So the APC UPS using the MH house battery bank does not need to be left plugged into a charger to keep it running when 120v is still on. It switches to inverter fast enough.
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EDIT--besides the modem for switching time, there is the gizmo the modem plugs into that converts from the fibre line to the house. Don't know about switching time for that. OK? That converter also is plugged into 120v, making the two things needing 120v.
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The OEM battery in the UPS recharges once the power comes back on? Or if you use a different battery, you recharge that with your converter or solar.
What did you have to do to connect the external battery to where the internal battery was?