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I am developing a Bluetooth tracking device that uses a single CR2032 battery. Would it be worth using instead a rechargeable CR927 (40mA) battery and a small solar cell, the size they put in watches?

Using this battery might save some space and the main advantage will be that the device will need battery changing less often.

How should I design the circuit? Should I use a charge regulator?

All components are rated at 1.8 to 3.6V. The circuit uses up to 20mA in full active state, and around 40uA in sleep state.

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closed as too broad by Asmyldof, uint128_t, PeterJ, Bimpelrekkie, Daniel Grillo May 13 '16 at 11:21

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ "Would it be worth"? How should we know what would be worth what in your thought universe? "How should I design circuit x or y?" is too broad for sure. Not to mention no specifics at all are given for anyone willing to go there to even work with. \$\endgroup\$ – Asmyldof May 12 '16 at 20:45
  • \$\begingroup\$ In terms of space the board will be round, 22mm diameter and not more than 3mm height including the battery. \$\endgroup\$ – Nick M May 12 '16 at 20:49
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    \$\begingroup\$ I don't want to charge the CR2032, but the CR927 battery. Just trying to figure out what type of charge regulator to use, what charge voltage and current to apply and so on. Ideally it should charge during the day from a SMD solar cell for up to 4 hours or so. \$\endgroup\$ – Nick M May 12 '16 at 21:29
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lithium coin cells can take a bit more abuse than their larger, more flammable cousins. with the limited current that a solar cell that size would generate, you could probably get away with a direct connection to the battery with a pv cell with a high enough voltage (and series shottky diode) and just put a small zener diode in parallel with the battery to limit the peak voltage. the powers we're talking about are so low that burning the extra milliwatts as heat shouldn't be an issue. Or if you want to get fancy, both TI and Linear have energy harvesting chips which will pull the maximum possible power from the solar cell and produce a regulated voltage to charge a battery/supercapacitor.

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  • \$\begingroup\$ Thanks for replying Tom. I really thought about using a diode and LDO to charge the battery from the solar cell, would prefer not doing that as it could make a difference in terms of battery life. I am looking at some ICs but can't figure out some things. For instance BQ24210 outputs 4.7v, this means I have to add a regulator otherwise it will kill the other ICs. Then there are so many other pins to connect, and the IC says 3.5v minimum are required on the power present pin, but the battery outputs 3V. So I feel like trying to learn chinese. \$\endgroup\$ – Nick M May 12 '16 at 23:24
  • \$\begingroup\$ the BQ24210's output voltage can be controlled from 3.65-5V with a resistor but if it's a 3V battery then it's not going to work anyway. There's nothing wrong with using an LDO, just use a solar panel with an output just a bit higher than the battery voltage (pv voltages are pretty stable vs light levels, it's the current that drops off) some LDOs have nA quiescent currents so you won't be wasting that much power. Check out the LTC3105, it'll work down to 0.25V and step it up to whatever voltage you want (max ~5V or so), just set the output voltage to the fully charged voltage of your battery. \$\endgroup\$ – Sam May 12 '16 at 23:39
  • \$\begingroup\$ I was looking at the KXOB22-01X8F solar cell from IXOLAR, thought of adding a LDO that outputs 3V. The thing is, I can get about 6 months out of a CR2032 battery (non-rechargeable), and if this setup will reduce the CR927 rechargeable battery life to less than 7-8 months then it's a waste of money and space. These batteries only last about 400-500 cycles, in theory that should be a full year so it's worth it, but if the solar cell will apply a continuous 3V+ for 5-6 hours each day, will it not reduce the battery life significantly? \$\endgroup\$ – Nick M May 12 '16 at 23:48
  • \$\begingroup\$ The cycle life is highly dependent on how much you drain the battery, what does the datasheet say? 500 cycles at 80% dod (depth of discharge)? it'll last way longer if you only drain it by 5% and fully charge it each day - besides, the published cycle life is the point at which you permanently lose some capacity (20%?) or at which point the internal resistance goes up by some amount, see what the fine print says. But seeing as you've got such low average power requirements, what about using a super capacitor? Some of them last for millions of cycles. \$\endgroup\$ – Sam May 12 '16 at 23:54
  • \$\begingroup\$ Can't find a super cap to match the size constraints, this is a very small device, like 23mm/0.9" in diameter and now I'm hoping to work on thickness while also improving battery life. 400-500 cycles is an optimistic average, I didn't start with the battery when designing this. Some counterfeit batteries will barely last 100 charges and the IR/capacity will drop much sooner than a "good" batteries. I'd rather design the circuit to work with any cell and get as much as possible out of it. \$\endgroup\$ – Nick M May 13 '16 at 0:04

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