Battery Power

Because battery voltage varies with charge, it must be regulated with some type of DC to DC Converters

PIC Specific Battery Backup Power Supply

Battery characteristics

"Battery characteristics" from _Portable Design_ 1998 July article by Jim MacDonald

Battery chemistry	Energy density (W·hr/kg)	Energy density (W·hr/l)	operating cell voltage (V)	discharge profile	number of recharge cycles[*]	self- discharge (%/month)	internal resistance	discharge rate	standard charging voltage	standard charge current	charge time (hr)
SLA: sealed lead acid	30	60	2	slightly sloping	500	3	low	< 5C	2.3	0.25C	24	after charging at 0.25C, ... The float voltage (approximately 2.25 V/cell) can be maintained indefinitely
NiCd: nickel cadmium	40	100	1.2	relatively flat	1 000	20	very low	< 10C	1.5	0.1C	16	NiCd batteries can be continuously charged at a standard 0.1C trickle rate indefinitely ... many manufacturers' NiCd batteries can be fast charged in 15 minutes ...
NiMH: nickel metal hydride	60	140	1.2	relatively flat	1 500	25	low	< 3C	1.5	0.1C	16	charge at C/10 for 16 hours; then the charge current must be reduced to C/40, or pulse-trickle charged
Li-ion: lithium ion	90	210	3.6	sloping	1 000	5	medium	< 2C	4.1 V or 4.2 V (depending on manufacturer)	0.1C	16	... after charging at 0.1C ... constant-voltage charging within 50 mV ...

Size vs Capacity

Battery capacity (in Ampere·hours) is described by the letter C. A battery with a C rating of 1.2 A·hr is capable of supplying a 1.2 A load for 1 hour. Also, when charging this battery at a C/10 rate (120 mA charging current), it takes a little over 10 hours to charge it.

Battery Type	Capacity (mAh)	Typical Drain (mA)
18650	1800 - 3500
14500	1050 - 1115
D	13000 - 15000	200
C	6000 - 7500	100
AA	2250 - 2400	50
AAA	1000 - 1050	10
N	650	10
9 Volt	500 - 550	15
6 Volt Lantern	11000	300

Load vs Time

The Eveready data book lists the following loads vs. hours, to a cutoff voltage of 1.0 volt for AA Alkaline batterys:

1500 ohms = 1850 hours
300 ohms = 340 hours
150 ohms = 160 hours
75 ohms = 82 hours
50 ohms = 54 hours
30 ohms = 32 hours
10 ohms = 7.8 hours
4 ohms = 2.3 hours

[*] number of recharge cycles until only 80% of initial charge capacity is available upon recharge. These figures are /very/ best case and probably double the real world value.

Lifetime cost comparison

From http://www.batteryspace.com/li-ionsinglecell.aspx (Thanks to Russell McMahon)

Chemistry	Voltage	Energy Density	Working Temp.	Cycle Life	Safety	Environmental Impact	Cost (based on cycle life x wh of SLA)
LiFePO4	3.2V	>120 wh/kg	-0-60 °C	>2000	Safe	Good	0.15-0.25 lower than SLA
Sealed Lead Acid	2.0V	> 35wh/kg	-20 - 40°C	>200	Safe	Not good	1
NiCd	1.2V	> 40wh/kg	-20 - 50 °C	>1000	Safe	Bad	0.7
NiMH	1.2V	>80 wh/kg	-20 - 50 °C	>500	Safe	Good	1.2-1.4
LiMnxNiyCozO2	3.7V	>160 wh/kg	-20 - 40 °C	>500	Unsafe without PCB or PCM, better than LiCo	OK	1.5-2.0
LiCoO2	3.7V	>200 wh/kg	-20 - 60 °C	> 500	UnSafe without PCB or PCM	OK	1.5-2.0

The environmental impact of batteries can be vastly reduced by responsible recycling. The cost of LiFePO4 may be overstated, but is still valid in comparison. E.g. all the cycle life numbers are best case, but the relative low cost of LiFePO4 remains if all cycle life figures are reduced.

Lance Allen

The internal resistance of any power source is a representation of that batteries ability to provide power to a load. This can be thought of as a battery capable of delivering infinite current at no voltage drop in series with a resistor, that resistor being the internal resistance. So a small 12volt battery will have a higher internal resistance than a big 12volt battery and a damaged battery will have a higher internal resistance than an undamaged battery.
You can determine internal resistance by loading the battery and watching the voltage drop across its terminals. If a 12 volt battery drops 1 volt under a load of 1 amp then its internal resistance is 1 ohm. Internal resistance will vary with the state of charge so compare batteries at a common condition, such as full charge.
Manufacturers data is usually comprehensive on a batteries ability to deliver power at different rates, temperatures etc.
A lead acid battery is fully charged at a terminal voltage of 12.6 volts, 25% discharged at 12 volts and flat at 10.8 volts. DO NOT discharge a 12 volt Lead Acid Battery below 10.8 volts or it will be damaged over time.
So (from manufacturers data) a Hitachi 12 volt 7 Amp Hr Lead Acid battery has a fully charged internal resistance of 25 milliohms and can supply a maximum current of 40 Amps for 5 seconds. The maximum charging current is 2.1 Amps.

Don Hyde says:

The non-rechargable (primary) batteries are easy to get your hands on and easy to use. They can be dangerous though, because they will deliver several amps at 3V, which will heat up a wire hot enough to burn your hand.
Don't ask how I know this.{ed: <GRIN> been there, have that scar. Many people have experienced this as a result of carrying batteries around in a pocket, then forgetting and tossing in loose change. Very serious damage can and has occured.}
As for the rechargables...
I mentioned exploding and catching fire... Lithium rechargables have a serious problem with thermal runaway while charging. As they charge, they warm up, and as they warm up, they draw more current, which makes them even warmer...
Lithium rechargables are usually "smart battery" packs, and the plastic and stuff includes temp sensors and usually part of the charger circuitry.
There just might not be any available as individual components since there is such an intimate relationship between the charging circuitry and the stuff in the battery pack, with a serious liability issue if it's not right. They are light, and have lots of capacity, otherwise the laptop and cellphone guys wouldn't be going to all the trouble and expense.
Replacement cellphone and laptop battery packs are readily available. Perhaps the easiest way to get one to play with would be if one of them sold an offline charger. They are expensive, though.