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I have a sensor in a remote area that draws 15 mA @ 6 V for 16 hours each day, powered by a 6 V, 1100 Wh low discharge battery. I'm trying to achieve around 2 years of autonomy.

Daily consumption is 1.44 Wh (6 V x 0.015 A x 16 hours), so I'll need around 1051 Wh for 2 years. Assuming a discharge of 70% of the battery before replacing it, my power budget is around 1502 Wh, too high for my 1100 Wh battery.

I have a small 6 V, 1 W solar panel laying around so I thought I could use it to charge my battery. I made a simple test with a small 6 V lead acid battery on a sunny day (not too bright) and measured 6.9 V @ 70 mA when charging, so on a full sunny day at the peak I guess I can get more than that. To be conservative, I'll consider 6 hours of sun each day @ 70 mA, or 420 mAh per day. My sensor draws 240mAh per day, so at least in spring/summer I could expect to not drain the battery and also charge it a little bit (I guess).

So my questions are:

  1. Will this small panel make any difference charging this battery?
  2. Is it OK to connect the solar panel directly to the battery? (I'll add one Schottky diode to prevent discharge during the night)
  3. Since the battery is expensive, is there any chance a faulty solar panel could destroy the battery (broken panel, short circuit...)?

Any other ideas are welcome. Thanks

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  • \$\begingroup\$ What kind of sensor do you use? Is it necessary that it functions 24/24h? \$\endgroup\$
    – Antonio51
    Mar 18 at 12:45
  • \$\begingroup\$ I guess the sensor is a "geological" (:-)) sensor as an accelerometer, so signals are "very low frequency" happening at only "some time". I guess also that the datas are saved on "disks". Probably, it can go to "sleeping" mode with a "watchdog" above "noise" ... This can save perhaps 10 times power. \$\endgroup\$
    – Antonio51
    Mar 18 at 14:10
  • \$\begingroup\$ @Antonio51: the whole system must be on for 16 hours each day, it is more than just the sensor. \$\endgroup\$
    – Geologic
    Mar 18 at 16:29
  • \$\begingroup\$ Frame challenge : consider a similar capacity 12V battery, on the grounds that a 12V solar charge controller is dead cheap and easy to find. (Then a buck convertor to step down to 6V) \$\endgroup\$ Mar 18 at 18:48
  • \$\begingroup\$ @user_1818839: to avoid vandalism i want to use a panel as small as possible, 12V solar panel are too big. \$\endgroup\$
    – Geologic
    Mar 21 at 17:08

1 Answer 1

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Will this small panel make any difference charging this battery?

  1. Yes.

Is it OK to connect the solar panel directly to the battery? (I'll add one Schottky diode to prevent discharge during the night)

  1. Probably no, you need to provide a datasheet or a least the Voc voltage for the cell to tell for sure. I suspect you'll end up with a small DC/DC, extra series diode or linear regulator to limit overcharge of the battery.

Since the battery is expensive, is there any chance a faulty solar panel could destroy the battery (broken panel, short circuit...)?

  1. Depends on the implementation. A Schottky diode would protect you against that edge case but with the cost of a small voltage drop.
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  • \$\begingroup\$ Well, it is a cheap solar cell found on ebay, not much information. Meanwille, i found a 6 V solar panel with these specs: Open Circuit Voltage: 7.09V Peak Voltage: 6.07V Peak Current: 200mA. That "peak voltage: 6.07V" seems small to be usefull to charge the battery, don't you think? \$\endgroup\$
    – Geologic
    Mar 21 at 17:02
  • \$\begingroup\$ Your dimensioning factor is unfortunately the opposite, the open circuit voltage. 7.09 V is however a good match for not boiling away your batteries. Heck, you could even consider an active load for it if you want to maximize battery lifetime and keep it in the 6.75 V range maximum for the sunniest summer week. If this is ok, I would look for one with even higher voltage to get some use from it in spring and fall. \$\endgroup\$
    – winny
    Mar 21 at 18:34

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