I've been looking at the various options for running an arduino and PIR sensor from a battery that will last several months. The average current draw will probably be less than 1mA.
Initially I looked at single cell 3.2v LiFePO4 batteries but it's difficult to find single cells with built in protection circuitry. Most are sold "unprotected" with the idea of building multi-cell battery packs with a dedicated balance charger/protection circuit PCB. LiFePO4 cells are sensitive to over discharging so you definitely need some kind of low voltage cutoff circuit otherwise the cells will be quickly ruined.
Protection modules for single LiPo cells are commonly available, but I haven't seen any that work at LiFePO4 voltage levels. I would prefer to avoid LiPo cells entirely because of their flammable nature.
Eneloop NiMh batteries have a low self discharge rate. 3 AA cells in series would give enough voltage to power an Arduino. Can strings be safely wired in parallel to increase the capacity?
If I have to go with a multi-cell LiFePO4 or lead acid solution what is the most efficient way to drop the voltage down to 3.3 or 5v?
Are there any super efficient drop-down converters that spend most of their time sleeping, draw a very small quiescent current and only wake up when the voltage across the output capacitor drops below some threshold?
The average current draw will probably be less than 1mA.
Are you sure?
An Arduino will normally draw 10s of mA, making it unsuitable for use for months on a single charge
Nick Gammon has some useful information about reducing the current drawn by an arduino, here.
Do you have a link to the PIR sensor?
I wouldn't have thought that they used less than 1mA.
JohnLincoln:
Do you have a link to the PIR sensor?
I wouldn't have thought that they used less than 1mA.
100uA is quoted here http://www.ladyada.net/media/sensors/PIRSensor-V1.2.pdf
The whole system will spend a large proportion of the time sleeping, so 1mA average is a realistic value.
Hello,
If you want to run an Arduino for longer periods, rechargeable batteries are not the best option. They normally have some significant self-discharge when compared with standard Alkaline or (non-rechargeable) Lithium ones. People forget that a cheap AA battery has over 2500mA and will last at least 5 years before any major self-discharge, that's great!
Other thing to consider when trying to power MCUs directly from batteries, is that some cell will reduce the voltage during it's life (http://data.energizer.com/PDFs/E91.pdf). For example, a 1.5V Alkaline cell is more likely a 1.6V brand new and will get down to 0.8V at the end of it's life.
Many electronic equipments won't work down to 0.8v per cell and you basically throw away half of the batteries energy. For that reason using a good quality step-up regulator circuit is highly recommended to extract all the juice from each battery.
Check out this board: Bitbucket, it has been designed from scratch to run on batteries. It has ultra-low consumption while sleeping and counts with a very efficient step-up regulator.
If you just want to start playing with step-up/step-down regulators, that can be a big challenge by itself. They are quite "delicate" circuits, especially if you're after the maximum efficiency, where every milliamp counts! For example, it's very difficult to breadboard a switching regulator and keep a good performance... better stick with some good quality, ready-to-use modules like those one: Pololu - Step-Up Voltage Regulators
Cheers