It's been asked here, but looking for clarity. How bad is it to undervoltage a 12-volt lead-acid battery?

SLI automotive starter batteries list RC (reserve capacity), but I think this a usable amperage that does not take battery life into account.

Since Starter batteries are not intended for deep cycle, what's a safe low voltage disconnect that preserves my batteries full life?

12v, 11v or 10v?

My battery is rated 70AH, 900CCA, 120RC.

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    \$\begingroup\$ Sure, you want to hear: never discharge below x V bit things are not as simple as that, especially not with relation to battery lifetime. There are many more factors which influence battery lifetime. Also, what is battery lifetime? Is a battery with 50% of it's nominal capacity "end of life"? Or is it that at 30% or 70%? \$\endgroup\$ – Bimpelrekkie Feb 20 '19 at 12:29
  • \$\begingroup\$ Even when you keep a battery at its "best" voltage all the time (like in a UPS application) the battery will still wear out. Doing "anything" to a battery like using it will decrease its lifetime further. All-in-all there are only general recommendations and no clear answers. For maximum lifetime: leave the battery at the shop. Yeah, crazy answer. Question is, what is the minimum usage of the battery you can allow, because that will give you the longest lifetime. \$\endgroup\$ – Bimpelrekkie Feb 20 '19 at 12:29
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    \$\begingroup\$ as it runs down the volts drop faster and faster. This means that it's not super critical as you're only drawing a little bit more going down to 10 vs 11. OTOH, since you're not getting a whole lot extra from 10v anyway, it might make sense to cut it off at that 11v point, or for that matter the same for 12v vs 11v, so long as you can live with the capacity. Keep in mind temp can affect the total charge, so you will have different amount of energy above n volts as temp varies, make sure to account for that. \$\endgroup\$ – dandavis Feb 20 '19 at 20:40

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