What is a Cycle?

equis · January 2013

Hello

I am reading about how long my batteries will last, as you know, it all depends on DOD and cycles.

If the battery is 100% SOC and I take it down to 50% then recharge it to 100% SOC that's easy, 1 cycle to 50%

What happens if I cannot fully recharge the battery for a couple of weeks (winter) and it only charges to 80% SOC

so from 80% SOC to 30% SOC and back to 80% SOC, does that count as a 50% cycle or a 70% cycle?

Also, what do you do in winter when you can't get that last 10 - 20% for weeks on end?
Will a battery be damaged if only charge to 80 - 90% SOC long term?

Thanks

Rhys

Vic · January 2013

Re: What is a Cycle?

Hi Rhys,

SOME batteries CAN take cycles down to 20 or 30% SOC, but generally, this results in fewer cycles until end-of-life. Personally, I would try to avoid any cycling below 50% on any of my batteries, which are rated at 3300 cycles to 50%.

In your example, the cycle from 80% to 30 % SOC is a cycle to 30% in my book. Many say that one need not fully recharge a battery bank to 100% more than once or twice per week -- I am in that camp. BUT it is not good to allow a battery bank to remain below about 70% SOC for more than one day, IMHO.

Battery charging seems to be one large gray area. Everything seems to depend upon everything else, and there are few absolutes, other than do not let Flooded batts run low on water, and do not let them sit at a low SOC for very long at all ...
Opinions, Vic

BB. · January 2013

Re: What is a Cycle?

But remember that cold batteries, in general, will last longer (aging wise, every 10C drop from ~25C is a doubling in life)--So if you cannot recharge them >80% and the batteries are very cold (say 0C), they are going to last longer even if at less than ideal "storage charge" (as I understand--I ain't no battery engineer).

-Bill

NorthGuy · January 2013

Re: What is a Cycle?

These cycle life figures are just wild approximations.

If you want to estimate battery life with irregular pattern, you can use AH-life instead of cycle life.

Say, you discharge your batteries in average to 50%. You look up manufacturer data for 50% DOD cycle life. Let's say it's 1500. If you bank has a capacity of 200AH then every 50% cycle is 200*50% = 100AH. Your AH-life is 1500*100 = 150,000 AH. You'll see that AH-life doesn't depend that much of the depth of discharge as does the cycle-life.

Now, when you discharged to 50%, you took 50% or 100AH. Then you charged back to 80% and then discharged to 50% again, you took 30% or 60AH. Sum these numbers up. When this sum gets to 150,000 AH, that's the end of the life.

When you discharge batteries, the acid gets converted to lead sulphate. Left unattended it will become crystallized. To prevent this, you need to charge the battery to 100%. When you do, lead sulphate is converted back to acid and crystallization is avoided. You probably don't need to do that every day. Opionions on how often you need to do that vary ...

gww1 · January 2013

Re: What is a Cycle?

I read a post on fork lift batteries and short opertunity charge, (say a thirty minute charge during the day) and it was mentioned that the battery manufacture stated this was a cycle. Better to run down and then do a 12 hour charge. I don't remember which forum or post I read this on. I read a post by chris olson that taking the batteries down over several low pv/wind production days and then catching up on one good production day is one cycle. I could probly find this post if needed and may be the same post I read the other statement on. Your thoughts.
Thanks
gww

BB. · January 2013

Re: What is a Cycle?

Yes, it does appear it would be better to let the battery get down towards 50% State of Charge then bulk back up ~80-90% SOC with the genset on the third day--Rather than running the genset every day.

Of course, with sunny weather, you are probably getting toward float every day--Which is exactly (possibly) the wrong thing to do. Stephen's information (from SMA) on not initiating a "full charge" until the battery drops below 80% SOC or so, may be the better way to get long(er) battery life.

What I think is bad is a Lead Acid Battery sitting at low SOC (I use 75% SOC or below, others may use lower or have different criteria--But, I am not a Battery Engineer). A day at 50% is not the end of the world--A week or a month at 50% SOC--That is probably not a good thing. Sulphates take time (hours to days) to begin to crystallize. It does not happen right away. One FAQ said that a "full recharge" can recover "most" of a battery's sulphates formed in multi-day period with less than full SOC operation. But they are looking at longer recharging/absorb times (and warm batteries help dissolve sulphate faster) (current is probably based on 25-75 AH or so wheel chair battery).

There are two types of sulfation: reversible (or soft sulfation), and permanent (or hard sulfation). If a battery is serviced early, reversible sulfation can often be corrected by applying an overcharge to a fully charged battery in the form of a regulated current of about 200mA. The battery terminal voltage is allowed to rise to between 2.50 and 2.66V/cell (15 and 16V on a 12V mono block) for about 24 hours. Increasing the battery temperature to 50–60°C (122–140°F) further helps in dissolving the crystals. Permanent sulfation sets in when the battery has been in a low state-of-charge for weeks or months. At this stage, no form of restoration is possible.

-Bill

vtmaps · January 2013

Re: What is a Cycle?

gww1 wrote: »

I read a post on fork lift batteries and short opertunity charge, (say a thirty minute charge during the day) and it was mentioned that the battery manufacture stated this was a cycle. Better to run down and then do a 12 hour charge. I don't remember which forum or post I read this on.

I have posted on this forum about what I read on Giant Battery's website. They advise to wait until you run the battery down before recharging.
--vtMaps

What is a Cycle?

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