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I have a working lightning system on my bike with wired front and rear lights, powered by a dynamo. The dynamo's plastic has broken and I have repaired it several times now, but now it is FUBAR.

Thinking about a replacement, I wondered if it would be possible to just connect some batteries to the dynamo cables. AFAIK the usual voltage for dynamo powered lights is 6 volts, so if I were to add 4 normal batteries (4 x 1.5 = 6) in series, and connect this to the power cables, would it work? Or are there other things to consider?

Both front and rear light are LED lights; the rear light has a capacitor to maintain illumination even when you stop.

I would use a battery holder like the one pictured below, and rechargeable AA batteries, since I have a lot of them already.

If you have any other suggestions on how to do this, I look forward to hearing them!

I want to do this because I have had issues with rim dynamos in the past, and just want a reliable power source; and I am too lazy to install a hub dynamo. It is also a cost issue; I already have the rechargeable batteries, but would have to buy a new dynamo. This is my city bike; it could be stolen at any time, so I do not want to invest a lot of money in components.

http://www.tandyonline.co.uk/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/4/x/4xaa-battery-holder.png

  • If the lamps were old-fashioned incandescent lamps it would be simpler -- you'd just need to match the battery voltage to the RMS output of the dyno. But with LEDs it's harder to say. Probably you could use batteries of approximately the same voltage, but it might not work, and it might set the whole bike (OK, only the lamp socket) afire. – Daniel R Hicks Jul 23 '14 at 16:33
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    (BTW, rechargeable penlight batteries are a PITA. And keep in mind that they run about 1.2v each.) – Daniel R Hicks Jul 23 '14 at 16:34
  • And rechargeables have almost no internal resistance so they are much more hostile to LEDs compared to alkalines. – RoboKaren Jul 23 '14 at 21:18
  • If the LED light is modern enough to have a standlight it's almost certainly got decent protection circuitry inside it. I'd try a 6-cell holder as well if you can and have a multimeter that can measure 500mA, because it's possible that the light will run better off a slightly higher voltage (lower current = longer battery life and 4.8V from 4 rechargables is a bit low). My IQ2 has all sorts of stuff in it, including a USB output, but it pushes the dynamo up to about 12V. I'm not going to connect it to 12V DC just to see what happens, though, it's a bit pricey for that. – Móż Jul 24 '14 at 4:50
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    Hmm ... well why are penlight batteries a PITA? Already made some coin cell to AA conversions for some of my other battery lights, always had good experiences. It is my first dynamo to battery conversion though ... Setting my bike on fire sounds unreasonable, except if there is a shortage somewhere, but there is not and I got separate plus and minus cables everywhere, no power goes over the frame. @Mσᶎ i will try a slightly larger voltage, I have a variable voltage AC DC converter and a multimeter at home. Thanks for the input! – Paul Weber Jul 25 '14 at 10:54
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Yes, a battery holder and switch from radio shack would accomplish this. Most dynamos are 6vac and can be replaced with 4xAAs or the 5vdc from a USB pack. You can ignore the issue about polarity as the dyno lights that I've worked are either incandescent and don't care or are LED but have built in blocking diodes and don't care.

Whether you would want to do this is another issue. I've gone the opposite direction and put a dynohub in my commuting bike.

Note: if you are handy with a soldering iron and multimeter, you can remove the diodes/rectifier (and likely super cap) from the LED lights and have them run in straight DC. You'd gain brightness especially when your batteries are running low because you would no longer have the voltage drop of the diodes or rectifiers, but you would need to check polarity and make sure you aren't drawing too much current.

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  • @Mσᶎ you will note that this was in my footnote where i specify the removal of rectifier and the direct connection to the DC voltage source that the OP wishes to use. – RoboKaren Jul 24 '14 at 4:12
  • Removing the rectifier should have no effect on a modern light since the LED will be controlled by a constant-current driver circuit. You might get longer run time if the regulator part of the circuit is switch mode (or it might just run hotter), but you'd need to look at the actual light you have. – Móż Jul 24 '14 at 4:42
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    @Mσᶎ if the OPs light uses a current limited circuit, there'll be no difference. Many cheap LED dyno lights only use a resistor as they know/assume that the dynohub source is itself current limited. They also tend to under power things for longevity. Also, the OP proposes to use 4xAA which if he uses Nimh may not give enough voltage at the tail end of the discharge curve. In that circumstance, removing the rectifier will yield better performance and longer run times. – RoboKaren Jul 24 '14 at 6:47
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    I just want to do this because I have had issues with rim dynamos in the past and want a reliable power source, and I am to lazy to install a hub dynamo. It is also a cost issue, I already have the rechargeable batteries, but would have to buy a new dynamo. This is my city bike, that could get stolen at any time, so I do not want to invest a lot of money in componets. – Paul Weber Jul 25 '14 at 10:47
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    Makes sense. Rim dynos are fussy. If this were my bike, I'd use a rechargeable USB power pack as the source of clean, easily rechargeable 5vdc. For the hookup, cut a standard usb cord in half and use the red and black leads. – RoboKaren Jul 25 '14 at 12:33
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I've done something similar as a temporary setup and had no issues.

I had a new bike with an incandescent headlight, capacitor-backed LED taillight and a cheap bottle dynamo. I took a USB cable, cut off the B plug, hooked up the power wires to the lights and plugged the other end into a USB power bank. I didn't even remove the dynamo – if you do that, the dynamo will get warm and drain the battery faster, but even so the power bank lasted for some 2 hours. I didn't test it for long, though, as I installed a hub dynamo and all-LED lights soon after.

Points to consider:

  • Voltage is unlikely to be an issue. Bike dynamos have a nominal voltage of 6V, but this depends on your speed and if you are going fast, the voltage can get significantly higher than that. Lights are designed to cope with that, thus 5–6V from a battery pack will not hurt them. I found the amount of light I got with the 5V from the USB power bank to be sufficient.
  • If you have LED lights, polarity may matter, depending on the circuitry inside your lights. If everything stays dark, swap the +/- wires and try again. There's no risk here – the dynamo is an AC source, which means the polarity is "wrong" half of the time, so the lamps should be able to cope with that.
  • Don't do as I did – remove the dynamo. The dynamo is the only part of the setup which may cause trouble (since dynamos aren't designed to be powered by a battery). They definitely will drain your battery faster (almost twice as fast in my lab tests) and transform that energy into heat – if you're unlucky the dynamo might overheat.
  • Personally I'd recommend rechargeable batteries over alkaline ones – I can recharge them when I have the chance to and thus don't get surprised by flat batteries when shops are closed or the next one is far away.
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In theory there's nothing wrong with that idea. In practice there are a few things you'll need to consider.

  • LEDs have a cutoff voltage below which they won't work, so you'll need to stay above that

  • You'll need a switch or something

  • This will all need to be waterproof

  • LEDs are directional, if you try to power them backwards they won't work

There's nothing inherently wrong with the idea, though, so I'd say go for it.

Or just buy some new lights, which is probably what I'd do. You can get a decent set of LED lights for $20.

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  • As RoboKaren indicates, most dynamos are AC. The LED lamps may be designed such that they depend on having AC, or may just blindly rectify the "juice" such that they won't notice that it's DC. One tricky issue is whether they depend on having AC to perform voltage regulation (not unlikely given that a dynamo has such variable output). If they depend on AC to enable the regulator then they may well self-destruct when powered with DC. – Daniel R Hicks Jul 23 '14 at 19:46
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    LEDs only care about current. You could feed 100vdc into an LED as long as the current was regulated. (Pendants will note the voltage drop is over the resistor, etc.). Most cheap dyno LED lights I've seen don't care much about efficiency and use either a simple resistor or LM618 after the rectifier. Battery powered lights are more power conscious and the better ones use switching circuitry. Since the OP is going from a dynohub to battery pack, he's safe. – RoboKaren Jul 23 '14 at 20:57
  • Sounds like you should listen to RoboKaren on this one, they seem to know more about this than I do. – stranger Jul 23 '14 at 21:27
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    Like @RoboKaren says, it's current that matters. And since most dynamos are effectively current sources (except the ones with in-built regulators) at one stage I ran a 5W white LED straight off a Schmidt hub dyno. It worked brilliantly since the LED needed 500mA and that's what the dyno puts out. – Móż Jul 24 '14 at 2:18
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Understand that an LED circuit consists of a voltage/current source (dyno or battery), a current limiter (resistor or something more exotic), and the LED itself.

"LED" means "light emitting diode", and a "diode" is a one-way switch -- a "rectifier" that, in other situations, can be used to convert AC to DC. So current only flows through an LED in one direction.

An LED has a "voltage drop" in the "forward" (conducting) direction of about about two volts. This means that no matter how much voltage you attempt to apply (up to the point that the LED catches fire), the voltage across the LED terminals will be about 2 volts, with the remaining voltage being "dropped" by resistance elsewhere in the circuit. So typically, for, say, a 6 volt supply, a series resistor will be added to the circuit to such that the voltage drop across the resistor would be 4 volts at the target current (where V(voltage) = I(current) * R(resistance)). This is a simplistic "constant current" power supply.

Since a dynamo does not produce a constant voltage (it inherently varies with speed), a simple resistance-based constant-current supply is not optimal. So circuitry is likely to include a transistor or two or three to regulate the current in a different fashion. Another approach, where AC power is available (as it is with a dynamo) is to "switch" the power so that current only gets through for part of the AC "cycle". (This is the scheme used by household light dimmers.)

So the issue here is not so much the LED itself, but the associated current-limiting circuitry. A simple transistor-based current limiter will probably work fine. A "switching" scheme will likely fail to work properly when fed DC, and there is some chance of circuit damage.

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  • I've yet to see an AC based switcher in a dynohub light. Much easier to rectify to DC and PWM there. If you can find one, I'd be surprised. Just not worth the effort given that the dyno AC voltage is higher than the LED forward voltage in almost all situations. – RoboKaren Jul 25 '14 at 13:01
  • See DIY circuit diagrams here: candlepowerforums.com/vb/… – RoboKaren Jul 25 '14 at 13:06
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I just can't think of any reason for your idea not to work, but to be sure, carefully examine both the dynamo and the lamps looking for any identification of operating voltage.

Otherwise, do a search for your Dynamo and light system specifications. If none of these yields any results, then, if you are unsure on voltage, do a simple test with pieces of wire and maybe electrical tape, adding batteries one by one until the brightness of the led resembles its normal level when connected to the dynamo.

LEDs are not supposed to be connected directly to a power source without a resistor in series, but it is almost sure that this resistors are already in place in the circuit board inside the lamp units, so I would not worry at all.

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  • Maybe worry a little bit, just out of an abundance of caution. – Scott Hillson Jul 24 '14 at 0:33
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    Being so scare of everything is what is causing the maker-spirit of this country (choose your own country) to go down the drain. It's very very very hard to hurt yourself with the voltages and currents involved here (of course says the girl who zapped herself with 240vac when she was 7 years old -- do not try THAT at home). – RoboKaren Jul 25 '14 at 16:49
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    Yes @RoboKaren. That's why I would't worry. I've been playing with lamps, bulbs, leds, batteries, and ac since I was 10. I have disassembled may cheap led flashlight/torches and all of them had a resistor in series with the led, wich simplifies using the circuir on any DC source with the same voltage. Even have replaced kripton bulbs with ready made LED equivalents and no problem has arised. – Jahaziel Jul 25 '14 at 18:24

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