Negative Pulse Technology - What is it, why is it better?

The major benefits Negative Pulse Technology Charging

Dislodges 0₂ bubbles and removes plate surface charge providing:

• Maximum plate surface area for maximum charge acceptance.
• Accurate battery voltage measurements.
• Safe fast charging of a battery near its hot and cold temperature extremes.

Prevents large crystal formations on the nickel plates in new batteries, so:

• More plate surface area available for charge acceptance.
• Prevents "memory effect" and dendrite growth problems.
• Lower plate resistance and less power loss during charge and discharge.
• Can more than double the battery's useful life.

Gradually restores the crystal structure thereby reversing "memory" in used batteries:

• Each "burp pulse" charge produces a gradual run time increase due to less battery voltage drop while the product is operating.

By switching a battery with "memory" to an NPT charger, the severity of the memory (i.e. voltage depression knee) caused by the old charger can largely be reversed. In most instances a significant increase in the product's run time is observed with the first recharge using an NPT charger.

Batteries -- General Characteristics

A battery's life can be defined as the number of charge/discharge cycles before it becomes damaged, unusable, fails or wears out.

1. Battery damage

The method selected for charging the battery is the most critical decision in preventing battery degradation and damage. Overcharging a battery produces heat and gas pressure stresses that degrade the battery. Batteries that are discharged too fast are also degraded by heat and pressure stresses. Under-voltage lock-out circuits can prevent battery over discharge, but this, too, contributes to memory effect problems by establishing its starting point. What happens if the charging method does not eliminate memory? The continued use of the product with an under voltage lockout circuit, over time, results in a battery that seems to charge, but does not power the product.

Use of unsophisticated charging techniques is the most common cause of NiCd and NiMH battery damage. Constant trickle charge methods for extended periods heat the battery and produce memory and dendrite crystal formations. Fast charge methods that end when overcharge is detected also heat the battery and produce memory and dendrites.

2. Unusable Batteries

NiCd and NiMH batteries with memory can actually have a near full-charge that AEGs cannot use. Memory causes a sudden voltage drop in the battery voltage. This sudden voltage drop occurs within a few minutes of power-up, and the product stops after what appears to be a complete recharge.  This characteristic is particularly problematic for AEGs because of the 'burst' nature of power demands when you pull the trigger.  That is why reading a battery's voltage level with a voltmeter in the field when you're having a battery problem is completely misleading.  Voltage may be present and readable, but when the fast surge demand of a trigger pull happens (load) the voltage suddenly disappears.

Dendrites form leakage paths that quickly drain a battery after a recharge. In both cases the battery can be classified as damaged, but technically it is unusable. A complete discharge to 1 volt per cell for multiple cell packs can remove the memory and dendrites, but this consumes charge/discharge life cycles of the battery. It is also very time consuming, and with further charging, the memory and dendrites will return. The NPT charging method prevents their formation.

3. Battery Failures

Unsophisticated overcharge methods allow the battery to get hot before the charge is ended. Heat caused by overcharging greatly accelerates the aging process in batteries. Batteries that are overcharged time and time again fail abruptly as a stressed battery has higher power losses which produce more heat and more stress. The process is self-perpetuating and self-accelerating. Battery capacity plummets in a short amount of time. The battery will die without warning.

4. The Battery Wears Out

NiCd and NiMH batteries are constructed from materials that have different coefficients of thermal expansion and different coefficients of compression. The positive and negative plates are composed of dissimilar metal compounds. The separator is a polymer (plastic) material. The electrolyte is a viscous liquid and the battery case is usually made of a plated steel alloy. Heat and pressure associated with the charging process cause mechanical stresses that cause batteries to wear out if heated plastic separators in the batteries expand more than the surrounding plate materials and the case. The soft, plastic separator is most vulnerable to the pressure that results from gas generation associated with overcharge.

The NPT Charging System

NPT based burp chargers have the most advanced charging technology for providing maximum battery performance and charge/discharge cycle life.

The basic principle of the NPT burp charging system combines a high-current charge with regularly spaced negative current pulses. With NiCd and NiMH cells this, in effect, burps the battery. During the application of the high-charge current, some oxygen is generated due to electrolysis of the electrolyte. The generated oxygen gas accumulates as "bubbles" attached to the cell plates, reducing the effective surface area and raising the internal impedance. This increased impedance reduces charge efficiency and causes heating during rapid charging.

When a momentary high-amplitude discharge pulse is applied, the reversal of the chemical process has a relaxing effect, burping the battery by stripping the bubbles from the cell plates. The negative pulse burp not only makes a high-current fast charge possible, but also increases the charge acceptance. Furthermore, in the case of both NiCd and NiMH cells, greater charge acceptance results in less heat generation, which in turn results in a higher charge acceptance.

The burp conditions the battery prior to the quiet time sampling period. The battery's exact charge level is calculated using the quiet window when the data is collected. Variations in NiCd and NiMH appear in the data, allowing the charging of both types.

Charge Termination

The NPT charger uses the most sophisticated charge termination methods available in a standalone battery charging controller. Unlike other charger controllers which terminate the charge when the battery is in a state of overcharge (the most popular being the negative delta voltage method), the  NPT burp charger employs Inflection Point Cut Off, which terminates the charge when the battery reaches 100% charge. This terminates the charge prior to the internal pressure rising significantly, thereby reducing the likelihood of premature battery failure due to venting. (Venting is the result of excessive internal battery pressure that causes electrolyte loss. It significantly reduces the life of the battery.) The NPT burp chargers incorporates variegated measurement techniques to ensure appropriate charge termination in both NiCd and NiMH cells.

The battery voltage is measured during the quiet window immediately following the discharge pulse. The no-load voltage measured here contains less noise and is a more accurate representation of a battery's true state of charge.

In addition to the Inflection Point Cut Off method, there are additional termination methods to ensure a safe termination of the charge:

1. Negative Derivative Termination. Unmatched cells may have a charge profile that obscures the normal inflection point. Negative Derivative Termination detects the maximum point of the charge curve and terminates the charge when the slope of the charge voltage reverses direction.

2. Fully-charged "rested" cells that have lost their surface charge are detected by observing a rapid increase in voltage when current is first applied. This condition is detected and the charge is terminated.

3. Fully-charged batteries which have not rested long enough to loose their surface charge, will frequently produce a negative initial slope. This condition is detected and the charge terminated.

4. Shorted cells or shorted contacts may be detected by observing that the charging voltage remains below a predetermined threshold during charging.

5. An internal timer will terminate the charge after a predetermined period of time has elapsed.

Maintenance Mode

Upon completion of a normal charge sequence, the controller enters a maintenance mode. The purpose of this mode is to keep the battery primed at peak condition, ready for use. It is noteworthy that the maintenance charging scheme used is NOT a low-current trickle charge, as is customarily found, but the same high-current pulse used during rapid charging. The difference is that the duty has been extended, thereby applying an equivalent average maintenance charge current. The charge/discharge pulses help prevent dendrite formations and help maintain proper crystal structure of the cell plates. Dendrite formation leads to shorted cells. Improper crystals structure contributes to memory problems.