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The charging circuitry on the Mayfly isn’t capable of charging 2 parallel battery packs simultaneously, so it’s not advised to connect two packs to the Mayfly if you ever connect a mucroUSB cable or solar panel to it because either of those will start the charging process.
The second LIPO connector on the Mayfly is for providing a direct connection to the battery for high-current accessories like a GPRSbee module. It’s not supposed to be used to provide a secondary battery input for the Mayfly.
However, if you take precautions to make sure you never connect a microUSB or solar panel, then theoretically it’s possible to connect two parallel batteries to power the Mayfly, but you run into the usual issues related to parallel battery operation, like making sure both batteries have identical capacity and are charged to the same initial voltage, along with other precautions. Given the dangers of short-circuiting LIPO batteries and causing a fire, EnviroDIY does not recommend that users connect more than one battery to their Mayfly at a time.
Also, if you connected 2 LIPO batteries at once, you can’t use the onboard LIPO charging circuitry on the Mayfly, so there’s no benefit to using LIPO batteries and you might as well just use a single high-capacity battery pack of a different chemistry (like lead acid or alkaline) if you’re looking for longer lifespan.