From Open Circuits
Many electronic devices run off batteries as their energy source. The ideal battery is a source of voltage, when connections are made to it current will flow, the amount depending on the connections. For the ideal battery the voltage will not fall when current is drawn, or time goes by. For real batteries none of this is true, but may be useful approximations. An improved model of a battery is that it is an ideal voltage source in series with an ideal resistor ( known as the internal resistance of the battery ). A battery is a collection of cells so a single cell battery is really just a cell, not a battery.
Some of the most important characteristics of batteries:
- Internal Resistance
- Energy content ( often indirectly measured by amp hours or mAh )
- Rechargeable? Primary ( not rechargeable ); Secondary ( rechargeable )
- Chemistry ( lead-acid, NiCd, NiMH, Lithium, etc ...... )
- Provides power to circuits. At any time the power is equal to the voltage of the battery times the current. When the battery is not connected, the current and the power are 0.
- We will often use a wall-powered power supply in place of a battery.
There are a surprising number of myths and urban legends about batteries (such as the "memory effect" near-myth and various ways of "fixing" it). Also the process of charging them up. Is it true that "almost all batteries don't perform to optimum until after 10 or so full charging cycles"?
 battery holder
With many modern electronic devices, the battery is often the single heaviest component, and one of the largest parts. Also, it is often the only part designed to be replaced periodically.
- coin cell battery holder: Are you designing or soldering a PCB and you need battery backup? PG31 GPS USB Dev Board has a few tips. The µWatch uWatch takes a completely different approach to connecting coin cells to its PCB.
- 9 V battery holder: ... any tips?
- AA battery holder: ... any tips?
- What kind of battery and holder is appropriate for powering a sewn-on LilyPad Arduino or wearable LED display or other sewn-on wearable electronics?
 battery test jig
I wish someone would design an open-hardware battery test jig. One I could program with various recharge protocols, to see which protocol is *really* the best for recharging secondary batteries. And one that anyone could build to repeat the experiment. Such a jig could conclusively prove or disprove the many myths and urban legends about batteries.
- Can I do that with the Smart Battery Charger with Graphical Data Logger ?
- Can I do that with the Ardustat ?
- * Yes, yes you can. DS
- "Team Toad: Battery Tests": empirical tests of real batteries with real motors.
 DIY UPS
The biggest batteries in my house are inside my UPSes. Alas, sometimes I think they are not quite big enough. Here are some links to people building custom UPS with bigger batteries than most off-the-shelf UPSes:
- Mike Howes. "Beating brownouts: building a super UPS". 2007. 
- "The DIY (or, if you must, "Ghetto") UPS"  via  "even though it makes rain look really cool, please do not stand in the rain holding the strobe."
- "max". "DIY HACK – How to make your own UPS for your computer and other peripherals during power outage". 2008. 
- "12V DC micro UPS system": DC input (up to 18 VDC), 12 V battery charger, and DC output (connected to either the input or the battery). ,
- BenjaminNelson. "48V UPS for DIY Solar PV System" 
- "jacquesl". "DIY UPS, completed". 
 Further Reading
- "Performance Measurements of Some NiMH Battery Chargers" by Matt Blaze
- "The Great Battery Shootout" by Dave Etchells
- Wikipedia: Battery (electricity)
- Wikipedia: memory effect
- sci.electronics FAQ: NiCd Batteries do NOT have "memory"
- "Digital Photo Myths Dispelled" by Dave Johnson
- "Dan's Quick Guide to Memory Effect, You Idiots"
- Battery University
- "Charging NiMH Cells" mentions that "If the charge current is 1/40th the ampHr rating of the cell (c/40), ... pressure will not build up. The cell can be kept in a state of trickle charge for a long period of time without damage."
- "Lithium-ion Battery Charging Basics" claims that lithium batteries should not be trickle charged.
- Instructables: "NiCd - NiMH PC Based Smart Charger - Discharger" by hosam_eldin
- Massmind: "Battery Power"
- "Your mobile phone is lying to you. So is your laptop." claims that "Both phone makers and cellular service providers want you to think that your phone is still pretty much full of charge even if it's almost half empty. For this reason, many of them tweak the charge meters to overestimate the remaining charge."
- "The big fat lie about battery life"
- [http://arstechnica.com/science/news/2009/03/lithium-breakthrough-could-charge-batteries-in-10-seconds.ars "Lithium breakthrough could charge batteries in 10 seconds" by John Timmer 2009
- "The best batteries in the world..." by ian 2006 ("rechargeable NiMH AA cells"?)
- No. 6 Dry CellThis is now sort of an old time thing
- "Deep Cycle Battery FAQ"
- (by Paul Hills ?) "Batteries". 2004.  (was:  ); also "Battery Monitoring" , with an emphasis on robot competitions.
- (by Constantin von Wentzel ?) "How Lead Acid Batteries Work" says that "One of the main reasons why lead-acid batteries dominate the energy storage markets is that the conversion efficiency of lead-acid cells at 85%-95% is much higher than Nickel-Cadmium (a.k.a. NiCad) at 65%, Alkaline (a.k.a. NiFe) at 60%, or other inexpensive battery technologies." Concludes that AGM batteries are better for marine application. 
- TimAnderson. "Easy Test of Battery Amp-Hours Capacity". 
- "PowerStream's Power Humor Page" 
- Isidor Buchmann. "Battery Fuel Gauge: Factual or Fallacy?" 2012. Describes a way to measure state-of-charge with a magnetic field sensor.
- Isidor Buchmann. "Be Aware of Risk Management in Batteries" 2012. Describes poorly understood characteristics of batteries: capacity fade, self-discharge, and temperature effects.