Will rechargeables replace the primary battery?

RFI Technology Solutions
Wednesday, 05 June, 2002

Compared with other power sources, electrical energy from non-rechargeable (primary) batteries is very expensive. To reduce cost, many organisations are switching to rechargeable (secondary) batteries. This article examines the advantages, limitations and economics of primary and secondary batteries.

Primary batteries have their advantages. Operational readiness, high energy density, long storage and instant readiness are just a few.

But advancements in rechargeable batteries have put power densities on a par with primaries. Some work will still be needed to obtain longer storage times and improve cycle life, issues that plague the ultra-high density rechargeable battery.

Figure 2 reveals the much lower energy cost rechargeable batteries provide. The analysis is based on the purchase price of a commercial battery pack and the number of discharge-charge cycles it can endure before replacement is necessary.

The cost does not include the electricity needed for charging, nor does it account for the cost of purchasing and maintaining the charging equipment. The comparison applies to batteries for commercial products such as mobile phones, two-way radios, laptops and video cameras.

The extreme right column evaluates the cost of the BB-390, a military NiMH pack, which is used in lieu of primary lithium sulphur dioxide. The cycle life of all batteries is calculated at best cases.

This takes into account the initial investment, fuel consumption where applicable, maintenance and eventual replacement of the equipment. The cheapest power source is the utility; the most expensive is primary batteries.

The fuel cell offers the most effective means of generating electricity but is expensive in terms of cost per kWh.

This cost becomes economical when compared with portable batteries, however. For vehicular and stationary applications, the fuel cell is considerably more expensive than the combustion engine.

The costing information is based on current estimates and assumptions.

Primary versus secondary batteries

Consumer market put aside, the largest users of primary batteries are defence organisations and emergency services.

High energy density, long storage and simple usage put aside, one of the most important attributes of the primary battery is combat readiness. No charging and priming is required before use.

Logistics are simple and portable energy can be made available at remote distribution points that are unmanned and have no electricity. Disposal is easy because little toxic material is used.

Because of one-time use, the cost of the primary battery is about 30 times higher than that of rechargeables. The pricing becomes even more exorbitant if the packs are replaced after each mission, regardless of length.

A General in the US Army said that half of the batteries discarded still have 50% energy left. Discarding partially used batteries is widespread because keeping track of these packs is time-consuming and awkward.

It is much simpler to issue fresh packs before each activity.

Reading the state-of-charge (SoC) of primary batteries is possible. The most basic method is measuring the terminal voltage but the result is inaccurate. A better method is counting the out-flowing energy units, also known as coulombs.

This requires a circuit and display on the battery. Due to high cost and inherent inaccuracies, especially during pulsed loading, this method is seldom used on primary batteries.

A more accurate SoC measurement is possible with quick-test instruments in which the chemical integrity of the battery is examined.

Each battery type requires a reference matrix, which can be stored in the designated adapters that are used for the battery interface. The test lasts a few seconds and is non-invasive.

During the last 10 years, armies and emergency response teams have gradually been switching to rechargeable batteries. The reasons are improvements in battery technology, better charge methods and more readily available charge power. But the most important single reason is cost.

In the US Army, rechargeable batteries have been used predominantly for training. Officials are now exploring the suitability for combat missions. Rechargeables have advantages that go beyond cost issues.

For one, the batteries can be re-used and do not burden the supply channels. In the absence of electric power, charging can be done through solar power, windmills and hand-crank generators.

Even kinetic power is being explored in which an electric generator is built in the sole of the soldier's boot.

Rechargeable batteries can keep communications going in areas where no supply of fresh batteries is possible.

Rechargeable batteries are not new to the armies - the Dutch Army has been using them for decades. Whereas the Dutch Army uses smaller packs for handheld devices, the US Army uses larger batteries for backpack equipment.

Beside chemistry and size, there are other differences in how the two armies manage the batteries in the field.

The US Army issues batteries with no maintenance program in place. If the battery fails, another pack is released, no questions asked. This has resulted in a high failure rate.

The Dutch Army, on the other hand, has moved away from the open fleet system by making the soldiers responsible for their batteries.

The change was made in an attempt to reduce waste and improve reliability. The batteries become part of the soldier's personal belongings.

Since adapting this new regime, the failure rate has dropped considerably and battery performance has increased. Unexpected down time has almost been eliminated.

It should be noted that the Dutch Army uses exclusively NiCd batteries. Each pack receives periodic maintenance on a Cadex battery analyser to prolong service life.

Batteries that do not meet the 80% target capacity setting are reconditioned; those that do not recover are replaced.

The US Army, on the other hand, uses NiMH batteries, which offer higher energy densities than NiCd but have a shorter service life.

Battery maintenance

With the switch to secondary batteries, some level of battery maintenance is required, a service that is best performed with a battery analyser. Here are some field results on the use of battery analysers:

At the conclusion of the Balkan War, the Dutch Army serviced all batteries at the Dutch Military headquarters using Cadex 7000 series analysers.

The army was aware that the packs were used under the worst possible conditions. Rather than a good daily workout, the NiCds were employed for short patrol duties lasting two to three hours per day.

The rest of the time the batteries remained in the chargers for operational readiness. The batteries were two to three years old.

The capacity on some packs had dropped from 100% nominal to 30%. With the analyser's recondition function, nine out of 10 batteries were restored to full service. The Dutch Army sets the target capacity threshold for field acceptability to 80%.

The importance of exercising and reconditioning NiCd batteries with an analyser is emphasised by another study carried out for the US Navy by GTE Government Systems in Virginia, USA.

To determine the percentage of batteries needing replacement within the first year of use, one group of batteries received charge only (no maintenance), another group was periodically exercised and a third group received recondition.

The batteries studied were used for two-way radios on the aircraft carriers USS Eisenhower, USS George Washington, and the destroyer USS Ponce.

With charge only (charge-and-use), the annual percentage of battery failure on the USS Eisenhower was 45% (see Figure 4). When applying exercise, the failure rate was reduced to 15%.

By far the best results were achieved with recondition. The failure rate dropped to 5%. Identical results were attained from USS George Washington and the USS Ponce.

Recondition is a secondary discharge that removes the remaining battery energy by slowly draining the cells towards zero volts.

The need to replace batteries decreases by three and nine-fold respectively when exercise and recondition is applied. These statistics were drawn from batteries used by the US Navy on the USS Eisenhower, USS George Washington and USS Ponce.

The GTE Government System report concluded that a battery analyser featuring exercise and recondition functions costing $US2500 would return its investment in less than a month on battery savings alone.

The report did not address the benefits of increased system reliability, an issue that is of equal if not greater importance, especially when the safety of human lives is at stake.

Battery analysers are also used to quick-test battery performance.

Cadex Electronics has introduced a technique to measure the state-of-health of a battery in three minutes. Based on inference technology, the Cadex Quicktest uses battery specific matrices that are derived through a 'trend learning' process using artificial intelligence.

The ability to self-learn enables the system to adapt to new battery chemistries without changing hardware.

Qucktest is available on the Cadex 7200 two-station and the 7400 four-station battery analyser/reconditioners. The system accommodates Li-ion, NiMH, NiCd and lead acid batteries; the required charge level is 20 to 90%.

If outside this range, the analyser automatically applies a brief charge or discharge. The matrix is stored in the battery adapters that also hold the battery parameters to configure the analyser.

Testing a battery with a properly learned matrix achieves an accuracy of ±5% on most batteries. Popular battery adapters include the matrix at time of purchase. For other batteries, the matrix can be obtained by running the analyser's Learn program.

Primary batteries will always be around, if only to run wristwatches, portable entertainment devices and flashlights.

While the primaries were once the only practical power source for portable applications, there is a shift towards rechargeables.

Ever since Neumann successfully sealed the NiCd battery in 1947, the era of rechargeable batteries had begun.

The 1990s brought many improvements in terms of higher energy densities and lower costs. But the portable world is not yet satisfied - we need smaller batteries that last longer.

Will the chemical battery retain its status or does the future lie in fuel cell or atomic fusion? Hype put aside, we are still years away from any practical alternative solution.

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