I always used 10.5V as the minimum for lead-acid, SLA and AGM batteries, but I have no idea if that's 50%...

Hi Gil. 10.5V is considered to be 95% - 100% discharged and the battery is considered completely dead. In fact, for most batteries, anything below about 11.3 volts is thought to cause permanent damage.

I agree Wally. The biggest takeaway here seems to be the reverse of what I've always been told. That in order to get the most life out of one's batteries, both in terms of longevity and max ah delivery, one should only sip the top few percent off of them.

People who use these batteries for off grid living generally do one cycle per day. They charge during the day and discharge at night, to be charged again the following day. What most off grid people want, is to buy the fewest number of batteries per year. So for the batteries on the graph, 50% discharge gives you 700 cycles or almost 2 years of battery life. But in order to discharge one's batteries less, one has to have a bigger battery bank (more batteries). So the question is, when does it start paying off to buy a big battery bank up front and only sip the top 20% off of them and when does it make more sense to buy fewer batteries at a time but buy them more often. According to this graph, you will always pay less overall, the more batteries you buy upfront and sip just the top of them.

Lets say a person needs 100ah per day. Lets say he can buy 100ah batteries for $200 a piece. If he plans to discharge them down to 50% he will need two batteries to meet his demands and they will last him almost 2 years costing him about $200 per year.

Now say he wants to only draw 25% off of them. He will then need twice as many batteries to meet his demand. 4 batteries will run him $800 up front but those batteries will now last him 1900 days/cycles or 5.2 years. So twice as many batteries lasts him well over twice as long when running them all together. $800 / 5.2 = $154 per year, a $46/year savings.

This trend holds true for all values above 50% discharge but not for anything below 50% discharge since the vector of the graph changes abruptly at the 50% point. According the the graph, after 50%. you're no worse off and perhaps even slightly better off discharging the batteries all the way down to 80%.

This is what surprised me. I've always been told the reverse, that you will get most of the batteries maximum life so long as you keep it above 50% and you only start to see rapid drop in longevity when going below 50%.