Details

In my other project, #Coin Cell Jump Starter, I'm trying to start my car with the energy extracted from a single coin cell. I may or may not succeed before the contest deadline, but I've learned a few things:

Coin cells may deliver a large amount of energy, but only over long periods (they deliver low power)
Supercapacitors can deliver high power for a short amount of time, but are plagued with high self-discharge
Nickel-based batteries (NiCd, NiMH) can deliver lots of energy at high powers, and have much lower self-discharge than supercapacitors

So, you can't efficiently charge supercapacitors directly from coin cells: the capacitors lose too much energy during the long charging times required to extract maximum energy from the cell. But, you can charge other batteries at least somewhat efficiently from coin cells.

One issue remains: how do you prove there wasn't a bunch of energy already in the battery you are charging? It turns out that you can (and according to NASA, should) store NiCd cells with their terminals shorted, in a zero state of charge. So, I discharged a bunch of 600mAh NiCd cells safely through a resistor, then made some aluminum shorting bars to ensure they were fully depleted:

After being shorted overnight, there is only negligible charge left in the cells. (Note: the cells pictured above are different ones than in the project photo. I have 6 sets of cells for various experiments. They all have been stored shorted like this.)

The Test

So, to test how well this all works, I built a constant-current switchmode charger to charge 4x AA NiCds from a CR2477 cell overnight. In my tool bin, I found an old AA-battery-powered screwdriver I had originally purchased to harvest the motor and gear train for a robot drive:

The battery holder says you should only use alkaline batteries, presumably so nobody puts lousy "heavy duty" cells in there. I'll still take a little pleasure in breaking the rules by using NiCd's instead.

How many screws can you drive with the energy from a CR2477? Watch the build logs to find out.

Project Logs

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19 Screws! (OK, maybe 3 screws?)
Ted Yapo • 01/07/2018 at 15:56 • 4 comments

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Not great, but something. If you count just getting them to start, it's 19, but really only 3 were properly driven.

The NiCd's were charged in about a day, so the current drain on the CR2477 was fairly high (for a coin cell), and not much energy was extracted to the NiCd's.

I have some others charged over longer periods, but I'm saving those for my #Coin Cell Jump Starter .

Control

After this test, I decided to see how many screws the same cells could drive if charged fully in a normal charger.

Answer: at least 27! That's all the holes I had. So, the coin cell charged the NiCd's to no more than 11%. I suspect it was less than that, though, since the screwdriver still had enough juice to remove all those screws after I stopped the video!

If I end up with some extra cells after the jump starter experiments, I might try this again.

I always thought this screwdriver was lousy, but I never tried it with anything but alkaline cells. With NiCd's, it's actually not that bad...
NiCd Charger
Ted Yapo • 01/06/2018 at 16:29 • 0 comments

I hacked a commonly-available MT3608 boost converter PCB into a constant-current trickle charger:

The MT3608 is an SOT23-6 boost converter IC from Aerosemi. The cheap PCBs incorporate a 25-turn pot to control the output voltage. The pot is used to sample a fraction of the output voltage for the feedback connection to the chip. I unsoldered the pot and replaced it with a fixed resistor in the negative output line. This is a common trick to convert switching converters to produce a constant current output:

Here's a partial schematic of the modification:

Using a constant-current output lets me limit the current drawn from the coin cell during charging. This is a key feature, since the amount of energy you can draw from the coin cell is a strong function of current: the lower the current, the more energy you can (eventually) get.

Since the MT3608 compares the feedback voltage to an internal 0.6V reference, the output current is equal to 0.6/R1. For the first test, I chose a 360-ohm resistor, for an output current of 1.7mA. Since the coin cell has a nominal voltage of 3.0, while the 4x NiCd's should be around 4.8 during most of the charge cycle, I expect a current drain from the coin cell of around 2.7mA. In reality, the cell voltage quickly dropped to around 2.5V, while the NiCd voltage is around 5.0V, so the input current is more like 3.4mA.

Before you (ab)use the MT3608 like this for other constant-current purposes (like LED driving), you should check that the feedback is stable. The way I'm using it, the charging NiCds act like an enormous filter capacitor on the output, so there is no problem.

A day later, and the cells are still charging. After the coin cell voltage drops significantly below 2.0V, I'll stop the charging and drive some screws!