 rpm

 

Now this is a sweeping statement and only applies to 'modern' engines on narrow boats (does that cover all the get outs) we run our engines, when cruising at somewhere between 1200 and 1600 rpm.

 

So assuming a ratio of three to one on the pulleys, the alternators are run at 3600 and 4800 rpm, not ideal but it does charge the batteries.

[Neil Smith]

So with the 2.5" alt pulley and the 5" engine pulley at 3000rpm that would give me 6000rpm. That sort of falls in with what I thought as the 8" would probably stall the engine under load, How about the electrical questions any ideas.

 

Neil

[Nickhlx]

If the " No, No " was directed at me, you have just referred to what I said as being the requirement !

 

I realise that most canal boat (modern) engines run at typically less than 2000 rpm, and that most alternators can be run at at least 10,000 rpm, but most power is being generated by 6000 rpm, which is what I was suggesting the gearing be set to when the engine is at its max rpm. Now it will be preferred to run it maybe a bit slower, and the bonus will be a quieter operation, but to get useful power out of the alternator it will need to be running at around 4000 rpm or more.

 

I remain convinced that the gearing will have to be between 2:1 and 3:1 for this case...

 

( or are we actually saying the same thing ?)

 

Nick

[by'eck]

I experimented with my Leece Neville 160 amp alternator aided by a 90 amp one when the boat was first commisioned. With a 3kW kettle load on the inverter (exceeding the max rated output of both alternators) I wound up engine revs until I got no further reduction in current drain from the batteries. At that point the 160 amp alternators output was measured using a clamp-on at 156 amps - good enough for me. Alternator revs were at that point were 3600 rpm taking account of pulley gearing.

 

In fact alternator output current tails off on a graph against rpm to an almost flat line as losses come into play, with the curve much steeper at the low end of their ouput range.

[Neil Smith]

Yes Nick if I use the 5" pulley it will be 2 to 1 so at 3000rpm on engine will give me 6000rpm on alternator so all good there. Now I need to sort out the wiring, any ideas.

 

Neil

[Nickhlx]

It was Bottle's " No, No" I was wondering about, but praps he was referring to my comment about him being "the expert"

 

 

Nick

[Neil Smith]

Yes I thought he was talking about him not being an expert, he is being humble as he has given very good advice in the past, so hoping he can steer me in the right direction re wiring my contraption up.

 

Neil

[bottle]

Hi

 

I was referring to the 'expert' bit.

 

I am definitely not the expert.

 

As for the connections yes positive to battery positive and the alternator frame to negative unless it has a dedicated negative terminal (some do). Both large

 

I do not know if this particular alternator self excites but wiring as per one in a boat or car then the D+ will need a switched supply (ign) with a bulb in the circuit from the battery positive terminal.

[Neil Smith]

So I could try the plus and neg first and if it does not start charging I will need to connect the D+ to a bulb via a switch from the + battery terminal.

 

Neil

[Alan Saunders]

120A x 14.8V = 1,776W = 2.38hp. Even at 50% efficiency a 4.8hp engine would be adequate but running the engine at 4.8/5.5 = 87% of its maximum seems sensible. OTOH, when charging lead/acid batteries, depending on the size (Ah) of the battery bank and state of charge you may never achieve 120A. The practical engineers solution is to try the highest ratio 8/2.5 = 3.2. i.e. if the engine idles at 800rpm the alternator will be well into its power curve at 2,560rpm; ith the engine at 3,000 rpm the alternator will be at 9,600 rpm, probably less than its rated maximum. If the engine cannot achieve this in your application then the above recommendations of 5/2.5 = 2:1 may be best.

 

You say the engine has 'variable speed on demand'. A far as I am aware this is only implemented on the 'inverter' generators (EU10i etc.) and will require more complex electronics than simply connecting plus and minus and a switched energising circuit.

 

Best wishes, please report back on your progress. It is exactly what I want; a quiet, independant petrol/LPG battery charger without the expense or the losses incurred by a 230V generator driving a battery charger.

 

Alan

[bottle]

So I could try the plus and neg first and if it does not start charging I will need to connect the D+ to a bulb via a switch from the + battery terminal.

 

Neil

 

Yes.

[Neil Smith]

Hi Alan,

 

Good advice about power needed but does that work in reality as I googled making a 12 v petrol Genny and one you tube clip said he used a 100 amp alternator and when he geared it up it laboured the engine and I think stalled sometimes, on the recoil I noticed 6 so might be HP.

I think you have to alow 1 or 1.5 HP extra for losses, but even then it looks like your figures plus the losses will still not struggle with my 5.5 HP.

 

Neil

[Neil Smith]

Decided, Going to fit the 8" pulley first and see what happens under load then if it struggles I can fit the 5" one or I have a 3 1/4" alternator pulley and that would give me just over 2.46 ratio so might be a good bet also.

[onionbargee]

You said you have "variable speed on demand " with a GX160, if you mean an eco throttle as per a Honda EU , you don't.

[rogeriko]

Dont forget to use thick cable and keep the alternator as close as you can to the batteries. If you measure the voltage at the generator it will be higher than the voltage at the batteries, that is the cable losses that you will have to deal with to get perfect charging.

[Neil Smith]

What cable section should I go for, I was going to have them about 3-4 metres long.

 

Neil.

[Ryeland]

Don't forget to connect to the batteries just before you start the engine, and disconnect just after. If the alternator becomes disconnected while it it running it could be damaged, so make sure the connections are secure. I have a similar setup, and I have to start the generator with the boat engine running, then let it warm up before it will take over the load when the boat engine is stopped.

 

Richard

[Neil Smith]

Don't forget to connect to the batteries just before you start the engine, and disconnect just after. If the alternator becomes disconnected while it it running it could be damaged, so make sure the connections are secure. I have a similar setup, and I have to start the generator with the boat engine running, then let it warm up before it will take over the load when the boat engine is stopped.

Richard

That is why I want to get the pulley sizing right so it will charge almost flat batteries on its own, what engine/alternator setup is yours and pulley sizes, it might be worth fitting a larger one on the alternator or a smaller one on the engine.

 

Neil.

[Neil Smith]

Right another question, should I put in an amp meter or a volt meter or both, and do they come off the D+ after the switch but before the lamp.

 

Neil

[bottle]

Volt meter goes to positive and negative on the battery.

 

You will know the voltage at the battery and can be used to 'guesstimate' the SOC of the battery after they have 'rested' (not being charged or discharged for a couple of hours)

 

The voltmeter could be connected the positive alternator terminal and the alternator neutral (casing), this would tell you the voltage at the alternator

 

An ammeter can be in the positive supply to the battery but has to be able to cope with the maximum ampere charge or discharge.

 

or

 

The ammeter can be of the shunt type, the shunt goes in the positive supply and (small) cables/wires are taken from the each end of the shunt to the meter.

 

The full amperage goes through the shunt but not the meter in this case

Edited February 15, 2015 by bottle

[Detling]

The ammeter can be of the shunt type, the shunt goes in the positive supply and (small) cables/wires are taken from the each end of the shunt to the meter.

 

The full amperage goes through the shunt but not the meter in this case

If using an electronic ammeter of the type frequently seen, most if not all will need a separate power supply for the meter when the shunt is in the positive lead. This is because they measure the voltage at the shunt with reference to their 0 volt supply and 12+ volts will fry them.