Trickle charging a button cell battery using an energy harvester?

Hi Elliott,

The problem is not the current but rather the voltage. In order to recharge a battery it is necessary to have a voltage that is higher than the battery voltage. The current that is then supplied by the charging voltage is determined by the internal resistances of the battery and the charging voltage supply. The other bump in the road for you will be that most button cell batteries are not designed to recharge. With the level of voltage and current available from your energy harvesting there is no danger but you may not get a very efficient recovery of energy. One solution that you might want to try is to use a super capacitor to store the energy from the harvesting. Here is one that Newark sells.

http://www.newark.com/illinois-capacitor/505dcn2r7q/super-capacitor-aluminum-elect/dp/02P5131

John

You can't get more energy out than you put in so the average power available for your LED is going to be about 0.15 X 1.1 = 0.165mW  which is not very bright !

If you used the capacitor John suggests it leaks some current (about 0.03mA according to spec) so you might hope to charge it to 1V but the average charge current will be only about 0.12mA.

To get up to 1V will take 5/0.00012 seconds = 41666 = 11.57 hours.

Now to work the LED you'll need some sort of voltage boosting circuit - Google "single cell led driver" and avoid anything with the words "joule thief" in the description (why  - because switching up converters are engineering and "joule thief" is snake oil speak !!!).

The boost circuit will be about 80% efficient (could be better if you do it well).

Suppose you keep your LED on until the cap voltage drops to 0.5V - the energy in the capacitor = (CxVxV)/2 so it starts with 2.5J and drops to 0.625J giving you 1.875J into the booster and 1.5J into the LED.

So after 12 hours of charging you can have 1.5W for one second, or perhaps more usefully 50mW for 30seconds.

MK

Hi Elliot,

michaelkellett has given us a very good analysis of how much power you have available from energy harvesting and the time lines involved. I assume that you are doing this for fun as there isn't a lot of practical application for such a small amount of energy. Instead of using a convertor as proposed by Michael you could charge 2 or 3 capacitors and then hook them in series to get the voltage that you need for the LED. In my experiments with some super capacitors I have been able to get 15 to 20 minutes of illumination before the voltage level drops too much. You are right though in that the voltage of a capacitor will begin to drop and without the chemical supply of energy that is provided by a battery it will be like slowly emptying a glass of water.

Sounds like a cool experiment.

John

Thanks for the help! I going to buy 3, 4v super capacitors. Heres the link: http://www.mouser.com/Search/ProductDetail.aspx?R=MAL219691113E3virtualkey59420000virtualkey594-MAL219691113E3.

AS of right now im pulling around 3.7v and about 35mA. I was curious, I really would like a much higher current I was researching into the use of transitors and how they seem to somehow amplify current. Could you explain if thats possible?! I beleive that would be greatly benifical to my project for school. And I could probably get a much higher lumen led if I can raise my current somehow!

You can't get energy from nowhere. If your power source is only 0.165mW that's the most power you can ever get on average. You can store it in capacitors and let it out in short bursts or use electronics to convert  a low voltage into a higher one, but you'll get less current at the high voltage.

When we talk about transistors amplifying current we mean that a small input current controls a larger output current but the power must come form an additional supply.

35mA @ 3.7V is 130mW or 787 times as much power as you are harvesting - if you want more drive to your LED you'll need a bigger harvester.

MK

Thats what I figured. Is it possible to wire up to harvesters in parallel series? Right now im using an ltc 3105 energy harvester demo board that I bought of digi key. IS it feasible to wire up another energy harvester that takes my 3v and boosts it to lets just say 9V? I know with that thought my current will drop dramatically due to ohms law but still, possible??

Hi Elliott,

The voltage of a power source times the current it is supplying is equal to the power. The laws o physics demand that power must be conserved. This means that we can raise the volts and lower the current or raise the current and lower the voltage but their product must always be equal under ideal circumstances. In the real world we always loose some power when we make voltage or current conversions. This power (energy) is lost to the environment in the form of heat.

You can add your extra energy harvesters in series to increase your voltage (current will stay the same) or you can add them in parallel to increase your current (voltage will stay the same). If one harvester is supplying 0.165 mW of power (energy) then 2 harvesters hooked in either series or parallel will produce twice that much or 0.33 mW.

Keep exploring and experimenting as this is the way new inventions and new ideas are born. The other person who has been helping you with this question, Michael Kellett, is one of the best Engineers on the site and you can take the information that he provides to the bank. He has helped me many times to better understand the circuits I was experimenting with.

John

Hey John and Michael

     So I found a type of storage evaluation board that sparked my interest. I may be completely reading this wrong, but the input current is 20mA and the capacity is anywhere from 300mAh to 8000mAh. With a input voltage of 2.55V and a output of 2.55V. Would this circuit work with my project? I really need to amplify my current and I saw this and it sparked my intrest... Heres the link: http://www.element14.com/community/docs/DOC-68623/l/evaluation-board-for-bq28550-r1

With a small change in my energy harvesting im pulling 3.3V at 40mA. Would this just take forever to charge? even if it gets all the way to 1000mA which is my goal?

This board is no use to you at all - the chip is for measuring the charge going in and out of a cell, requires a micro to operate it and is not recommended for new designs !

I think you need to study some basic stuff about electricity and batteries.

Can you post a schematic or some pictures of your set up and how you are measuring the output from it and it might be possible to give you some more suggestions.

MK

Hi Elliott,

I agree the michaelkellett  about the board you have listed. I  am curious how you are arriving at your stated 3.3 volts with 40 mA from your energy harvester. Are these published numbers or are you actually measuring these values as the harvester works into a load? It would be fun to see some diagrams and pictures of your project.

John

Hi Elliott,

I agree the michaelkellett  about the board you have listed. I  am curious how you are arriving at your stated 3.3 volts with 40 mA from your energy harvester. Are these published numbers or are you actually measuring these values as the harvester works into a load? It would be fun to see some diagrams and pictures of your project.

John