Performance analysis/review of Kentli PH5 Li-ion 1.5V AA battery

In my previous blog post, I tore down the Kentli PH5 battery – a Li-ion battery that has an internal 1.5-volt regulator that allows for terrific voltage stability… up to a point. In terms of data collection, so far I have collected 55+ runs of data logs (248 MB of text files!) and still do not quite have all the data I want. As for the data that I do have, I will be disseminating them with as much thoroughness as possible.

Voltage vs. load current

As expected, the voltage output of the PH5 remains quite stable, up until roughly 2.1 amps where the voltage sags noticeably until the regulator goes into overcurrent protection mode.

A maximum load capacity of 2.1 amps seems to be a bit… limiting. That said, I have not done tests on the PH5’s transient load capacity, as it would require more automated control than what I currently have available.

Another issue with having such a flat discharge curve is that any device that performs fuel gauging using voltage alone will report 100% capacity, until it suddenly shuts down. This could be a big problem for digital camera users, as they will have no indication that their batteries are running low, until the device abruptly stops working. If the camera was writing an image to its memory card when the battery died, it could cause the image to be corrupted, or worse, damage the file system on the card!

Voltage vs. state-of-charge

Unless you are running the battery at a high discharge rate, the output voltage will be flat at 1.5 volts before abruptly brickwalling and dropping to zero immediately at the end of discharge. At a high load (in the case of the graph below, at 2 amps), the voltage remains flat until the very end of the discharge cycle (99% depth of discharge for my test run), where it quickly tapers off and drops to zero.

Capacity vs. load

This is the big one, and it took a lot of work to get this data, especially at low loads (48+ hours of continuous logging is just asking for Murphy’s Law to come into play). I used almost 50 discharge runs to create the graph below.

This is where things get… interesting. I was expecting the capacity to peak at low currents then taper off as the load current increases. Instead, I noticed a definite ‘hump’ in capacity around the 250 mA mark (reaching a maximum of 1700 mAh / 2550 mWh), and only after that point did I see the expected downward slope in capacity, reaching 1200 mAh (1800 mWh) at the 2 amp mark.

This data brings forth some very interesting conclusions. The PH5’s capacity is inferior to its Ni-MH counterparts (even the relatively crappy ones), and at higher discharge rates it has similar capacity to that of an alkaline at the same load, albeit with much better voltage stability than the Ni-MH or alkaline chemistries.

Other findings

Although I won’t go into too much detail for the next few points (I haven’t gotten quite enough data to be presentable), there are some other issues with the battery that I think should still be mentioned.

One issue is the amount of heat the battery gives off at high loads. At 2.1 amps, I had to use a fan to blow cool air onto the DC-DC converter just to prevent it from entering its over-temperature shutdown mode. Although the converter itself can tolerate elevated temperatures, the Li-ion cell inside will not; the uneven heating that the cell will encounter could potentially degrade its lifespan in the long run.

Another problem is efficiency. At 1 amp, the DC-DC converter is about 75% efficient, and is only 65% efficient at 2 amps. I have not tested the converter’s efficiency at lower loads yet, but I doubt it will achieve more than 85-90% efficiency.

A potential issue with this battery is self-discharge. The buck converter remains active all the time, unless the converter or the Li-ion protection circuit enters a protective shutdown state. I have not had a chance to fully charge an unmodified battery in order to perform a long-term self-discharge test, but I will create another blog post for that, if/when the time comes.

Conclusion

Overall, I’m on the fence when it comes to this battery. Its innovative design does provide unparalleled voltage stability, but its low capacity even at moderate discharge rates dampens the fun significantly. Additionally, the 2.1 amp discharge limit could prove to be a bottleneck for some high-drain applications; this, coupled with the cell’s tendency to shut down abruptly when the internal cell runs empty could potentially cause file system corruption for digital cameras that have not been designed to handle such sudden power interruptions.

Also, the batteries are very costly. At about $10 per cell, you may want to think twice about replacing all your current disposable and rechargeable batteries with these newfangled Li-ion ones. Don’t forget the charger either, as a special charger is required to make contact with a recessed terminal on the top of the battery.

Overall, this cell is… interesting. Just don’t expect a miracle in a steel can.

Pros:

  • Excellent voltage stability, even at high loads
  • Li-ion chemistry allows for a very lightweight cell, even with the addition of a DC-DC converter
  • High output voltage could allow some devices to run more efficiently

Cons:

  • Low capacity – provides a mere 1200 mAh (1800 mWh) @ 2 amps, and up to 1700 mAh (2550 mWh) @ 250 mA (even alkaline batteries can do better than this)
  • Abrupt shutdown when the battery is overloaded, overheated, or over-discharged
  • Runs hot at high loads (and therefore is fairly inefficient)
  • 1.5 MHz converter and unshielded inductor can cause excessive EMI (electromagnetic interference) in sensitive devices
  • Expensive! Costs approximately $10/cell
  • Requires proprietary charger

Bottom Line: This is a niche product and should not be considered a universal replacement for alkaline or Ni-MH AA batteries.

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Teardown of Kentli PH5 1.5 V Li-Ion AA battery

June 17, 2015 – Performance analysis/review HERE!

After having an entire month of dormancy on this blog, I’m finally beginning to cross off the blog posts on my “Pending” list.

Last year, I made a blog post talking about Kentli’s lithium-ion based AA battery that has an internal 1.5 volt regulator. The first order never arrived, and the second one had arrived a few months ago but I never got to actually taking one of the cells apart. That changes today.

Cell overview

The battery itself looks like a regular AA battery, except for the top positive terminal. There’s the familiar ‘nub’ that constitutes the 1.5 volt output, but also has a recessed ring around it that provides a direct connection to the Li-ion cell’s positive connection for charging.

 

After peeling the label, we are met with a plain steel case, save for the end cap that appears to be laser spot-welded. Wanting to take apart the cell with minimal risk of shorting something out inside, I used a small pipe cutter to gently break apart the welded seam. Two revolutions and a satisfying pop sound later, the battery’s guts are revealed.

Battery internals

The PCB that holds the 1.5 volt regulator is inside the end cap, with the rest made up of the Li-ion cell itself. Curiously enough, the cell inside is labeled “PE13430 14F16 2.66wh” which is interesting in more than one way. First of all, the rated energy content of the cell is less than what’s on the outside label (2.66 watt-hours versus 2.8), and the cell inside is actually a Li-ion polymer (sometimes called a “Li-Po” cell) type; I was expecting a standard cylindrical cell inside. Unfortunately, my Google-fu was unable to pull up any data on the cell. I might attempt to do a chemistry identification cycle on the cell and see if TI’s battery database can bring something up.

Battery circuitry

The end cap’s PCB uses a Xysemi XM5232 2.5 A, 1.5 MHz synchronous buck converter to provide the 1.5 volt output. According to the datasheet, it is a fully integrated converter with all the power semiconductor components residing on the chip itself. The converter is rated for 2.5-5.5 volt operation, well within the range of a Li-ion cell. Additionally, it has a rated Iq (quiescent/no-load current) of only 20 microamps. The buck converter’s 2.2 microhenry inductor is magnetically unshielded which may cause some increased EMI (electromagnetic interference) emissions, but I don’t have the equipment to test this.

I was looking around for the battery’s protection circuit, and found it on the flex PCB that surrounds the Li-ion cell. It uses a Xysemi XB6366A protection circuit which, like the buck converter, is a fully-integrated device; there are no external protection MOSFETs for disconnecting the cell from the rest of the circuit.

Performance analysis

December 14, 2015 – After extensive and detailed analysis (148 MB of text files!), I’ve analyzed the battery’s voltage and output capacity, which can be viewed HERE (lots of pretty graphs; check it out!).

The data doesn’t stop there. Over a long, long period of time I’m tracking the battery’s self-discharge as well. Those posts are available here (Part 1), here (Part 2), here (Part 3), and here (Part 4).