Capacitors? Why not NiCd's?

A project log for Coin Cell Jump Starter

Starting a car with a CR2477 coin cell

Ted YapoTed Yapo 12/17/2017 at 04:245 Comments

So @EricH gave me an interesting idea.  He asked if using an intermediate step with another set of capacitors could help with the energy transfer problem.  If you could find large-valued capacitors with low self-discharge, you could take a long time to charge those with a coin cell, then charge the supercapacitor quickly from the intermediate caps. It sounds like it could work, but I don't think the right capacitor exists for this intermediate step.

What about using a rechargeable battery as the intermediate energy storage?  This gets interesting.  Everyone seems willing to allow an electrochemical capacitor as intermediate storage, so why not a rechargeable battery? (I'll refer to the coin cell as a "cell" and the rechargeable battery as a "battery" in the discussion below).

Let's forget about any technical problems for a minute and consider the contest judges and spectators.  You have to convince them somehow that you're not running anything from energy pre-stored in the battery.  Since state-of-charge is very difficult to measure accurately, I'm not even sure I wouldn't be cheating with most battery chemistries.  The exception is NiCd, which can and should be stored with the terminals shorted and at a zero state of charge.  It's how NASA stores their NiCd cells, as detailed in this technical report on NiCds.  So, if I take a couple of AA NiCd's that have had their terminals shorted for a few days, then verify there is 0V across them, I think I can make a convincing argument that there's no energy hidden up my sleeve.

OK, so there's a way to verify that all the energy is coming from the coin cell.  What are the properties of a NiCd battery? 

Overall, they sound like a good intermediate reservoir for energy storage.  They have a much lower self-discharge rate than supercapacitors, so can be charged slowly from a coin cell without terrible losses (a DC-DC converter is still required).  Then, once charged, they can be drained very quickly to charge the supercapacitor before supercap self-discharge becomes an issue.

What are the drawbacks?  First, the energy will be going through two DC-DC converters, so losses get compounded there.  Also, there's the charging efficiency of the NiCd's. Wikipedia mentions that at a C/10 charge rate, you have to apply around 1.5C of charge to fully charge a NiCd (equivalent to a 33% loss of energy).  The NASA TR, however, shows that this ratio is a strong function of temperature (P. 13).  With battery temperature near 0C, the ratio approaches 1, so much less energy is lost in charging.

So, can I take a 1.7Ah LiSOCl2 cell, charge some 1000mAh AA NiCd's, then use the NiCd's to charge a 67F capacitor to 14V?  Here's how everything stacks up:

StorageCapacity (mAh)Energy (J)
2x NiCd AA10008600
3x NiCd AA100012960
4x NiCd AA100017280
67F capacitor @14V -6566

From the TL-5935/P datasheet, it looks like I can get the full 1.7Ah from the cell at 10mA, which is the maximum recommended continuous drain.  I also estimate that the cell voltage will remain stable at around 3V for the entire discharge.  Discharging at this rate will take 7.08 days.  Assuming the 1% per day self-discharge rate for NiCd's, I might lose 600J during this week. Assuming a 33% loss due to NiCd charging inefficiency (which might be improved by cooling), and a 70% DC-DC converter efficiency, I end up with 8961 J in the NiCds, which almost fits in 2 AA's. I'll call it 8600 which just fits.

In order to charge the capacitor to 14V with the 8600J in the NiCd battery, I need a DC-DC converter with around 76% efficiency.  Since the NiCd's can tolerate high current drain, I can use an efficient off-the-shelf converter, so that might not be too difficult to achieve.

It ain't pretty, and there's not much slack to play with, but it looks possible to start a car with a coin cell again.

EDIT: I found that charging efficiency for NiCd cells is nearly 100% until they reach 70% capacity.  So, I can gain a lot of efficiency by using more NiCd cells and charging them to less than 70% capacity.  Nice.

Also of interest is the PowerSonic NiCd Catalog.  I need to pour over those graphs.


jaromir.sukuba wrote 12/17/2017 at 20:08 point

Can't you just start the car with Ni-Cd battery once charged? They would probably cry a little, but that's what we want to see here.

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Ted Yapo wrote 12/17/2017 at 20:42 point

I thought about this.  I think you'd need 12 NiCd cells in series to have enough voltage (14.4 nominal).  To charge that many cells, they'd have to be very small, much less than an AA - I think I can only charge 4x AA NiCds at most (I think).

Lets say you can get 100C current from a NiCd.  To get 100-200A to crank the starter, the cells then need 1-2Ah capacity, which is 1-2x a high-capacity AA.

So you can't charge enough NiCds of the size required to provide the instantaneous power.

To start the car with minimum energy, you want something with a high power/energy density ratio.  Supercaps even beat NiCds in this respect.

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Ted Yapo wrote 12/17/2017 at 20:46 point

Now, could I charge many small NiCd's in parallel, then connect them in series to directly charge the supercapacitor?  That might be a way to go.

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jaromir.sukuba wrote 12/17/2017 at 22:28 point

Perhaps you may want to build another DC/DC converter to make the charging controllable and most effective, transferring as much juice to supecaps as possible.

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Ted Yapo wrote 12/17/2017 at 23:07 point

@jaromir.sukuba Yep, I think I need another converter.  I might try to hack one of those cheap, lousy generic "boost converters" you see everywhere.  I have a pile of various ones in a drawer.

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