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Topic: Minimum LiPo mAH capacity needed for solar powered project (Read 202 times) previous topic - next topic


I'm thinking of powering a wireless outdoor remote weather sensor node connected to my Arduino-based weather station via solar panels, and am wondering what capacity LiPo battery I'll need to assure that it stays on all night on even the longest and coldest nights.

I haven't yet decided whether the remote sensor node, which will likely have a DHT22 for humidity & temperature, an LDR for light levels, and perhaps an MQ-2 gas sensor, will be based on a small-form Arduino such as the Pro Mini or Nano, with an RF24+ handling the communication with the base station, or on a NodeMCU module that uses an ESP8266 for both the microcontroller and WiFi (the base station already has both WiFi & RF24+, so either would work).

This means, of course, that its power requirements will vary depending on which MCU/Wireless solution I use, and whether it'll have an MQ-2 sensor, which uses a fair amount of power to heat up the element.

I'm obviously new to solar/LiPo, but from what I've read, it appears that I'll need the following to make this work:

Solar Cell (I've ordered a cheap 6V/1A panel on eBay)
-->Solar LiPo charger circuit
-->LiPo battery (3.7V, mAH tbd)
-->3.7V-5V DC-DC power boost circuit
-->Sensor node (5V input)

Lots of components there! Did I get this right?

Anyway, I'm wondering, what's the lowest capacity LiPo battery I should consider, for the lowest power-consuming version of the remote sensor node (which I assume is the Pro Mini + RF24+ w/o MQ-2 sensor), and for the highest power-consuming version of it (which I assume is the NodeMCU w/MQ-2 sensor)?

FWIW I live in the NE USA, to give an idea of what kind of sunlight we get here, and how cold it gets. In the dead of winter days can be as short as ~10 hrs long and completely overcast, so I'm guessing that the battery, which I assume loses efficiency when it's cold, would have to have enough power to last several days when solar output is this low and it's this cold, as it trickle charges on overcast days, until the next sunny day comes along.

I'm just looking for ballpark estimates, nothing too detailed. Better yet, if someone could direct me to a good site that helps you estimate a project's power requirements and what kind of battery and solar panels you'll need to keep them running 24/7, and the various technologies and boards you need to make this work, that would be really helpful.


You need to measure the average current consumed by your entire project in mA and multiply that by the number of hours of darkness, to get mAh. Double that number for safety and buy a battery with that capacity (C).

LiPo batteries are a poor choice. They MUST be charged by a LiPo battery charger, and MUST be protected against over-discharge by another circuit, or they will be very quickly destroyed. Many LiPo battery chargers do not work properly when the battery is simultaneously being used to power a circuit.

NiMH batteries are much safer, much more tolerant of abuse and can be trickle charged indefinitely, at C/20 mA (where C is the battery capacity in mAh).

For advice on constructing low power Arduino projects, see this excellent post.


Also mind the solar cells are mostly rated for a direct sun exposure (no clouds), otherwise you may get a fraction of the power claimed (ie. the current).


Thanks! I've never done a solar-powered project so LiPo was my initial choice, but it also occurred to me that one of its main advantages, light weight, didn't really benefit a stationary device.

Are most portable cell phone charger/batteries based on NiMH? Since they already put out 5V, there's no need for output conversion, although I assume I'd still need some sort of board to connect the panel to these. Plus, they're pretty cheap. I have several lying around that I could use.


Normally, when building a Solar powered project, the solar panel is the last thing you buy, as it has to be sized based on total project power consumption, latitude , elevation and how reliable you want the project to be.
5 consecutive days of cloudy weather is a good starting point for reliability.
Nimh cells are the easiest to use , as they are the most forgiving in respect of overcharge and undercharge.


The panel I bought was just over $1 on eBay, almost an impulse buy. At this stage I'm just planning on playing around with components to get a feel for how this works, but I was hoping for some back of the envelope ballpark idea of what kind of battery capacity I'd need. Once I get the hang of it, I'll start to get more serious about capacity needs and all that. Given that I might work with LiPo, though, I realize that I have to be careful with what I connect to it, so nothing explodes or catches fire.


Its impossible to answer you question without knowing what the battery has to power, and what the power consumption rate is.
If you are trying to power some kind of weather node then you need to measure its power consumption.


I was hoping for some back of the envelope ballpark idea of what kind of battery capacity I'd need.
See the first sentence of reply #1.

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