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I'm looking at buying batteries, either 12v AGM or LI-Ion, for use in a backup power system. I have a choice between a Lifeline 8D AGM battery rated at 255 Ah @ 20 hr, or a Victron LI-Ion rated at 160 Ah @ 20 hr.

Because the depth of discharge between the two is different (50% for AGM vs 80% for LI-Ion), I have to wonder if the Ah rate takes into account voltages at different states of charge.

I seem to recall that Ah ratings are based on how long a battery can produce a given ampacity before it drops below 12v. Does that mean an AGM battery rated for 255 effectively only gets 127.5 Ah because of a max 50% discharge depth? Or is it 255 total Ah before it reaches the point at which it's no longer producing 12v? Or is depth of discharge just a consideration for battery life?

I can obviously get much more for my money with the Lifeline AGM 8D vs the Victron Lithiums, but I want to make sure I'm getting the full advertised capacity.

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  • \$\begingroup\$ Usually rated at CC loads , thus 20h rate to 0% SoC to maximize Ah capacity and not to maximize cycle life. \$\endgroup\$ – Sunnyskyguy EE75 Mar 12 '17 at 23:07
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    \$\begingroup\$ In 12V lead acid batteries, the Ah rating is usually based on a 20 hour discharge time, and a 10V ending voltage. So in your case, what it means is that you should be able to discharge the battery at 12.75A for 20 hours before the output voltage drops to 10V. As you note, it is not good to actually discharge your lead acid battery to this extent, because its usable life will be shortened. AGM is probably a better choice for backup power, unless your load will discharge the battery very rapidly. AGM can be float charged indefinitely in standby applications. LiIon is more complicated. \$\endgroup\$ – mkeith Mar 12 '17 at 23:19
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If an amp-hour rating is accompanied by a description of test conditions, then yes, one of those conditions is the voltage at end-of-capacity, i.e. the discharge voltage drop.

An amp-hour rating which is not accompanied by a description of test conditions is best treated as a bit of enthusiastic sales-speak. Unless the 'backup power system' is coincidentally similar to a documented battery test, your own test results will have to wait until you actually purchase batteries.
The results depend on your power system control electronics, after all.

On a lighter note, some clock batteries in my experience had exactly the lifetime at load that the manufacturer's amp-hour rating predicted!

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  • \$\begingroup\$ Lifeline AGM deep cycle batteries have an extremely good reputation, and are considered very high-end. The capacity is based on 20 hour discharge rate with 10V end voltage, which is industry standard for deep cycle lead acid batteries. This information can be verified for this specific battery at the lifeline website. It also gives discharge time for several other discharge rates. lifelinebatteries.com/products/marine-batteries/gpl-8dl \$\endgroup\$ – mkeith Mar 13 '17 at 6:13

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