Solar Design

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Solar powered Wi-Fi Pi hive monitor.


WARNING! This prototype is still being developed. It is not guaranteed to completely work yet. Use at your own risk.


  1. Operate through a week of rain (battery life of 10 days).
  2. One day of sun provides 7 days of operation (operate unit 1 day and charge batteries 6 days worth).
  3. Low profile (solar array fits on the top of a 10 frame Langstrough hive.)
  4. Affordable. Readily available parts.
  5. Simple. Easy to build and maintain.


Significant power savings can be achieved through software. See Software:Solar Power for details.

Under Clocking

Why doesn't this work?


  1. Disables the LAN9512
  2. Shuts down USB devices

On the Model B, the Ethernet and USB ports are provided by the LAN9512. This chip consumes almost 200mA. The LAN9512 datasheet explains some of its power saving modes: SUSPEND2, SUSPEND1 and SUSPEND0.

To disable the device (and reduce current from 470 ma to 200 ma):

echo 0x0 > /sys/devices/platform/bcm2708_usb/buspower 
echo 0x1 to the same file to wake up it.

See this StackExchange post for details.

This technique reduces the Model A current from 200 ma to 120 ma even though it doesn't have the LAN9512.

This also shuts down any USB devices which also saves power, but this also means that the devices (Wi-Fi) must be initialized when bus power is turned on.

Turn off HDMI

This saves about 20 ma.

/opt/vc/bin/tvservice -off

GPIO serial port

USB to TTL Serial Cable

USB to TTL Serial Cable

When using the Model A (or turning off the ethernet/USB ports on the Model B), this USB to TTL Serial Cable will allow connection to the console.

From AdaFruit:

USB to TTL Serial Cable - Debug / Console Cable for Raspberry Pi $9.95

From Amazon:

Elesonic Vomaxtech Pl2303hx USB to TTL to Uart Rs232 COM Cable Module Converter $5.99
Kootek PL2303HX USB to TTL to UART RS232 COM Cable module Converter $2.65


Power calculations

Average power consumption

Pi A: 126 ma @ 5V = .63 W
Scale: 70 ma @ 8V = .56 W

Assume PS efficiency is 85% 1.19 watts/.85 = 1.4 watts
1.4 watts / 6 VDC = .23 amps
.23 amp x 24 hours = 5.6 AHr

derate 42 AHr to 30 AHr
30 AHr / 5.6 AHr/day = 5.3 days

derate 200 AHr battery to 100 AHr
100 AHr / 5.6 AHr/day = 17 days

Model A Pi, USB hub, 2 TEMPerHUM, Edimax Technology Co., Ltd EW-7811Un 802.11n Wireless Adapter [Realtek RTL8188CUS]
110 ma 282 seconds 370 ma for 18 seconds
((110 ma * 282 sec) + (370 ma *18 sec))/300 sec = (31,020+6,660)/300 = 126 ma avg

Charge rate


5.6 watts (4x consumption)
9.8 watts (7x consumption)
5.6 watts x 24 hours = 134 watt hours
134 watt hours / 25 watt = 5.4 hours at maximum output (11 hours at 50% output
Derate panel to 20 watts = 6.7 hours
(note: 25 watts is maximum output and that probably won't ever happen - especially if the panel is flat on the top of the hive. A 45 watt solar panel wouild probably be necessary to achieve the goal of 7 days operation from one day of charging.


Internal wiring of power supplies, fuses, test points.
Solar power supply 1.png

Parts List

Meters, fuses, switches
Enclosure Polycase WH-18-01 $25.29
Battery Connector NAPA 784640 $2.79
25 Watt Solar Panel ALEKO® 25W Monocrystalline Solar Panel $59.00
MC4 Solar Panel Connectors Excelvan Pairs HC4 PV10C $1.80
Li-ion Battery Charger ProDC2DC Item ID #090627 $10.99
Li-ion Battery Charger ProDC2DC Item ID # #090626 $6.49
5 VDC Power Supply ProDC2DC $6.70
Fuse Holders Panel Mounted 5x20mm Fuse Holder $0.62
Test Points Digi-Key J117-ND $0.86
USB to TTL Serial Cable PL2303HX USB To TTL $2.99
Schottky Diode 40V 3A SCHOTTKY DIODE NTE $1.43
SPST On-Off Switch
NO Pushbutton Switch


1. Battery Amp Hour ratings are theoretical. A 200 AHr battery in theory would supply 200 amps for 1 hour or 1 A for 200 hours. In practice this is far from true - there is a better chance of getting closer to 1 A for 200 hours than 200 amps for 1 hour. The point is, the AHr rating of the battery must be derated (depends on the rate of discharge and temperature).
2. 6 V golf cart batteries are better deep cycle batteries (thicker plates) than 12 V marine deep cycle which are a compromise. Battery life depends mostly on Depth of Discharge (DOD) - the bigger DOD, the shorter the battery life.
3. A solar "Charge Controller" is probably not the optimal solution - Something like this is not recommended:
40 Watt solar array:
$54 on ebay
$95 on ebay, probably find locally (Sam's Club)
Solar Charger/PS:
$7 WiFi Adapter: $10
(This is about $170 not counting shipping/taxes.) May need gain antenna: $30

Raspberry Pi Model A vs Model B

The SMSC LAN9512 USB Hub and Ethernet Controller.

LAN9512 Data Sheet

Misc Info on power reduction

Solar powered Wi-Fi Pi hive monitor with dish antenna.