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I have a small breadboard that has a L293D motor controller and 2 DC motors. The board is hooked up to a Nordic DK and everything on the PWM side is fine. The breadboard is powered by the development board's 5 V pin but I am ready to move to the next step of a PCB so I will need a battery.

I have two 2032 coin cells that were taped together and have jumpers for positive and negative but whenever I try to use it as the power source on the breadboard it fails to work. I checked it with a multimeter and it's throwing out at least 5.6 V.

Is there something I am missing? Are lithium batteries not applicable for this? Can I still make a PCB with what I have now?

PeterJ
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KellysOnTop23
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2 Answers2

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Your coin cells have far too little peak current capacity to run anything but the tiniest sort of motor. As a result the voltage is probably much lower under load. Your L293D being a bipolar bridge will also have very high loss - probably in excess of a 1-volt drop by the time you count both top and bottom switches.

Further, your develoment board may not be designed to handle the (lightly loaded) voltage of two coins cells in series, so you may have already damaged it.

If you want an "easy" way to replace a 5v supply with a battery, you might consider using a USB powerbank, though they can have various sorts of turn-on behavior and some may turn themselves off below a minimum current draw. Doing it yourself is likely to either require a number of AA cells to get well above the target voltage even at end-of life, followed by a linear regulator. Or better your can use switching regulator or potentially a boost converter from a lower battery voltage (which incidentally is what a USB power bank is - some buck regulate from 2 lithium cells, others boost from 1, both typically cells good for well over an amp).

Chris Stratton
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  • snap, well i hope I haven't damaged it. The motors i wanted to use are the small little DC ones that are found in the micro servos but I am guessing that's still a little to big for the coin cells... – KellysOnTop23 Dec 16 '15 at 04:05
  • is there any way to control dc motors with small sized batteries? size is a pretty big part of it all? – KellysOnTop23 Dec 16 '15 at 04:32
  • Depends on what you mean by micro servos, what voltage rating, and how much load they are under. There is a lot of micro-RC gear out there which runs on 100-200 mAH high-discharge-rated lithium cells (powering the flight as well as the servos), but those are typically motors wound for direct usage on a single cell, and they use FET rather than biploar bridges to drive them. There's a huge difference between the instantaneous current you can get out of a tiny 10-20C lipo vs what you can get from a lithium coin cell, but they require care in application. – Chris Stratton Dec 16 '15 at 04:51
  • A single cell LiPo can be pretty small but I've never seen any around the size of a coin cell. For the best chance, take a look at the datasheet for your motor and find what it's peak draw current can be, as well as it's startup current and make sure whatever batteries you're looking at can support that. – NathanielJPerkins Dec 16 '15 at 04:52
  • Your classic "IR mall kiosk helicopter" (aka picoZ) will have a < 100mAH lipo that in a good example weights less than a CR2032, but size is hard to compare as the shape is different - definitely thicker, and either square or rectangular. If I understand the original market was bluetooth earpieces. You do not want to short out or overcharge or mechanically damage these! – Chris Stratton Dec 16 '15 at 04:56
  • I appreciate the help!! I haven't been able to find what servo's or dc motors i have here (they came in a pack) but do you think that two AAA batteries would cover a fair portion of most small dc motors so at least so i can move off the development board for a bit? – KellysOnTop23 Dec 16 '15 at 05:48
  • Maybe the motors by themselves at reduced speed and torque, but *definitely* not with the L293D and only barely with an FET driver. You probably need three alkalines or four NiCd/NiMH. And that L293D really needs to be swapped out for an FET bridge. – Chris Stratton Dec 16 '15 at 07:09
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You need to look up the data sheet for the cr2032 cells. Pay attention to the series resistance and the maximum current rating of those cells.

When you understand why they won't work in your particular application, you can then choose a more appropriate battery style and size.

Dwayne Reid
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