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Solar powered mobile phone battery charger


Aug 21, 2012
Aug 21, 2012

I am Matt and I live in the UK. We have a store here called Lidl and a few months ago I was bought a Rocktrail Dynamo Solar-Powered LED Torch which supplies power to a ZNB branded 3.6V 110mAh Lithium-Ion(LIR2450) coin battery (a bit thicker than a motherboard coin battery) by either the solar panel or the pullcord at the bottom (Dynamo).

I recently saw a project to Reuse Old Mobile Phone Battery for LED lighting which got me thinking about my solar torch.

Well I have a Nokia BL-6F 1200mAh 3.7V battery and was wondering if I have to modify the solar panel circuit board to allow charging the 3.7V battery and what parts I may need. I do have an account on RS-Online and I live close to a Maplin store so I can get components from either/both.

I haven't used the battery for a long time, but it still reads 3.76V on the multimeter. I was also wondering what the limitations/problems would be from charging via solar panel/dynamo (I know how charging from a PC's USB port vs Mains power decreases how long my mobile phone lasts). Would the same thing happen here?

If you are able to advise me I would be very grateful.

Rocktrail circuitboard pictures




I have read the following quote at this link

As far as I know 3,7V and 3,6V rechargeables are the same thing. Those are the mean/average voltages under load and it has to do with fully-charged-open-circuit-voltage. 10 years ago li-ions were only charged up to 4.1V and the mean/average voltage under load was 3.6V. A few years later the manufucterers changed this to 4.2V to incease the capacity of the batteries. Obviuosly this alse made the mean voltage under load increase.
Even today though it still makes sense to only charge your batteries (18650s for example) only up to 4.1V as this drastically increases the life expectancy (number of cycles) of the batteries while only reducing the capacity by ~10%. Many hobby chargers allow to do this.

Does this mean I should be able to use the solar charging circuit as is, I will only have problems with undercharging? If so then I guess I am going to need something along the lines of a joule thief to boost the output. I haven't done any electronics since school over 10 years ago, so I will probably open a few new thread with other questions I have so I can get started along a winter of electronics projects.

Thanks and regards,



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