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I have always been confused when it came to how much charge does a battery charge. Let's say, a phone battery: It says 1900 mAh @3.7 v. Now i know it goes up to 4.2v, but those 1900 mAh are available in the 2.5v ( cut off voltage i think) - 4.2v area or the 1900mAh are available in the entire 0v-4.2v, meaning that some of the battery s energy remains unused, right? Let's say that i want to power a 1A led from this specific battery, it will be on for about 2 hours? Also, on car batteries there are writings like 12v, 55Ah, 550A. Does this mean that this specific battery can output 55Ah for 10 hours?

I am sorry if the question sounds dumb, but i am just trying to learn...

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    \$\begingroup\$ You are confusing energy with power. Do a search here and there are several questions about it. Regarding you lithium batteries, the capacity is rated between two voltages, not 4.2 down to 0 but 4.2 down to about 3 V. Check the datasheet for the battery in question m, it's specified for sure. \$\endgroup\$
    – winny
    Commented Apr 8, 2017 at 20:11
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    \$\begingroup\$ For the lead-acid battery, 55Ah would mean 1A for 55 hours. But lead acid batteries don't last so long if run flat, so it's best to assume only about half the rated capacity if you want a long life. The 550A is the maximum current that the battery can produce for just a few seconds - such as when starting a car. \$\endgroup\$
    – Simon B
    Commented Apr 8, 2017 at 21:07
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    \$\begingroup\$ A battery does not store current. A battery rated in 'mAh' is storing milliampere-hours, i.e. it's storing electrical charge. \$\endgroup\$
    – Whit3rd
    Commented Apr 8, 2017 at 23:21

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A figure like 550 A means that the battery is capable of supplying a total of 550 amperes for a short period of time like a quick triggering of the car starter. Now, if you only draw 1 A out of a 55 Ah battery it will be able to supply the current for a total of 55 hours. Likely, if you draw 2.75 A it would last (55/2.75 = 20 hours, regardless of voltage.

The figure amp-hour (Ah) is a product of the amount of charge available in the battery. Charge like in coulomb or electrons. Since 1 ampere equals 1 coulomb during 1 second then the ampere measurement is the rate of charge flow per unit of time. Then the product of (charge count)/time X time = (charge count) you end up with your figure of amp-hour turn out to be how many coulombs is available in your battery. Why manufacturers don't express the battery capacity in coulomb is beyond my comprehension, but on a scientific standpoint that is what it is.

One should not confuse ampere x hour (Ah) with the available energy. Both are interrelated but the later involve the voltage. The product of charge count x voltage is a representation of energy. Hence, if you have a 12 V, 55 Ah battery, the total energy available (theoretically) would be 12 V X 55 A X 1 hour = 660 watt-hours = 0.66 kWh of energy. Or, also 2.38 mega newton-meters or 568 kilo-calories or 1.75 mega foot-pounds. Something to ponder on. :)

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    \$\begingroup\$ 550 error 1st sentence... \$\endgroup\$
    – Paul Uszak
    Commented Apr 8, 2017 at 21:23
  • \$\begingroup\$ @PaulUszak what's wrong with the first sentence? I don't see any error. \$\endgroup\$
    – Chupacabras
    Commented Apr 10, 2017 at 5:17
  • \$\begingroup\$ "...n car batteries there are writings like 12v, 55Ah, 550A. Does this me..." - OPs car battery is 55 Ah which is a typical size. The 550 figure is the cold crank capacity. A 550 Ah is double the largest leisure battery I've ever seen. I guess it could be a North Korean submarine battery :-) \$\endgroup\$
    – Paul Uszak
    Commented Apr 10, 2017 at 9:52
  • \$\begingroup\$ Absolutely right, a typical car battery would not have 550Ah capacity. the calculations are OK but I was way out of range on that figure. Typical car battery would be more in the range of 50Ah and 550A could be the surge capacity at start. thanks for the correction. \$\endgroup\$
    – Fred Cailloux
    Commented Apr 12, 2017 at 1:34
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    \$\begingroup\$ @Fred, you still have errors after your edits. (1) 1 A from a 55 Ah battery will last more than 1 hour. (2) The coulomb is amp-seconds, not Ah. (3) 'A' or 'amp'. 'C' or coulomb. 'W' or watt. SI units named after a person use capitals in the "symbol" but lowercase when spelled out. SI also recommends a space between the numbers and the units. It improves legibility. (4) Use [Enter] x 2 for paragraph breaks. \$\endgroup\$
    – Transistor
    Commented Mar 16, 2018 at 23:44

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