VOISS - Pi-UpTime UPS 2.0

Written by Rich Morin.

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Precis:  notes on the Alchemy Power Pi-UpTime UPS 2.0

See Also:  VOISS - PiJuice HAT, VOISS - Power, VOISS - Power HATs, and VOISS - Power Management

Note: As of early 2020, the VOISS project and F123Light were discontinued. For details, see VOISS - Status.

As discussed in the VOISS - Power HATs overview, Alchemy Power manufactures several power management products. This HowTo discusses my efforts to create a VOISS system, incorporating their Pi-UpTime UPS 2.0 Hardware Attached on Top (HAT).

Note: Alchemy’s proprietor, Percy Kawas, has been very responsive to my (many!) requests for more information. I wish more company representatives were as supportive as he has been.

Background

The Pi-UpTime UPS 2.0 (UpTime) is designed to work with full-size Raspberry Pi (RasPi) computers. It holds two (e.g., 3600 mAh) 18650 Lithium-ion cells, wired in parallel. Consequently, it can run on one cell (with reduced battery lifetime). It can also be configured to work with Lithium iron phosphate (LiFePO4) batteries.

The UpTime contains a TLA 202X. This is a four-channel 12-bit analog to digital converter (ADC) which monitors battery temperature, as well as the input, output, and battery voltages. A set of Python scripts is available to use this data. I haven’t had a chance to read or try out this code, but I presume that it’s a reasonable starting point for alert messages, shutdown logic, etc.

Note: A card shipped with the UpTime says: “Ensure [that] both batteries are fully charged or have [the] same amount of charge before you insert them.” So, you’ll need a way to pre-charge the batteries. Also, because the power supply must handle both running and charging current, Alchemy recommends using a 3.5A unit with a USB-C cable.

Here is some ordering information, generally including the ancillary charges (e.g., handling, shipping, taxes) I paid. All told, these additions added about $100 to the cost of the VOISS system:

Battery Notes

The 18650 designation covers a wide variety of batteries. Some have built-in overcurrent protection; others do not. Some have flat tops; others have button tops. Some have high current capacity but limited power capacity (and vice versa). Making matters worse, neither Alchemy Power nor any of its resellers say much about what this device needs. Grumble.

The Alchemy FAQ indicates that the Pi-UpTimeUPS and PiZ-UpTime should use unprotected batteries. Percy says that most unprotected batteries have flat tops, but either the button top or flat top style will work. This leaves us with the question of current vs. power capacity, which I’ve attempted to estimate, as follows…

The 3B+ would like to have a 12.5W (5V at 2.5A) power source, but the 18650 only supplies 3.6V. Assuming 85% conversion efficiency (90% is typical), a single cell would have to provide about 4A (12.5 / 0.85 / 3.6 = 4.08) in order to yield 12.5W. (Using two cells, we could cut this amount in half.) In any event, a pair of cells that can supply 4A should be just fine. This means that we can optimize for high power capacity, which the (3600 mAh, 10A) Samsung batteries listed above provide.

Mounting Notes

Due to the batteries and holders, the UpTime is quite a tall HAT. Unfortunately, this precludes the use of any normal RasPi enclosure. So, I ordered the ModMyPi modular enclosure (plus two spacers) from the PiHut. The 3B+ sits in the bottom of this enclosure, below the UpTime. This approach seems to be a useful approach for prototyping, but it wouldn’t serve the needs of blind end users. So, I’m looking into a few other approaches.

The 40-pin GPIO connector on the 3B+ provides enough friction to hold boards together under light handling and vibration, but it’s better to add some bolts and/or spacers for strain relief. The 3B+ has four mounting holes, large enough for a 4-40 (or 2.5M) bolt. FWIW, I’m using 3/4” long nylon spacers, 1” long bolts, etc. One of the nuts didn’t fit well next to the battery holder, so I put the bolts in with the heads on top.

Power Consumption

Note: Estimating power consumption on a real-world system is a chancy business. A lot depends on what apps, daemons, and peripherals are involved. So, the following should only be taken as rough estimates (YMMV).

A Lithium-ion cell produces about 3.6 VDC. So, accounting for some conversion inefficiency, a pair of 3600 mAh cells can supply nearly 5000 mAh at 5 VDC (2 * 3600 * 3.6 * 0.9 / 5.0 = 4665.6). According to Power Consumption Benchmarks, a 3B+ needs about 350 mA when idle and 1000 mA at full throttle. Because we won’t be using the HDMI port and can shut down some daemons, we can guess 333 mA as the idle current and three times that for normal operation.

This tells us that a 3B+ can idle for about 15 hours, perform useful work for several hours, and combinations thereof. Of course, we can also shut the system down entirely for periods when we don’t expect to be using it, carry a power bank and/or power supply to recharge the batteries, etc. All told, this should give us reasonably acceptable battery lifetimes as a starting point.

However, it should be possible to increase the idling battery lifetime. For example, some daemons which consume a lot of CPU activity could be suspended and some power-hungry devices could be turned off. However, it will take some research to find out which things are worth turning off and on, let alone how to do so.

One notion I’m playing with is to detect the presence of a keyboard. If none is detected, maybe we can assume that the system is quiescent and shut down assorted daemons and such. Obviously, this will take some fiddling to get right…

Wiring Notes

The 3B+ and the UpTime bolt together into a tidy little brick, which you may or may not wish to enclose in some manner. Regardless, you’ll be dealing with a number of cables. Some of these will only be used occasionally or by certain users. My development setup, in any event, includes the following cables:

ContentSourceConnectorSink
audioRasPi3.5mm phoneheadphones, speakers, etc.
controlswitchtwo-pin socket“Pi Power” pins on the UpTime
powerwall wartproprietarypower socket on the display
powerwall wartUSB Type C“Pwr In” pins on the UpTime
USBkeyboardUSB Type AUSB socket on the RasPi
USBmouseUSB Type AUSB socket on the RasPi
videoRasPiHDMI Type Ainput socket on the display

Note: The control switch cable is a rather specialized component, so I had to put one together myself. Basically, it’s just a small on/off (SPST) switch, wired to a two-pin socket. It replaces the jumper that normally bridges the “Pi Power” pins.

The GPIO pins that stick up from the Pi-UpTime board are about 0.19” long. This is far shorter than the 0.32” pins used on the 3B+ and might prevent them from being used (reliably) to attach another HAT. Percy is looking for a GPIO socket with longer pins. Meanwhile, any 40-pin socket with long pins can be used as an extender.

Resources

The FAQ and the data sheet answered most of my questions. Again, Percy has been very helpful.