Electronic – Understanding Batteries and Loads

batteriespower supply

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?

Best Answer

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!

Related Solutions

Finding the Amp hour of battery

The Black & Decker Firestorm 12 Volt drill / screwdriver FS1202D seems to match your description, I have the same product. This model uses a 12 Volt NiCd 1.2 Ampere-hour battery pack.

If your experiments indicate a much lower ampere-hour rating, that could be due to battery aging, charging issues or the battery being loaded beyond the current / continuous operation parameters it is rated for.

If you could provide the screwdriver model number, a more definitive answer could be obtained.

Electronic – Battery Ampere-hour rating vs Battery Amps (not an experienced user)

Your hunch that batteries have a current limitation is correct. In general, it's hard to tell the current rating [A] from capacity [A·h]. You have to look it up in the datasheet. A lot depends on the design of the battery.

For example: coin cells with 500mAh capacity may have only 3mA max current.
Another (opposite) example: automotive starter battery with 40Ah capacity may have 500A max current.

Lead-acid batteries are interesting in this respect, because there are two distinct types.

Starter lead-acid batteries are designed specifically to deliver high peak current for a short period of time. Deep discharge, however, dramatically shortens the life of a starter battery. So, it's not suited for routine operation at high depths of discharge. Your typical starter battery in the automobile works at very shallow depth of discharge.
Deep cycle lead-acid batteries are designed (as name suggests) to discharge further. But they can not provide as much instantaneous current.
Here's an example datasheet for a deep cycle battery. Have a look at the nominal capacity on p.1. Notice that capacity depends on discharge current (i.e. the rate of discharge).

Depth of Discharge    Starter Battery    Deep-cycle Battery

100%                  12–15 cycles       150–200 cycles

50%                   100–120 cycles     400–500 cycles

30%                   130–150 cycles     1,000 and more cycles

(Source.)

p.s. If you want to read-up, here's an excellent web site on batteries - Battery University.

Best Answer

Related Solutions

Finding the Amp hour of battery

Electronic – Battery Ampere-hour rating vs Battery Amps (not an experienced user)

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