1

**Batteries & Solar / Re: Lead acid batteries are backwards from what i've always been told**

« **on:**August 04, 2018, 07:19:26 PM »

That's an interesting observation, but I suspect one difference is in the definition of "life expectancy" - you are looking at it from a power delivered point of view but length of service is an important criteria. For example, I imagine most people rate their car battery in terms of how long it lasts not how many amp-hours (number of engine starts) it delivered over its life. Also, as your calculations show, the numbers change with different depths of discharge - your example showed more power delivered at a 90% DOD versus 50%, but the reverse is true at 30% DOD: 48,000 AHr vs 36,000.

If we use 100 AHr again, his illustration comparing two batteries at 40% DOD vs. one battery at 80% works out that two batteries at 40% DOD will provide 88,000 AHr over 1100 cycles but two consecutive single batteries at 80% yields 36,000 AHr each, times two batteries, is 72,000 AHr over 450 cycles. So running a single battery to 80% DOD to end of life and replacing it gives you 900 cycles and 72,000 AHr vs. 1100 cycles and 88,000 AHr. It would take 2.44 single batteries to yield the same power output of a pair over its life. One variable here is: how long is a cycle? That depends on rate of discharge and rate of recharge.

I think the take-away from his article and graph is lower DOD is a Good Thing (tm), but beyond 50% the importance diminishes as long as you don't damage the battery. I have heard the 50% number given as a maximum-do-not-exceed number to prolong life, and the graph seems to reinforce that. Prolonging life and delivering maximum energy may not be the same thing.

Wally

If we use 100 AHr again, his illustration comparing two batteries at 40% DOD vs. one battery at 80% works out that two batteries at 40% DOD will provide 88,000 AHr over 1100 cycles but two consecutive single batteries at 80% yields 36,000 AHr each, times two batteries, is 72,000 AHr over 450 cycles. So running a single battery to 80% DOD to end of life and replacing it gives you 900 cycles and 72,000 AHr vs. 1100 cycles and 88,000 AHr. It would take 2.44 single batteries to yield the same power output of a pair over its life. One variable here is: how long is a cycle? That depends on rate of discharge and rate of recharge.

I think the take-away from his article and graph is lower DOD is a Good Thing (tm), but beyond 50% the importance diminishes as long as you don't damage the battery. I have heard the 50% number given as a maximum-do-not-exceed number to prolong life, and the graph seems to reinforce that. Prolonging life and delivering maximum energy may not be the same thing.

Wally