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I would like to know if the schematic below would work to keep a 12V SLA battery at or close to float voltage to be used as a backup battery? R1 will limit the charge current to the battery, but am I right in thinking it is calculated by R1 = (Vin-Vzener) / Icharge and how much should the trickle charge current be?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Almost, Icharge=(Vin-Vbatt)/R. Think about what will happen if you battery is severly depleated, say 5 V. \$\endgroup\$ – winny Feb 1 at 11:22
  • \$\begingroup\$ The desired level of charging current would depend on information you have not given us. Please provide a link to the datasheet of the battery. Tell us about the temperature range of operation. How often and how much you will discharge the battery? Charging the battery to a maximum voltage of 13V is probably not going to achieve a full charge, but you should also link to a datasheet for the zener diode. \$\endgroup\$ – Elliot Alderson Feb 1 at 13:24

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