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07-10-2016

Charging batteries rapidly and safely

Steve Taranovich

EDN

Any hobbyist can charge a battery quickly, but can you do it without an explosion, excessive heating or major degradation in battery cycle life?

Well many companies have managed fast charging techniques that typically use specialized algorithms. These algorithms take into account the chemistry of the battery and some sort of non-standard charging rate curve. Many device manufacturers and wireless operators are now providing a minimum two-year warranty on smart phone devices setting 800 cycles as the battery cycle life of the battery.

I have yet to see many, if any, published lifetime test results from any fast charger supplier with popular battery types. One of these companies willing to publish life tests is Chargetek. They, in conjunction with Potential Difference, Inc., have managed to safely return a 51% charge in 25 minutes. Their technique is based upon proven patents, chemical analysis, and the all-important confirmation of the technology by extensive testing.

Their technique uses batteries ranging from Lithium Ion 18650 cells to 20,000Ahr batteries. Their technique is also applicable for lead-based batteries like Absorbent Glass Mat (AGM), Sealed Lead Acid (SLA) and maintenance free. Lithium cobalt oxide and Lithium iron phosphate are also able to use this rapid charge technique.

I spoke to Lou Josephs, CEO of Chargetek and we discussed their proprietary software that can be customized for the customer’s application. One nice feature of their patented and proven algorithm is that they can rapid charge two large battery banks from a single charger. Each bank will be independently regulated.

Concurrent charging

Josephs explains how this works:
The battery charger has two ports, one for each battery pack. Battery pack 1 and battery pack 2 are alternately charged and discharged. In the figure below, the battery voltage curve is depicted in green, the battery charging current is depicted in red and the battery discharge current is depicted in purple. See Figure 1.

A positive voltage slope (voltage increase) is generated during charge. A negative slope (voltage decrease) is generated during discharge. The flat area is the rest period of the charging cycle. The positive charging current amplitude can be three to four times the magnitude of a conventional charger due to our patented algorithm.

Charging batteries rapidly and safely
Figure 1. Battery pack 1 and battery pack 2 are alternately charged and discharged.
In the figure the battery voltage curve is depicted in green, the battery
charging current is depicted in red and the battery discharge current
is depicted in purple. (Image courtesy of Chargetek).

The industry standard charging current is based upon the amp hour (Ahr) rating of the battery and is denoted by C. For example, if each battery pack Is 500 Ahr, then the standard charge rate would be in the range of C/3 - C/5 resulting in a charging current range of 100 – 170 amps. There would be variations depending upon the characteristics of a particular battery.

For this particular application, the charge current would be set to 2C (1000 A) with a 50% duty cycle resulting in a charge rate of C or 500 amps. The discharge current would subtract from this slightly. The resulting charge time would be approximately 1.1 hours. This charge time is two to three times faster than what would be attainable using a conventional charger. With a conventional charger, the charge current would be limited to a maximum of C/3 or 170 amps.

The cost savings would be significant since there is only one charger required for two battery packs. In addition, at least twice as many battery packs could be charged in the same time. In summary, our charger has four times the charge capacity of a conventional charger.

Limitations of batteries during a charge cycle

There are three intrinsic battery limitations during a charge cycle;

  1. Maximum battery voltage
  2. Maximum battery temperature
  3. Maximum allowable charging current

By exceeding any of these parameters the results can be undercharging, overcharging, overheating or physical degradation. The challenge is to maintain a significant charge acceptance while charging at an elevated current.

Conventional high current charging methods can cause concentration polarization and electrochemical polarization.

The challenge

In order to return 51% capacity (71% total capacity) for a 3000 mAhr capacity cell in 25 minutes, an average current of 3.7 amps is required:

Average current = 51% × amphour capacity × (60 min/25 min) = 3.7 amps

The average manufacturer recommended charging current is 0.883 amps.

How does Chargetek’s algorithm provide the current required while maintaining battery safety and life? Their algorithm circumvents the battery’s physical limitations:

  • Reduces electrochemical polarization by providing regular rest periods to allow the ions to disperse evenly between the two electrodes
  • Concentration polarization is eliminated by applying a comparatively short duration reverse pulse, either preceding or following the positive charge pulse
  • Temperature, voltage, and charge acceptance are continually monitored, and from that feedback, the parameters of the charging algorithm adjust continually

The algorithm

By reducing heat generating charge acceptance problems and electrochemical polarization, Chargetek’s algorithm is able to recharge at exceptionally high currents. See Figure 2.

Charging batteries rapidly and safely
Figure 2. Critical waveform parameters (Image courtesy of Chargetek).

Chargetek’s rapid charge technique is comprised of three fundamental components:

  1. Charge current pulse: The amplitude (IC) and duration (tC) is depicted in red. A charge current of two to three times the amp hour rating of the battery is typically employed.
  2. Discharge current pulse: The amplitude (ID) and duration (tD) is depicted in blue. The magnitude of this current is equal or greater to the magnitude of the charge current. The time duration is a fraction of the charging current.
  3. Rest time: The battery current is zero (tR).

During the entire charging process, battery temperature, rate of temperature change, battery voltage and current are continually monitored and modulated by the proprietary PDI

Chargetek software. The parameters of the algorithm are adjusted in real time during the charge.

Applications

Chargetek’s patented fast battery charge technology is the key to the growth of electric vehicles and users of cell phones, laptop computers, power tools, etc. are not stranded without power, fast charging off-the-shelf batteries in minutes, without overheating.

Competitive Advantage:

  1. Electric automobile charging stations need fast battery charge times similar to gas refueling, using off the shelf batteries. Chargetek’s 20 minute time is close while the state of the art is 40 minutes.
     
  2. Among industrial vehicles, 95% of forklift trucks are forced to swap 1 ton batteries between shifts as fast battery charging alternatives require about 3.5 hours but regularly overheat and damage the batteries, releasing toxic fumes. Chargetek eliminate these problems and enable up to 75% savings for industrial rapid charging and double the revenue for automotive charging stations.
     
  3. Owners of devices such as cell phones, laptops, and power tools are often stranded waiting several hours to recharge. Chargetek’s 20 minute fast battery charge can solve that problem from a 110 volt outlet.

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