Radio battery backup on the CHEAP! Use a 120v 30-40 amp relay, not a 12v relay or else it doesn't switch over fast enough... I couldn't find ANY examples online so I
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Backup Battery Controller (BG-Bac) — Blue Guard Innovations
POE SWITCH WITH BATTERY BACKUP DSB-98 9-PORT PULSAR - PoE Switches with 32 Ports support - Delta
Amazon.com: LM YN LM YN Battery Emergency Switch Control Module DC12V Max 150W Power Failure Automatically Switches to Backup Battery Suitable for Network Equipment, Electrical Equipment etc. : Electronics
switches - Power switch (backup battery voltage is higher than main line voltage) - Electrical Engineering Stack Exchange