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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
  • \$\begingroup\$ @winny Thanks very much, I forgot about the case where battery is depleted. \$\endgroup\$ – BdT141 Feb 18 at 8:19
  • \$\begingroup\$ @ElliotAlderson I'm thinking a 12V 7.2Ah battery like this one -> link. I'm not going to discharge the battery too much, it's just there for back up if the power goes out and will then only be used for max 6hours supplying max 1A. I see from the link the float voltage is 13.5V to 13.8V. If need be I can get a zener closer to that voltage if it's make that much of a difference. Point is I don't want to have to buy a fancy LA charging IC. \$\endgroup\$ – BdT141 Feb 18 at 8:19
  • \$\begingroup\$ @BdT141 Not sure how long you have been into engineering, but as the years passes for me the more I realize how much of everything is spent on and optimized for the boundary or edge conditions like this one. Also, that super cheap R to limit the current will always highlight this above mentioned problem. \$\endgroup\$ – winny Feb 18 at 8:40

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