Batteries and solar chargers

When a rechargeable battery goes dead on one of your gadgets, you can usually plug in a charger and bring it back to life. If those batteries are AA or AAA cells, you can even buy nonrechargeable ones to keep you going until the old cells are charged up again. But what if you’re out where there are no AC outlets and no stores?

One solution is to tuck your dead batteries into a GaiaM solar battery charger, only $20, and leave it in the sun. The charger uses solar power to recharge a pair of AA or AAA cells.

Click the link above to go over to the GaiaM web site and buy the charger.

According to GaiaM, the US-only distributor, the unit can fully charge two AAA cells in 1-3 hours and two AA’s in no more than 5 hours, even on cloudy days. It is available now.

The SP 1300 weighs just 23 ounces, making it all the more likely that you will carry it with you to places where you need it most.

For another $12 you get four rechargable batteries as well. Gaiam put all the brands to the test, using a pair of identical Mini-Mag AA flashlights, and found their NiMH rechargeables outlasted the top alkaline battery by almost 30 minutes.

Despite the TV ads touting advanced technology, alkaline batteries have not gotten significantly better. But rechargeable NiMH batteries, which have always excelled in terms of environmental impact, have pulled ahead when it comes to performance as well as economic return.

A NiMH rechargeable battery delivers 2,200mAh and will do that about 500 times before you throw it away. The best alkalines deliver approximately 1,800 to 2,000 mAh — then they get tossed. We did the math and found 4,000Ah of power costs 1,425 with throwaway alkalines and only 61 with rechargeable NiMH batteries, including charger and electricity.

The Nickel-Metal Hydride batteries excel at hard use and deliver steady voltage with no sag all the way down to 10% of charge. This makes them a better choice for heavy-discharge uses like digital cameras or remote-control toys, where you actually get to use more of the stored energy than any other battery type will deliver. There is no memory effect from short cycling and no toxic content. Because they self-discharge over 3-6 months, these cells aren’t a good choice for emergency lights.

Rapid-testing of batteries (BU41)

When studying the characteristics relating to battery state-of-health (SoH) and state-of-charge (SoC), some interesting and disturbing effects can be observed – the properties are cumbersome and not linear. Worst of all, the parameters are unique for every battery type. This inherent complexity makes it difficult to create a formula for rapid testing that works for all batteries.

In spite of these seemingly insurmountable odds, battery rapid testing is possible. But the questions are asked, how accurate will the test results be and how will the system adapt to different battery types. Instrument cost and ease-of-use are also concerns. This paper evaluates currently used methods, which include the load test, AC conductance test and the six-point test developed by Cadex.
The load test
The load test provides important battery information consisting of open battery voltage, voltage under load and internal resistance. nickel-based batteries should always indicate an open terminal voltage of about 1.1V/cell, even if empty. The electro-chemical reaction of the different metals in the cell generates this voltage potential. A depressed voltage may indicate high self-discharge or a partial electrical short.

A lead-based battery must always have a charge and the open terminal voltage should read 2.0V/cell and higher. If below 2 volts, a sulfation layer builds up that makes a recharge difficult, if impossible. An open terminal voltage of 2.10V/cell indicates that the battery is roughly 50% charged.

The voltage of a lithium-based battery can, to some extent, indicate SoC. A fully charged cell reads about 4.0V/cell and a partially charged cell measures between 3.0 and 4.0V/cell.
The load test applies a momentary load, during which the voltage is measured. Voltage over current equals the resistance. More accurate results are obtained by applying a two-stage load. Figure 1 illustrates the voltage pattern of such a two-stage load test.

The AC conductance test
An alternative method of measuring the internal battery resistance is the AC conductance test. An alternating current of 50 to 1000 Hertz is applied to the battery terminals. The battery’s reactance causes a phase shift between voltage and current, which reveals the condition of the battery. AC conductance works best on single cells. Figure 2 demonstrates the relation of voltage and current on a battery.

Some AC resistance meters evaluate only the load factor and disregard the phase shift information. This technique behaves similar to the pulse method in that the AC voltage is superimposed on the battery’s DC voltage and acts as brief charge and discharge pulses. The amplitude of the ripple is utilized to calculate the internal battery resistance.

There are some discrepancies in the resistance readings between the ‘load test’ and ‘AC conductance test’. The differences are more apparent on marginal than on good batteries. So which reading is correct? In many aspects, the AC conductance is superior to the load test, however, one single frequency cannot provide enough data to evaluate the battery adequately. Multi-frequency devices are being developed but their complexity rises exponentially with the number of frequencies used.

Resistance measurement, as a whole, provides only a rough sketch of the battery’s performance because various battery conditions affect the readings. For example, a battery that has just been charged shows a higher resistance reading than one that has rested for a few hours. An empty or nearly empty battery also exhibits elevated internal resistance. To obtain reliable readings, a battery must be at least 50% charged.

Temperature further affects the internal resistance readings. A hot battery reads a lower resistance than one at ambient temperature or one that is cold. In addition, the chemistry, the number of cells connected in series and the current rating (size in mAh) of a battery influence the results. Many batteries also contain a protection circuit that further distorts the readings.

The Cadex QuickTest
Cadex Electronics has developed a method to measure the state-of-health (SoH) of a battery in 3 minutes. QuickTest uses a patent-pending inference algorithm to fuse data from 6 variables, which are: capacity, internal resistance, self-discharge, charge acceptance, discharge capabilities and mobility of electrolyte. The data is combined with a trend-learning algorithm to provide an accurate SoH reading in percent. Figure 3 illustrates general structure of such a network.

QuickTest is built into the Cadex C7000-Series battery analyzers and services nickel, lithium and lead-based batteries for two-way radios, cell phones, laptops, scanners and medical devices. The analyzers are user-programmable and also perform battery priming, reconditioning, fast-charging, life-testing and boosting functions.

QuickTest uses battery specific matrices that are obtained with the analyzer’s trend learning process. The ability to learn allows adapting to new batteries in the field. The matrices are stored in the battery adapters and automatically configure the analyzer to the correct battery setting. The adapters commonly include the matrix at time of purchase. If missing, the matrix can be added in the field by scanning two or more batteries on the analyzer’s Learn program. The required charge level to perform QuickTest is 20-90%. If outside this range, the analyzer automatically applies a brief charge or discharge.

What is the definition of state-of-health and when should a battery be replaced? SoH reveals the overall battery conditions based on the above mentioned variables, which are capacity, internal resistance, self-discharge, charge acceptance, discharge capabilities and mobility of electrolyte. If any of these variables provide marginal readings, the end result will be affected. A battery may have a good capacity but the internal resistance is high. In this case, the end SoH reading will be lowered accordingly. Similar demerit points are added if the battery has high self-discharge or exhibits other chemical deficiencies. The battery should be replaced if the SoH falls below 80%.


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