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I use an FMA PowerLab 8. You can charge 2 packs in parallel. When starting the charge, it asks you how many packs you have connected in parallel and will double the amperage to the packs.
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The packs need to be close to the same level of discharge, but as long as they were used together on the same flight, that should be no problem.
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If you have a large enough power supply, the FMA Powerlab will charge up to 9 batteries in parallel.
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