New Information:

I'm editing this log to reflect some new testing information. I received 2 purple Efest 14500 Hi-discharge batteries yesterday (2017-07-10). When I put them to the same testing as the other batteries I noticed that something was wrong -- the battery holder started melting at the minus terminal. Apparently, the crimped connection at that terminal had become higher resistance and causing a lot of heat to build up at that junction. After I replaced the cheap Chinese battery holder with a similar cheap Chinese battery holder I repeated the testing on all of the batteries. The new results are contained in this log. I must note that the MPD battery holders are probably the ones to get -- the contacts are a single piece of flat metal that insert directly into the PCB holes -- no chance for high resistance connection unless the contact tension is weak. And I'll bet that the plastic will withstand higher temperatures. You get what you pay for.

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PaulV and I have hashed out an agreement on the "Hat-like" UPS. He wanted the UPS to provide at least 3A @ 5V. I did not see any reason to provide more than 2A @ 5V. He won the argument and we are proceeding with the Hat. I've updated the log that deals with the Hat, so see that log for reference.

There are a few restrictions on the battery if you are going to provide that kind of current. I performed a series of experiments to see if any of my batteries would be able to provide more than 2.5A. Here's the results:

• Experiment #1: Initial VBAT=4.0V, Final VBAT=2.75V. Battery: Efest 14500 700mAh V1 (red). Load = 2Ω (2.5A). UPS duration: 6 minutes 18 seconds. Note: My wall wart could only provide 4.75V to a 2Ω load, so I reduced the UPS target voltage to 4.67V for all subsequent testing.
• Experiment #2: Same as Experiment #1, but the battery was a Keeppower 14500 800mAh (black). Duration: 0.1 second. (I think that the battery's protection kicked in and opened the circuit.)
• Experiment #3: Same as Experiment #1, but the battery was an Effest 14500 650mAh (purple). Duration: 8 minutes 52 seconds.
• Experiment #4: Initial VBAT=4.2V, Final VBAT=2.75V. Battery red Efest 14500. Duration: 8 minutes 49 seconds.
• Experiment #5: Same as Experiment #4, but battery was purple Efest 14500. Duration: 10 minutes 46 seconds.

When the booster is providing 3A @ 5V from the battery near exhaustion (2.75V) the current drawn from the battery will probably exceed 6A. Therefore you will need to get a battery that is rated above 6A continuous discharge current ( the "purple" Efest 14500 IMR battery is rated for 6.5A continuous, and 9.75A pulsed.)

There are a couple of take-aways from this. The type of battery that you intend to use is a big factor in the UPS performance. If you need the UPS to provide high currents then it would be wise to obtain a battery that uses IMR chemistry and IS NOT PROTECTED. If your load is less than 1A @ 5V then almost any 14500 Li-Ion battery will do the job.

The higher load current forced us to employ a poly fuse with higher current/voltage ratings, and a bigger footprint. The UPS Hat version PCB has been ordered from OSH Park and should be back from fab in a couple of weeks.