Charging NiMH cells from a hub dynamo

[Richard Bates]

With a desire to complete a cyclecamping trip with no depenence upon
mains power I would like to build a home-brew kit to charge NiMH cells
from my SON when the lights are not being used.

I already own equipment to make decent use of solar power.

My knowledge of physics and electronics is extremely rusty.

I'ver googled unsuccessfully for a how-to in English (I found a German
one at http://www.nabendynamo.de/akku.htm but the automatic
translation into English is rather more humerous than it is useful!)

http://www.mark-ju.net/bike_ride/equipment/charger.htm also gives some
useful information (see [4] below), although I'm not aiming for
anything as sophisticated as this.

Does anybody know of a useful source of information?

Additionally, to get me started, can anybody answer me a few questions
please?

1) The SON can generate 3W 6V AC. When converted to DC, would I be
correct in thinking that my usable supply is going to be around
(6*sqrt2) = 8.5V 3W DC?

2) And therefore a sensible number of NiMH cells, assuming 1.2V per
cell, to use would be 7 (possibly 6, which is a nicer number)

3) I need a rectifier to convert my generated AC into useable DC. A
bridge rectifier is more efficient than a half-wave rectifier. How
would I know what is a suitable rectifier specification?

4) For the purposes of charging cells, a smoothing component is not
necessary?

For an easier but slightly more impractical solution, I see no problem
with physically disconnecting the lamps whilst the battery is
connected; and vice-versa

No doubt I'll have more questions further into the project.

Thanks to anyone who can help.


--
Amazon: "If you are interested in 'Asimov's I-Robot',
you may also be interested in 'Garfield - The Movie'.
... erm, how do they figure that one out?

 

[m-gineering]

Richard Bates wrote:
>
> With a desire to complete a cyclecamping trip with no depenence upon
> mains power I would like to build a home-brew kit to charge NiMH cells
> from my SON when the lights are not being used.
>
> I already own equipment to make decent use of solar power.
>
> My knowledge of physics and electronics is extremely rusty.
>
> I'ver googled unsuccessfully for a how-to in English (I found a German
> one at http://www.nabendynamo.de/akku.htm but the automatic
> translation into English is rather more humerous than it is useful!)
>
> http://www.mark-ju.net/bike_ride/equipment/charger.htm also gives some
> useful information (see [4] below), although I'm not aiming for
> anything as sophisticated as this.
>
> Does anybody know of a useful source of information?

AFAIK NiMh chargers use the charging history to decide when to stop
charging. Powered by a hubdynamo with frequent stops they get confused
and will overcharge the cells.
No doubt Andreas will chime in if this is incorrect
--
---
Marten Gerritsen

INFOapestaartjeM-GINEERINGpuntNL
www.m-gineering.nl

 

[Chris Heys]

On Sat, 08 Jan 2005 19:19:57 +0000, Richard Bates
<[email protected]> wrote:

>With a desire to complete a cyclecamping trip with no depenence upon
>mains power I would like to build a home-brew kit to charge NiMH cells
>from my SON when the lights are not being used.
>
>I already own equipment to make decent use of solar power.
>
>My knowledge of physics and electronics is extremely rusty.
>
>I'ver googled unsuccessfully for a how-to in English (I found a German
>one at http://www.nabendynamo.de/akku.htm but the automatic
>translation into English is rather more humerous than it is useful!)
>
>http://www.mark-ju.net/bike_ride/equipment/charger.htm also gives some
>useful information (see [4] below), although I'm not aiming for
>anything as sophisticated as this.
>
>Does anybody know of a useful source of information?
>
>Additionally, to get me started, can anybody answer me a few questions
>please?
>
>1) The SON can generate 3W 6V AC. When converted to DC, would I be
>correct in thinking that my usable supply is going to be around
>(6*sqrt2) = 8.5V 3W DC?
>
>2) And therefore a sensible number of NiMH cells, assuming 1.2V per
>cell, to use would be 7 (possibly 6, which is a nicer number)
>
>3) I need a rectifier to convert my generated AC into useable DC. A
>bridge rectifier is more efficient than a half-wave rectifier. How
>would I know what is a suitable rectifier specification?
>
>4) For the purposes of charging cells, a smoothing component is not
>necessary?
>
>For an easier but slightly more impractical solution, I see no problem
>with physically disconnecting the lamps whilst the battery is
>connected; and vice-versa
>
>No doubt I'll have more questions further into the project.
>
>Thanks to anyone who can help.

I've used my Shimano Nexus hub dynamo to charge 5 x AA (6.6v, around
1.3v per cell) NiMH cells for over a year now. I'm aware that by
coverting from AC to DC you get a larger max voltage, but didn't
really take it into account when putting my circuit together.

I used a full wave rectifier (general 20p jobbie from Maplins
www.maplin.co.uk ). I have a semi-complex switching system to enable
me to run 6W of lights off the battery and dynamo or 3W just the
battery or dynamo.

I've found the system works well, giving me plenty of light if
required whilst providing charged AA cells for my GPS, MP3 player and
via a car charger, power for my phone and PDA. I wouldn't worry too
much about charging currents, voltages, time etc... and just go for
the simple approach. OK, it may reduce the life of your cells, but
I've used mine for a year with no noticable decrease in performance
(recently tested amp/volt output of each cell - found to be more than
adequate.) and at around £1 a cell for 2400mA/hr it's not a problem to
replace them every now and again.

Full details including a circuit diagram can be found at:
http://www.asme12.dsl.pipex.com/cycling/lighting.htm

Please excuse the website, it's on my long list of 'projects to
complete'.

If I were to do it again I would spend the time putting in a smoothing
capacitor. (if like me you run your lights DC) as at low wheel revs it
seems to flicker more than an AC setup (possibly due to recifier
reaction time??)

Let me know if I can be any more help e.g to the specifics of
parts/construction

cheers,
chris

 

[Ambrose Nankivell]

In news:[email protected],
Richard Bates <[email protected]> typed:
> With a desire to complete a cyclecamping trip with no depenence upon
> mains power I would like to build a home-brew kit to charge NiMH cells
> from my SON when the lights are not being used.

Sounds like fun. I was going to recommend finding a (smart) charger with a
6V DC input, but I don't know if it'd be easy to find. My very good & cheap
1 hour AA charger from Maplins takes 12V in, as do the others I found. Still
not entirely sure whether it's a good suggestion, though, as it might be
fussy about how smooth the power is.

A

 

[Richard Bates]

On Sun, 09 Jan 2005 00:20:21 +0100, Andreas Oehler
<[email protected]> wrote:

[useful stuff snipped, thanks)

>The biggest problem in charging NiMH cells from a dynamo is to detect,
>when the cells are fully charged and need to be disconnected. A rough
>calculation of time might help: Empty 2000mAh cells will need about 4
>hours of riding to be fully charged. In any case charging should be
>stopped, if at least one cell gets noticable warm.

How about if i use the generator output to charge a lead-acid battery?
I have a spare Cateye 6V battery, the mains charger for which outputs
12V 500mA.

Is the SLA battery more resilient to abuse from overcharging?


--
Amazon: "If you are interested in 'Asimov's I-Robot',
you may also be interested in 'Garfield - The Movie'.
... erm, how do they figure that one out?

 

[David Martin]

On 9/1/05 9:58 am, in article [email protected],
"Richard Bates" <[email protected]> wrote:

> On Sun, 09 Jan 2005 00:20:21 +0100, Andreas Oehler
> <[email protected]> wrote:
>
> [useful stuff snipped, thanks)
>
>> The biggest problem in charging NiMH cells from a dynamo is to detect,
>> when the cells are fully charged and need to be disconnected. A rough
>> calculation of time might help: Empty 2000mAh cells will need about 4
>> hours of riding to be fully charged. In any case charging should be
>> stopped, if at least one cell gets noticable warm.
>
> How about if i use the generator output to charge a lead-acid battery?
> I have a spare Cateye 6V battery, the mains charger for which outputs
> 12V 500mA.
>
> Is the SLA battery more resilient to abuse from overcharging?
>

I'm tempted to build a small wind generator for the allotment greenhouse
that would charge a car battery, which I could then use for heating
germination trays in the spring, automatic watering and light for the
evenings.

Trying to find a suitable generator is problematic though, and I'm not too
sure aboutthe electrics either.

Maybe I should risk a post in u.r.sheds?

...d

 

[Paul - xxx]

David Martin composed the following;:
> On 9/1/05 9:58 am, in article
> [email protected], "Richard Bates"
> <[email protected]> wrote:
>
>> On Sun, 09 Jan 2005 00:20:21 +0100, Andreas Oehler
>> <[email protected]> wrote:
>>
>> [useful stuff snipped, thanks)
>>
>>> The biggest problem in charging NiMH cells from a dynamo is to
>>> detect, when the cells are fully charged and need to be
>>> disconnected. A rough calculation of time might help: Empty 2000mAh
>>> cells will need about 4 hours of riding to be fully charged. In any
>>> case charging should be stopped, if at least one cell gets
>>> noticable warm.
>>
>> How about if i use the generator output to charge a lead-acid
>> battery? I have a spare Cateye 6V battery, the mains charger for
>> which outputs 12V 500mA.
>>
>> Is the SLA battery more resilient to abuse from overcharging?
>>
>
> I'm tempted to build a small wind generator for the allotment
> greenhouse that would charge a car battery, which I could then use
> for heating germination trays in the spring, automatic watering and
> light for the evenings.
>
> Trying to find a suitable generator is problematic though, and I'm
> not too sure aboutthe electrics either.
>
> Maybe I should risk a post in u.r.sheds?
>
> ..d

u.r.caravanning and motorcaravans often have this discussion too ..

--
Paul ...
http://www.4x4prejudice.org/index.php
(8(!) Homer Rules ...
"A ****** is a ******, no matter what mode of transport they're using."

 

[Richard Bates]

On Sun, 09 Jan 2005 09:58:34 +0000, Richard Bates
<[email protected]> wrote:
>How about if i use the generator output to charge a lead-acid battery?
>I have a spare Cateye 6V battery, the mains charger for which outputs
>12V 500mA.

Excuse me for thinking aloud here about my own post but there seem to
be a couple of other interested people here...

SLA seems not to be a good idea since SLA's like to be fed a constant
voltage, whereas NiMH likes to be fed a constant current.

It seems to me then that if the SON is a constant current device, NiMH
cells are more sutable.

Also, I understand that in the event of accidental short circuit
(bypassing the diodes), the current from NiMH is less likely to melt
the dynamo's internal wiring.

--
Amazon: "If you are interested in 'Asimov's I-Robot',
you may also be interested in 'Garfield - The Movie'.
... erm, how do they figure that one out?

 

[Tosspot]

David Martin wrote:
> On 9/1/05 9:58 am, in article [email protected],
> "Richard Bates" <[email protected]> wrote:
>
>
>>On Sun, 09 Jan 2005 00:20:21 +0100, Andreas Oehler
>><[email protected]> wrote:
>>
>>[useful stuff snipped, thanks)
>>
>>
>>>The biggest problem in charging NiMH cells from a dynamo is to detect,
>>>when the cells are fully charged and need to be disconnected. A rough
>>>calculation of time might help: Empty 2000mAh cells will need about 4
>>>hours of riding to be fully charged. In any case charging should be
>>>stopped, if at least one cell gets noticable warm.
>>
>>How about if i use the generator output to charge a lead-acid battery?
>>I have a spare Cateye 6V battery, the mains charger for which outputs
>>12V 500mA.
>>
>>Is the SLA battery more resilient to abuse from overcharging?
>>
>
>
> I'm tempted to build a small wind generator for the allotment greenhouse
> that would charge a car battery, which I could then use for heating
> germination trays in the spring, automatic watering and light for the
> evenings.
>
> Trying to find a suitable generator is problematic though, and I'm not too
> sure aboutthe electrics either.

Try a chandlery, they must have that sort of thing.

 

[John Wilton]

"Richard Bates" <[email protected]> wrote in message
news:[email protected]...
> On Sun, 09 Jan 2005 09:58:34 +0000, Richard Bates
> <[email protected]> wrote:
>>How about if i use the generator output to charge a lead-acid battery?
>>I have a spare Cateye 6V battery, the mains charger for which outputs
>>12V 500mA.
>
> Excuse me for thinking aloud here about my own post but there seem to
> be a couple of other interested people here...
>
> SLA seems not to be a good idea since SLA's like to be fed a constant
> voltage, whereas NiMH likes to be fed a constant current.
>
> It seems to me then that if the SON is a constant current device, NiMH
> cells are more sutable.
>
> Also, I understand that in the event of accidental short circuit
> (bypassing the diodes), the current from NiMH is less likely to melt
> the dynamo's internal wiring.
>
> --
> Amazon: "If you are interested in 'Asimov's I-Robot',
> you may also be interested in 'Garfield - The Movie'.
> ... erm, how do they figure that one out?

I think that I am correct in saying that NiMH only 'prefer' constant current
as it is then easier to ensure that they are not overcharged which is
something that will rapidly kill them.

One charging option involves starting from a known state of charge (in
practice this almost certainly means 'flat') and calculating the required
amount of charge ie 2000 mAh = 200 mA for 10 hours plus a bit to cover the
inefficiency of the system. It is not impossible to do this with a variable
current and there are 'coulomb counting' chips available for this purpose
but things are probably getting more complicated now.

The other option is using a so-called smart charger to detect the voltage
dip that occurs when the cells are full but this relies on a constant,
relatively high, charge current.

John

 

[Steven M. Scharf]

Richard Bates wrote:

<snip>

You're assumptions are all correct. A 1A bridge rectifier connected to
six cells in series, with a current limiting resistor, is all that is
required. 300mA/cell is fine, that's about the minimum commercial
charger you can buy these days.

You're not going to be able to know when the batteries are fully charged.

Slightly more complex, you could hook the output of the bridge rectifier
to a switching regulator, i.e. Linear Technology LTC1504A - 500mA Low
Voltage Step-Down Synchronous Switching Regulator. Set the output
voltage to 5V. Then use one of those USB powered AA NiMH chargers.

Don't use a non-switching regulator, as the losses are too high. The
switching regulators have an efficiency of over 90%.

 

[Steven M. Scharf]

Steven M. Scharf wrote:

<snip>

Forgot to mention, if you want information on the best NiMH chargers for
AA cells, click over to "http://nordicgroup.us/chargers/".

This is a very small sub-set of all chargers, but AFAIK, it's all the
chargers that allow individual cell charging, that are reasonably priced.

The Panasonic BQ-830 has the USB input option. Not sure if it is sold in
the UK though. If you go the route of regulating the output of the
dynamo down to 5VDC, this would be good charger to use.

There are other USB-only chargers, but these are not on listed on this
web site.

 

[Mark Thompson]

> I'm not sure how you could
> manage this with a variable, intermittant current.

[Stupid question from layman]

Does the SON provide a steady current when the bikes moving at trundly
speed or more ( >10mph )?

If he wants it for touring, could he not just take it off charge when 'stop
starting', and whack it back on charge when doing a steady speed for hours
on end?

 

[David Martin]

Tosspot wrote:
> David Martin wrote:
>
>> I'm tempted to build a small wind generator for the allotment greenhouse
>> that would charge a car battery, which I could then use for heating
>> germination trays in the spring, automatic watering and light for the
>> evenings.
>>
>> Trying to find a suitable generator is problematic though, and I'm not
>> too
>> sure aboutthe electrics either.
>
>
> Try a chandlery, they must have that sort of thing.
>

That might require spending real money which goeas against the
principles of the allotment shed...

...d

 

[Richard Bates]

On 9 Jan 2005 17:33:36 GMT, Mark Thompson
<[email protected]> wrote:

>If he wants it for touring, could he not just take it off charge when 'stop
>starting', and whack it back on charge when doing a steady speed for hours
>on end?

Good idea, but not my style of touring! I take the kitchen sink when I
cyclecamp, and even the slightest hill would bring my speed down to a
pitiful 5mph or less


--
Amazon: "If you are interested in 'Asimov's I-Robot',
you may also be interested in 'Garfield - The Movie'.
... erm, how do they figure that one out?

 

[Ambrose Nankivell]

In news:[email protected],
Richard Bates <[email protected]> typed:
> On 9 Jan 2005 17:33:36 GMT, Mark Thompson
> <[email protected]> wrote:
>
>> If he wants it for touring, could he not just take it off charge
>> when 'stop starting', and whack it back on charge when doing a
>> steady speed for hours on end?
>
> Good idea, but not my style of touring! I take the kitchen sink when I
> cyclecamp, and even the slightest hill would bring my speed down to a
> pitiful 5mph or less

I suggest a carbon fibre kitchen sink.

Ob on-topic:

I picked up a cheap dynamo set as a gift at the L(to your new job)BS, and
the proprietor insisted I had a look at his hub dynamoes while I was getting
it. Probably wouldn't be much help with esoterica, though. Russell's on
Stafford Street, Walsall is the place, though.

A

 

[James Annan]

Toby Douglass wrote:

> With NiMH, the charger detects that the battery is full by monitoring
> the batteries response to the input current; there's a certain behaviour
> exhibited when the battery is charged.

But only with a suficiently smart charger. As others have already said,
a dumb trickle(ish) charge will work well enough in practice, and NiMH
are getting cheaper all the time. So long as the dynamo doesn't really
fry them it should work fine.

James
--
If I have seen further than others, it is
by treading on the toes of giants.
http://www.ne.jp/asahi/julesandjames/home/

 

[Jon Senior]

James Annan wrote:
> But only with a suficiently smart charger. As others have already said,
> a dumb trickle(ish) charge will work well enough in practice, and NiMH
> are getting cheaper all the time. So long as the dynamo doesn't really
> fry them it should work fine.

Definitely the best suggestion. Avoid wrapping them up too well
(Exposure to the wind will help keep the temperature low) and then
charge them at something like C/10 (ie. 2000mAh should be charged at
200mA). They'll take a long time to charge, but it's a much simpler
setup than messing around with smart charging.

If you've got the confidence to go down the smart charger route though,
I've got one of the Maxim smart charger ICs (Don't remember which),
which you're more than welcome to.

Jon

 

[David Lowther]

"Steven M. Scharf" <[email protected]> wrote

<snip>
> > Slightly more complex, you could hook the output of the bridge rectifier
> > to a switching regulator, i.e. Linear Technology LTC1504A - 500mA Low
> > Voltage Step-Down
> > Synchronous Switching Regulator. Set the output voltage to 5V. Then use
> > one of those USB powered AA NiMH chargers.
<snip>
> The Panasonic BQ-830 has the USB input option. Not sure if it is sold in
> the UK though. If you go the route of regulating the output of the dynamo
> down to 5VDC, this would be good charger to use.
<snip>

Bear in mind that most USB devices do nothing useful
without being told what to do by a PC.

Any USB device that functions normally, merely because
it has 5V applied to its input, is a device that seriously violates
the USB standard.

If you can find one then it would be fine for use on the bike
but I wouldn't plug one in to my PC ;-)

This link has some useful info on charging NiMh batteries.
http://www.powerstream.com/NiMH.htm

Dave.

 

[[email protected]]

David Lowther wrote:

> Bear in mind that most USB devices do nothing useful
> without being told what to do by a PC.
>
> Any USB device that functions normally, merely because
> it has 5V applied to its input, is a device that seriously violates
> the USB standard.

This is not true. The USB standard sets limits for the amount of
current a device can draw, 500mA (though in reality you can't even set
the trip point for over-current to less than 1 amp on any of the USB
controllers I've worked with).

There are probably a hundred different devices that use just the power
from a USB port, with no connection to the data lines. As long as they
don't draw more power than the USB port can supply, it's just fine.

> If you can find one then it would be fine for use on the bike
> but I wouldn't plug one in to my PC ;-)

Whatever. But please don't believe that doing so would have any effect
on your PC at all.