Advertise on Bikeforums.net



User Tag List

Results 1 to 7 of 7
  1. #1
    Senior Member
    Join Date
    Jun 2008
    Posts
    78
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Battery backup to 6V gen?

    Hi, I'm running a 6V, 3W light setup (Union brand). What I'm interested in is adding battery backup for illumination at slow/no speed, as well as a redundant system in case of gen. failure, plus if I just want to be lazy and let Duquesne light do all the illumination work, lol.

    I happen to have 10, Ni-mh batteries that output ~1.35V each. I was thinking that I'd wire 4 of them in parallel with the gen, and at lower speeds, the batteries would "win" the voltage war and drive the lights. At higher volts, the gen would win.

    Three Q's:
    1. Do I need to put a diode coming off the batteries? (At high speeds, the gen would otherwise attempt to "charge" the batteries, and I'm not aware if there'd be an overcharge concern.)
    2. Do I need to put a diode coming off the gen?
    3. Does this sound otherwise workable?

  2. #2
    Bill
    Join Date
    May 2007
    Location
    HIGHLANDS RANCH, CO
    My Bikes
    Specialized Globe Sport, Specialized Stumpjumper FSR Pro
    Posts
    630
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Just hooking the batteries directly might work but in a very limited way.
    1. Batteries may overcharge depending on the generator characteristics and lead to premature battery failure.
    2. When stopped the batteries would discharge into the generator as well as the light so not good.

    You do need some sort of isolation between battery and generator to properly control charging and discharging. A diode is a simple solution to discharging into the generator.

    Another important question to answer is: Does the generator put out AC or DC? AC is OK for the light but not so for the battery.
    [SIGPIC][/SIGPIC]
    Eventually you will reach a point when you stop lying about your age and start bragging about it. - Will Rogers

  3. #3
    Senior Member
    Join Date
    Mar 2005
    Location
    South Florida
    Posts
    726
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Why not just isolate one from the other with a couple of cheap switches from Radio Shack?

  4. #4
    Senior Member
    Join Date
    Jun 2008
    Posts
    78
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Thanks for the responses.

    Didi a bit of looking on the 'net and Sheldon Brown covered this already here: http://www.sheldonbrown.com/dynohubs.html

    Sounds like he just hooked them up in parallel and let the gen charge the batteries. The only thing I wondered was if an unregulated charge of the batteries might harm them (recall from my flying days that Ni-Cds are subject to a peculiar little phenomenon known as "thermal runaway.")

    Pretty sure it's DC because I already tested output w/ a voltmeter and got good results...imagine it'd have a hard time making heads or tails out of AC current in DC setting.

  5. #5
    ol' Icebeard jbabic's Avatar
    Join Date
    Jul 2008
    Location
    Cleveland, OH
    My Bikes
    Dynamic Crossroad 8
    Posts
    148
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    You could try these:

    Altronix AL624 6v/1.2a power supply
    Altronix SMP3 6v/2.5a power supply
    Altronix SMP5 6v/4a power supply

    Just keep in mind that they are designed for charging SLA batteries, not nicd or nimh. I think a hub dynamo could replace the transformer input to these units and charge the batteries during riding time with lights off.

  6. #6
    Scott n4zou's Avatar
    Join Date
    Jun 2006
    My Bikes
    Too Many
    Posts
    2,393
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Here's how you do it.

    4 diodes form a bridge rectifier converting AC to DC current.
    S2 disconnects the batteries from the bridge rectifier and dynamo.
    S1 turns the LEDs on and off.
    The 4 Ni-MH batteries must be solder tab type with no possibility of poor or open connection.
    Paralleled resistors are used to obtain the required values.
    350mA LED is used in the headlight and the 36mA LED is used in the taillight.
    The LEDs must not use more current than the dynamo can produce. The small excess current recharges the batteries as you ride compensating for stops when the dynamo is not producing power. Closing S2 and opening S1 allows rapid recharging of the batteries at 500mA while riding.
    Last edited by n4zou; 10-28-08 at 10:07 AM.
    [SIGPIC]http://www.bikeforums.net/image.php?type=sigpic&userid=57360&dateline=1197386754[/SIGPIC]
    It's easier to pick a Yankee tourist than a bail of cotton.

  7. #7
    Scott n4zou's Avatar
    Join Date
    Jun 2006
    My Bikes
    Too Many
    Posts
    2,393
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by bcubed View Post
    Thanks for the responses.

    Didi a bit of looking on the 'net and Sheldon Brown covered this already here: http://www.sheldonbrown.com/dynohubs.html

    Sounds like he just hooked them up in parallel and let the gen charge the batteries. The only thing I wondered was if an unregulated charge of the batteries might harm them (recall from my flying days that Ni-Cds are subject to a peculiar little phenomenon known as "thermal runaway.")

    Pretty sure it's DC because I already tested output w/ a voltmeter and got good results...imagine it'd have a hard time making heads or tails out of AC current in DC setting.
    The batteries act as there own voltage regulators. As output voltage from the dynamo increases there internal resistance also increases. The resistance reaches a point where the dynamo is so loaded down that it can't produce more voltage. 4 series connected rechargeable Ni-MH batteries will limit voltage to 5.2 volts.
    [SIGPIC]http://www.bikeforums.net/image.php?type=sigpic&userid=57360&dateline=1197386754[/SIGPIC]
    It's easier to pick a Yankee tourist than a bail of cotton.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •