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This battery is rated at 12V, and it says it provides 7Ah of power.

Does that mean that it can provide 12V at the terminals for 1 hour if 7 amps are being drawn from it?

Also, what does it mean when someone say some appliance is X watts. Like a 60 watt bulb or a 200 watt ice crusher? Doesn't the amount of watts depend on the voltage applied and the appliance's internal resistance?

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    \$\begingroup\$ Period over which Ah rate applies varies with standardised and battery chemistry. Lead acid batteries may be rated at 10h rate or lower. eg a 7 Ah battery will notionally supply 0.7A for 10 hours, but less than 7A for one hour. A LiIon battery MAY be rated at the 1 hour rate. \$\endgroup\$ – Russell McMahon Dec 1 '13 at 10:06
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The datasheet is INCORRECT! but your understanding is not so far off...

The battery capacity of 7Ah, x its voltage (12V), indicates its stored energy not power. If you need 7A for 1 hour you need to read the fine print, to see if that was the capacity at its "1C" rate (1x its capacity, or drain it in an hour)

Oh wait, there is no fine print on that page, unlike a proper datasheet. So if you need that current you take your chances or look elsewhere.

The Yuasa datasheet shows 7Ah at the 20hour rate, falling to 6.4Ah at the 10 hour rate. So that battery from a reputable manufacturer is rated to last 20 hours at 350ma, falling to 10 hours at only 640 ma. It also tells you that the endpoint they measured to is 1.75V per cell, or 10.5V. But at least it tells you what you're getting.

Lead acid batteries can be tuned for different purposes including high current, so it's possible that the advertised battery will source 7A for 1 hour ... if the vendor is trustworthy. If they are, they may be able to supply a proper datasheet on request. Can't hurt to ask!

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    \$\begingroup\$ The Ah is neither a unit of energy or power, it is a unit of electric charge: 1Ah = 3600 Coulombs. The product V * Ah gives the energy in watt-hours (1wh = 3600 Joules). \$\endgroup\$ – Alfred Centauri Dec 1 '13 at 12:04
  • \$\begingroup\$ oops : you are absolutely correct Sir! \$\endgroup\$ – Brian Drummond Dec 1 '13 at 12:56
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Yes you understood it correctly, a 12V,7Ah batteries is supposed to provide 7A @12V during one hour.

Regarding regarding consumption in Watts, it is the current multiplied by the voltage the device is supposed to run on. If your 60 watt bulb is supposed to work on 250V it will draw 0.24A when powered that way. That's the basic theory. There are some subtleties though:

Inductive loads: for some devices powered by AC, consumption is not just amp times volts, it's I * V * cos(phi). Phi being specific to the device itself. Those devices are called inductive loads. Most of them are devices containing coils, such as transformers, motors etc...

Batteries capacities: beware, in real life you wont be able to draw your 7A during a whole hour from your 7Ah battery, because:

  • When your battery will be near empty, it won't be able to provide both 12V and 7A
  • Batteries age and tend to loose capacity during their life
  • Batteries manufacturers are often a bit optimistic with their specs
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12 volts battery: max voltage is around 13.1V - 13.3V & min voltage is around 11.1V - 11.3V

7 AH: Battery can deliver less than 7 Amps for an hour or 1 Amp for 7 Hours.

If the battery is fully charged it contains max voltage and 7 AH capacity. As u go on discharging the voltage and capacity go on reducing.

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