Electric Engines

[Hesp]

Thought I'd look at the power requirement for my own boat - 27' 9000lb (4000kg). Seems a bit low, can anyone see a mistake?

Power required to push boat at certain speed + prop losses + motor losses = power required from battery.

Force to push boat at 5 knts (2.5m/s) = 85 lb (40kg or 400N - from computer).

Sorry, I'll have to do this in metric.

Power = force x speed = 400 x 2.5 = 1000W or 1KW

Lets say prop efficiency = 66%

Power at shaft = 1/0.66 = 1.5 KW

Lets say 90% motor efficiency, which I believe is now attainable

Power from battery = 1.5/0.9 = 1.7 KW

So at 12v current I = Power/Volts = 1700/12 = 142 Amps!!

Can I travel at 5knots for an hour with 2 (allowing for 50% discharge) 12v 150 Ahr batteries? That sounds fine for my requirements.

John.

[htom]

I think you'd do better to have (say) four 75Ah batteries in series, and run the motor at 48VDC, to cut the I^2R losses (heat in the wiring between the batteries and motor). I suspect that's part of how Solomon etc are getting some of their efficiency.

[htom]

I think you'd do better to have (say) four 75Ah batteries in series, and run the motor at 48VDC, to cut the I^2R losses (heat in the wiring between the batteries and motor). I suspect that's part of how Solomon etc are getting some of their efficiency.

[htom]

I think you'd do better to have (say) four 75Ah batteries in series, and run the motor at 48VDC, to cut the I^2R losses (heat in the wiring between the batteries and motor). I suspect that's part of how Solomon etc are getting some of their efficiency.

[Hesp]

Yes, in fact motors of the power required seem to run at at least 72v
http://www.electriclaunch.com/drives.html
and the Solomon at 144v I think
http://solomontechnologies.com/installs/301.htm

John

[Hesp]

Yes, in fact motors of the power required seem to run at at least 72v
http://www.electriclaunch.com/drives.html
and the Solomon at 144v I think
http://solomontechnologies.com/installs/301.htm

John

[Hesp]

Yes, in fact motors of the power required seem to run at at least 72v
http://www.electriclaunch.com/drives.html
and the Solomon at 144v I think
http://solomontechnologies.com/installs/301.htm

John

[Bryan Mehus]

John,
How, or where did you obtain the "force required to push the boat at 5 knots"?


Slip of the propeller is directly proportional to the drag, i.e. zero drag zero slip, or 100% drag 100% slip. A lot of factors enter into the calculation of drag, including wetted surface area, hull shape, draught, windage etc. I don't know how to make this calculation, and would be interested in how you did it, as well as the "force required" calculation.
Thanks
Bryan

[Bryan Mehus]

John,
How, or where did you obtain the "force required to push the boat at 5 knots"?


Slip of the propeller is directly proportional to the drag, i.e. zero drag zero slip, or 100% drag 100% slip. A lot of factors enter into the calculation of drag, including wetted surface area, hull shape, draught, windage etc. I don't know how to make this calculation, and would be interested in how you did it, as well as the "force required" calculation.
Thanks
Bryan

[Bryan Mehus]

John,
How, or where did you obtain the "force required to push the boat at 5 knots"?


Slip of the propeller is directly proportional to the drag, i.e. zero drag zero slip, or 100% drag 100% slip. A lot of factors enter into the calculation of drag, including wetted surface area, hull shape, draught, windage etc. I don't know how to make this calculation, and would be interested in how you did it, as well as the "force required" calculation.
Thanks
Bryan

[Bob Adams]

The USN has figured out how to move big boats with electric motors,now if I could just figure out where to put the damn reactor....

[Bob Adams]

The USN has figured out how to move big boats with electric motors,now if I could just figure out where to put the damn reactor....

[Bob Adams]

The USN has figured out how to move big boats with electric motors,now if I could just figure out where to put the damn reactor....

[Frank Wentzel]

John

I am puzzled about the power level you believe is necessary to drive your 9,000 lb boat at 5 knots. I have a 28 foot Egret-type sharpie that dispaces about 3,500 lb. I use a 9.9 hp Honda in a well for auxiliary power. I have to run about 2/3 throttle to get 5 to 6 (estimated) knots. That sounds like at least twice the calculated power requirement for your 9,000 lb boat. (The question in my case is what percentage of rated horsepower does the Honda put out at 2/3 throttle.)

/// Frank ///

[Frank Wentzel]

John

I am puzzled about the power level you believe is necessary to drive your 9,000 lb boat at 5 knots. I have a 28 foot Egret-type sharpie that dispaces about 3,500 lb. I use a 9.9 hp Honda in a well for auxiliary power. I have to run about 2/3 throttle to get 5 to 6 (estimated) knots. That sounds like at least twice the calculated power requirement for your 9,000 lb boat. (The question in my case is what percentage of rated horsepower does the Honda put out at 2/3 throttle.)

/// Frank ///

[Frank Wentzel]

John

I am puzzled about the power level you believe is necessary to drive your 9,000 lb boat at 5 knots. I have a 28 foot Egret-type sharpie that dispaces about 3,500 lb. I use a 9.9 hp Honda in a well for auxiliary power. I have to run about 2/3 throttle to get 5 to 6 (estimated) knots. That sounds like at least twice the calculated power requirement for your 9,000 lb boat. (The question in my case is what percentage of rated horsepower does the Honda put out at 2/3 throttle.)

/// Frank ///

[Hesp]

Bryan, I got the force required from the hull design software I use - Prolines 98. I chose a reasonable speed to give a low force (see below) and a round number for ease of calculation. Other forces at given speeds are:

1 knt 4 lb 2 kg 20 N
3.25knt 31 lb 14kg 140
5.4 110 lb 50kg 500
7 410 lb 185kg 1850
7.6 579 lb 262kg 2620

You can see why I chose 5knts, 7.6 knts requires 10 KW (14BHP) output from the prop. You can also see that large engines are for those who want to go that extra half Knot. All these figures are approximate, & I've done much rounding up/down, but they should give a good indication.


I got the prop efficiency from Skene's Elements of Yacht Design, Propellor Chart B (P141 in my edition). I must admit, you have cast doubt in my mind regarding this figure. For starters in my haste I wrongly chose an efficiency from the high end of the scale - they run from 0.3 to 0.7. A more reasonable figure might be 50 percent efficiency. Higher efficiency might be obtained on a power boat with a large, slow turning prop.
The chart is entered with a figure obtained using RPM, BHP & Hull speed. I assume (but I could be wrong) that 'efficiency' as quoted in the chart, is power out divided by power in. So although we then go on to calculate slip, it is already included in efficiency.

Frank, much depends on speed. As you can see from the figures above & in my previous post, at 5 knts we need a force of 400N, and if we increase the speed slightly to 5.4 knts we need 500N. 25 percent more force, and therefore power.
Also, without looking at a power curve for your outboard we don't know what power output you've got at a given RPM. And then, you can rev the 200 BHP engine in your SUV to max revs in nuetral, but it won't be producing much power.
Was the prop on the outboard selected to push a 3500lb 28' boat? Outboards tend to come with props designed for small boats, your prop might not be the most efficient for the job in hand.

When all's said and done, I got data from a computer program and a book, both well respected. They & I could be wrong.
Given the amount of pollution internal combustion engines in boats produce, I think this is an idea well worth persuing. No doubt the engine manufacturers will tell us it won't work, but I believe it's only a matter of time before we see a massive increase in the number of electric powered boats.

John

[Hesp]

Bryan, I got the force required from the hull design software I use - Prolines 98. I chose a reasonable speed to give a low force (see below) and a round number for ease of calculation. Other forces at given speeds are:

1 knt 4 lb 2 kg 20 N
3.25knt 31 lb 14kg 140
5.4 110 lb 50kg 500
7 410 lb 185kg 1850
7.6 579 lb 262kg 2620

You can see why I chose 5knts, 7.6 knts requires 10 KW (14BHP) output from the prop. You can also see that large engines are for those who want to go that extra half Knot. All these figures are approximate, & I've done much rounding up/down, but they should give a good indication.


I got the prop efficiency from Skene's Elements of Yacht Design, Propellor Chart B (P141 in my edition). I must admit, you have cast doubt in my mind regarding this figure. For starters in my haste I wrongly chose an efficiency from the high end of the scale - they run from 0.3 to 0.7. A more reasonable figure might be 50 percent efficiency. Higher efficiency might be obtained on a power boat with a large, slow turning prop.
The chart is entered with a figure obtained using RPM, BHP & Hull speed. I assume (but I could be wrong) that 'efficiency' as quoted in the chart, is power out divided by power in. So although we then go on to calculate slip, it is already included in efficiency.

Frank, much depends on speed. As you can see from the figures above & in my previous post, at 5 knts we need a force of 400N, and if we increase the speed slightly to 5.4 knts we need 500N. 25 percent more force, and therefore power.
Also, without looking at a power curve for your outboard we don't know what power output you've got at a given RPM. And then, you can rev the 200 BHP engine in your SUV to max revs in nuetral, but it won't be producing much power.
Was the prop on the outboard selected to push a 3500lb 28' boat? Outboards tend to come with props designed for small boats, your prop might not be the most efficient for the job in hand.

When all's said and done, I got data from a computer program and a book, both well respected. They & I could be wrong.
Given the amount of pollution internal combustion engines in boats produce, I think this is an idea well worth persuing. No doubt the engine manufacturers will tell us it won't work, but I believe it's only a matter of time before we see a massive increase in the number of electric powered boats.

John

[Hesp]

Bryan, I got the force required from the hull design software I use - Prolines 98. I chose a reasonable speed to give a low force (see below) and a round number for ease of calculation. Other forces at given speeds are:

1 knt 4 lb 2 kg 20 N
3.25knt 31 lb 14kg 140
5.4 110 lb 50kg 500
7 410 lb 185kg 1850
7.6 579 lb 262kg 2620

You can see why I chose 5knts, 7.6 knts requires 10 KW (14BHP) output from the prop. You can also see that large engines are for those who want to go that extra half Knot. All these figures are approximate, & I've done much rounding up/down, but they should give a good indication.


I got the prop efficiency from Skene's Elements of Yacht Design, Propellor Chart B (P141 in my edition). I must admit, you have cast doubt in my mind regarding this figure. For starters in my haste I wrongly chose an efficiency from the high end of the scale - they run from 0.3 to 0.7. A more reasonable figure might be 50 percent efficiency. Higher efficiency might be obtained on a power boat with a large, slow turning prop.
The chart is entered with a figure obtained using RPM, BHP & Hull speed. I assume (but I could be wrong) that 'efficiency' as quoted in the chart, is power out divided by power in. So although we then go on to calculate slip, it is already included in efficiency.

Frank, much depends on speed. As you can see from the figures above & in my previous post, at 5 knts we need a force of 400N, and if we increase the speed slightly to 5.4 knts we need 500N. 25 percent more force, and therefore power.
Also, without looking at a power curve for your outboard we don't know what power output you've got at a given RPM. And then, you can rev the 200 BHP engine in your SUV to max revs in nuetral, but it won't be producing much power.
Was the prop on the outboard selected to push a 3500lb 28' boat? Outboards tend to come with props designed for small boats, your prop might not be the most efficient for the job in hand.

When all's said and done, I got data from a computer program and a book, both well respected. They & I could be wrong.
Given the amount of pollution internal combustion engines in boats produce, I think this is an idea well worth persuing. No doubt the engine manufacturers will tell us it won't work, but I believe it's only a matter of time before we see a massive increase in the number of electric powered boats.

John

[Hesp]

[This message has been edited by Hesp (edited 03-07-2001).]

[Hesp]

[This message has been edited by Hesp (edited 03-07-2001).]

[Hesp]

[This message has been edited by Hesp (edited 03-07-2001).]

[Bryan Mehus]

John,
Looking at your examples, I can see how the "force required" figure can be so low. It is quite a curve i.e. over 200 times the force to go 7.6 times as fast. We all know that standing on a dock, you can push a 5 ton boat away by yourself, but if it is a house boat, and you're on the lee side of a light breeze, it is another matter.
I think that it may be wise to use a force figure based on the hull speed of the boat as the upper end of the speed requirement. Is your hull speed around 8 knts?

I could be wrong, but I believe that it would probably be more efficient to use a specially designed separate propeller to charge the battery bank. That is because the propeller designed to drive the boat will not work as well being driven.

If you are designing from scratch, how about using a variable frequency drive and a three phase induction motor. I remember(vaguely)a chart showing weights of various DC motor and drive combinations, compared to AC motor and drives with respect to electric vehicles. The DC units were twice the weight of the AC, and half the power. Since that particular research was done('80s)there have been great strides in the area of AC variable frequency drives. Then use a dedicated generator/prop,wind gen.,and solar system to charge the banks. Just some thoughts, that may be out to lunch!
BTW, first warm, sunny day of the century 4 us.
Bryan

[Bryan Mehus]

John,
Looking at your examples, I can see how the "force required" figure can be so low. It is quite a curve i.e. over 200 times the force to go 7.6 times as fast. We all know that standing on a dock, you can push a 5 ton boat away by yourself, but if it is a house boat, and you're on the lee side of a light breeze, it is another matter.
I think that it may be wise to use a force figure based on the hull speed of the boat as the upper end of the speed requirement. Is your hull speed around 8 knts?

I could be wrong, but I believe that it would probably be more efficient to use a specially designed separate propeller to charge the battery bank. That is because the propeller designed to drive the boat will not work as well being driven.

If you are designing from scratch, how about using a variable frequency drive and a three phase induction motor. I remember(vaguely)a chart showing weights of various DC motor and drive combinations, compared to AC motor and drives with respect to electric vehicles. The DC units were twice the weight of the AC, and half the power. Since that particular research was done('80s)there have been great strides in the area of AC variable frequency drives. Then use a dedicated generator/prop,wind gen.,and solar system to charge the banks. Just some thoughts, that may be out to lunch!
BTW, first warm, sunny day of the century 4 us.
Bryan

[Bryan Mehus]

John,
Looking at your examples, I can see how the "force required" figure can be so low. It is quite a curve i.e. over 200 times the force to go 7.6 times as fast. We all know that standing on a dock, you can push a 5 ton boat away by yourself, but if it is a house boat, and you're on the lee side of a light breeze, it is another matter.
I think that it may be wise to use a force figure based on the hull speed of the boat as the upper end of the speed requirement. Is your hull speed around 8 knts?

I could be wrong, but I believe that it would probably be more efficient to use a specially designed separate propeller to charge the battery bank. That is because the propeller designed to drive the boat will not work as well being driven.

If you are designing from scratch, how about using a variable frequency drive and a three phase induction motor. I remember(vaguely)a chart showing weights of various DC motor and drive combinations, compared to AC motor and drives with respect to electric vehicles. The DC units were twice the weight of the AC, and half the power. Since that particular research was done('80s)there have been great strides in the area of AC variable frequency drives. Then use a dedicated generator/prop,wind gen.,and solar system to charge the banks. Just some thoughts, that may be out to lunch!
BTW, first warm, sunny day of the century 4 us.
Bryan

[Phil Young]

Honda have just brought a car onto the Australian market which is petrol/electric. Uses battery when they have enough juice. Uses petrol to generate electricity when need extra power or batteries are low. Recharges battery when coasting/slowing. Does not rely on external electricity input. I think the batteries are NiMH whatever that is rather than ordinary lead/acid. One just finished a promotional drive from Brisbane to Sydney I think, about 500miles, on 1 tank of petrol, 40 litres or so. Maybe the technology could be adapted, give it a few years and start hunting around the wreckers yards.

[Phil Young]

Honda have just brought a car onto the Australian market which is petrol/electric. Uses battery when they have enough juice. Uses petrol to generate electricity when need extra power or batteries are low. Recharges battery when coasting/slowing. Does not rely on external electricity input. I think the batteries are NiMH whatever that is rather than ordinary lead/acid. One just finished a promotional drive from Brisbane to Sydney I think, about 500miles, on 1 tank of petrol, 40 litres or so. Maybe the technology could be adapted, give it a few years and start hunting around the wreckers yards.

[Phil Young]

Honda have just brought a car onto the Australian market which is petrol/electric. Uses battery when they have enough juice. Uses petrol to generate electricity when need extra power or batteries are low. Recharges battery when coasting/slowing. Does not rely on external electricity input. I think the batteries are NiMH whatever that is rather than ordinary lead/acid. One just finished a promotional drive from Brisbane to Sydney I think, about 500miles, on 1 tank of petrol, 40 litres or so. Maybe the technology could be adapted, give it a few years and start hunting around the wreckers yards.

[Dale Harvey]

If useing a sailing charge system, the prop shaft needs to run in an oil filled tube with thrust bearings and seals as opposed to the conventional rubber cutlass bearing. There was a company up in Maine makeing such a system for Lobsterboats. Advertised in National Fisherman. I think they patented it. The only current problem with fuel cells, is getting them to run only on gasoline. Currently the oil companies know that it would be far too easy to modify them run on water and cut them out of the loop. Again, the technology to solve all our problems is there, but the money pigs won't let us use it.

[Dale Harvey]

If useing a sailing charge system, the prop shaft needs to run in an oil filled tube with thrust bearings and seals as opposed to the conventional rubber cutlass bearing. There was a company up in Maine makeing such a system for Lobsterboats. Advertised in National Fisherman. I think they patented it. The only current problem with fuel cells, is getting them to run only on gasoline. Currently the oil companies know that it would be far too easy to modify them run on water and cut them out of the loop. Again, the technology to solve all our problems is there, but the money pigs won't let us use it.

[Dale Harvey]

If useing a sailing charge system, the prop shaft needs to run in an oil filled tube with thrust bearings and seals as opposed to the conventional rubber cutlass bearing. There was a company up in Maine makeing such a system for Lobsterboats. Advertised in National Fisherman. I think they patented it. The only current problem with fuel cells, is getting them to run only on gasoline. Currently the oil companies know that it would be far too easy to modify them run on water and cut them out of the loop. Again, the technology to solve all our problems is there, but the money pigs won't let us use it.

[Ross Faneuf]

The drive system is the Evolution drive; they're located in Rockland ME. Quite expensive, but very well done. They're usually at the Maine boat show (2 weeks from today).

[Ross Faneuf]

The drive system is the Evolution drive; they're located in Rockland ME. Quite expensive, but very well done. They're usually at the Maine boat show (2 weeks from today).

[Ross Faneuf]

The drive system is the Evolution drive; they're located in Rockland ME. Quite expensive, but very well done. They're usually at the Maine boat show (2 weeks from today).

[Hesp]

Bryan, I agree that it would be better to have a dedicated charging prop. Preferably one that could be used when conditions were right. The aquair http://www.ampair.com/ would be good but only produces a few amps. I have a self feathering Max prop to reduce drag whilst sailing - I wouldn't be happy when sailing upwind to divert some of the boat's motive power into electric generation. Downwind in a fresh breeze would be okay. Whilst I am a great fan of wind and solar power, it looks like the only way to recharge in hours rather than days/weeks is 110 or 240v mains (ideal if berthed in a marina), or an onboard generator.

Perhaps fuel cells hold the key. I believe that later this year Coleman are to market a portable fuel cell generator based on the Ballard fuel cell. I don't know what fuel it will use, and this might be an issue on a boat.

I like the idea of connecting the motor direct to the prop shaft - no gearbox with its (3%?) loss, expense and maintenance. But for max efficiency we need a slow turning prop, say 800-1000rpm. Although the Shadow Drive turns at a reasonable rpm, most motors will have max revs way above this I should imagine. I suppose one way round this would be to use an oversize motor which produced more torque at lower revs, but its an expensive option.

Bryan, you briefly touch on the advantages of AC over DC, if you can give more info on motors, or point me in the right direction it would be appreciated. As you will have guessed I have minimal knowledge of electric motors, but I think it is something worth looking into.

Yes, I did conveniently overlook the windage on the boat. And the force required to motor through oncoming waves. These forces would be added to the required motive force at a given boat speed. I am asking myself how an electric motor compares to having no engine, rather than how it compares to a diesel engine.

..........

Frank, is your Egret type sharpie a double ender? In an idle moment I drew a 28' double ended sharpie weighing 3500 lbs. At a given speed force required is - 1knot-5lbs, 2knt-9lbs, 3knt-22lbs, 4knt-48lbs, 5knt-88lbs, 6knt-158lbs, 7knt-264, 7.4knt-317lbs.

Phil, I think the hybrid car you refer to does can use mains to charge the batteries, and this is cheaper. But as you say there is petrol engine that charges the batteries. The engine runs at a constant speed for max efficiency. NiMh batteries are Nickel Metal hybrid, more expensive, but more watts hours per kg.
For those interested in pv solar panels, NiMh batteries, fuel cells and more, an interesting company is Energy Conversion Devices http://www.ovonics.com/

John.

[Hesp]

Bryan, I agree that it would be better to have a dedicated charging prop. Preferably one that could be used when conditions were right. The aquair http://www.ampair.com/ would be good but only produces a few amps. I have a self feathering Max prop to reduce drag whilst sailing - I wouldn't be happy when sailing upwind to divert some of the boat's motive power into electric generation. Downwind in a fresh breeze would be okay. Whilst I am a great fan of wind and solar power, it looks like the only way to recharge in hours rather than days/weeks is 110 or 240v mains (ideal if berthed in a marina), or an onboard generator.

Perhaps fuel cells hold the key. I believe that later this year Coleman are to market a portable fuel cell generator based on the Ballard fuel cell. I don't know what fuel it will use, and this might be an issue on a boat.

I like the idea of connecting the motor direct to the prop shaft - no gearbox with its (3%?) loss, expense and maintenance. But for max efficiency we need a slow turning prop, say 800-1000rpm. Although the Shadow Drive turns at a reasonable rpm, most motors will have max revs way above this I should imagine. I suppose one way round this would be to use an oversize motor which produced more torque at lower revs, but its an expensive option.

Bryan, you briefly touch on the advantages of AC over DC, if you can give more info on motors, or point me in the right direction it would be appreciated. As you will have guessed I have minimal knowledge of electric motors, but I think it is something worth looking into.

Yes, I did conveniently overlook the windage on the boat. And the force required to motor through oncoming waves. These forces would be added to the required motive force at a given boat speed. I am asking myself how an electric motor compares to having no engine, rather than how it compares to a diesel engine.

..........

Frank, is your Egret type sharpie a double ender? In an idle moment I drew a 28' double ended sharpie weighing 3500 lbs. At a given speed force required is - 1knot-5lbs, 2knt-9lbs, 3knt-22lbs, 4knt-48lbs, 5knt-88lbs, 6knt-158lbs, 7knt-264, 7.4knt-317lbs.

Phil, I think the hybrid car you refer to does can use mains to charge the batteries, and this is cheaper. But as you say there is petrol engine that charges the batteries. The engine runs at a constant speed for max efficiency. NiMh batteries are Nickel Metal hybrid, more expensive, but more watts hours per kg.
For those interested in pv solar panels, NiMh batteries, fuel cells and more, an interesting company is Energy Conversion Devices http://www.ovonics.com/

John.

[Hesp]

Bryan, I agree that it would be better to have a dedicated charging prop. Preferably one that could be used when conditions were right. The aquair http://www.ampair.com/ would be good but only produces a few amps. I have a self feathering Max prop to reduce drag whilst sailing - I wouldn't be happy when sailing upwind to divert some of the boat's motive power into electric generation. Downwind in a fresh breeze would be okay. Whilst I am a great fan of wind and solar power, it looks like the only way to recharge in hours rather than days/weeks is 110 or 240v mains (ideal if berthed in a marina), or an onboard generator.

Perhaps fuel cells hold the key. I believe that later this year Coleman are to market a portable fuel cell generator based on the Ballard fuel cell. I don't know what fuel it will use, and this might be an issue on a boat.

I like the idea of connecting the motor direct to the prop shaft - no gearbox with its (3%?) loss, expense and maintenance. But for max efficiency we need a slow turning prop, say 800-1000rpm. Although the Shadow Drive turns at a reasonable rpm, most motors will have max revs way above this I should imagine. I suppose one way round this would be to use an oversize motor which produced more torque at lower revs, but its an expensive option.

Bryan, you briefly touch on the advantages of AC over DC, if you can give more info on motors, or point me in the right direction it would be appreciated. As you will have guessed I have minimal knowledge of electric motors, but I think it is something worth looking into.

Yes, I did conveniently overlook the windage on the boat. And the force required to motor through oncoming waves. These forces would be added to the required motive force at a given boat speed. I am asking myself how an electric motor compares to having no engine, rather than how it compares to a diesel engine.

..........

Frank, is your Egret type sharpie a double ender? In an idle moment I drew a 28' double ended sharpie weighing 3500 lbs. At a given speed force required is - 1knot-5lbs, 2knt-9lbs, 3knt-22lbs, 4knt-48lbs, 5knt-88lbs, 6knt-158lbs, 7knt-264, 7.4knt-317lbs.

Phil, I think the hybrid car you refer to does can use mains to charge the batteries, and this is cheaper. But as you say there is petrol engine that charges the batteries. The engine runs at a constant speed for max efficiency. NiMh batteries are Nickel Metal hybrid, more expensive, but more watts hours per kg.
For those interested in pv solar panels, NiMh batteries, fuel cells and more, an interesting company is Energy Conversion Devices http://www.ovonics.com/

John.