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I have a battery which indicates 12V25Ah/10hr. What does 10hr mean here? I guess "hr" stands for "hour". But 25Ah already means if battery is fully charged and if I load 12V 25A (=300W) to it, it should last for an hour (correct me if I am wrong).

  1. What does "/10hr" mean here?
  2. How many watts I can load to use the battery for an hour?
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It means that the battery has a capacity of 25 Ah when discharged in 10 hours. 25 Ah is 25 Amps for 1 Hour which is equivalent to 2.5 Amps in 10 Hours.

So if you load the battery with 2.5 Amps it will last 10 Hours.

If loaded with a higher current usually battery capacity decreases so that is why the 10 Hours is mentioned, it results in a higher battery capacity making the battery look "better".

How many watts: simple 2.5 Amps x 12 V = 30 Watts and that for 10 Hours.

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    \$\begingroup\$ "If loaded with a higher current usually battery capacity decreases"- does it mean if you load 2.5A it will last 10 hours, but if you load 5A it will last LESS than 5 hours? Meaning, Amper x n is not proportional to hours / n ??? \$\endgroup\$
    – Farrukh
    Dec 1 '15 at 15:15
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    \$\begingroup\$ Yes that is what it means, you should be able to see the exact numbers in the datasheet of the battery. This effect mainly exists when you load the battery heavily. For a very long discharge like 100 hours (light load) or more you would not notice much change in capacity. \$\endgroup\$ Dec 1 '15 at 15:20
  • \$\begingroup\$ Actually, long (low current) discharges matter as well; batteries, like any other real pieces of a circuit, have internal resistance that will drain them over time. A battery that delivers 2.5A to a circuit in 10 hours may only deliver 1A over a period of a year. As Icy mentioned, you'd have to look at the specific battery's discharge curve to find optimum use. \$\endgroup\$
    – ArmanX
    Dec 1 '15 at 19:23
  • \$\begingroup\$ @DavidRicherby I believe the intended meaning is "if you apply a load of 2.5 Amps", meaning that the Amps are being used as a measure of the discharge rate, not the capacity. \$\endgroup\$ Dec 1 '15 at 21:55
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    \$\begingroup\$ At the other extreme you have your self discharge curve too. If you try to draw 0A out of a battery, the naive result would be that the battery would last forever. However the self discharge on a NiCd for example, would deplete it in a few weeks. \$\endgroup\$
    – Aron
    Dec 2 '15 at 1:19
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I concur with what others have said above. Battery capacity is somewhat dependent on discharge current At higher discharge current, the battery capacity decreases.

Here are discharge curves that illustrates this. The same battery was discharged at different rates.

enter image description here ( source )

This chart is for a Li-ion battery. A chart for a Lead-acid battery would also show the decrease in capacity.

Note that in this case, C doesn't stand for °Celsius. In the chart above, C stands for discharge rate normalized by battery capacity. 1C means such rate that will discharge a battery in 1 hour. 0.5C and 4C correspond to 2 hours and 15 minutes, respectively. Consider a 12Ah battery. For such battery 0.5C, 1C, 4C correspond to 6A, 12A, 48A, respectively. This kind of normalization helps to abstract away the size of the actual battery, which makes it easier to look at other aspects. (More here.)

update: Here's another similar chart.

enter image description here

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  • \$\begingroup\$ If I understand correctly, the different curves here correspond to different temperatures. How do they illustrate difference in capacity vs current? \$\endgroup\$
    – Ruslan
    Dec 2 '15 at 5:27
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    \$\begingroup\$ @Ruslan In this case, C doesn't stand for °Celsius. In this chart, C is a discharge rate normalized by battery capacity. 1C means such rate that will discharge a battery in 1 hour. 0.5C and 4C correspond to 2 hours and 15 minutes, respectively. Consider a 12Ah battery. For such battery 0.5C, 1C, 4C correspond to 6A, 12A, 48A, respectively. This kind of normalization helps to abstract away the size of the actual battery, which makes it easier to look at other aspects. (More here.) \$\endgroup\$ Dec 2 '15 at 5:40
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The capacity of a battery depends partly on how fast you discharge it. A specification like "25Ah/10hr" says that the battery has a capacity of 25 amp-hours when it is discharged at a rate that takes it from fully charged to fully discharged in 10 hours — in other words, at \$\frac{25Ah}{10 hr} = 2.5 A\$.

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    \$\begingroup\$ In other words, if you try to discharge it in less than 10 hours, its capacity will be reduced : at 25A you'd be lucky to get half an hour. \$\endgroup\$ Dec 1 '15 at 14:57
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It means it will be 25Ah if discharged over 10 hours. Battery capacity reduces if they are discharged fast. Most battery capacities are quoted on a 20 hour discharge rate.

You would have to look at the battery's discharge curve to see what capacity it has at a 1 hour discharge rate to find the maximum current you can draw for 1 hour.

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A lead battery has problems with load due to internal resistance. That's why the capacity needs to be given dor a duration. The capacity left is as follows:

  • From 10 to 6 hour the reduction is almost a straight line from 100% to 94% after that it gets bad....
  • 5 hour discharge you have only 92%
  • 4 hour makes 88%
  • 1 hour has only 58%
  • 250AH hours vs Ah

As a rule of thumb: a 25kg lead can charge 1kWh. So a battery of i.e. 8kg can deliver around 280Wh (guessing a bit .... 0.5kg housing, 0.5kg acid) this is almost independent of what type lead battery it is since its based on the amount of lead used. This can be useful if you don't know if you have a traction or a starter battery.

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    \$\begingroup\$ Please try and use proper punctuation\capitalization, this is a Q&A style forum for professionals\experts. Please also review the site tour electronics.stackexchange.com/tour Thanks \$\endgroup\$
    – Voltage Spike
    Jun 14 '16 at 22:56

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