The Protection Circuit Module (PCM) was designed by Poly-Quest engineers to greatly reduce the chance of accident while charging Lithium-Polymer batteries.

Each cell in a lithium-polymer battery pack must be limited to a maximum of 4.2 volts during charging, and ONLY battery chargers specifically designed for lithium polymer cells may be used.  A pack of 2 cells is series (2S) would be properly charged at 8.4 volts maximum, for example. The greatest risk of fire associated with lithium polymer batteries is from applying too much voltage during charging. In most accidents, the user simply makes a mistake when setting the number of cells in series, for example choosing "3S" - 12.6 volts charging - when charging a "2S" pack which should only receive 8.4 volts, maximum.

The Apache SMART charger was introduced last year in an effort to eliminate those fires caused by over-voltage charging, and has been remarkably successful.  It contains the world's most accurate detection system for improper charge settings, and will refuse to charge a pack which does not match the voltage settings on the charger.  It is the main line of defense in Lithium Polymer charging safety.

However, the Apache SMART charger can only check to see that the total charge voltage is correct for the total number of series-connected cells in the pack.  And no charger can determine the state of each cell in a pack when charging multiple cells. So that leaves another avenue for charging accidents: cell imbalance within the pack itself.  A lithium-polymer pack of 3 cells, for example, could gradually drift out of balance due to small differences in the cells at the beginning, due to use beyond recommended discharge rates; over-capacity discharge, or even due to impact damage at some time.

If one cell in a 3S pack has lower voltage than the other two, for example, any lithium charger will attempt to charge the whole pack to 12.6 volts. But when the two higher cells reach 4.2 volts, the charger can't know that the low cell is still at 4.1 volts, for example. So it will continue charging, trying to reach the proper 12.6 volts for the pack. The two higher-voltage cells will therefore be overcharged, which may damage them or even cause a fire if gas generated inside the cells builds to the point that the cell envelope ruptures.

(caution:  Lithium Polymer cells should always be charged and stored within a fire-safe container.  Never leave lithium polymer cells in any environment which may exceed 130 degrees farenheit)

NOTE:  PCM Guard Version II has been on sale at AIR CRAFT since Aug 16th, 2004.  The description below refers only to the new VII model.  For description of original PCM-Guard, see bottom of this page.

The Poly-Quest PCM-Guard^TM is a voltage-limiting device that prevents single-cell over voltage charging in the case of cell imbalance.  It is made specifically to work with Poly-Quest Lithium Polymer battery packs, which have the factory-installing multi-connector cell tap to match the PCM-Guard. Connected between lithium-polymer charger and Poly-Quest battery pack, PCM-Guard limits charge voltage to 4.2V per cell, allowing a short-time maximum of 4.35 volts to reduce possibility of false warnings.

If PQ-PCM-Guard detects that charge voltage to any one cell exceeds the limit, the PCM-Guard electronically disconnects the pack from the charger and the warning LED will light.

PCM-Guard is available in three configurations:
* 2S dedicated
* 3S dedicated
* 4S dedicated (see notes at bottom on 4-Cell Pack availability)

Each PCM-Guard can only be used with battery packs of the corresponding type: "2S" PCM-Guard for "2S" Poly-Quest battery packs, for example. Or the "4S" PCM-Guard can only be used with "4S" packs.

The PCM-Guard should work with any proper Lithium-Polymer charger, but we STRONGLY recommend the APACHE SMART Charger for greatest possible safety. We cannot guarantee that the PCM Guard will funtion properly with any particular lithium charger, except Apache (SEE NOTES AT BOTTOM PAGE for info on non-Apache Chargers).

PCM-GUARD V2 SPECIFICATIONS (any sold after Aug 16, 2004)
* Maximum Input Voltage : 18V
* Maximum Charge Current : 5A (increased from 3A in Ver I)
* Overcharge Protection Voltage : 4.35V ± 0.025V
* Overcharge Protection Release Voltage : 4.15 ± 0.060V
* Dimensions : 55.0mm X 35.0mm X 5.0mm (W*H*D)

We STRONGLY recommend that charging ALWAYS be monitored.  If you decide not to heed this warning, please be especially sure that the Lithium pack is restrained inside a fireproof container, and located in an area without flammable materials, during charging.

THE PCM GUARD MUST BE CONNECTED/DISCONNECTED IN THIS ORDER:

CONNECT FOR CHARGING:

• Connect Charger to 12V power source (car battery or high-quality switching power supply)
   
• Connect Lithium-Polymer Battery to PCM Guard via the Multipin-Connector
   
• Connect PCM Guard to Charger Output
  WARNING!  Ignore the charger polarity symbols on the PCM Guard label.
  Red Wire = POSITIVE ( + )
  Black Wire = NEGATIVE ( - )
  

  

DISCONNECTION:

• DISCONNECT CHARGER FROM POWER SOURCE
   
• DISCONNECT PCM GUARD FROM CHARGER
  (Li-po is still attached to PCM Guard at this point)
   
• DISCONNECT Li-Po BATTERY from PCM GUARD

In most cases, the operation will be completely automatic.  The red LED on PCM-Guard VII will light continuously when connected and charging is in progress.  If voltage exceeds 4.35V for any cell, the LED will start blinking, then disconnect the charge circuit to the high cell electronically and LED will go off.  In that case, disconnect the pack and measure cells/rebalance as instructed below.

The PCM-Guard cannot guarantee that damaged cells will be properly charged.(see notes at bottom).

Air Craft offers the PCM-side multi-connector (AC-CONNMULTI-PCM) which mates with the multi-connector on each Poly-Quest pack. This allows users to attach their preferred connectors for metering/balancing the pack. Any such connectors attached must be fully insulated such that they cannot short inadvertently.

Note that these procedures require knowledge of electronic circuits, and entails risk of equipment damage and personal injury if connections are made improperly.  If you have ANY doubt about your ability to safely diagnose and re-balance packs, refrain from the attempt. Air Craft accepts no liability for accidents arising from improper use, nor do we claim that the instructions and cautions below are sufficient for all possible circumstances.

Checking Individual Cell Voltage

The diagrams on left show the wiring for the Multi-Connector attached to Poly-Quest Lithium-Polymer BATTERY. The connector has a single red wire (B1+), and number of black wires equal to the number of series cells in the pack. (2S pack has two black wires, labeled B1- and B4-) To check voltage (or balance charge) for a single cell, connect voltmeter (or 1-cell charger) so that the target cell is spanned: EXAMPLES: (cells are counted from top down) *Check cell #2 in 2-Cell Pack >  connect to B1- and B4- *Check cell #1 in 3-Cell Pack >  connect to B1+ and B1- *Check cell #3 in 4-Cell Pack >  connect to B2- and B3- NOTES *Avoid checking cell balance on fully discharged packs, if possible.  Best readings will be obtained when the pack is at least half charged. *If there is any evidence that the pack or a single cell has been damaged by impact, dispose of the pack immediately after fully discharging via safe means.  No device can insure safe charging of an impact-damaged cell, nor a pack with damaged connections between the cells. *When using the multi-port balancing connector on Poly-Quest pack, NEVER connect to more than one cell at a time to check voltage, nor attempt to charge more than one cell at a time. *If you find cell imbalance, charge each cell individually through the multi-port connector at 1S rate (3.7 cell volts, 4.2 charge volts), one cell at a time.  Make sure that all cells in the pack are fully charged during balancing (don't forget a cell!). During balancing, the main pack wires (the two larger black/red wires which power the model) must be open circuit.  Do not attach any device to these while using the multi-port balancing connector.

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