Replacement Battery:
  1. Battery Pack
  2. Soldering Cells

Replacing a Battery Pack with Consumer AAA Rechargeable NiMH Batteries

Over the last six months, our cordless landline phone (AT&T model EP5632) has gradually started dropping calls and displaying the message “SEARCHING FOR BASE”. At the same time, the battery life decreased and the handset battery eventually stopped holding a charge. In this case, the inability to find the base was related to the weak battery providing inadequate power to the antenna.

The original battery pack (103 89-0429-00) lasted almost three years. The Lenmar CB0103 replacement battery pack appears to have lasted only half that time. This is possibly due to the replacement pack having sat on the retail shelf for a long time before I purchased it. Alternatively, it might have been a bad batch of cells.

My family retains a landline phone because our youngest child is not yet old enough to own a mobile phone. That will change soon enough, so I had hoped that the current landline phone would continue working long enough to be the last we would ever own. Furthermore, we are generally happy with the phone and I dislike adding salvageable equipment to the landfill.

Instead of taking a chance on another unreliable replacement battery pack, I decided to look inside the existing one. The plastic shell consists of two parts held together with transparent tape that is easily removed with a razor blade. Inside, there are three industrial tabbed cells of the same length and diameter of consumer AAA cells, without the bump on the positive terminal.

CB103 phone battery

CB103 phone battery

Given the identical cell size, it distresses me that the phone manufacturer didn’t simply mold a AAA battery holder into the handset. This consumer-friendly feature would have allowed the end user to replace the individual cells using off-the-shelf consumer batteries. The cynic in me believes the proprietary package is to force customers to purchase from AT&T for profit maximization. The idealist assumes this is a safety feature to prevent errant installation of alkaline cells or mixed chemistries that might catch on fire when recharged from the base.

Regardless, the phone’s battery pack is simple enough that a hacker with qualified electrical experience and tools could make a replacement. Using a multimeter, I measured the polarity of the cells. Additionally, I examined the flat metal tabs and wires that connect the cells.

Inside original battery pack showing connections and welds

Inside original battery pack showing connections and welds

Note the thin metal strip is attached with two tiny indentations and no visible solder. Possibly this was accomplished with some form of electrostatic welding. Also worth noting -- I am impressed with the attention to insulating material.

Using a fine point marker, I labeled the batteries (as shown in the first photo in this article) in polarity and connectivity order. To avoid damaging the handset or causing a fire, it is critical that the battery arrangement is identical to the manufacturer’s pack. Speaking of which, the phone warns me not to do exactly what I’m about to do.

Phone contacts ATT EP5632 handset

Phone contacts AT&T EP5632 handset

CAUTION

In all seriousness, this article deals with very real fire and chemical hazards. You could burn your house down or cause bodily injury or worse, simply to save a few dollars on a battery pack. Assembling and/or soldering a battery pack must only be attempted by adults with appropriate knowledge, skills, and tools, and who are willing to take full responsibility for any outcome.

Proceed at your own risk.

Buying AAA Batteries

The label on the battery pack indicates that the battery chemistry is NiMH (nickel metal hydride). It is critical to use the same type of battery as the phone charger was designed for.

I purchased a four pack of Rayovac NiMH AAA cells (PL724-4) from Fry’s Electronics for $9.99. The official AT&T battery pack costs $19.95 (plus shipping) for three AAA cells. Knock-off battery packs are as low as $6 each, plus shipping.

Rayovac AAA platinum rechargeable NiMH

Rayovac AAA platinum rechargeable NiMH

The advantages of using off-the-shelf consumer batteries are:

The advantages of ready-made battery packs are:

All reasons except the first translate into potentially higher safety and reliability, but that’s not guaranteed. There have been examples in the media of OEM battery packs catching fire due to manufacturer’s defects.

Next, I created a prototype battery pack using two pieces of insulated wire (stripped on the ends) and low-adhesion masking tape. Before installing the experimental pack, I confirmed that the output polarity was correct and the voltage (nominal 3.6) matched the original battery pack. If the voltage is less than 3.3 volts, then one or more of the batteries are backwards or miswired.

Test battery pack with tape

Test battery pack with tape

In the photo above, the green wire on far left of photo connects the top and bottom cell. The purple wire connects middle and bottom cell.

I placed the black plastic shell around the batteries and installed it into the phone. It fit snugly and worked well. My wife was content at this point.

The problem with stopping here is that masking tape is not a reliable method of connecting wires. As the tape ages, the wire may come loose. Intermittent contact can result in dropped calls or a scrambled headset. Worse still, one of the wires could migrate to an incorrect connection point and cause a fire or chemical spill.

That means it is time to permanently attach the wires. I initially considered silver conductive epoxy. During my research, I ran across a lot of articles on soldering batteries, particularly in the R/C (radio controlled) community. This convinced me to give soldering a shot.

Let’s see how that turned out.