Lead acid batteries are backwards from what i've always been told

Started by vwflyer, August 04, 2018, 01:04:43 PM

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I was reading this article about AGM batteries and noticed the graph relating to cycles and depth of discharge.


Now what I've always been told is that if a deep cycle battery is discharged bellow 50% you will significantly reduce it's life, ie reduce the number of available cycles. Most plans for how much battery capacity you need figures on discharging down to 50%. So if you calculate that you will need 100ah per day of storage you should buy at least 200ah worth of batteries. This will keep your batteries above that magic 50% number and significantly prolong the life of your batter.

Indeed, the graph does so a sharp change in direction right at that magic 50% mark. But after looking at it for a few minutes, it dawned on me that what the graph is showing is exactly opposite the above premise. To me at least, the graph seems to be saying that the first 50% of your battery is where the number of cycles takes the biggest hit and the second 50% has less impact on the number of cycles available to you.

So I crunched a few quick numbers taken from the graph. If I take a 100ah battery and drain it 50%, I get 50ah out of it per cycle and 700 cycles. 50ah x 700 cycles yields 35,000ah over the life of the battery.
If I only sip 30% off the top I get 30ah per cycle and 1600 cycles. 30 x 1600 is 48,000ah out of the battery during it's life. That's a whopping 52% increase in power harvested over the life of the battery.

On the other hand, if I discharge the battery to where there is only 10% left in it at the end of the day (I'm discharging it by 90%) it still gives me 400 cycles, not that many fewer than only taking 50%. In fact, 90ah x 400 cycles gives me 36,000ah over the life of the battery. That's more than I get when I discharge it down to only 50%.

So the graph is definitely showing that the first 50% of the power taken out of the battery takes the biggest tole on the battery's life. The shallower line means smaller changes in discharge result in larger changes in number of cycles and the steeper line bellow 50% means larger changes in DOD result in smaller changes in number of cycles. If you are going to pull it down to 50%, you are not losing anything to keep on pulling it down to 70%, 80% or even 90%. I wondered if this graph was wrong so I looked for more graphs online and they all say roughly the same thing.

Am I missing something or have I been misled all these years?


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.



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



Quote from: gil on August 05, 2018, 03:07:51 AM
I always used 10.5V as the minimum for lead-acid, SLA and AGM batteries, but I have no idea if that's 50%...

Yes, 10.5v gives a better and long life st the batteries. I use the same config for my solar regulator.

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QuoteI 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%.