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Solar charging a single NiMH AAA battery

NAV50500
Prodigy 30 points
Other Parts Discussed in Thread: BQ2004, BQ2002, TPS61200

Hi,

I would like to use a small solar cell to charge a single NiMH battery (between 500 and 1000mAh).  The solar cell's ouptut is about 1.5V, so I'd probably use 2/3 of them and boost the output to a regulated 5V.  The end-product will be used to recharge batteries while on outdoor hiking trips, meaning that the solar cells will be mounted on top of the hiking bags.

I've considered the bq2004, but would prefer not to have an external constant current regulator.  Are there any chipsets in the TI portfolio that does not require external current sources?  I suspect not, so I'll continue with the asumption that I'm going to use a bq2004.

1)  If I walk under a tree and the solar voltage drop, what would the effects be, assuming that the battery has been charging for some time by then.  I.e. the input voltage will be zero, and the battery voltage will be higher.

 

2)  For batteries from different manufacturers, ranging from 500mAh to 1000mAh, will a C value of 1 be acceptable?

 

3) In order to keep the solution cheap and easy, I would prefer to skip temperature sensing.  Can I simply connect TS to ground?

 

Finally, if I'm asking stupid questions and you know of a resource that will save you time by my reading it, please post a link?  I'll gladly read up on the subject, on which I'm less informed than I'd like to be, rather than wasting your time by asking basics all the time.

  • 0
    TI__Guru*** 148090 points

    First, we don't recommend charging nickel cells from an intermittent source such as a solar panel.  Nickel cells require a constant, uninterrupted charging current to be able to detect fast charge termination.  You can probably just look on wikipedia for a description of how fast charge is terminated on nickel cells, but all of them assume and require a constant current source to be able to detect the small change in internal resistance of the pack.  Missing this signal and continuing fast charge on a full pack is dangerous.  You should rather charge lithium or lead acid batteries from a solar cell.

    I would recommend the simpler bq2002 over the bq2004.  Its EVM has a current source design shown.  This will be the simplest way to charge your battery.

    1) Most charger designs have a diode in series with the input voltage.  So, when there is no input source (no light), the batteries will be disconnected and not drained.

    2) 1C is usually acceptable.  You need to stay above C/3 to accurately detect fast charge termination.

    3) The TS input is still active even if you don't want it.  You cannot just connect it to ground.  Instead of using a 10k thermistor for example, just use a 10k resistor and mount it on the board next to the IC.

    You can use this app note with the TPS61200 to generate the 5V: http://focus.ti.com/lit/an/slva345a/slva345a.pdf