Sunday, 20 November 2011

Using Li-Ion and Li-Po Batteries in Your Own Projects (without getting ripped off or blown up)

Those with whom I've been discussing circuit design lately have, it seems, been surprised to learn that there's a dead-easy way to use Li-Ion and Li-polymer ('Li-po') batteries in your own designs. Lithium-ion batteries give a nice 3.7V supply to any project and are available in a nice range of mAh capacities, ranging from small coin-cells, to AA- and AAA-imposters, to thin square or rectangular packs. The latter are used in all kinds of things like MP3 players, iPods, and little USB keychain picture-frames (if you see those at discount stores for under $2, grab as many as you can -- they're worth it just for the battery! -- but see the end of this post for more on that).

Lithium-ion cells, as I understand it, have some specific charging requirements which sounded scary to me at first, especially with stories of burning laptops, exploding white-hot batteries and the like.. but when handled properly are no worse than using alkaline or Ni-MH cells and they don't suffer from the memory problems for which the latter were infamous (apparently newer ones don't have memory problems -- I haven't really researched it myself).

From what I've read the short-list of Lithium rechargeable safety tips are:

  • don't drop them (it breaks the internal short-circuit/overheat protection circuit);
  • don't solder to them (search 'spot-welding coin cells' for the right way to attach pins yourself);
  • don't charge them using Ni-MH or other types inappropriate for Lithium rechargeables.

Interestingly, the LIR series of rechargeable coin-cells, in addition to being drop-in replacements for their non-rechargeable brethren, also have the ability to deliver much higher current. The LIR2032 for instance is rated at about 70mAh peak supply current, while regular CR2032s are only able to supply about 0.2mA continuous drain(!?) [anyone know the absolute max rating? specs sheets I've found don't say], making them not really suitable for anything other than where you typically see them -- in blinky-light dollar-store gadgets and for CMOS backup. [Edit: hmm, I just noticed that the standard CR2032 apparently has a higher capacity than the LIR2032 according to the linked specs... I'd guess there's some general tradeoff going on here wrt. long endurance vs. high power...]

Anyway.. to the point -- Dallas-Maxim makes a pair of really convenient single-chip solutions, the MAX1551/1555, to charge any single-cell Lithium-ion or Lithium-polymer battery that is just so easy to use, there's no point in using alkaline or other batteries unless you have good reason to. These ICs take care of the whole charge cycle, and even have dual DC inputs, one for USB and the other for a DC power supply. The IC switches between whichever provides the best power at the moment -- or just ground one input if you're only using the other. Combine this IC with a USB-mini socket on your project, even if you're not using USB comms, and you have a dead-simple way to recharge your device. I personally prefer the MAX1555 variant as its CHG/ output hooked through a resistor to the battery's positive terminal VBUS gives a handy charge indicator:

Example Li-ion/Li-po charger using MAX1555 and USB-mini jack. Heck, leave out the caps if you're really cheap, it still works.
Design-in-progress w/MAX1555 and salvaged Li-po from a USB picture-keychain thingie, happily charging
[Blatant Plug: the board above uses all SMT parts save for the jumpers, which I wouldn't have attempted myself, but my colleague Chuck Rohs (the world's only lead-free engineer, HAR HAR) has a purely awesome PID controller for a souped-up toaster oven reflow station (that's just one application). Take a look at that board up close, the soldering job is bee-yootiful. You should buy his reflow oven kit :)]

Bargain Tip: search on eBay (or alibaba or other wholesaler if you are doing quantity) and you can find Li-polymer square packs in the 180mAh-600mAh range for $2 USD or under per cell. LIR2032 rechargeable coin-cells are also to be found online, again at prices $2 or less per unit if you shop around. Don't buy from anyone charging significantly more than this, you wouldn't believe the markup some online [US-based, sorry] retailers will charge on these things! Generally, if the website says they "specialize in batteries", they really are specializing in overcharging for batteries (har har, I made a pun). :)

No comments:

Post a Comment