Charging 12V SLA with bike dynamo / generator

[scwhiteley]

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

 

[MikeMl]

Why do you need 12v? I get great music out of AM_FM radios, XM radio, MP3 player, all of which operate on voltages less than 5V?

 

[scwhiteley]

Why do you need 12v? I get great music out of AM_FM radios, XM radio, MP3 player, all of which operate on voltages less than 5V?

The stereos I have built (and am building) amplify the line signal from such sources (in my case an iPod, but any of the other sources you listed would work) to power one or more speakers (the spec sheet for the TDA7240A amp I'm using in this project touts ~14W output for the 12V supply voltage I'm using). The idea in this particular case being that I can rock out while I ride my bike without having to use headphone's (danger danger).

 

[audioguru]

The TDA7240A bridged amplifier has an output at clipping of 14W into a 4 ohm speaker when the "12V" battery is fully charged at 13.8V.
The amplifier is about 60% efficient so its power from the battery is 23.3W. Therefore the current from the battery is 1.69A and stereo requires 3.38A.

I know that music is never full blast continuously (except acid rock is) but continuous power will kill the charge in your tiny battery in about 10 minutes.

 

[scwhiteley]

I think I grasp what you're saying. I wasn't sure about the practical output I was getting when the stereo was in use, I just rattled off a number from the figure in the spec sheet to indicate to MikeMI that I was powering more than just headphones. (I would like to learn more about the calculations you performed though.)

The stereo I've built for this project seems to draw around .7 - .8 A at loud volume, probably above the level I would normally use it at. The spec sheet for the battery claims an hour of use at 0.72 A. I've not yet tested this empirically, but I've definitely let it run for a half hour to forty minutes or so and it seemed to be running pretty well. It is indeed a tiny battery (also cheap!), which is one of the reasons that I'm looking to help juice it with some pedal-power. Any thoughts on my dynamo quandary, Audioguru?

 

[audioguru]

If you have only one channel of a bridged TDA7240A driving an 8 ohm speaker then the max RMS power at clipping is about 8W when the battery is fully charged at 13.8V. With an efficiency of 60% then the power from the battery is 13.3W and the current is 0.97A.
The 1.2Ah battery is rated with a 120mA or 60mA load. It will last only a few minutes.

The datasheet lies about the output power because the volume control is turned up way too high causing severe clipping distortion to be a horrible 10%. My power calculation is when the amplifier is barely clipping with very low distortion.

Your battery is weak and your dynamo is also weak. Pedal for an hour to charge the battery so the amplifier can play for a few minutes before you must pedal for another hour again.

 

[shokjok]

May I suggest a smaller radio directly on the handlebar? Keep in mind that the dynamo voltage will be noisy given the differences in tire rotation speed. What about a second dynamo, one on each fork? You can wire them in series for a higher ( and more reliable) voltage output, connected to a voltage regulator powering the radio. The second dynamo may need to be isolated from the conductive bike frame.
Twenty-three years ago, I built a bike utility trailer with a detachable AM/ FM/ cassette car radio with a 6W output, powered by a garden tractor battery and recharged by a solar panel between the speakers ( recent addition). I still have this innovative trailer. I'm taking it this weekend to the Canadian Rockies. My experiences with dynamos were not favorable.

 

[Boncuk]

World's first ghetto blaster on wheels

Hi scwhiteley,

I suggest to rewind the bike dynamo to supply 11VAC/1A. With a bridge rectifier your output voltage will be 11V*1.141-1.4V=14.154V.

At 1A output current you'll have to pedal at a speed of not less than 50kmh for the time you want to fill the world with noise and recharge the battery sufficiently or omit the extra weight and supply the amplifier directly from the dynamo-rectifier circuit.

Try your personal capabilities on an Ergo-meter first.

Pedaling 100W you're done within 15 minutes. (Just as a rule of thumb).

Boncuk