Charging 12V SLA with bike dynamo / generator - diyAudio
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Old 29th July 2010, 05:43 AM   #1
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Default Charging 12V SLA with bike dynamo / generator


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...

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Old 29th July 2010, 07:51 AM   #2
Mooly is offline Mooly  United Kingdom
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I've no idea what a bottle dynamo puts out but imagine it's not very suitable as it will be designed to try ! and maintain a constant low voltage across the bulbs over a wide range of speed. It's never going to provide the benefits you seek charging an SLA. I think the internal magnetic losses are deliberately very high to provide this effect... it's no good if the output keeps increasing the faster you pedal as the bulbs would blow.
Anything more than a few tens of milliamps would require quite a lot of mechanical energy, in other words the generator becomes much harder to turn and that would literally rip the tyres to bits... and you would notice the drag and unequal pull on one side.

SLA,s require a constant voltage low ripple supply for best results and life.
The lower voltage quoted such as 13.5 volts is for 24/7 float charging such as in alarms etc.

Yuasa's little red book of batteries explains all
Yuasa - industrial batteries, automotive batteries, motorcycle batteries, golf batteries, mobility batteries.

Last edited by Mooly; 29th July 2010 at 07:56 AM.
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Old 29th July 2010, 08:07 AM   #3
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Location: Missoula, Montana you're tellin' me there's a chance.
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Old 29th July 2010, 09:56 AM   #4
AndrewT is offline AndrewT  Scotland
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the axle mounted contraption built into the spokes of the wheel is an alternator.
The tyre rubbing (bottle shaped) dynamo is a (pulsed) DC generator. Just like the dynamo in older vehicles. It needs a regulator, but that is easy. Just limit the maximum voltage to {battery +1 or 2volts}.
regards Andrew T.
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Old 29th July 2010, 12:05 PM   #5
Mooly is offline Mooly  United Kingdom
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Originally Posted by scwhiteley View Post you're tellin' me there's a chance.
Anythings possible and do able, but this is just too enificient. The magnetic losses of a bottle dynamo are purpose designed to be high to limit the "power" that can be supplied to a load so as to stop bulbs blowing... it's a very crude self regulating system... and you just won't get enough from it to be worthwhile IMO.

A switching regulator could be used to provide a constant voltage of say 15 volts DC from say 3 to 30 volts input, but again the limitation is the power that's actually available, maeaning only a few 10's of ma at best for charging.

A solar panel for charging SLA's would be better depending if the suns out.
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Old 29th July 2010, 08:38 PM   #6
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Originally Posted by Mooly View Post
A solar panel for charging SLA's would be better depending if the suns out.
A small wind turbine maybe? After all, the faster you go, the higher the air pressure. Though a propeller strapped to a bike would look silly... and it's probably unsafe too.
"Audio grade" components simply means that they failed at a more critical job.
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Old 29th July 2010, 10:18 PM   #7
amt is offline amt  United States
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A friend of mine fooled around with this idea years ago and it proved to be very inefficient (as already stated) and not worth the bother. Sorry.

He and I both bike with LED lights and on occasion, small T-amp based mobile sound. We use standard 7aH SLAs, which although heavy, are more efficient than the frictional loss of a generator and PITA of charging system maintenance. Even better would be lighter Lithium Ion batteries but they are a bit pricey.

One 7aH will run a small T-amp and/or lights for several rides. In your case, just buy a bigger battery to accommodate your amp hour requirements. Mine crams nicely into the bottle cage.

You will use less energy by adding some static weight and reducing your rolling resistance and rotational weight, which the generator simulates an increase of.

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Old 30th July 2010, 02:32 AM   #8
star882 is offline star882  United States
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Connect a proper generator to the wheel and design the regulator to output about 12.8v (charge maintain) normally and up to 14.4v (full charge) for regeneration. Or use a powerful motor/generator and do a hybrid conversion.
"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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Old 9th August 2010, 03:37 PM   #9
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10 years ago a friend who worked at NASA on a project using Ultracapacitors, wrote an engineering note about his bicycle. (you need to go to the pdf version for more info)

Ultracapacitor powers bicycle light - 2000-09-28 07:00:00 | EDN
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Old 11th August 2010, 06:44 AM   #10
dangus is offline dangus  Canada
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I'm pretty sure the bottle style "generators" put out AC. I once hooked one to a step-up transformer and ran a 120V table radio directly from the bike. Most generators are rated at 5 or 6 watts, which ought to be enough to develop useful charging current. FWIW, there was an article or letter in Wireless World on improving bike generator performance with a capacitor to compensate for the inductance or something. I may have that in a a binder.

Full wave rectifier, then a switching converter with current limiting (if that's necessary) might be the ideal approach, if a bit complex. But an autotransformer or voltage doubler and a resistor to limit charging current may be "good enough". If you're concerned about overcharging, maybe a typical car battery charger regulator can be adapted. Google scr battery charger. If your microcontroller "skillz" are "mad", use one as a voltage monitor (red/green LED to indicate flashing red low, flashing green normal; solid green charging, solid red overcharging) and charge controller.
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