Hi folks,
I have a some questions about dynohubs and batteries that I hope you can help with.
On my 15 mile commute to work on unlit undulating bike paths I average 14mph. Temperature is never below freezing.
I'd like a dynamo powered light setup incorporating a not-too-large battery pack that ensures full light output whenever the hub is spinning too slow to achieve this, while remaining relatively power neutral, beginning and ending each ride at a similar charge level without intervention on my behalf.
To this end I figure I could have three Cree XML2s in series, with a parallel 2000mAh 8.4v (7 x 1.2V) NiCad battery pack, powered from an SP 8 series generator hub.
The XML2 datasheet suggests that they draw 500mA at 2.8v. With the hub supplying roughly 500mA and the battery supplying roughly 2.8V (8.4V/3) per LED, I expect the current draw would be very similar whether running from generator or battery power, thereby preventing the LEDs drawing current from the battery while the hub is supplying full power.
Using the 3xLED series plot on Pilom's power vs speed chart as a guide, I believe my average speed would likely only marginally exceed that required to fully power the LEDs. Factoring in the self-discharge rate and 50-70% charge efficiency of nickel batteries, I would expect negligible overcharging.
http://www.pilom.com/BicycleElectronics/MultiLedCompHub.gif
I don't mind tinkering and trial and error, nor overly care if the battery only survives one winter, however I would prefer to have a better understanding so that I can purchase suitable components.
Essentially I'd really appreciate your clearing up my assumptions and explaining how the generator, battery and LEDs would likely interact in practice, factoring in all the things I've inevitably overlooked and don't understand.
The sort of questions that come to mind:
What would the combined load look like that the hub would see in this configuration? How dependent is it on the battery charge level, impedance, and capacity?
Would an empty battery situation draw most of the current from the generator, leaving little for the lights?
Do the LEDs somehow clamp the operating voltage? Enough to prevent battery damage? Or perhaps all the way down to their combined 500mA Vf of 2.8v, meaning at full power the three LEDs would restrict voltage to 8.4V (3 x 2.8V), thereby leaving no voltage overhead for the 8.4V (7 x 1.2V) battery pack to sufficiently charge?
If the battery pack is unlikely to charge in parallel, how about instead running it in series with the LEDs? Pros? Cons?
I'm led to believe NiCad is most suitable, but might NiMh actually be the preferred battery chemistry in this specific application?
Might a smaller low power hub (e.g. SP 9 series) provide sufficient power (approx. 6w) and be preferred in this application (to help prevent possible battery overcharging)?
I do apologize for the barrage ... without an understanding of electrical theory it's been difficult for me to even frame the questions let alone find the answers in the public domain.
Thanks,
Glenn
I have a some questions about dynohubs and batteries that I hope you can help with.
On my 15 mile commute to work on unlit undulating bike paths I average 14mph. Temperature is never below freezing.
I'd like a dynamo powered light setup incorporating a not-too-large battery pack that ensures full light output whenever the hub is spinning too slow to achieve this, while remaining relatively power neutral, beginning and ending each ride at a similar charge level without intervention on my behalf.
To this end I figure I could have three Cree XML2s in series, with a parallel 2000mAh 8.4v (7 x 1.2V) NiCad battery pack, powered from an SP 8 series generator hub.
The XML2 datasheet suggests that they draw 500mA at 2.8v. With the hub supplying roughly 500mA and the battery supplying roughly 2.8V (8.4V/3) per LED, I expect the current draw would be very similar whether running from generator or battery power, thereby preventing the LEDs drawing current from the battery while the hub is supplying full power.
Using the 3xLED series plot on Pilom's power vs speed chart as a guide, I believe my average speed would likely only marginally exceed that required to fully power the LEDs. Factoring in the self-discharge rate and 50-70% charge efficiency of nickel batteries, I would expect negligible overcharging.
http://www.pilom.com/BicycleElectronics/MultiLedCompHub.gif
I don't mind tinkering and trial and error, nor overly care if the battery only survives one winter, however I would prefer to have a better understanding so that I can purchase suitable components.
Essentially I'd really appreciate your clearing up my assumptions and explaining how the generator, battery and LEDs would likely interact in practice, factoring in all the things I've inevitably overlooked and don't understand.
The sort of questions that come to mind:
What would the combined load look like that the hub would see in this configuration? How dependent is it on the battery charge level, impedance, and capacity?
Would an empty battery situation draw most of the current from the generator, leaving little for the lights?
Do the LEDs somehow clamp the operating voltage? Enough to prevent battery damage? Or perhaps all the way down to their combined 500mA Vf of 2.8v, meaning at full power the three LEDs would restrict voltage to 8.4V (3 x 2.8V), thereby leaving no voltage overhead for the 8.4V (7 x 1.2V) battery pack to sufficiently charge?
If the battery pack is unlikely to charge in parallel, how about instead running it in series with the LEDs? Pros? Cons?
I'm led to believe NiCad is most suitable, but might NiMh actually be the preferred battery chemistry in this specific application?
Might a smaller low power hub (e.g. SP 9 series) provide sufficient power (approx. 6w) and be preferred in this application (to help prevent possible battery overcharging)?
I do apologize for the barrage ... without an understanding of electrical theory it's been difficult for me to even frame the questions let alone find the answers in the public domain.
Thanks,
Glenn