12.1v is about 55% for long life that's about as low as you should go

State of Charge	Sealed or Flooded Lead Acid	Gel battery	AGM battery
100%	12.70+	12.85+	12.80+
75%	12.40	12.65	12.60
50%	12.20	12.35	12.30
25%	12.00	12.00	12.00
0%	11.80

Every time you run below 50% you are seriously reducing the life of the battery.

mickm wrote:

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Every time you run below 50% you are seriously reducing the life of the battery.

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 It's not like a line in the sand.   The deeper the discharge, the fewer the number of charge/recharge cycles the battery is likely to tolerate.   Many a battery tech sheet says a deep cycle battery discharged to 20% state of charge will only last 500 cycles.   At that rate, down to 20% once a week, the battery will only last 10 years.

I had a low tech Flooded deep cycle down to under 11.5 Volts quite regularly (Engel fridges run all the way down to about 11 volts) and that battery lasted 6 years.

Best advice given so far is to recharge as soon as possible.   

Iza

-- Edited by Izabarack on Tuesday 23rd of June 2015 06:52:17 AM

The other thing to be aware of is that discharging to 20% provides DOUBLE the power to discharging to 60%, so you only need half as many batteries and only need half the number of cycles to break even.
Work life out in kWhs of power delivered, not number of cycles for a real comparison.

Cheers,
Peter

Actually Peter, the thing is that continually discharging your battery to 20% will GREATLY reduce the life of your battery... by 90% in fact!!!

Just google 'AGM battery discharge depth vs cycles' and you'll see the same thing everywhere.

The lower you discharge your batteries, the shorter their life.

The longer you leave them even partially discharged, the shorter their life.

To maximise your battery life, keep them as close to fully charged as possible.

Yes JayDee, in that situation it's best to disconnect it as you have done. Just put them on charge overnight to make sure they're fully charged.

If they're AGM or other type of deep cycle batt then they'll be fine for a few months.

If they're the old style of non-sealed lead acid batts with the caps for topping up water, then it'd be wise to put them on charge overnight once a month.

Whether you discharge to 20% or 60% before recharging, it is still a cycle. A cycle is defined as a period of discharge followed by a period of charge, ideally to full capacity.

While a battery is discharging, the sulphuric acid, which is the electrolyte, chemically reacts with the lead plates to create lead sulphate, which builds up on the surface of the plates.

When the battery is recharged, most of the lead sulphate is converted back into sulphuric acid and lead, but some of it will crystalize and will not reconvert.

The deeper you discharge a battery, the thicker the layer of lead sulphate build up, and the less of the lead sulphate that will convert back into sulphuric acid when the battery is recharged.

The longer a battery remains in a state of even partial discharge, the more of the lead sulphate on the plate surfaces will crystalize, which is why a battery left discharged for a long time will no longer accept a charge, because too much of the lead sulphate will have crystalized and no longer be able to be reconverted to electrolyte and acid.

 

So the bottom line is that the more deeply you discharge your battery, the fewer discharge charge cycles it will endure, in other words, the shorter it's life.

 

Ondabeach wrote:

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Whether you discharge to 20% or 60% before recharging, it is still a cycle. A cycle is defined as a period of discharge followed by a period of charge, ideally to full capacity.

So the bottom line is that the more deeply you discharge your battery, the fewer discharge charge cycles it will endure, in other words, the shorter it's life.

 

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 All cycles are not equal, Steve.   A cycle to 20% is not equal to a cycle to 60%.    To talk cycles and try to define life in cycle terms is mostly meaningless.   Yes, during each recharge event, a little less lead sulphate is converted, as I explained above.   The rate at which the degradation occurs is pretty well linear to the amount of conversion that occurs during a single event where the battery is discharged from some charge level.   A little use, a little degradation; a lot of draw and a lot more degradation.   So the number of "cycles" is just a count of the number of charges the battery receives after a discharge of some value; nothing to do with some prediction of "life".

Crystalisation of lead sulphate is a slow process and can largely be avoided by not leaving a battery in a discharged state any longer than possible.   Crystalised lead sulphate effectively forms a crust on the plates and does not allow the electrolyte to contact the active surface of the plates.   Hence the useful advice that a battery should be recharged as soon as possible after any level of discharge, and given a regular tickle up when in storage situation.

 

Iza

-- Edited by Izabarack on Wednesday 24th of June 2015 04:05:46 PM

Ondabeach wrote:

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So the bottom line is that the more deeply you discharge your battery, the fewer discharge charge cycles it will endure, in other words, the shorter it's life.

 

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This is exactly the same as saying if you drive to the next city instead of just driving to the shops you will get less TRIPS from a tank of petrol.

A battery works in much the same way.

The graph you posted has the information to calculate the differences. Measuring the life in Ahs or Whs will indicate the power the battery delivers in its life - like a car, we are interested in fuel range, not how many trips we have.

Shallower discharges give slightly better life it is true, but that may require buying and carrying several batteries instead of just one.

To get back to the OP, discharging your battery to a low state will not DAMAGE it, it will simply use more of the total fuel that it can deliver over its lifetime.

And do recharge it fully as soon as possible. Leaving it flat will damage it.

 

Cheers,

Peter

-- Edited by Peter_n_Margaret on Thursday 25th of June 2015 06:32:35 PM

-- Edited by Peter_n_Margaret on Thursday 25th of June 2015 06:42:59 PM

Ondabeach wrote:

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........the manufacturers use 'cycles' as a yard stick because it's the most definitive and succinct method they have.

Any time that a battery is not fully charged then there will be some lead sulphate present, and some of it will be crystalizing.

So, more time a battery spends near fully charged the longer it will last.

 

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 Manufactures use graphs of cycles as page fillers in their ads.   Cycles as per your definition are categorical but graphs of cycles add a dimensional component to the advertising.   Interpretation of the graphs as an indication of how much power a battery can store and subsequently deliver allow a pretty good definition of the design capacity of a battery.   "Life" is a pretty meaningless attribute to apply to a battery as it suggests some time component.   In my example earlier, 500 deep discharge cycles out of a battery may take 10 years to complete yet most batteries degrade to the point of throwaway as a function of plain old age, well before 10 years.   Uninterrupted Power Supply batteries, even newer technology ones, are replaced after a relatively short period despite being held at and charged to a full state all their "life".   Are you suggesting that batteries that are always full charged, that is, never cycled, will last forever?

Iza

Ondabeach wrote:

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....., to use a fuel economy analogy, ...............

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 That's an analogy that does not work, Steve.   Fuel usage varies in accordance with several factors other than how much you use before filling the tank again.   A water analogy might work if you could explain how water disappears if you empty more each time before you refill.

Iza

I have never had such a reaction from so many men before. Must tell my husband.
Cheers
Kim

These battery threads really are a popcorn lovers dream, there is certainly no shortage of them, they go on interminably, everyone is an expert, and, at the end of the discussion everyone is either confused or angry, perhaps a little of both.

Love it.

 

Just a few urban myths for ya Pete

https://www.google.com.au/search?q=agm+battery+cycle+discharge+curve&biw=1280&bih=623&tbm=isch&tbo=u&source=univ&sa=X&ei=5CiPVZSdIoTl8gXM6YPgBw&sqi=2&ved=0CCMQsAQ#imgrc=_

Look like all the world's battery manufacturers are pretty clueless Pete, you could save them a fortune in all that silly testing they do.

 

Amp Hour lifetime delivery comparison for a 100 amp hour AGM battery based on data from battery manufacturers.

***********************************

Discharge down to 20%

500 cycles @ 80 Amp Hours = 40,000 Amp Hours

***********************************

Discharge down to 50%

1100 cycles @ 50 Amp Hours = 55,000 Amp Hours

***********************************

40 x 1.375 = 55

So only discharging your batteries down to 50% will give 37.5% more usable power during the lifetime of a battery compared to discharging down to 20%.

Not my opinion, just cold hard scientific fact, proven not by me, but by the companies that make the batteries.