NiMH, Li-Ion or Li-polymer? What to consider…

Nickel-Metal Hydride, Lithium Ion, and Lithium Polymer are the three main battery chemistries dominating the portable electronics space. All three types are capable of supplying the high power demand of portable applications. To make an optimal battery selection for a design one has to have a full understanding of the application and the unique strengths and weaknesses of each battery. Basically one needs to match the specific chemistry characteristics of the battery to the real life portable application’s usage specifications. So voltage, cycles, load current, energy density, charge time and discharge rates to temperature ratings, discharge profiles, charging cycles, expected shelf life and transportation requirements. Here is a brief overview of the different chemistries and how they match up to some of the real life usage specifications.

Nickel-Metal Hydride (NiMH):

Nominal cell Voltage: 1.25 V
Cycle durability: 500-1000 Cycles
Energy/weight: 100 Wh/kg
Energy/size: 140-300 Wh/L
Power/weight: 250-1000 W/Kg
Charge/discharge efficiency: 66%
Energy/consumer-price: 1.40 Wh/US$
Self-discharge rate: 30%/month
Optimal load current: <0.5C
Maximum aggregate voltage: 12.5 (10 cells)
Charge Time: <4 Hours

NiMH

Lithium Ion (Li-Ion):

Nominal cell Voltage: 3.6/3.7 V
Cycle durability: 1000-1200 Cycles
Energy/weight: 160 Wh/kg
Energy/size: 270 Wh/L
Power/weight: 1800 W/Kg
Charge/discharge efficiency: 99%
Energy/consumer-price: 2.8-5 Wh/US$
Self-discharge rate: 5-10%/month
Optimal load current: <1C
Maximum aggregate voltage: 25.2 (7 cells)
Charge Time: <4 Hours

Li-Ion

Lithium Polymer (Li-polymer):

Nominal cell Voltage: 3.7 V
Cycle durability: <1000 Cycles
Energy/weight: 130-200 Wh/kg
Energy/size: 300 Wh/L
Power/weight: 2800 W/Kg
Charge/discharge efficiency: 99%
Energy/consumer-price: 2.8-5 Wh/US$
Self-discharge rate: 5%/month
Optimal load current: <1C
Maximum aggregate voltage: 25.2 (7 cells)
Charge Time: <4 Hours

 Li-Polymer

Knowing the battery specifications is important but not enough to make the ideal battery selection for a portable device. One has to consider the following real life usage specifications.

Temperature Range:

Firstly, here are some typical operating temperature ranges:

Consumer: -20°C to +60°C (-4°F to + 140°F)
Industrial/Military: -40°C to +85°C (-40°F to +185°F)
Automotive: -40°C to +115°C (-40°F to +239°F)
Geophysical: 0°C to + 150°C (32°F to +302°F)

The cell capacity changes with temperature and these variations can be quite large. Higher temperatures allow for a reduction of the batteries internal resistance, this increases the effective capacity of the battery. However a continuous exposure to high temperatures will shorten the cycle life of the battery along with increasing the self-discharge rate (cell degradation). At lower temperatures the cell capacity is reduced, also batteries have a tendency not to perform well in cases were instantaneous pulses of power are required mostly do to the increased internal resistance. One has to look at the average and extremes of their operating temperature rang and compare them to the various temperature curves provided by the manufacturer. Also the batteries have a tendency to heat up when charging so one has to take this into consideration when creating the PCB layout for the design.

Discharge Rate:

A discharge rate also has an effect on the amount of heat generated by the battery. The larger the load current the more heat generation exists. Also the batteries will perform better with a more uniform discharge pulse rate. Spikes in the power profile should be minimized; this goes for all three chemistry’s..

Charging Regimens:

The batteries heat up as they are charged. For example a typical NiMH battery will increase by +25 C while a Li-Ion will only increase +10 C with the same constant-current, constant-voltage charge method. This has to be considered especially for consumer portable devices.

Expected Shelf Life:

One has to consider how long the device will be in storage before being used. This makes a huge difference between NiMH and Lithium batteries. A NiMH battery could be completely dead within half a year without even being used. Lithium batteries have much better performance in this consideration.

Transportation Guidelines:

Transportation requirements are especially important for lithium based batteries, since there are strict regulations imposed on their transportation. Sometimes one has to two different battery designs one for local distribution and one for overseas.

Overall there is plenty of material available on the Internet and through the different battery manufactures that should help you make the optimal battery decision. The important part is just to know that that decision has to be made and that it’s not as simple as choosing based on price.

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11 Comments on “NiMH, Li-Ion or Li-polymer? What to consider…”

  1. Anonymous Says:

    Very nice guide!

  2. peter Says:

    what kind of measurement is Wh/$
    the common unit is $/Wh

    • John Artiuch Says:

      Wh/$ just tells you how many Wh you will get for your buck :) It quickly shows you that you get more Wh with Lithium batteries comparing to Nickel-Metal Hydride.

  3. fatooh Says:

    very good and helpful information, thanks

  4. Tweak Says:

    helpful guide, thanks


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