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Charging 12V SLA with bike dynamo / generator


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Hello,

I've found a number of posts regarding this (or related) topics, but I still have a few questions. This project is part a bigger project that involves a bicycle stereo powered by a 12V 1.2Ah battery. The current setup requires a wall charger to recharge the battery, but I'm looking into using pedal-power as well.

The dynamo I picked up is a common, super cheap "bottle" style dynamo. After mounting it to my bike, I found it produces about 8 to 10VAC (not attached to any load) when I pedal at a moderate pace. I can get it to max out at around 12VAC, just as I start to break a sweat.

If I understand everything correctly, to charge the 12V SLA, I need to apply slightly more than 12VDC to the battery.(In fact, according to the charging instructions on the battery casing, I need to supply 13.5 - 13.8 V for stand-by use, or 14.4 - 14.7 V for cycling use, although I'm not entirely sure what the difference is.)

Here's what I've done with the dynamo so far:

I hooked up the dynamo to a 1A max bridge rectifier and put a 33 ohm resister across the DC end. At a moderate pedal (I'm guessing around 10-15 mph), it generated about 6V @ 200mA

I also tried using a simple full-wave voltage doubler (using 2 diodes and 2 220uF caps), also with the 33 ohm load, this generated 7V @ 220mA (please keep in mind, these are all fairly rough measurements made with a multimeter strapped to my handlebars)

I upped the load to 1000 ohm and tested the dynamo again with the bridge rectifier and the voltage doubler, yielding 8V @ 7mA and 24V @ 25mA respectively

So I guess the main question here is, is the dynamo generating enough juice in the first place? And if it is, how should I use it to charge the battery? I've researched a number of charge control ICs available on the internet, and it seems the main issue to worry about when charging a SLA battery is over-charging (although that doesn't even seem to be that big of an issue when compared to other battery chemistries). I'm not sure that my little dynamo can generated enough power to damage (or even charge for that matter) the battery. Some of the other posts and websites I've gone over suggest simply applying the DC power right to the battery leads and calling it good... Perhaps I can use the voltage doubler and attach it right to the battery? I'm a little wary of using a voltage regulator or a charge control IC simply because it may rob me of some of the precious few volts and amps I'm producing.

Okay, sorry for being so long winded, but I'm definately reaching the point where my own knowledge is running dry and I need to depend on you guys and gals. (I've never taken a class on electronics, but I think I might soon... literature suggestions?)

Finally, a word on the actual intended use for the battery. I'm nearly done with a small stereo that will go on the rear rack of my bike. (Not that I don't love my current bike stereo, but there's always room for improvement.) It appears that the stereo draws between .7 and .8 A at full volume, so in theory, it should last over an hour before it needs to be recharged (no problem, I rarely ride for more than a consecutive hour in a day, and I can plug it in at night). It seems clear(ish) to me that the dynamo could never actually power the stereo totally by itself. In other words, I would still eventually have to charge the battery from the wall (unless perhaps I rode around for hours with the stereo off and the battery charging). But perhaps the dynamo can augment the life of the stereo on a single charge? Maybe this is the most I can hope for, or maybe I can get a better dynamo...

Thanks,
SCW

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What's the current rating of the dynamo?

To be picky, it isn't a dynamo but an alternator because it produces AC not DC.

Standby or float charge, just means that you can connect 13.8V to the battery and leave it there forever in a day and no harm will be done to the battery. The trouble is, if the battery is flat it will take ages to float charge, days if not longer. To charge the battery in hours, a constant current needs to be applied until the battery voltage reaches 14.4V to 14.7V and the current falls below a certain level, then the voltage shoul be reduced to 13.8V and the battery can the be left connected continuously on standby.

Have you read the datasheet carefully? The battery is only rated to the capacity when it's brand new and the discharge current is very low, probably something like 20mA in this case, if the discharge current is increased the capacity is reduced, at 0.8A the Ah rating will probably be near half the rated capacity so I doubt it will last an hour. I think you need a larger battery and dynamo.

The rectified voltage seems a little low. I suspect the capacitors are is probably too small for the the current you're drawing which is causing ripple and the average voltage to be lower. The minimum capacitor size  depends on the frequency which you don't know and the maximum acceptable ripple votlage, which depends on what's connected to it.

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Bottle alternator, got it.

As for the current rating, not sure. Google seems to indicate that these "bottle alternators" (bartenders?) generate anywhere from 6V/3W to 12V/6W, but I think that has something to do with the characteristics of the bike lights used in the intended circuit, but maybe that info is worth something. I picked mine up from the local free-cycles (http://www.strans.org/freecycles.html). Don't worry, I donated. It came in a greasy zip-lock bag along with the shoddy head and tail light originally included in the set. If you're referring to printed spec data that might have been stuffed into the original packing, no dice. As for trying to rate it personally: I stuck a multimeter on my handlebars, rode around in circles, and switched out circuits and resistors. Any rating I've got, I've posted. But I'll get back on that horse and get some more if you think it would help.

From what you post, it sounds to me like that the charger in my application would need to have a charge controller (supposing that the alternator was beefy enough to run the IC and charge the battery.)

The battery specs claim that it can be run at 0.72 A for an hour, we'll see how it holds up.

I think you're right, I do need a bigger battery and a bigger alternator (mostly what I come across are pretty cool looking hub dynamos that are rated at 6V/3W, other than that, there's not much out there) Also, I want to keep everything as small and light and as cheap as possible... while still working superbly, of course.

The 220uF capacitors I chose were essentially a shot in the dark. The paper I was going off at the time claimed that for a 60Hz application, I needed somewhere between 1.0 - 200uF. additionally, applications around 10kHz would need somewhere between .02 and .06uF of capacitance. I have no idea what frequency this alternator alternates at, and I'm sure it changes quite a bit depending on speed, so I went big. I don't have or know how to use an oscilloscope, but I'd like to learn. As for the maximum acceptable ripple voltage, the doubler was only connected to a resister and the enigmatic inner workings of my digital multimeter...

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