Cheap small 12V Li or Alkalines?

I am thinking of a stack of watch batteries. Since you want solder tabs, there might be a problem in the application of keeping them in contact with just spring pressure. On the other hand, I have seen batteries that do appear to be made out of several coin cells, so maybe the manufacturers have a jig that can reliably spot-weld the cases.

Browsing through Digi-Key, lithium coin cells might work:

Price (USD per assembly for 1000 assemblies) mAh Qty Cell D x H * Pana Energizer 48 4 BR1225 12.5 x 10 2.45 --- 55 4 CR1616 16 x 6.4 2.12 2.13 55 4 CR2012 20 x 4.8 2.19 2.24 90 4 CR2016 20 x 6.4 0.64 0.69

165 4 CR2025 20 x 10 0.61 0.80 *Of entire stack

Keystone and MPD both have holders that will hold 2 of these coin cells. Two holders will add anywhere from $1.08 to $2.32 to each assembly, depending on how many you buy. With the CR2025 batteries you might get away with about $1.70 per assembly.

Is this the one going in series with the power wire to the LCD, or something else?

Matt Roberds

How much voltage drop can you tolerate? I've measured a 10% drop for a single 2016 Li battery drawing 19 mA. If you're drawing 3 mA (you said "a few mA" in your OP), that works out to just a 0.2 V drop for a 12V stack of 4 batteries.

You can also get a holder for 4 2016's from Digikey. Its 0.89" diameter is bigger than you wanted, you can decide if you could live with that or try a smaller diameter battery. It's overall height is

0.62", considerably less than the 2" you wanted.

The holder is Digikey part # BH800S-ND, cost is $0.43 each in quantities of 1000. It has tabs that fit into a pc breadboard; they are not what is known as a standard "solder tab" but they can be soldered to nonetheless.

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Mark

Joerg wrote:

Hello Joerg,

The holder is for two 2032's. 2016's are half the thickness, so twice as many should fit.

More to the point: since I happen to own a couple of these holders, I have now gone ahead and inserted four 2016's into one of them. The holder and batteries are sitting here in front of me as I type this; the batteries are held securely in place. (A thin wall surrounds most of the holder's circumference, preventing sideways slippage. A metal clip on the top prevents slipping out that way.)

I might as well test the assembly under load while I'm at it ...

Open circuit voltage is 12.75 V.

I added a 4k load (to draw 3 mA): Right away, the voltage is about 12.5 V and continually dropping. After 5 minutes, it's at 11.61 V. After 10 minutes, it's at 11.39 V. (Note, this has used up 0.5 mAh of the 80 mAh life rating.)

Don't know if that voltage drop is acceptable to you; does your application have a continuous or intermittent load? Guess you'd need to test them in your own device of course.

Mark

Hello Matt,

Unfortunately most coin cells are for low drain applications only. Their internal resistance is so high that the voltage begins to collapse at low currents, sometimes at less than 1mA. Maybe for liability reasons, I don't know.

No, just for simple electronic circuits that can't easily work from a low voltage cell. Sometimes a converter can be provided but other times it's so cost critical that this extra 50c or so in cost isn't in the cards. Often it is stuff that will only use one battery over the lifetime so it won't pay to run through hoops to accept a CR123 or something like that. Also, in most places a CR123 can cost as much as a

12V remote control battery and be less available at local stores.

Regards, Joerg

Hello Mark,

The 2016 would be ok. It was one of the batteries under consideration but it would take four of them.

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It seems this holder is for two so 12V would require a couple of them. Not necessarily a problem but when I look at the data sheet it seems the cells can easily slide out sideways or up.

Anyway, thanks for the hint. I'll check out holders some more.

Regards, Joerg

Hello Mark,

That's cool.

Thanks for the info. That wasn't very clear in the drawing. They showed a thin wall but only half way around.

I wonder why this droop occurs. Strange, because it seems that after less than 5 minutes they went under their spec'd voltage.

It's mostly continuous. The voltage drop would be ok but not if this continues at 0.3V every 5 minutes. Looking at the data sheet it could just be the initial droop they all have:

Regards, Joerg

It's a little over half way around, enough to hold things in place.

Yes, most batteries I've ever looked at run below the spec while under load. With compactness and portability comes a lack of regulation on the voltage.

Since the drop rate had slowed considerably between the first and second 5-minute intervals, and given the graph in the Energizer datasheet, it seems likely that the drop does not continue at this rate. A test to verify this would be necessary, of course.

Good luck!

Mark

I'm using that same "power supply" (two 2032's) for a small home-built photosensor. It provides the reverse bias to a photodiode. Everything (batteries, photodiode, resistors, and 4-position switch) fits on a

1.5" square PC breadboard. All that's missing is a display for the readout; must use a separate DVM for that.

Mark

I've abused CR2032's pretty heavy before, and they seem to last OK. Application was a small demo card driving 16 LED's sequentially, where when the button was pressed, each LED lit one at a time and stayed on. Cycle time for all the LED's to light and then shut off a second after the last one lit was about 4 seconds. A small AVR MCU controlled the thing, which drew just under 200 mA when the last LED lit. Power was from 2 CR2032's in series, in one of those 2 cell holders you mention. They lasted for hundreds of cycles, the only thing that really drained them is if we forgot to take out the batteries after use, since when I wrote the code, was having problems getting the MCU to go to sleep and have an interrupt vector wake it up when the button was pressed. No time to debug, so we just accepted a 4 mA continuous drain.

Thanks Joerg. It seems whenever I do a search on displays like this I can't find anything under $50. At $18-20, these are more attractive to a home hobbyist like myself. Making a small compact voltmeter just might become my next project ...

Thanks,

Mark

Hello Mark,

In case you need a panel meter that can operate without the usual tight tolerancing of 5V +/-0.25V check these out:

For example their SP200 runs from 3.5V to 5.25V and the SP400 from 3.5V to 7.5V. Someone here in this NG had suggested them to me.

Regards, Joerg

Hello Mark,

When you call them they speak perfect rightpondian. Nice, haven't heard that in a long time.

Regards, Joerg