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I am building a battery backup system for a DC appliance that runs on 6V.

I have a 6V sealed lead acid battery to serve as the backup source.

However I read that a lead acid battery at full charge has slightly more voltage than its rated at. E.g. a full charge lead acid 6V battery would rest at 7.15V

At this voltage it would be damaging for the appliance so a voltage regulator is desirable.

However many voltage regulators like LM317 give a drop of about 1.5V. So, at full charge the battery's 7.15V would get regulated to 5.65V which is lower than the required 6V. Not to mention it would drop more as the battery discharges.

I'd also wish to put a transistor switch between the battery and the appliance to turn it off when battery is discharged to prevent a deep discharge. Again that would lead to some voltage drop.

What is the best regulator and switch to use in this scenario? A Zener diode?

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    \$\begingroup\$ There are more modern LDO (Low Drop-Out voltage) linear regulators available. Or use a buck-boost switching regulator. \$\endgroup\$ – MarkU Jul 21 '16 at 21:26
  • \$\begingroup\$ how much current does your load take? many switching regulators have and under voltage lock out (UVLO), some LDOs have this features too \$\endgroup\$ – Wesley Lee Jul 21 '16 at 21:41
  • \$\begingroup\$ @wesley 300ma max \$\endgroup\$ – Allahjane Jul 21 '16 at 21:48
  • \$\begingroup\$ quick look at digikey gives many options with less than 0.4V drop @500mA for less than a dolar \$\endgroup\$ – Wesley Lee Jul 21 '16 at 21:54
  • \$\begingroup\$ You make it sound like 6.0 volts is a crucial value. Like you need a voltage reference (6.00 volts) as well as it being a power source. \$\endgroup\$ – Sparky256 Jul 21 '16 at 22:43
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For this situation, you want a SEPIC regulator. This type of regulator can take input voltages higher or lower than the output voltage, which is perfect for your situation where you want to regulate voltages from 6.9V (fully charged 6V lead-acid) down to a little less than 6V (discharged) to a flat 6V. They are sold on Digikey, Mouser, TI, etc..

Unfortunately, they are a little pricey. Possibly a better solution is to get a 12V lead-acid battery and any old 6V regulator. If you are going to regulate the voltage down, any battery voltage will do...

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  • \$\begingroup\$ Good find but i want to keep this thing compact and hence as few external parts to regulator as possible. Initially i was going to use lm7506 to avoid space taken by external resistors on lm317 \$\endgroup\$ – Allahjane Jul 22 '16 at 19:10
  • \$\begingroup\$ 12v battery would have worked but that means increasing the charging voltage as well since the max output of current charger is 12v (which is pretty standard and easy to find) \$\endgroup\$ – Allahjane Jul 22 '16 at 19:12
  • \$\begingroup\$ @allahjane You can find integrated SEPIC regulators for $15: you input the battery, it outputs the voltage. \$\endgroup\$ – dpdt Jul 22 '16 at 20:57

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