renewable power for rechargeable batteries - planning advice needed

i'm building a project that will need to be outdoors for extended periods of time (days - weeks), and be able to occasionally deliver 1-2A @ 5v for a few seconds at a time (turn on a servo). I'm trying to decide on what type of batteries to use and the best way to recharge them. Right now I'm thinking a combination of 2-3 of these 3.5W solar panels (Large 6V 3.5W Solar panel [3.5 Watt] : ID 500 : $45.00 : Adafruit Industries, Unique & fun DIY electronics and kits) and maybe a micro wind turbine (Amazon.com).

I'd dump whatever power that cobbles together into a bank of 4 D-Cell NiMH batteries. I'm looking at NiMH over LiPo because they seem to come in larger capacities, don't have a problem with water, and are apparently easier to charge?

This is where I start to get lost. Regardless of battery source, can I charge on-line? Or do I need two separate banks of batteries and circuitry to toggle between them so that one's active, and one is charging?

I'm sure there's a ton of stuff I'm not even thinking to ask - so please, thoughts on my specific questions, as well as anything else you think appropriate. I'm a software guy that's dabbled quite a bit with Arduino, meaning I know enough to be dangerous, but that's probably it.

With lipo batteries you can get boards that charge the batteries in place. For example, Adafruit sells these:

True, though that's still less capacity than the 4 D-cells would be, plus I'm a bit concerned about water. There's a risk of this project getting fairly wet at some point, and I'd prefer to avoid a fire in that case.

Definitely an option though.

I would use a small lead-acid battery unless there is a good reason not to. They are very resilient.

I would be very surprised if a wind turbine would repay its cost.

...R

See this project for a remote solar-powered water tank monitor. The design principles are clearly described and should be useful for your project The finished Arduino wireless water sensor | Lifeboat Farm

For batteries you could look at the following options for different batteries and some simple ideas for charging

The benefit of LiFePO4 is life time number of discharges, and safety. They are being designed for ElectricVehicles
LiIon has good energy density, safety issues, and lifetime issues, but not as bad as NiMH
NiMH has lifetime discharge memory
All rechargeable batteries can be used in series - that is solar panel to LiFeP04 charger -> LiFePO4 batteries -> your board.
However you need to understand the voltage range of the battery technology - for LiFePO4 its 3.7V fully charger to about 2.5V for LVD (Low Voltage Disconnect) to preserve the life of the battery.
Basically unless you use a standard battery pack that does this for you, you are going to need to do some engineering.
There are some SolarLed Lighting products that have integrated solar panels and LiFePO4 batteries that if you can live with the power budget might do everything for you.
lowes.com "Portfolio 3-Pack Black Solar-Powered LED SpotLights" - by my experiment the solar panel is under powered and the batteries on a full daily charge/discharge cycle are loosing capacity.

I'm looking at a project and specifying

  1. Total power budget for 7 days - this sizes the battery
  2. Expect there to be 7 days of inclement weather with no solar and when the sun returns recharge in 4hrs - this sizes the solar panel
    The big issue with standard Arduino boards I've seen is they are not built for battery powering - the DC regulators and always on LEDs - so my criteria is for the hardware to be put to sleep - and have a low quiescent power usage of less than 0.5mA.
    The Teensy3.1 has some powering metrics that are in the uA and a good starting point for a battery powered board.

Back some years ago, I rolled my own Mega2560 board and interfaced it to a cellular module which required bursts of 1.2A@5V for transmit and 700mA for Rx. Overall it took 1minute to make a cellphone connection and transmit data. The cellphone was on a 1hr reporting schedule, the quiescent power of the board asleep was about 0.7mA
I powered it off a 2.2ALiIon battery and I think a 6Watt solar panel and its been very successful, it was a lot of work in software to manage the power.
Arduino software/libs - which prioritizes ease of understanding and is fantastic for accessibility - however so far I've not seen any priority for power management. From what I've seen using yield() makes it challanging to create algorithims to do power management. TinyOs.net "wireless networking" has some documents together on power management algorithms - however they are engineering concepts. Linix 3.8+ is also evolving power management capabilities

Nick Gammon has a FAQ on ways to reduce power, starting with dropping the use of development boards: Gammon Forum : Electronics : Microprocessors : Power saving techniques for microprocessors

MichaelMeissner:
Nick Gammon has a FAQ on ways to reduce power, starting with dropping the use of development boards: Gammon Forum : Electronics : Microprocessors : Power saving techniques for microprocessors

great resource, thanks. i plan on using a pro mini instead of a regular uno, which does a much better job, but there's a lot of stuff there i'll need.

neil12:
For batteries you could look at the following options for different batteries and some simple ideas for charging
http://www.batteryspace.com/lifepo4cellspacks.aspx
The benefit of LiFePO4 is life time number of discharges, and safety. They are being designed for ElectricVehicles
LiIon has good energy density, safety issues, and lifetime issues, but not as bad as NiMH
NiMH has lifetime discharge memory
All rechargeable batteries can be used in series - that is solar panel to LiFeP04 charger → LiFePO4 batteries → your board.
However you need to understand the voltage range of the battery technology - for LiFePO4 its 3.7V fully charger to about 2.5V for LVD (Low Voltage Disconnect) to preserve the life of the battery.
Basically unless you use a standard battery pack that does this for you, you are going to need to do some engineering.
There are some SolarLed Lighting products that have integrated solar panels and LiFePO4 batteries that if you can live with the power budget might do everything for you.
lowes.com “Portfolio 3-Pack Black Solar-Powered LED SpotLights” - by my experiment the solar panel is under powered and the batteries on a full daily charge/discharge cycle are loosing capacity.

I’m looking at a project and specifying

  1. Total power budget for 7 days - this sizes the battery
  2. Expect there to be 7 days of inclement weather with no solar and when the sun returns recharge in 4hrs - this sizes the solar panel
    The big issue with standard Arduino boards I’ve seen is they are not built for battery powering - the DC regulators and always on LEDs - so my criteria is for the hardware to be put to sleep - and have a low quiescent power usage of less than 0.5mA.
    The Teensy3.1 has some powering metrics that are in the uA and a good starting point for a battery powered board.

Back some years ago, I rolled my own Mega2560 board and interfaced it to a cellular module which required bursts of 1.2A@5V for transmit and 700mA for Rx. Overall it took 1minute to make a cellphone connection and transmit data. The cellphone was on a 1hr reporting schedule, the quiescent power of the board asleep was about 0.7mA
I powered it off a 2.2ALiIon battery and I think a 6Watt solar panel and its been very successful, it was a lot of work in software to manage the power.
Arduino software/libs - which prioritizes ease of understanding and is fantastic for accessibility - however so far I’ve not seen any priority for power management. From what I’ve seen using yield() makes it challanging to create algorithims to do power management. TinyOs.net “wireless networking” has some documents together on power management algorithms - however they are engineering concepts. Linix 3.8+ is also evolving power management capabilities

cool, thanks.

i’ll have to do some digging around the charging requirements of those LiFePO4 batteries.

fwiw, an arduino pro mini + http://www.rocketscream.com/blog/2011/07/04/lightweight-low-power-arduino-library/ gets you into the uA range pretty quickly.