Hi,
i want to know the difference between AMP and AMPH. i want to drive 300 watt inverter from rechargeable battery so, which battery should i use?????
Hi,
The term "Amp" is usually used to refer to "Ampere" which is a unit of electrical current. That tells you how much current is being pulled from your battery. Knowing this then helps with the other term "AMPH" which is used to refer to "Ampere Hours" which is the product of current times time when time is measured in hours.
For example, for a 120 watt load running off of a 120v battery the load current (Amps) is 1 amp, and that's because the current is the wattage divided by the voltage:
Current(Amps)=Wattage(Watts) divided by Voltage(Volts)
or as written in the literature:
I=P/E
So to find your current (amps) you take the wattage and divide by the voltage. Thus if your battery is 12v and you have 120 watts you have:
current=120/12
which of course equals 10, which means it draws 10 amps.
Now Ampere Hours tells you how much current you are drawing from the battery over time, and this is used to calculate the run time of a given product running off of a given battery. This is what you really wanted to know, but you might note that you need to calculate the current itself first. You then divide the battery rating in Ampere Hours (AHr or Ahr) by the current.
Given 300 watts and a 12v battery, we first calculate the current:
current=300/12=25 amps
then divide the Ampere Hour rating by this 25, and say we have a 50 Ampere Hour battery, then we have:
Time(Hours)=50 AmpereHours/25 amps
or simply:
Time=50/25=2 hours
Thus a 50 Ampere Hour battery would keep the 300 watt load running for 2 hours.
There's a little catch here though. The efficiency of a 300 watt inverter or any other inverter for that matter is not 100 percent, so that the energy it takes in does not equal the energy it puts out as some is wasted as heat. Inverters have gotten much better over the years through advances in technology, but they still are not perfect. Also, the batteries are often over rated because they are often rated at lower than normal current draws. To make up for these problems and others, a fudge factor of 70 percent is a good idea, or to put it another way, we will try to obtain a battery that has 30 percent more capacity then we need.
This isnt hard to calculate, as we just have to increase the size of the battery calculated by 30 percent, which means we multiply it by 1.3 and that's about it.
Ok, so we have a 300 watt inverter. We will be using a 12v battery. The current is:
300/12=25 amps
We want it to run for at least 2 hours. This would mean we would need a battery of:
25*2=50 Ampere Hours
but with the fudge factor above we have to multiply by 1.3, so we get:
50*1.3=65 Ampere Hours
So here we found out we needed a 65 Ampere Hour battery. If this was a life support system we would probably go with a higher rating than that, like 100 Ampere Hours, and have a backup or two ready just in case.
These calculations where based upon a typical Lead Acid battery, but what you should do before buying is check the manufacturers data sheet to make sure the rating for the battery is the rating for the current draw that you will actually be using. The rating is not a constant, but goes down with higher current draw levels. This must be checked or the calculations above would be meaningless.