# maximum power output while charging battery

6 Posts
Dec 11, 2011 01:24 pm
maximum power output while charging battery

Recently I have constructed a permanent magnet generator that generates 368W at 400 RPM at no load and when loaded with
battery(12V, 135AH) produces only 56W at same RPM. The charging current is be very low which is appreciable for fully charged battery but the PMG easily produces more than 100W(13V, 8A) at 200RPM unloaded.

How can I extract more power from generator while charging a battery so that battery can utilizes the optimum power from the generator when available? Could anyone suggest some ideas...

97 Posts
Dec 11, 2011 11:44 pm
Re: maximum power output while charging battery

Simon, your question is confusing, because you state that the PMG generates 368 watts under no load.  Since wattage is a product of both voltage and current ( P(watts) = E(voltage) x I (current), you must have a current flow through a load to be able to measure wattage.  So, how can you possibly determine wattage with no load?  Generally speaking, a generator produces an open load voltage that is higher than that when it is under load, but the current is determined by the amount of power that can be extracted from the wind, the efficiency of the PMG, and the losses due to resistance of the wiring in the generator and in connective wires to the load, and the voltage would be clamped at or near your nominal battery voltage, since you are using the battery as a load.  In order to get more power from the PMG, you would need to have less resistance in the windings of the PMG, or increase efficiency of the turbine blades, or have greater strength of the permanent magnets or their coupling to the windings.  Increasing the length of the turbine blades is an easy way to capture more power from the wind, since it is dependent on swept area.  Just my suggestion.  :-)  Jon C.

6 Posts
Dec 12, 2011 01:56 pm
Re: maximum power output while charging battery

hello Jon,
Actually the short circuit current shows up with 17 amp at around 400RPM, but the battery charges with only 4.45 amp at the same RPM. Now there is not much wind at the site and the battery charging rate is very less.
Is there any way so that battery could extract more power from the generator at even low RPM. Right now it powers itself up to 56W at 400 RPM. it would have been great if the power level was same at 200RPM.

97 Posts
Dec 12, 2011 03:20 pm
Re: maximum power output while charging battery

Hello again, Simon.  You simply do not have enough turns of wire on the PMG to generate over 12v at 200rpm.  The only other way to get more voltage would be to DC - DC convert the output to a higher voltage, but then you would suffer losses in the converter and wiring, and realize a smaller output wattage overall.  You could, as I said earlier, put larger, more efficient blades on the turbine, or increase the efficiency of the turbine itself.  If you don't have sufficient wind at your site to spool up the turbine at adequate speed, you might raise the mast height or find a better location for the turbine.  I got the impression that your PMG is homemade?  Might I suggest buying a commercially made wind turbine that may satisfy your charging requirements?  Jon C.

6 Posts
Dec 13, 2011 12:33 pm
Re: maximum power output while charging battery

hello again Jon...
Actually i am working in building a 300W wind turbine system using as much as resources available locally. So most of the components are homemade. You mentioned increasing the number of turns but that would increase the stator size. Right now the winding thickness is 1.8mm.
Would it be possible to slightly decrease the winding thickness and then increase the number of turns in order to increase the voltage.
Actually the generator needs to generate 150W at 400RPM. Mine just reaches half of it. I am having trouble figuring out the problem..

97 Posts
Dec 13, 2011 11:54 pm
Re: maximum power output while charging battery

Hi Simon.  Yes, increasing the number of turns of wire in the PMG will increase the voltage, since each turn contributes to the resultant output voltage.  If you must decrease wire size in order to accomplish that, or keep the same size wire, there will be an effect of increased resistance in the windings, and a subsequent voltage drop under load due to power losses in the smaller wire, so it is a tradeoff........more voltage out, but more internal losses.  This may or may not work for you, and I think that you will have to experiment with this to determine the results.  I wish you good fortune in your project.  Jon C.

6 Posts
Dec 14, 2011 05:47 am
Re: maximum power output while charging battery

97 Posts
Dec 14, 2011 04:24 pm
Re: maximum power output while charging battery

You are most welcome, Simon.  :-)

55 Posts
Jan 15, 2012 05:36 pm
Re: maximum power output while charging battery

Have you put that unit into the wind yet? I have a post here. Once I put mine into the wind even the high wind days I would only get 3.5 volts. Hub and blade drag. Take a look at my wind pic.

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