I'm new, and most of my experience is putting together simple circuits from diagrams, (and I don't even understand what half the prohibited topics are), but I'm hoping you can help with my simple project.

The problem: I have an emergency dynamo powered radio with nicad batteries. It's has sat so long the batteries are dead.

Proposed solution: Since I only need a few minutes of run time I'd like to replace the nicad batteries with a supercapacitor. Then however long it sits when I need it it's easily charged and usable, however briefly.

The 2 AA size Nicad batteries are already wired and a switch changes things from charging mode to operating mode. Can I simply replace the batteries with a power supply supercap (or three) configured in the same polarity as the batteries?

In reviewing everything I can find from searching the forum to the math my small electronics library it looks doable... or am I really off?

 

Everything with energy inside it will leak - even super capacitors. The best solution is to unpower the device when it is not in use and keep several sets of fresh batteries for when you need it.

 

It depends upon how well regulated the output of the dynamo is. If the dynamo is using the NiCds to regulate the voltage then a super cap wouldn't work directly. In that case you could them with an added zener to clamp the voltage at perhaps 3V.

Super caps may not give you the desired run time. How large are the caps you want to use and what current does the radio draw? Also what is the minimum operating voltage of the radio?

 

Thanks for your responses!

Gee, papabravo.. what fun is that?

Hey crutschow- It's a little tiny radio that only draws about 350 mA and I'm thinking I could squeeze in three or four 4F backup power supply caps. I haven't yet removed the nicads and tested the dynamo voltage output.. though there's obviously rectification of the ac dynamo already.

 

Oops... voltage would be 2.5 to 3....

 

Thanks for your responses!

Gee, papabravo.. what fun is that?

Hey crutschow- It's a little tiny radio that only draws about 350 mA and I'm thinking I could squeeze in three or four 4F backup power supply caps. I haven't yet removed the nicads and tested the dynamo voltage output.. though there's obviously rectification of the ac dynamo already.

350 mA is a tiny little radio!??! At 3v that is almost a watt.

 

You must be an enginneer Papabravo... guess it's the same wattage as my idea!

Practicality isn't a consideration... if toy makers can make capacitor driven airplanes that make one spectacular flight and are then are lost forever I know I can get enough energy to power that "little" radio for a few minutes. Plus I'm just reading what the external power supply label says... reality often differs from manufacturors recomendations.

 

Chemical batteries give much higher energy density - more power per volume and per weight - than a capacitor and I believe they leak far more slowly as well.

For example, a big capacitor is 1F. At 3 volts, that's 3 amp-sec of power, or just 3/36 mAh. A typical AA is 2500mAh or more.

If you want the battery to charge up faster, just get one with a much lower capacity. I've seen solar light batteries are as low as 300mAh, which would charge up 8 times faster than the typical one.

 

thanks wayneh... maybe I could two minutes of operation (I saw some 10F 2.5 volts supercaps that would fit giving me 40F) .. more than enough. (and then I can throw the radio in a drawer and it willl be ready to go if there ever is an actual emergency!)

Time to proceed... nothing ventured nothing gained!

 

What do those shake charge flashlights use? I assume they use super caps, but I also assumed that the hand crank emergency radios used super caps too before I read this thread.

 

I'm almost certain mine has a small battery, but I'll double check. Built-in voltage regulation is a big advantage to a battery.

Found it! It's labeled NiMH, 40mAh. That's the shaker. I'm not in the mood to crack open my little crank light right now, but my money's on there being a battery in there.

 

I'm almost certain mine has a small battery, but I'll double check. Built-in voltage regulation is a big advantage to a battery.

Found it! It's labeled NiMH, 40mAh. That's the shaker. I'm not in the mood to crack open my little crank light right now, but my money's on there being a battery in there.

Wow, ok. I learn something new every day!

 

Shake light; LED, magnet, coil, diode, 2 ea CR2032 coin cells[ non rechargable],& jr super cap. After batteries finally died, no amount of shaking produced any light, After dissasimbly, with just coil, magnet & LED, it will flash for each pass of magnet, or about 41 μAhr/flash.
My emergency crank radio runs about 5 min on spring power after a minute of cranking.
I'm surprised at Ni-Mh batteries beeing used- complete discharge is not recomended.