There has been a LOT writting about the Backup Battery in the Kyocera 7135. Lots of pissing, moaning, groaning and supposition. The purpose of this post is to clear up some of the mystery and provide some facts.

First - it is a battery. Specifically, it is a Panasonic ML-920. Very similar to CR series Lithium batteries, but this is a rechargeable Manganese-Lithium. (read: Don't replace it with a CR-1010!)
It produces a little over 3V when fully charged.

Second - the battery is located in the display portion of the phone. It is opposite of the speaker-phone speaker. That is to say, when looking at the display screen, it is to the left of the ear-piece. It is hard soldered to the single ribbon connector that runs the display, ear-piece, and speaker-phone speaker.

Modes of failure:

Any battery can go bad over time. There are several factors that can contribute to speed that this occurs.

This battery relies on the phone charging to get it's charge. Those that continually swap their batteries, never actually charging the phone, never allow the backup battery to fully charge. As Mr. Mackey would say "This is bad."

Extremes in temperature. Usually heat more so than cold. Even though this is a sealed cell, heat will force the electrolytic out, drying the cell.

High discharge/charge cycle count. This will kill any battery. The more times a battery is completely discharged and the recharged, the shorter its lifespan will be.

Suggested reading, pictures, etc: 1, 2, 3.

Although I have replaced several of these batteries (proving that they are the backup battery), one thing that I have had occur twice that counters this is replacing the phone board.

I have had 2 phones that would hard reset with the battery pulled for less than 5 seconds. I replaced the phone (RF) board, and they both stopped hard resetting until the battery was out for at least 30 minutes. I have no explanation for this, other than to say that it is fact, and verifiable.

My only supposition is that the charging controller is on the RF board. As I stated in the above post, the backup battery does not charge until the main battery has fully charged.

Good topic, Hash! One other thing worth mentioning in regards to the backup battery is that the only component that can be powered off that battery is the Infineon 128 Mb SDRAM chip, which goes into standby mode with self-refresh mode enabled. The backup battery cannot power the micro-Processor, the display, or any of the phone components. Therefore, if you remove the main battery you will go through a soft reset upon supplying power to the phone (to initialize the Processor and all other peripherals), even if the backup battery was able to maintain the SDRAM content. To avoid any type of reset when swapping batteries it is recommended that you supply external power during the procedure. The backup battery is really only meant for emergencies when the main battery becomes discharged and an external supply is not available.

Thanks Dr. D.

You make several good observations in your reply. However, I didn't include these in my original post, as they were outside of the scope of my original writing.

My post was written for the user whose backup battery was no longer functioning. This would predicate their already knowing what the backup battery was for. It's scope was to provide information to that user for them to use in resolving their issue or/and prevent it from (re)occurring. Additionally, I kinda hoped the post would make it to the FAQ section...

My first "reply" I made to my own thread was done intentionally for that very reason. It was originally part of the first post. However, I felt that it bordered on being "off-topic" and it certainly would have been distracting to the original information.



It is not always possible to have external power, especially when you're out in the field. There has been many a time I have told a dozen or so people on a conference call "My battery's gonna die soon - I'll call you back after I swap it out." This is while I'm wiring up a cross-connect or splicing fiber in the middle of nowhere - miles away from the nearest 110V outlet.

Additionally, I just like to pull the battery for 10-15 seconds at least once a week. I find that when I do that, my goofy problems and forced resets are greatly reduced. I certainly wouldn't do this if everytime I pulled the battery, I had a hard reset.

Just to clarify this for some of the not-so-techies.... 128 Mb is "Mega-bit". That is not MB "Mega-Byte". 128Mb=16MB

It should also be pointed out that when used as a reference to memory size, M and K do not have the same meaning as when used as units of metric measurement. In the metric system K implies a multiplier of 1,000, while M implies a multiplier of 1,000,000. When used as references to memory size, K implies a multiplier of 1,024, while M implies a multiplier of 1,048,576.

Yes. It would be clearer to call it 128Mib or 16MiB.

See Definitions of the SI units: The binary prefixes .

Still assuming 8b=1B, both 16MiB = 128 Mib and 16MB = 128Mb are correct so I don't understand what the fuss is about.

Are there posts with instructions/How-To on changing the battery.

My wifes 7135's backup battery is dead. My 6035's battery is still hanging in there though.

Like Hash mentioned in his first post, the backup battery is not really a user-servicable item. To get to it you have to disassemble pretty much the entire phone, including the top (display) half of the clamshell, which is rather tricky to do without breaking or damaging anything. Even if you manage to do that, you will need to desolder the old battery from a flat ribbon cable and reattach the new one to the same, again using soldering. Doing this without melting the insulation on the ribbon cable is not trivial either and requires decent equipment and good soldering skills. It's definitely not advisable to the average consumer.

No, it shouldn't - at least not here. This thread is titled "7135 Backup Battery".


Ummm.. NO - It wouldn't. It may be more technically correct - but it certainly wouldn't be "clearer".
The average person expects memory and storage to be quantified as MB - Mega-Bytes.
Not "Mb" or "MiB" or "Mib" or any other generally obscure nomenclature. That is "what the fuss is about."
And, while it is wonderful that you understand all of this, and could never be confused by not noticing the difference between B and b - my point was that it could be confusing.
Now, you have added to that potential for confusion by bringing up yet another entire set of unit nomenclature.

All of this effort b/c I pointed out 128 Mega-bits equaled 16 Mega-Bytes. *Sheesh*

Thank you.

Can we get back to discussing the Backup Battery now? PLEASE

It's actually not that difficult. Some skill with the soldering iron is required, though. Also, some patience.

Obviously, before starting this you will need to have acquired a new ML-920 battery - with solder tabs attached.

You don't actually have to disassemble the entire phone - just the display. However, first-time, this is no cake-walk in and of itself.

1 - Backup your data (either hotsync or to SD card) and pull the main battery from your phone.

2 - Remove the Aluminum Escutcheon (P/N TMSRP10154). This is the approx 2" x 3" metal trim piece covering the outside of the display half of the clamshell. Heat and a wide flat prying tool helps make this less painful.

3 - Remove the (2) #6 Torx screws found under the above cover, near the hinge.

4 - CLOSE THE PHONE (if it's not already), and start to pry the display case apart, starting from the hinge.

DO NOT attempt to pry at the top (opposite of the hinge) of the display. That won't end well.

There are 2 keeper clips per side, located 30mm and 63mm from the hinge edge, respectively. Remember, the cover is plastic, but the other piece is a Magnesium alloy. One will give - the other just breaks.

5 - Once the plastic cover is removed, disengage the top-center of the interconnect cable from it's keepers. Remove the ear-piece and speaker - put them in a safe place. Carefully pop the PLC style connector from the main display. Remove the main display and also place in a safe place.

6 - Now - carefully peel up Speaker phone/Ringer contact tab. It is held in place with a non-drying adhesive. Heat is usually not necessary here, but keep the idea as an option.

Once the flex interconnect cable is is detached from the upper clamshell, peel it back. It will still be attached to the internals of the main part of the handset. With some care, this will not be a problem.

7 - Carefully, peel off the grey foam covering the battery. This is also a non-drying adhesive, and you will want to reuse this to keep the battery from going "clack-clack" in your ear every time you move.

8 - Now that the battery and it's terminals are exposed, you can un-solder the old and solder in the new. Please observe polarity. Referenced to the top of the phone (when assembled): Positive is up, Negative is down.

BTW: The entire Flex Interconnect Cable (with battery) is available for ~$50 + S/H. The P/N is TMSRP10155. I've seen them on eBay for under $5...

9 - Assemble the phone in reverse order.

Ummm... don't forget the earphone and speaker