Battery Chemistry

The battery chemistry that will be best for your device depends on the device and the application. Many devices will simply not allow you swap chemistry types. The big exception to this rule is power tools where you can use NICD, NIMH, or Li-ion battery chemistry types.

Let’s take a more detailed look at: NiCd, NiMH, Li-ion, Li-Po, and Reusable Alkaline batteries.

By the numbers we get the following:

	NiCd	NiMH	Lead Acid	Li-ion	Li-ion polymer	Reusable Alkaline
Gravimetric Energy Density(Wh/kg)	45-80	60-120	30-50	110-160	100-130	80 (initial)
Internal Resistance (includes peripheral circuits) in mΩ	100 to 200¹ 6V pack	200 to 300¹ 6V pack	<100¹ 12V pack	150 to 250¹ 7.2V pack	200 to 300¹ 7.2V pack	200 to 2000¹ 6V pack
Cycle Life (to 80% of initial capacity)	1500²	300 to 500^2,3	200 to 300²	500 to 1000³	300 to 500	50³(to 50%)
Fast Charge Time	1h	2-4h	8-16h	2-4h	2-4h	2-3h
Overcharge Tolerance	moderate	low	high	very low	low	moderate
Self-discharge / Month (room temperature)	20%⁴	30%⁴	5%	10%⁵	~10%⁵	0.3%
Cell Voltage(nominal)	1.25V⁶	1.25V⁶	2V	3.6V	3.6V	1.5V
Load Current – peak – best result	20C 1C	5C 0.5C or lower	5C⁷ 0.2C	>2C 1C or lower	>2C 1C or lower	0.5C 0.2C or lower
Operating Temperature(discharge only)	-40 to 60°C	-20 to 60°C	-20 to 60°C	-20 to 60°C	0 to 60°C	0 to 65°C
Maintenance Requirement	30 to 60 days	60 to 90 days	3 to 6 mos⁹	not req.	not req.	not req.
Typical Battery Cost (US$, reference only)	$50 (7.2V)	$60 (7.2V)	$25 (6V)	$100 (7.2V)	$100 (7.2V)	$5 (9V)
Cost per Cycle(US$)¹¹	$0.04	$0.12	$0.10	$0.14	$0.29	$0.10-0.50
Commercial use since	1950	1990	1970	1991	1999	1992

1: Characteristics of commonly used rechargeable batteries

Internal resistance of a battery pack varies with cell rating, type of protection
circuit and number of cells. Protection circuit of Li‑ion and
Li-polymer adds about 100mΩ.
Cycle life is based on battery receiving regular maintenance. Failing to apply
periodic full discharge cycles may reduce the cycle life by a factor of three.
Cycle life is based on the depth of discharge. Shallow discharges provide more cycles
than deep discharges.
The discharge is highest immediately after charge, then tapers off. The NiCd
capacity decreases 10% in the first 24h, then declines to about 10% every 30
days thereafter. Self-discharge increases with higher temperature.
Internal protection circuits typically consume 3% of the stored energy per month.
1.25V is the open cell voltage. 1.2V is the commonly used value. There is no difference
between the cells; it is simply a method of rating.
Capable of high current pulses.
Applies to discharge only; charge temperature range is more confined.
Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
Cost of battery for commercially available portable devices.
Derived from the battery price divided by cycle life. Does not include the cost of
electricity and chargers.

Since the year 1800 when the first voltaic battery was invented portable battery power has been a fascination by many. In 1991 Sony commercialized the first lithium-ion battery and in 1999 lithium-polymer came out commercially with PDAs.

But what are the differences of the two chemistries and in terms of your PDA battery which one is better? In a nutshell the two chemistry types are similar but one benefit that lithium-polymer offers is that enables slim geometry that allows it to fit in small places like a PDA.

Let's look inside the battery technology a bit more!

Lithium-ion Battery:

* The lightest of all metals
* The greatest electrochemical potential
* The largest energy density for weight.
* The load characteristics are reasonably good in terms of discharge.
* The high cell voltage of 3.6 volts allows battery pack designs with only one cell versus three.
* It is is a low maintenance battery.
* No memory and no scheduled cycling is required to prolong the battery's life.
* Lithium-ion cells cause little harm when disposed.
* It is fragile and requires a protection circuit to maintain safe operation.
* Cell temperature is monitored to prevent temperature extremes.
* Ccapacity deterioration is noticeable after one year (whether the battery is in use or not).

Lithium Polymer Battery:

* The lithium-polymer differentiates itself from the conventional battery in the type of electrolyte used (a plastic-like film that does not conduct electricity but allows ion exchange – electrically charged atoms or groups of atoms).
* The polymer electrolyte replaces the traditional porous separator, which is soaked with electrolyte.
* The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety and thin-profile geometry.
* Cell thickness measures as little as one millimeter (0.039 inches).
* Can be formed and shaped in any way imagined.
* Commercial lithium-polymer batteries are hybrid cells that contain gelled electrolyte to enhane conductivity.
* Gelled electrolyte added to the lithium-ion-polymer replaces the porous separator. The gelled electrolyte is simply added to enhance ion conductivity.
* Capacity is slightly less than that of the standard lithium-ion battery.
* Lithium-ion-polymer finds its market niche in wafer-thin geometries, such as PDA batteries.
* Improved safety – more resistant to overcharge; less chance for electrolyte leakage.

Tag: Battery Chemistry

What Battery Chemistry Will Be the Best for My Device?