First of all, let me clarify that with normal 14500 cells in general, a heavily loaded (with attached USB devices) or tasked (running all cores to the max) Raspberry Pi Model 3 will not run for long on these cells, if at all. Regardless of the capacity (between 700 to 900 mAh) available for these cells, they cannot supply the required current levels.

I measured that with these standard cells, the cell voltage will collapse within seconds with currents approaching 1.5A, which is no good for a Model 3.

However, there are 14500 type high discharge capacity cells available. These so called IMR (Lithium manganese oxide, also called LMO) cells can supply currents in the 8A or more range, and I wanted to test if our HAT-like UPS equipped with these cells can actually support a loaded and tasked Model 3.

In our efforts to see what is available for a Model 3 UPS support, Bud and I did not find many (if at all) that we thought could support current levels for the Model 3.

Even with these high discharge IMR type cells, we’re still talking about a UPS function in the minutes only, certainly not hours.

So for all practical matters, this UPS configuration is intended only to ride-out small Brown-Outs, swapping out wall-warts, moving the RPi to another wall socket, etc. and still provide a safe power down after a serious power deficiency has been detected. I will personally only use this UPS while I’m testing hardware and software on my Model 3, which is typically located on my desk. I use the same setup with my own UPS (Described here : https://www.raspberrypi.org/forums/viewtopic.php?f=37&t=145954) for the other RPi models, and it saves me from doing something stupid and still preserve whatever script or settings I’m working on.

The main power in my country and location (the Netherlands) is excellent with virtually no issues, so this UPS can also be used for more traditional applications like websites and servers, and still provide a clean and safe power down.

The high power RPi Model 3 considerations and specifics

The minimum requirement for the UPS to supply an RPI Model 3 running at full speed together with some peripherals connected to the USB ports was determined by me to be at least 2.5 Ampere. The minimum safe input voltage for the RPi family is published by the RP Foundation to be 4.75V.

To supply a 2.5A payload with a minimum of 4.80V to the RPi, means that the booster will have to deal with cell voltages ranging from 4.2V, the top-off charge level, and the minimum safe discharge level of 3.0V. Realistically speaking, the cell will rapidly drop from 4.2V to 3.7V with a serious load attached. The most critical test however is at the lower cell voltages. At a cell voltage of 3.0V, the booster will require a whopping 5.2A from the cell to supply 4.8V at 2.5A to the Model 3!

Normal 14500 cells, regardless of their capacity, cannot supply these high current levels. It should be clear to anybody looking for a UPS for a Model 3, that any design or commercial offering that does not include high current or IMR cells, simply cannot work. At the same time, a booster chip or circuit that cannot handle these currents is off limits as well, or will result in failure, at best and hopefully not cause a fire or something melting.

This was the reason for the costly (in terms of ordering components to try, several manufactured PCB’s and shipping cost of components to me in Europe) and also time consuming journey Bud and I went on to try fulfill this requirement.

At these high current levels, there are some serious challenges to overcome. The most important ones are dealing with high currents and the heat. Bud designed the UPS PCB’s specifically to channel the heat away from the booster chip, and made sure that there is enough PCB trace capacity to handle these currents and keep weird side-effects at bay.

During my measurements, the surface temperature of the booster chip topped out at 60 degrees C, with a room temperature of 24 degrees.

Another often overlooked element is the battery holder. Any contact resistance will lead to heat. I tried a normal cell holder with PCB contact pins.  The end contacts connecting to the cell terminals, melted right through the holder. Even the fully metal version I ended up using, the Keystone 2222 AA aluminum battery holder, got warm. This fully metal holder also helps to absorb the heat from the discharging cell. I measured that to be about 44 degrees C. Because this holder is all metal, some care must be taken to isolate it from the PCB surface.

The test setup

I used the following setup to measure various key elements.

First of all, I disconnected the UPS from the RPi. The only connections between the two were the i2c bus signals SDA, SCL and GND. This allowed me to run a special version of the UPS monitor script with elaborate logging facilities while also piping that information to the monitor (my PC).

The RPi would not be fed by the UPS and the software would not switch it off, of course. This allowed me to capture a log of all activities for later analysis. The RPi itself was powered through the normal micro USB connector and a wall-wart supply.

To power the USB,  I used my Lab Power Supply connected to a USB breakout board that connected by a USB cable to the micro USB of the UPS. The Lab Supply was adjusted to deliver 5.10V measured at the input of the UPS (at the C8 capacitor), with a 1.0A load, to compensate for voltage drops due to the lead length. For higher currents, there would still be a voltage drop due to the cables, but that would be the same if you use a wall-wart.

The cell holder was connected through two short wires of 5 cm with a 0.5mm core to the UPS. The positive connection was through a 1% 0.01 Ohm shunt, so I could measure the charging and discharging levels of the cell with a DMM in the milli-Volt setting.

I also connected a DMM to the cell directly, to measure the cell voltage and compare that to the voltages reported by the UPS PIC processor. I found that the reported PIC voltage levels were 50-60mV below that of my DMM readings. This is more than accurate enough for the intended use.

My DC Load in the Constant Current Mode was connected to the output pins (The P-1 adapter socket) of the UPS. With these high currents, the voltage measurement of my DC Load (it has no sensing) were not going to be accurately representing the voltage at the UPS output, so I used my bench DMM to measure the true output level of the UPS.

I also used a temperature probe to measure the temperatures of various critical parts, junctions or chips.

To get a visual indication of the charge process, I modified my UPS to have an LED connected to the STAT output of the charger. The LED is on when the charger is operating. This circuit is shown on the datasheet of the MCP73831.

Before I ran a test, I would first let the charger fully end the charge cycle (until the LED went out), before I tested that cell. In my case, the maximum charging current I measured was about 400mA. This is also due to tolerances of the charge programming resistor of 2K7 5% that I used.

This is important to know, because this charging load will come on top of the load already required by the RPi, so when you are selecting a wall-wart or other supply, add at least 400mA to the requirement.

Another change I made to my UPS version was to the fuse rating. I used a 2.5A PTC fuse.

One word of caution: There must be a fuse on any(!) UPS or supply that feeds power at the P1 connector. The reason is that this connection bypasses the fuse on the RPi. If you don’t supply a fuse on that P1 power line, there is no working over-voltage protection for the RPi, because the TVS mounted on the RPi to protect against that situation, should be protected by a fuse. If there is none, the TVS can burn a hole in the PCB or explode and with it probably take the rest with it on its way to bit heaven.

The top-off voltage for the charger is factory set at 4.20V. After that cycle is terminated, the cell will be slightly discharged by the electronics on the UPS. To compensate for that, the cell is “trickle charged” through R4, a 2K4 resistor and one half of D1, a BAT54C dual Schottky diode. This small current compensates for a large portion of the battery current drain from the PIC and booster, but doesn’t actually cause the cell to charge.

Before we move to the actual tests,  I should also mention that the switch-over from mains to cell causes the input voltage to the RPi to drop from 5.1 to 4.8V. The RPi will not blink an eye nor drop a beat to this change in the supply. It happens very fast and the output capacitors (C4 and C5 and optionally C10 and 12) will smooth that transition somewhat. I did not have C10 and C12 installed myself.

The test results

First of all, let me clarify again that even with high discharge rate 14500 cells, a heavily loaded or tasked RPi Model 3 will not run for long. The 700 to 900mAh capacity available for these cells is not enough for serious UPS applications. The UPS will only keep the RPi running for a few minutes.

Following are tests with three different 14500 cells.

UltraFire LC 14500

This is a normal discharge rate cell. Mine came in a blue package. It has 900mAh capacity.

This was the first cell that I tested and obviously, it did not work. The cell voltage collapsed immediately with a load of about 1.5A. I should note that I use this cell in my own UPS designs for my RPi Classic Model B, and Model 2B and have been using it for well over a year. It is sufficient for these models but absolutely lacks the power for the Model 3 at high loads.

Efest IMR14500

The Efest IMR14500 cell I purchased has a red color is brand new, never used. Watch out for the color and the actual specifications, because these cells come with different specifications and colors. I purchased a set of these high discharge cells specifically for this application. The cell has a 700mAh capacity.

As was expected, this cell worked fine. I got a good 8 minutes of power at a constant 2.5A load. The RPi under normal operation will never cause a constant 2.5A load, due to the processor and the USB bus device activity. The power consumption can actually vary a lot.

The 2.5A is therefore a kind of worst-case scenario. The cell had no problems supplying 5.2A at 3.0V. There is not much to gain to let the voltage drop lower, by the time the cell is below 3V, the capacity drops off very quickly. You may be able to get a few more 10’s of seconds, but for this particular application it is not needed.

TrustFire IMR14500

I also purchased a set of Golden TrustFire IMR14500 with 700mAh capacity specifically for this application. They are also brand new, never used.  As expected that cell also worked fine, it provided power for 10 minutes. Also this cell could easily supply the needed 5.2A at 3.0Volt.  
 

So what is the maximum current we can really draw from this UPS?

Well, I didn’t try. I only have one working UPS version so I don’t want to risk it at this moment. Besides, the 2.5A PTC protection fuse will most likely blow and it takes quite some time to let it “heal” itself.

Both the IMR cells I tested can supply upwards of 8A, so that’s not the limitation. I have no doubt that for shorter periods, especially when an RPi is used, instead of a constant current DC load, the power demands can be a lot higher, momentarily. The booster chip has thermal protection, so it will be fine with a higher load.

Enjoy!

paulv