Replacing the Rechargeable Batteries on FRS/GMRS Motorola Radios

In our family, we use the Motorola series of FRS/GMRS radios on almost every occasion when we leave the house.  This can be in the grocery store or at an amusement park.  Our current favorite unit is the T5950, which has:
These were about $60 a pair with charger and batteries, and have a three-bar battery gauge.  With experimentation I found that if the voltage is greater than 4.28V, all three bars light up.  More than 3.47V, two bars will light up.  Below this latter voltage the radio chirps regularly, and only one bar will light up.

After a few years use, we noticed the batteries do not last longer than a few hours, so I wanted to replace them.  A search on the web shows that the battery packs are about $12 each (plus shipping), and their capacity is still only 600mAh, so I decided to investigate a home-brew battery replacement for these units.  I pealed back the sticker on one battery pack and discovered that the hidden elecronics are very simple and can be easily reproduced.  One terminal of the charger connections connects to the negative terminal of the battery pack, and the other terminal connects to the positive terminal via a blocking diode.

Photograph of the inside of the battery pack with the outer sticker removed.  Notice
the small fuse (near the left side) and the diode (middle of picture).  The part number
on the original pack is HKNN4002B.

These are the items you will need to do this do-it-yourself replacement:
  1. Begin by cutting a 1.5"x1.5" piece of the paper.  This will serve as your substrate.
  2. Cut the strip of sticky copper tape into three pieces.  They are 1", 0.75" and 1.5" in length respectively (all 0.75" in width).
  3. Insert the three batteries into the radio, and verify that it powers up.  If it doesn't you may have to bend the metal tabs out to meet the battery terminals.
  4. Take the 1.5" section of copper tape, and make a cut into it in the middle of the long side.  Cut only half way through the width.
  5. Remove the paper backing, on the tape, and fold the tab over so that the tape forms a 'b' shape.  The top arm of the 'b' is not sticky on either side because you folded the tab over.  The bottom belly of the 'b' is sticky.
  6. Tape the bottom (belly) part of the 'b' onto a corner of the paper substrate (top copper tab in the pictures below).  Have the non-sticky top arm stick out of the edge.
  7. Insert the top arm of the 'b' under the negative terminal of the battery.  Since you folded the tab under, both sides of this leg are exposed copper and will make good contact with the battery and the radio terminal.
  8. Tape the 0.75" square piece of copper tape in the opposite corner of the paper (see below).  The two previous squares now form the charging contacts for the new pack.
  9. Take the remaining copper strip and make a 'P' with it by cutting half way through the long side and folding it back.
  10. Insert the leg of this 'P' under the positive terminal of the right most battery (not sticky), and glue the exposed tape onto the paper substrate (bottom picture).
  11. Solder the diode to the latter two copper tabs.  Cathode ring toward the positive battery terminal as shown in the bottom picture.  Put the diode in the crease formed by the gap between the two batteries.
  12. Snap cover back on and test in charging stand.  You may need to bend out the contacts in the cover to touch the two copper tabs on the paper substrate.

Half way through the battery replacement.  note that one of the copper strips
is been inserted between the negative terminal of the left most battery.  The
unit is now ready for the installation of the diode.

Picture of the completed replacement.  After this, you just snap the cover back on
and test the radio in the charger.

The original battery packs are only 600mAh units.  The new ones are more than three times the capacity (I used NiMH), and should last us a few days between recharging with even daily use.  Remember to recycle your used batteries as they contain Cadmium. 

After some use it became clear that the circuitry in the charger needed to be modified to provide more charge current for the batteries.  Tracing the circuit, I came up with this schematic.

Circuit diagram of the charger.  Each radio is charged on a different half cycle of the AC waveform.

To increase the charging current, I shorted R3 and R6 with a jumper and replaced R2 and R5 with a 10 Ohm resistor.  This quadruples the charging current from the standard 39mA to 155mA (both measured).  It is still below the C/10 slow charge rate, so it should be safe to leave the radios charging for extended periods.

Send me e-mail if you do this replacement.  I would like to hear about it.

On a somewhat related note, I tend to collect somewhat unusual wrist watches, so I decided to buy this pair of radios on ebay for $25.

Cool Dick Tracy wrist watch radios that I bought on ebay.  They have
FRS/GMRS channels with privacy code.  The latter feature is
unusual among wrist watch FRS radios.

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