Computing stuff tied to the physical world

New solar setup

In Hardware on May 15, 2012 at 00:01

Time for another experiment, this time combining my small solar panel with the 3.4 mAh Lithium battery which seems to work so well. The circuit I’m going to try is as follows:

JC s Grid page 16

Here’s the construction, cozily attached to the back of the solar cell:

DSC 3128

Same solar cell, I think it can supply up to 4.5V @ 1 mA in full sunlight.

The tricky bit is that the rechargable lithium cell needs to be treated with care. For maximum life, it needs to be hooked up to a voltage source between 2.8V and 3.2V, and the charge current has to be limited.

Note that the 1 kΩ resistor is put in series with the battery not only to charge it, but also when taking charge out of it. Seems odd, but that’s the way the datasheet and examples show it. Then again, with a 10 µA current draw the voltage drop and losses are only about 10 mV. A diode bypass could be added later, if necessary.

The diode after the regulator has the nice effect of dropping the 3.3V output to an appropriate value, as well as blocking all reverse current flow. There is no further circuitry for the regulator, since I don’t really care what it does when there is too much or too little power coming from the solar cell. Let’s assume it’s stable without caps.

It all looks a bit wasteful, i.e. linearly regulating the incoming voltage straight down to 3.3V regardless of PV output levels and discarding the excess. But given that this little 3V @ 3.4 mAH battery has already supported a few days of running time when fully charged, maybe it’s still ok.

I’ll let this charge for a day or two.

  1. Any comments on good resources for working with batteries (Books, websites, etc.) I wanted to roll my rechargeable setup with some eneloop NiMH batteries but didn’t know where to start…

  2. The 1kΩ in series with the lithium cell might be a result from safety regulations to limit current to nondestructive levels (lithium cells tend(ed) to explode and burn if mistreated). At least with lithium primary cells which are quite often used as backup for RTCs, this is a requirement (from IEC 60950 electrical safety). Maybe this is the same for rechargable cells also.

  3. I didn’t consider using a diode to drop the voltage coming out of the 3.3V regulator, that’s a great idea to reduce power! Also connecting the solar output directly to the LDO input basically nullifies the drop of the LDO.

    Tim, Wikipedia has great articles on the various battery chemistries which explains their charging cycles. Recharging multiple cells simultaneously is not recommended because they may charge at different rates and it is difficult to detect the end of the charge cycle. That said, NiMH batteries are generally safe at 0.1C (C being the battery capacity so 190mA for an eneloop AA). I’ve never been able to find documentation on what Sanyo recommends for keeping a battery topped, but Duracell recommends 0.0033C and Panasonic agrees but also says “Trickle charging is not recommended for NiMH batteries”.

  4. An unfortunate drawback of these cells is they are rated for 1000 charge/discharge cycles (10% discharge, then charge). Since they are through hole solder, they don’t lend themselves to easy replacement. So we can expect ~ 2.75 years service out of them. I guess one could go with a CR2032 cell and holder, but those cost like $5 each (amazon and ebay). I don’t trust those sources to send a rechargeable cell instead of a discharge only cell.

    Nothing like trade-offs in design…. (imagine if we could have it all)

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