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I'm considering putting a small lithium battery on my Super OSD Pro boards. The battery would provide power to the RTC. I am wondering how small can you get a battery? I don't really care about capacity, I expect current draw to be less than 1µA, so it's really only size I'm looking for.

Best solution would be one that is removable, because I'd like people to be able to replace the battery when it does go flat.

And I'm open to suggestions on how to control the power to the main bus which has some heavier current draw devices which would overwhelm the 1µA for the RTC (the RTC is provided by a PIC24F in deep sleep mode.) I'm thinking of a power monitor, but it needs to be able to switch at least 100mA for the MCU when it's running full speed.

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CR2032 (20mm x 3.2mm) is the cheap standard for coin cells, if you want smaller there are several other CRxxyy type cells (e.g. CR1220) that are meant for this sort of thing.

The crudest design could just be a diode-OR with the power supply, before or after the regulator depending on the voltages/regulator type, otherwise you could implement something like in this article.

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  • \$\begingroup\$ Wouldn't a diode drop 0.7V or 0.3V? Also, I suspect a SOD package would be bigger than a dedicated switching IC, and you'd need two diodes wouldn't you? \$\endgroup\$ – Thomas O Nov 2 '10 at 22:41
  • \$\begingroup\$ check the datasheet, but I'm pretty sure the PIC24 doesn't need the full supply voltage to keep the RTC rolling. Keep high-current devices shut off in firmware when the main power supply is missing. \$\endgroup\$ – markrages Nov 3 '10 at 0:44
  • \$\begingroup\$ @Thomas: As mark mentions, you can be fairly loose with the supply voltage if you're just running the RTC. If you're running the PIC at 5V, it's trivial as you just have a forward diode from the coin cell to the supply. If it's 3.3V or thereabouts, it will be more difficult as the typical Li/Mn02 battery may exceed that when new. \$\endgroup\$ – Nick T Nov 3 '10 at 14:42
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Check out this table with button cell battery specs. There is one 1.55V battery sized 11,6 x 5,4mm @ 190mAh. If you're trying to save space 'horizontally' on the board, just stack 2 of these vertically: that gives you 3.1V and 190mAh. That's about what you'd get from an almost twice as wide CR2032.

Note: I haven't checked battery type, availability or cost, you'll have to check those yourself.

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  • \$\begingroup\$ Just a small correction, stacking two batteries in series would get you to 3.1V if each one is 1.55V but it would not double the capacity so you'd still have 190mAh if that is what each battery is. \$\endgroup\$ – Andrey Nov 3 '10 at 12:41
  • \$\begingroup\$ As Andrey pointed out, you don't increase both the voltage and capacity when putting batteries in series-just the voltage. Also, you linked to the Dutch Wikipedia - I'm 50% Dutch, but can't read that. This page is the closest analog on the English Wikipedia. Finally, the battery you mentioned is an alkaline battery, which isn't rechargeable like the CR2032. I like the stacking idea, though! Do you have a reference for a stacking battery holder? \$\endgroup\$ – Kevin Vermeer Nov 3 '10 at 15:08
  • \$\begingroup\$ Woops, you're right about the capacity, sorry. CR2032 isn't rechargeable either, to my knowledge? The request was for batteries to be replaceable anyway, not rechargeable :-) Don't have a reference for a retail holder, but that should be easily achievable with a little DIY. Unless you're planning to produce a large amount of boards, of course... \$\endgroup\$ – Vincent Van Den Berghe Nov 3 '10 at 15:41
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If size is an issue I would go for a tabbed rechargeable lithium cell. Since it's rechargeable it doesn't have to be user-replaceable so it can be soldered to the PCB. Since it doesn't require a holder it can be kept compact; a holder may otherwise add up to 200% to the volume).
You don't specify how low the battery should last, but the Panasonic ML-414 is only 4.8mm diameter and 1.5mm thin yet at a capacity of 1.2mAh it should last 50 days if you draw only 1\$\mu\$A.
This is a drawing of the tabbed version.

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