Battery Technology:

Lithium-ion versus Ni-MH

There is no question that Lithium-ion batteries play an important role in the Modem Army and will continue to do so for the foreseeable future.

Used for the correct application they have more power density, are smaller and have less weight than most other battery chemistries.

Lithium-ion batteries predominantly are used in controlled applications, such as lap-tops, cellular phones, camcorders and some communications equipment. By in large all of these power sources tend to be small in power (Ah) capacity.

Due to their size, Lithium-ion batteries undoubtedly have been a major contributing factor to the miniaturisation of many electronic items.

Placed outside of their dedicated applications, Lithium-ion rechargeable batteries have been the topic of many safety issues.

Used as stand alone packs in the aero modelling world they have caused countless fires, through charging incorrectly and especially after the models have crashed where the battery became damaged.

Nickel Metal Hydride batteries have found a niche market where large Ah capacity batteries are required both in and out of controlled environments such as the auto motive industry: They are predominantly used in hybrid electric cars where safety is paramount and the British Army for various field operations.

When used as a stand alone battery Ni-MH is a much safer chemistry than Lithium-ion for the following reasons:-

1. If wires connecting the battery to equipment are accidentally damaged there is no risk of damage to the battery or safety issues concerning personnel.

2. Should the battery be accidentally submerged in water there is no risk of safety issues concerning personnel.

3. Should the battery be accidentally damaged there is no risk of safety issues concerning personnel.

The HAWK. 50Ah Ni-MH batteries have been in operation as stand-alone batteries within the British Army for over 3 years both in extreme temperatures and hostile conditions without one failure and more importantly no safety issues.

To summarise both Lithium-ion and Ni-MH have a role to play, with each ideally suited for their specific tasks.

 

Large Multiple Cell Lithium-ion Batteries.

The construction of a large multi-cell Lithium-ion battery can potentially in high temperatures become an extreme safety hazard combined with poor performance.

Many papers have been published regarding commercially available off the shelf (COTS) small capacity Lithium-ion rechargeable batteries, which are warning of potential problems if handled incorrectly both in charge or discharge mode.

Let us ftrst examine the potential problems of the (COTS) Lithium-ion rechargeable batteries.

 

Ageing.

A unique drawback to the Lithium-ion battery is that its life cycle is dependent upon ageing from time of manufacture (shelflife) regardless of whether it has been charged or not and to a lesser extent on the number of charge/discharge cycles. This drawback is not widely publicized.

The speed by which the ageing process takes place is subject to temperature: the higher the temperature the faster it will age.

Temperatures above 55°C can significantly reduce the life expectancy by as much as 60% and, furthermore, this loss is none recoverable.

The capacity loss manifests itself in increased internal resistance caused by oxidation and crystallisation. Eventually, the cell resistance reaches a point where the pack can no longer deliver the stored energy although the battery may still have ample charge. (Simply put the battery will not deliver heavy loads.)

 

Charging.

Lithium-ion batteries have a nominal voltage of 3.6V and typical charging voltage of 4.2V. The charging procedure is one of constant voltage with current limiting. This means charging with constant current until a voltage of 4.2V is reached by the cell and continuing with constant voltage applied until the current drops close to zero. (Typically, the charge is terminated at 7% of the initial charge current.) Lithium-ion batteries cannot be fast-charged.

Li-ion batteries are not as durable as Ni-MH and NiCad designs, and can be extremely dangerous if mistreated. At a typical 100% charge level at 25 degrees Celsius, Lithium­ ion batteries irreversibly lose approximately 20% capacity per year from the time they are manufactured, even when unused. Every (deep) discharge cycle decreases their capacity. The degradation is sloped such that 100 cycles leave the battery with about 75 to 80% of the original capacity.

 

Discharge.

Some lithium-ion batteries fail due to excessive low discharge. If discharged below 2.5 volts per cell, the internal safety circuit opens and the battery appears dead. A charge with the original charger is no longer possible. Some battery analyzers feature a boost function that reactivates the protection circuit of a failed battery and enables a recharge at a price namely a reduced overall capacity.

However, if the cell voltage has fallen below 1.5V/cell and has remained in that state for a few months, a recharge should be avoided because of safety concerns.

 

Large Ah Lithium-ion batteries.

As mentioned earlier in the charging section, Lithium-ion cells have to be strictly controlled when charging in order to maintain the correct top end charge characteristics. All Batteries are built up from a number of cells which in the case of large Ah batteries are constructed into packs. The number of cells connected in series dictate the voltage per pack therefore in order to increase the size of the battery capacity these packs are connected in parallel .Two potential problems can develop :-

1. The cells can develop a fault whereupon the pack becomes unstable.

2. The packs can become mismatched with each other.

The more the packs are combined to create the larger Ah battery, the more the risk of mismatch will become.

Strict control management has to be implemented in order to avoid the cells from becoming unbalanced. Furthermore this control management has to perform through a wide range of temperatures-and should this fail the end result at best would render the battery inoperative and at worst become potentially dangerous.

The process of mismatch within multiple cell packs when constantly charging and discharging is a gradual process and problems do not manifest themselves in the early stages. The first signs would be a reduced Ah capability, and if allowed to continue particularly with Lithium-ion, could result into a serious safety hazard with catastrophic consequences.

There is a serious safety issue concerning the use, - and particularly the transporting of batteries constructed with re-chargeable Lithium-ion cells. New regulations specific to the transportation of Lithium-ion rechargeable cells and batteries that are constructed with them became effective on 1st January 2003.

These are strict safety requirements introduced by lATA, DOT and other associated Government bodies, worldwide. For reference refer to the lATA Dangerous Goods regulations 49CFR173.185 as well as packing instructions 903 & 912.

Lithium content within a battery may not exceed more than 8 grams per battery or a total of 25 grams for 3 batteries. These figures mean that any battery with a rated ampere hour capacity of 6Ah or more cannot qualify or meet these new regulations.

To calculate the approximate amount of Lithium content in a battery multiply the rated capacity of the battery by 0.3 then multiply by the number of cells in the battery pack.

At the present time, car manufacturers actively developing electric vehicles will not use large capacity Lithium-ion batteries due to safety reasons and state that they are at least ten years away from solving the problem of management control. Most electric cars are using Ni-MH batteries in large Ah format due to their safety record and durability.

When large amounts of Lithium are combined in a closed environment the potential of a catastrophic event can only leave the mind to wonder what the outcome would be.

If we look at a typical performance data specification and factor in both size and temperature (ageing) coefficients the results will be as follows:-

1. 50Ah Lithium-ion battery@ 25°C =50 x 80% = 40 Ah over 1 year
   100 cycles @ 40Ah = 40 x 20% = 8 - 40 = 32Ah
   100 cycles@ 32Ah = 32 x 20% = 6.4-32 = 25.6Ah
   
   This shows that the battery @ 25°'C with 200 cycles of recharging over 1 year could be approximately 50% of its original capacity.

2. 50Ah Lithium-ion battery@ 60°C =50 x 60% = 30 Ah over 3 months
   100 cycles@ 30 Ah = 30 x 20% = 6-30 = 24Ah
   
   This shows that@ 60°C with 100 cycles of recharge over 3 months that the battery could be less than 50% of its original capacity.

These figures has been extrapolated from papers presented by the Battery University, History of the Lithium ion battery and Determining Factors that Affect the Ageing Behavior of High Power Lithium-Ion Cells Argonne National Laboratory Operated by The University of Chicago and Cycle-Life Studies of Advanced Technology Development Program Gen 1 Lithium Ion Batteries which was prepared for the U.S. Department of Energy Assistant Secretary for Energy Efficiency and Renewable Energy (EE) Idaho Operations Office Published March 2001.

Regulation reference sources ICAO Technical instructions (2003-2004 Edition), lATA Dangerous Goods Regulation 44 Edition, the IMDG Code as well as the US HMR pursuant to the final rule by RSPA.

With regard to cycle life. A full cycle constitutes a discharge to 3V/cell. When specifying the number of cycles a lithium-based battery can endure, manufacturers commonly use an 80 percent depth of discharge.

This 80% figure artificially increases the manufacturers claims on number of cycles achieved. e.g. 600 cycles would in reality be 480 full discharge cycles.

Using the same 80% figure will also reduce the capacity of the battery e.g. 50Ah @ 80% = 40Ah deliverable.

If we recalculate the original figures with this 80% depth of discharge it is as follows:-

1. 50Ah Lithium-ion battery utilising 80% depth of discharge = 40Ah
   40Ah Lithium-ion battery@ 25°C = 40 x 80% = 32 Ah over 1 year
   100 cycles@ 32Ah = 32 x 20% = 6.4-32 = 25.6Ah
   100 cycles@ 25.6Ah = 25.6 x 20% = 5.1-25.6 = 20.5Ah
   
   This shows that the battery @ 25°C with 200 cycles of recharging over 1 year could be less than 50% of its original capacity.
   

2. 40Ah Lithium-ion battery@ 60°C = 40 x 60% = 24 Ah over 3 months
   100 cycles@ 24 Ah = 24 x 20% = 4.8-24 = 19.2Ah
   
   This shows that@ 60°'C with 100 cycles of recharge over 3 months that the battery could be approximately 38% of its original capacity.

 

Storage.

As earlier explained the Lithium-ion battery starts to age from the day it is manufactured. The amount of permanent capacity loss the battery suffers during storage is governed by the state of charge and the temperature.

Manufacturers recommend that they are stored at 15°'C (59°F) and at 40% charge.

Constant monitoring has to be implemented whilst in store to maintain the battery at a 40% state of charge, allowing it to drop will result in permanent damage.

 

Comparisons between Lithium-ion and Ni-MH

	Lithium-ion 50Ah Battery	Ni-MH 50Ah ( HAWK) Battery
100% depth of discharge based on 600 cycles	480 actual cycles	600 actual cycles
Life expectancy @ 60°C to 50% capacity	3 months	2 Years
Transport certification	Class 9 Dangerous Goods	None
Ageing from date of manufacture (100% charge)	20% per year @ 25'C	No losses
Charging maximum temperature	45°C	65°C
Minimum temperature for charging	0°C	-25°C
Losses after 100 cycles @ 25°C	20%	2%
Losses after 100 cycles @ 60°C	62%	4%
Cell safety circuitry	Required	Not required
Safe discharge temperature	45°C	70'C
Safety description	Hazardous	None Hazardous
Damage due to deep discharge	Yes	No
Storage requirements to prevent permanent damage	15°C @ 40% charge	No requirement
Fast charge capability	No	Yes

Lithium-ion information extrapolated from internet
Hawk information based on actual results over a 3 year period.