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Having blown a PiModules UPS Pico I played around with the LiPo battery 450mAh as it has an embedded PCB (Over Charge, Over Discharge, Over Current) inside. After connecting it directly to pins 4 & 6 of the RaPi, I turned on the AC power supply and measured the current into the LiPo & its voltage across it while checking for overheating, etc. There was no heating & 25 minutes later it was charged at 4.2V with the current starting at 1.5 (3C !) & dropping to 0.9A (2C).

On disconnecting the AC the RaPi stayed on with the small, multicoloured image indicating low power showing from the start. With a steady current of 0.7A the RaPi was functional for 8 minutes, with the mouse going slow at 6 mins. The keyboard stopped working at 10 mins. At 20 mins the screen went blank but when I plugged the AC in all the open files reappeared.

I understand that a few LiPos have exploded but was that before the embedded PCB was added? I have read that they can be charged/discharged between 100 & 500 times and am tempted to test this battery in this way. I would set up a thermistor next to the LiPo &, if the temperature went to say 30°C, would shut down the experiment automatically.

If the experiment worked it would make a very cheap UPS system!

See the comments I have made about the impartiality of @laptop2d below her or his answer before dismissing my idea out of hand.

14 June - At the moment I am working on the idea that the LiPo will register that the 5v power supply has failed & instruct the RaPi to use the 'sudo shutdown -h now' instruction. I am still try to find an instruction that will save & shutdown any open documents like LibreOffice, automatically. The LiPo will be used for, say, 30 seconds before being cut-off from the RaPi automatically with a timer circuit & then recharged for a short time when the 5V power supply restarts. At this point the LiPo voltage can be monitored by an ADC like the MCP3008. This would be a useful add-on for a RaPi.

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    \$\begingroup\$ This absolutely cannot be recommended as a practice. I have been involved in battery qualification testing for consumer products, and we generally do tests like this (abusive tests) to make sure the cell is not especially susceptible to failure. Rarely will a cell fail if you do this once or twice. But you are playing with fire if you do it repeatedly. The 30C limit will help, but the issue is that over-charge could create latent flaws inside the cell that will cause failure later, possibly during discharge. Bottom line: don't do it. \$\endgroup\$ – mkeith Jun 1 '16 at 21:06
  • \$\begingroup\$ This is a bad idea, as other have/will explain. But the question is a good one! Hopefully you won't receive undeserved downvotes... \$\endgroup\$ – bitsmack Jun 2 '16 at 3:05
  • \$\begingroup\$ The charging rate was 3C @ mkeith & not 30C. See comments/questions lower about the embedded PCB. \$\endgroup\$ – walter1957 Jun 2 '16 at 7:38
  • \$\begingroup\$ Have not got many votes to lose @bitsmack ! \$\endgroup\$ – walter1957 Jun 2 '16 at 7:40
  • \$\begingroup\$ Somebody downvoted me. C'est la vie.... \$\endgroup\$ – walter1957 Jun 2 '16 at 20:11
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If your going to use a battery to power your rasp-pi, make sure you use a battery charger circuit between it and the pi. Just because it semi-works doesn't mean its safe.

For charging, you need a constant current and then constant voltage. Your raspberry pi can't do this on its own. If you don't use the appropriate times and voltages you will significantly reduce the lifetime of the battery if not cause it to overheat.

For discharging, you need to boost the voltage up to 5V to ensure the raspberri pi is operating properly. The battery needs to shut down before it reaches 3V. Your raspberry pi can't do this on its own either.

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  • \$\begingroup\$ Para1-Is the embedded PCB a recent addition? How effective is it as it implies it will protect against electrical misuse.. \$\endgroup\$ – walter1957 Jun 1 '16 at 19:51
  • \$\begingroup\$ Para2 - The RaPi voltage was 4.3 at the start & 4.5V after 25 mins. Is that not constant enough. I recorded the Current & Voltages while charging in 5 minute intervals. I did try it with a diode & the current was lower. \$\endgroup\$ – walter1957 Jun 1 '16 at 19:56
  • \$\begingroup\$ Para3 - I was not expecting the RaPi to function perfectly during the AC outage but it would have given me time to save everything & close it down correctly. I was considering an ADC to measure the LiPo voltage or a potential divider so that, when the LiPo reached 3.5V, say, the divider would allow a low enough voltage for a GPIO to read as 'low' & shutdown the RaPi correctly. \$\endgroup\$ – walter1957 Jun 1 '16 at 20:02
  • \$\begingroup\$ @walter1957 diodes don't limit current, you would have to add a circuit or IC that handles all of the charging for you. Some voltage regulators will do constant current (that means it vary's the voltage as the battery charges). Otherwise you will have to do a lot of homework and build a battery charger yourself. Please look at the graphs in the links I provided. It would be very foolish to run the rasberry pi lower than its rated voltage. \$\endgroup\$ – Voltage Spike Jun 1 '16 at 22:07
  • \$\begingroup\$ The diode, with a forward voltage drop of 0.8V lowers the input voltage to the LiPo to about 4.0V which is why the charging current was lower. The charging current is 0.3mA (C 0.75 which is more reasonable) \$\endgroup\$ – walter1957 Jun 2 '16 at 7:21
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Instead of messing around with a bare lipo battery why don't you consider getting one of the plethora of available "power banks". These have the necessary internal lipo chargers and have the necessary DC to DC converter to raise the output voltage to 5V.

Do be aware that there is quite a range of quality on these devices. Some of the cheap ones do a crappy job of regulating the 5V output and it may drop to 4.7 volts when you apply a load. I've had good luck with units manufactured by ANKER.

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  • \$\begingroup\$ I have a 3350mAh Anker but it cannot be connected to a power supply & the RaPi at the same time. \$\endgroup\$ – walter1957 Jun 2 '16 at 7:20

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