cycling batteries

[maxomill]

hi guys

ok I get the cycling part , like 30 % and generally no more that 50%.

on my old system 24v with 4 100ah bats .

while running on solar during the day my draw could briefly drop the bats to 80 % soc and some times 70% soc and then quickly  charge up again

under normal cloudy day conditions this would also happen .

my 2 big questions are  generally speaking ,   every time I drop down to a 90% soc  , is that 10% of a cycle?

do 10 minicycles for use of a better word equal 1 battery cycle life

 

think about it in a solar set up your soc is up and down all day depending on your load

 do all the "minicycles"  tally up and eat your batteries lives or do I understand this all wrong

 make me clever

cheers

 

[PaulF007]

Hello @maxomill as I understand it you are correct in your assumption with regards to the cycle. Every time your soc drops there is a chemical reaction that takes place and as a result it is a cycle. When you say it goes to 70% and quickly goes back , I assume that you are looking at your BMV? 

[Chris Hobson]

9 hours ago, maxomill said:

while running on solar during the day my draw could briefly drop the bats to 80 % soc and some times 70% soc and then quickly  charge up again

This 70-80 % SOC is this as reported by the Axpert? If so you can ignore it all that is happening is the inverter cannot accurately determine SOC and what is happening is you are going from charging to discharging and the reported SOC drops rapidly only to restore rapidly.

If this is reported by a BMV then you can count this as a "mini cycle" as you call it.

I think a cycle is defined as a SOC that drops below 70% (75% according to some sources). My BMV reports 88 cycles after 2 years of  daily use and my batteries are tired so obviously not all cycles are equal.

If one looks at the chemistry of a lead acid battery they should last forever. So what is causing them to age? Lead is continually shed from the plates. This is more rapid during overcharging. So that equalise charge  restores your battery but shortens its life a bit. These lead flakes accumulate at the bottom of the battery in the sediment trap. Eventually there is a conductive layer of electrolyte which causes a "soft short". During charging this short "disappears" only to re-establish on discharge.

The formation of lead sulphate causes the plates to expand and charging causes them to contract but not back to their original size. Thus over time the plates "grow" and end up making contact with the detritus layer at the bottom of the battery.

If we look at which cycles are the most destructive. Rapid discharges are more destructive than slow discharges and shallow discharges are better than deep discharges as there is less lead sulphate formation. Charging at higher temperatures shortens battery life. This is partially overcome by changing charging voltages based on ambient temperatures.

 

 

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On a chemical level, when you discharge you disassociate the H2SO4 (sulfuric acid) into water and sulfur. On the positive plate, the reaction creates two water molecules and some lead sulfate. On the negative plate it creates lead sulphate, hydrogen, and some free electrons (this last part is the bit you want :-) ). On recharge, the sulfate that forms on the outer layer of these plates is reversed and the water recombines with the sulfur, which turns it back into sulfuric acid.

So that is why the SG goes up and down. When it discharges, you get closer and closer to a point where the electrolyte is pure water (SG=1), and as you recharge, you again get closer to the heavier SG of the original sulfuric acid mixture.

The chemical reaction doesn't reverse fully. A tiny fraction is never reversed and that part of the lead plate becomes nonreactive forever, but it takes a while before you notice because you still have plenty of material left.

Lets assume, as a thought experiment, that 1% of the used material is lost on each cycle (it's probably much less than this, but this keeps the math easier). Now suppose you discharge the battery to 30% DoD, and then back up to 100%. You will then lose 1% of the 30%, or 0.3*0.01 = 0.003, or 0.3% of the capacity. So after one cycle, your battery has 99.7% of its original capacity left. If you repeat this cycle, then of course the battery drops off by a rate that accelerates over time. It accelerates because you don't normally compensate for the losses. Your BMV is still programmed to believe that the battery is a 100Ah battery (for argument's sake), so next time you take it down to 30% DoD, you'll be taking out 30Ah out of a battery that was only 99.7Ah (instead of 100Ah), so technically you discharged it to 30.1% DoD the second time, and again you lost that 1%, or 0.31%, leaving you with 99.39%. And so on and so forth.

Now if you were taking it down to 50%, then of course you'd be losing 1% of 50%, and the acceleration would be much faster too.

But these aren't the only factors. The ability of a battery to reverse the chemical reaction also depends on how soon afterwards you do it. If you wait until the next day, then you are much more likely to lose some of the active material, if you do it immediately you will lose much less. I don't understand the exact math around that, but I suspect that this too isn't a linear relationship, in other words, if you can recharge twice as often you will likely more than halve the deterioration rate (this you and I will have to research, this is a gut feeling at the moment).

In other words: so-called mini/micro cycles are much less damaging than some people think.

If you take the batteries down to 30% DoD twice a day, but always finish the day with them fully recharged (for example), it might well be that you won't do much worse than the guy who drops them to the same DoD overnight and leaves them there until tomorrow. On face value it might look as if you're getting twice the cycle life out of them... there really is a lot of factors that go into this.

[maxomill]

thanks guys

this subject came from my old setup and my readings came from my charge controller. clearly you all have a lot of faith in the bmv and you have my curiosity going . I think I might have to get one to keep up with the jones

my new infini/axpert v series now runs completely in self consumption mode so that my batteries don't get cycled

 

[Chris Hobson]

4 hours ago, maxomill said:

my new infini/axpert v series now runs completely in self consumption mode so that my batteries don't get cycled

If that is the case then you probably don't have to

4 hours ago, maxomill said:

..... get one to keep up with the jones