[aprssig] re:Solar charging

Jim Lux jimlux at earthlink.net
Fri May 6 13:31:56 EDT 2005

Scott wrote:
Slightly different topic this time - I'm interested in solar charging for
small systems, probably with NiMH batteries.  I suppose a very small SLA
might work, too.  I want to put 1-wire weather station OpenTrackers in
mast-mounted boxes with small PV panels.

The average power consumption of the system can be kept to under 50 mW.  The
tracker (with an LDO regulator installed) maybe 6 mA at 5 volts, including
the weather sensors.  I figure a 6" by 6" or smaller panel should be able to
do it, but I'm not sure about the efficiency of small charging circuits.
Does anyone have any good example circuits?  I think I saw a Maxim appnote a
while back that did something similar - I'll have to check.  I figure the
battery pack will probably be 6 or 7.2 volts.

One thing to think about on such a device is the behavior over 
temperature.  Not only does the required battery charging current/voltage 
change with cell temperature, so does the output of the solar cells, and, 
of course, the electrical properties of the regulator/charger.

If average power consumption is 50 mW, that's about 4.8 Joules over 4 
days.  Solar insolation can be taken as an average of about 700 W/square 
meter, for 8 hours a day (but clearly, you need to make sure you can do 
winter, etc.).  Figure 10% for solar panel efficiency, and you get 
70W/square meter.  Assume you need to accumulate 4 days worth of energy in 
1 day, so you need 0.6W, or about 0.009 square meters (about 85 square 
centimeters), but that's before you take charge/discharge efficiency into 
account (perhaps 80%?) and charger efficiency (perhaps 65%).

I get 165 cm2, which is compatible with your 6x6 panel (225 cm2), so on a 
gross energy budget estimate, you're ok.

The real challenge won't be efficiency, but operating over temperature 
without inadvertently killing your batteries from over charge or over 

Jim, W6RMK

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