Battery

Basics

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 ).

Some of the most important characteristics of batteries:

• Voltage
• Internal Resistance
• Energy content ( often indirectly measured by amp hours or mAh )
• Rechargeable? Primary ( not rechargeable ); Secondary ( rechargeable )
• Size
• Weight
• Chemistry ( lead-acid, NiCd, NiMH, Lithium, etc ...... )

Uses

• 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 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.

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.

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."