Choosing a DC Motor Question

[Wellington]

After much research I'm just getting started building my first stirling engine and have come full circle only to realise that building one from junk is the most sensible way to go. I want to charge 12v batteries. How do I select the correct sized and rated dc motor to charge a 12v gel battery? do I first need to know the average rpm of the engine under average power input? And how many volts above 12v should I charge a 12v gel battery? Do I need some kind of over charge protection and or auto off switch? A laymans explanation would be good if anyone can advise. PS. My electrics knowledge consists of knowing that amps x volts = watts and nothing more.
.Wellington.

[Alfista]

Wellington,
I will give you the benefit of my experience, and what is experience ? According to Mark Twain, it is "that marvelous thing that allows us to recognize our mistakes when we make them again."

If you could see my collection of unused motors and useless generators, then you would know what I mean. You are asking all of the right questions !

First, I want to reassure you that if you can get it to turn at all or produce any electricity at all, then there is probably a way to manipulate the output for 12v battery charging.

I recently put up this video on youtube

[youtube]https://youtu.be/RPZmmxDZyJg[/youtube]
It is a micro steam power plant. The generator is a Dayton permanent magnet motor 4Z524B : 180vdc at 1725 rpm, armature amperage 2.8A.  It will produce something close to 180vdc at 1725 rpm, and about 18vdc at 173 rpm. This is a .5 hp motor. I will be adapting this one to my large Stirling engine as I have decided that I could use a slightly more powerful motor / generator for this micro steam plant. In any case, you should be able to find a PMM rated at 180vdc at a relatively low rpm. After some experimenting, if you find that the Stirling engine just does not have enough power to turn the generator in a satisfactory way, then you may want to find some way to either reduce friction or find greater mechanical advantage through gear or pulley selection or direct drive. Lastly, you may have to resort to some method of limiting current or boosting voltage. Neither of which is terribly difficult to do.

The best solution may be to make your own alternator as the lynxsteam guy has done, an axial flux alternator driven directly from the crankshaft.
[youtube]https://m.youtube.com/watch?v=4CaRJVajOQc[/youtube]

I have seen this style of alternator used to good effect on a few Stirling engines. The lynxsteam guy has some videos on sizing and building the alternator. This is also a popular topic among the wind turbine builders on fieldlines.com

[Wellington]

I have seen this style of alternator used to good effect on a few Stirling engines. The lynxsteam guy has some videos on sizing and building the alternator. This is also a popular topic among the wind turbine builders on fieldlines.com

Yep, Ive seen the lynx steam channel and his monotube boiler and rocket stove etc. Hes got the best most practical channel ive seen. People say his monotube boiler is a safe type of boiler but intuition tells me if that mono tube springs a leek it will pour water direct onto the flames :( . (I cant see how or why a monotube is considered safe for this reason???) I understand a monotube is much lower pressure than a regular steam boiler but as someone noted to me: 130 psi is still enough to potentially kill you when you concider people often die from car tyres exploding at much less pressure than this). I will hopefully have a steam system one day for water distillation but it will be minimal size and pressure and just enough to provide drinking water from seawater. Your steam system look great. I only wish I had begun tinkering earlier in life to produce stuff like that. Think ive found my natural calling in life now even though I have two left hands. :)

[Bluechip]

Hi Wellington

The charging voltage is temperature dependent, about 13.5 ish volts usually for a 12V battery. See table:

http://www.powerstream.com/SLA.htm

Then you need to decide the charging current, a typical figure is to charge at the 20 hour rate; ie. if you have a 40 AH battery it should be charged at 2A max. It is permissible to charge at the 10 hour rate ie' 4A in this example but you will eventually shorten the service life of the battery unless it's designed to be used that way.

What you are doing is not trivial in an electrical sense, I have made numerous chargers and my favourite method is to use a ST L200 ic. OK up to 2A ( more with external circuitry ).

It's produced to make battery chargers from amongst other things, and is easy to use. Maybe not so for someone new to electronics though.

Generally speaking a battery is first charged with current limiting and then it then goes into a 'constant voltage' float charge, The L200 does this.

You don't say where you are ?? Country ??

Dave

[Wellington]

Hi Wellington

The charging voltage is temperature dependent..........

Dave

Thanks for the tips bluechip. Ive seen a few projects on youtube of stirlings made from junk charging 12v car batteries like approtechie channel. I notice they have outputs around the 10w mark. How long does it take to charge a car battery with these kind of outputs???
Geeze....all this electrical theory on lynx steam channel is above my head. I should have mentioned I can barely wire a plug. Think I will have to come back down to earth and rethink my plans and stick to the micro scale stuff as its becoming clearer than even a micro scale project is going to be a mountain to climb especially if I try to build a mini axial flux alternator. The more I think about it the more I think it is wiser to go biomass compact/portable 12v scale as trying to replace the grid with grand ideas is no small task and would result in a lot of expense and maintanance. Learn how little you need rather than how much you want as the old saying goes. My goal now is to create a 12v system made from junk that can run leds, 12v converted toaster heater and a 12v heated motorcycle jacket from a 12 - 48v battery bank. I know that motorcycle heated clothing can run for at least 6hrs on a 12v battery and get you through a nights cold weather but my question now is how long would it take to charge such a system with a stirling made from junk? Idearly I want a daily routine where I charge once every twenty four hrs and get enough power for the following day. Any ideas on if my revised plan is doable and what charging times I can expect???
.Wellington.

[Bluechip]

Wellington

Assuming 13.5 Volts output, 10W gives some 0.75A nominal.
Going back to the 20 hour rate bit, it suggests that is sufficient to charge a battery no larger than 15AH.
In fact it will, it will just take longer.
How long? That depends on the state of (dis) charge of the battery.
A bit more:
Generally speaking a Lead Acid battery should not be discharged lower than it's half charge point. For a 12V nominal battery this is as near as dammit 10V.
Another bit:
The unit of electrical charge is the Coulomb. It is Current x Time. ie. Amps x Seconds.
Say I have a battery with a terminal voltage of 13.00V. I now take a current of 1A from it for 1 hour.
I have taken (1A x 3600Secs) = 3600 Coulombs of charge from that battery.
So what happens if I now charge it at 1A for 3600secs ? Back to 13.00V ??? Nope, nowhere near!
There is a thing called the Coulomb Penalty / Over-head etc. etc.
It's about 1.4 or so.
I would have to charge it at 1.4A for 3600 Secs. or any combination that gives the same result ...
The chemical reaction in a SLA is not very efficient.

So, unless you know all the details of what you want to do, your question cannot be answered except in very general terms.

Bugger, ain't it ...

Regards

Dave

[Ian S C]

I'v been playing around with a couple of my motors, I burned out the 12v fan on the radiator of one motor while I was trying for maximum power. both the generator, and the alternator that I have been using were peaking at 38 volts. I dug out another DC motor( 36V, 2.2A, 2750 rpm), this one with the right ratio on the belt pulleys gives me about 10 volts under load, more testing to be done. The motor is about 150 mm long, and 50 mm diameter, with an 8 mm shaft at one end, and a 10 mm shaft at the other.
The axial type alternator is very useful as the armature, and stator can be adjusted to alter the load, by opening or closing the gap between them.
Ian S C

[Wellington]

The axial type alternator is very useful as the armature, and stator can be adjusted to alter the load, by opening or closing the gap between them.
Ian S C

dam, that's a good idea. I did not think of that. good tip.
Wellington

[Wellington]

Wellington

Assuming 13.5 Volts output, 10W gives some 0.75A nominal...................
............
Dave

All good advice bluechip, thanks. the more I think about it the electrical side is the most vulnerable and so having a huge alternator is a bad idea. much better to have muttiple smaller units that provide for living needs. I'm going down the path of ultimate frugality and want to figure out just how few watts a single person can live on per day.
Wellington

[Wellington]

I'v been playing around with a couple of my motors, I burned out the 12v fan on the radiator of one motor while I was trying for maximum power. both the generator...................Ian S C

sounds like your out of pocket. but better than dropping an engine on your big toe like I just did. I was wondering how effective 12v systems are at chargine mobiles etc. The youtube vids ive seen usually output about 5 watts. How does do these outputs compare to charging a phone with a commercial phone charger from a 230v socket? I assume a stirling outputting 5 watts would take an awful lot longer to charge a phone than a regular phone charger on a home socket right?
Wellington

[Ian S C]

A phone would take (if it's like mine) .55W, at 4.75V, so 12V, 5W will charge about 10 phones after you sort the voltage out. With a couple of extra volts, and 5W, you could charge a car battery, that's about the same as the normal DIY battery charger. Most phones etc have Lithium batteries these days, and I think they need a bit of care in their charging. As long as the volts and Watts are the same as the mains charger, it will take the same time, don't up the power, you'll just kill the battery.
Ian S C

[Wellington]

A phone would take (if it's like mine) .55W, at 4.75V, so 12V, 5W will charge about 10 phones after you sort the voltage out. With a couple of extra volts, and 5W, you could charge a car battery, that's about the same as the normal DIY battery charger. Most phones etc have Lithium batteries these days, and I think they need a bit of care in their charging. As long as the volts and Watts are the same as the mains charger, it will take the same time, don't up the power, you'll just kill the battery.
Ian S C

Ok so your answer sounds like a gadget charging stirling using a 12v car battery is a very doable and possibly saleable project. Idearly I would want to have an idiot proof product that nobody can damage the battery or gadget and a system that can be plugged in and forgotten about safely. Given that I know nothing about electrics can you help me out by describing what a 12v car battery stirling system would look like in terms of the components I will need to research to make it idiot proof and safe? I'm hoping I can just buy what I need off the shelf at an electronics store. I assume a 12v deep cycle battery would be the best most idiot proof option.
Regards...Wellington

[Ian S C]

Basicly the power supply is every thing after the diodes in a battery charger if you have a DC supply, you would need to have about 17 volts at a bit over 5 Watts for a 5 Watt battery charger, the shaft power of the motor would need to be able to produce at least 10 Watts quite comfortably to be effective as a useful power unit. It depends on what you want to use the battery for as to which type you use, but for most uses other than starting a car the deep cycle battery is the most useful. Get good batteries, they will last for years. Another type is the traction battery from a forklift truck, you get them second hand(if you are lucky), I, heard of one person who asked about them, and was told that yes he could have them, but take the lot! It seems that in USA where he was they have to pay big money to dispose of them, he was still using them 10 years later on his wind turbine.
Ian S C

[Wellington]

Basicly the power supply is every thing after the diodes in a battery charger if you...........................
Ian S C

Thanks Ian, You should write a book on stirlings as a complete laymans guide is much needed. Thaks again
Wellington.

[Ian S C]

One place you can get quite a good little PM motor is an old battery drill, there are hundreds of them with dead batteries, you find them a garage sales/car boot sales.
Ian S C