Battery Amp Hours Rating

[Hobo]

Hi everyone. First post here at SPT.

Concerning the rated amp hours for a 200 AH (@ 20 hours) battery... Is that rating down to 100% discharged? If it is, then if you can't discharge below 50%, does that mean it is really a 100 AH battery? If that is true, then why are batteries rated beyond their useful power?

[Sunking]

Yes the rating is for 100% discharged at the 20 hour discharge rate if that is what the battery is rated at. So a 200 AH rated at 20 hours can deliver 10 amps for 20 hours until it is fully discharged. However that same 200 amp hour battery can only deliver 70 amps for 1 hour until 100% discharged making the same battery a 70 AH battery.

As to your question applies to lead acid Pba chemistry, not others like Lithium, Nickel, Alkaline, etc...

[RCinFLA]

The deeper you repetitively cycle a lead acid battery the shorter its longevity will be.

Attached is one manufacturer's data for a particular model battery. These curves are not the same for all lead-acid batteries but general trend line is similar, just different number of cycles on vertical scale. Deep cycle batteries with thicker plates can take more deep cycle. Do a total discharge on a typical car battery and it will be severely damaged. You might get 2-4 rapidly diminishing capacity, total discharge cycles from a car battery which have very thin plates.

The chart does not specify their criteria for 'life' but a typical number is 70% to 80% of rated capacity delivered at end of life.

[Hobo]

Thank you RC for the file, yes, we are talking about a deep cycle application here, and not a starting type battery.

So a 200 AH rated at 20 hours can deliver 10 amps for 20 hours until it is fully discharged. However that same 200 amp hour battery can only deliver 70 amps for 1 hour until 100% discharged making the same battery a 70 AH battery.

OK, that is understood. Peukert's Law, I believe.

What I was trying to get at... If a deep cycle battery is discharged below 50%, we know the internal chemistry creates sulfation deposits, and limits the surface area of the plates creating loss of capacity. It does not take too many times before the damage is beyond repair. So a 200 AH battery is not really a 200 AH battery in real life is it?

Is the realistic available power from a 200 AH battery only 100 AH @ 20 hour rate, if @ 50% DOD?

I've got to understand this in order to move on to the next thing, as it involves applying a defined load to the battery.

[Sunking]

Hobo is has to do with the number of cycles vs depth of discharge, and that is it... The deeper you discharge, the fewer charge/discharge cycles you will get.

20% is the recommended DOD as it is a economical compromise and allows for 2.5 days of real autonomy before you reach the 50% DOD mark. You really do not want to go down to 50% unless necessary as at that point the lead sulfate crystals harden on the plates and cannot be reversed.

Note that all deep cycle batteries are not created equal. One of the best is Rolls Surrette 4000 series where at 20% DOD gives approx 5000 cycles. Others can go as low 100 to 300 cycles at 20%.

Yes you are correct about Peukert Law

Bottom line it is economics. vs design. An excellent RE battery capable of lasting 10 years will cost you about $200 to $230/Kwh At 20% DOD, where a cheap battery that only last 1 to 2 years at 20% DOD will cost $100/Kwh or less. You get what you pay for.

[Hobo]

Hobo is has to do with the number of cycles vs depth of discharge, and that is it... The deeper you discharge, the fewer charge/discharge cycles you will get.

20% is the recommended DOD as it is a economical compromise and allows for 2.5 days of real autonomy before you reach the 50% DOD mark. You really do not want to go down to 50% unless necessary as at that point the lead sulfate crystals harden on the plates and cannot be reversed.

OK I get it now... the number of cycles in relation to DOD is the perspective, not the AH available at a certain DOD.

The 5 day fudge factor (2.5 days autonomy) formula makes sense when I look at it using a different formula: (Daily Wh/24V) /.2 = Ah @ 20% DOD per day. The end result is the same Ah bank.

[RCinFLA]

As to Peukert Law, I am not a big fan of it. Normal battery useage is not a constant current draw. Usage profile is a random mix of high current draw for short periods and periods of lower current draw. A sizable percentage of the Peukert's effect, for flooded lead-acid batteries, at moderate to high current draw is local electrolyte depletion which recovers (remixes) in a few minutes when current draw is reduces.

Battery Rs heating is unrecoverable loss. Lead sulfate plugged plate pores, that occur more during high discharge rates, may not recover until battery is recharged.