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Peukert Law, Batteries, and You
Time for another STICKY THREAD. This is more bad news for you off-grid battery folks. Peuket law is like Death and Taxes, you will pay, and for 99% of you a lot more than you have too. Peukert Law simply states for flooded lead acid batteries: "The higher the discharge rate (Amps) the less capacity the battery actually has". For example your 100 amp hour battery can be as low as 30 AH if discharged at the 1 hour discharge rate of 100 amps. Or as high as 160 AH at the 100 hour rate of 1 amp.
In theory only with batteries:
Amp Hours = Amps x Hours
Hours = Amp Hours /Amps
Amps = Amp Hours / Hours.
Well that theory only applies at the discharge rate the manufacture specifies the battery at which is typically 20 hour rate. So for example if you have a a 100 AH battery the 20 hour rate is 5 amps 5 amps = 100 Amp Hours / 20 Hours). From that formula a layman would assume if you had a 100 AH battery you should be able do discharge that battery at 1 amp for 100 hours, or 1 hour at 100 amps. YOu would be wrong, very WRONG, Mr Peukert will not allow it. If you were to discharge that 100 AH battery at 1 amp Mr. Peukert smiles and will makes your battery a 133 AH battery meaning you can discharge it for 133 hours at 1 amp. On the other hand if you were to discharge it at 100 amps or the 1 hour rate Mr. Peukert frowns and robs you with only allowing you 30 AH or a mere 18 minutes when you expected 60 minutes. Surprise!
Here is Mr Peukert in real terms a Rolls 30H125. It is a 125 AH 12 volt battery. Look at the Discharge Rate table. At 100 hours it is a 166 AH battery. At 1 hour it is a 45 AH battery.
To make life simple over the years I and other battery professionals have come up with a generic Peuket derating Correction Factor called C. It can be used to ball park determine or adjust if you will for your actual discharge rate where True Amp Hours = C x Rated AH @ 20 hour rate. To bget actual numbers will have to come from the specific battery manufacture and model number. But this will get you close for preliminary design.
1 Hour = .3
2 Hour = .5
3 hour = .6
4 hour = .65
5 hour = .7
6 hour = .75
8 hour = .8
10 hour = .85
12 hour = .9
16 hour = .95
20 hour = 1
24 hour = 1.05
36 hour = 1.1
72 hour = 1.25
100 hour = 1.30
So for example if you have a 100 AH battery and discharge at the 2 hour rate (50 amps), then 100 AH x .5 = 50 AH.
Time for another STICKY THREAD. This is more bad news for you off-grid battery folks. Peuket law is like Death and Taxes, you will pay, and for 99% of you a lot more than you have too. Peukert Law simply states for flooded lead acid batteries: "The higher the discharge rate (Amps) the less capacity the battery actually has". For example your 100 amp hour battery can be as low as 30 AH if discharged at the 1 hour discharge rate of 100 amps. Or as high as 160 AH at the 100 hour rate of 1 amp.
In theory only with batteries:
Amp Hours = Amps x Hours
Hours = Amp Hours /Amps
Amps = Amp Hours / Hours.
Well that theory only applies at the discharge rate the manufacture specifies the battery at which is typically 20 hour rate. So for example if you have a a 100 AH battery the 20 hour rate is 5 amps 5 amps = 100 Amp Hours / 20 Hours). From that formula a layman would assume if you had a 100 AH battery you should be able do discharge that battery at 1 amp for 100 hours, or 1 hour at 100 amps. YOu would be wrong, very WRONG, Mr Peukert will not allow it. If you were to discharge that 100 AH battery at 1 amp Mr. Peukert smiles and will makes your battery a 133 AH battery meaning you can discharge it for 133 hours at 1 amp. On the other hand if you were to discharge it at 100 amps or the 1 hour rate Mr. Peukert frowns and robs you with only allowing you 30 AH or a mere 18 minutes when you expected 60 minutes. Surprise!
Here is Mr Peukert in real terms a Rolls 30H125. It is a 125 AH 12 volt battery. Look at the Discharge Rate table. At 100 hours it is a 166 AH battery. At 1 hour it is a 45 AH battery.
To make life simple over the years I and other battery professionals have come up with a generic Peuket derating Correction Factor called C. It can be used to ball park determine or adjust if you will for your actual discharge rate where True Amp Hours = C x Rated AH @ 20 hour rate. To bget actual numbers will have to come from the specific battery manufacture and model number. But this will get you close for preliminary design.
1 Hour = .3
2 Hour = .5
3 hour = .6
4 hour = .65
5 hour = .7
6 hour = .75
8 hour = .8
10 hour = .85
12 hour = .9
16 hour = .95
20 hour = 1
24 hour = 1.05
36 hour = 1.1
72 hour = 1.25
100 hour = 1.30
So for example if you have a 100 AH battery and discharge at the 2 hour rate (50 amps), then 100 AH x .5 = 50 AH.
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