Battery ampage
 

Hi im a begginer in this rc stuff and i wondered if someone could help me out please.How do I check ampage on battery packs---eg ---if the pack says 3000mah how do i go about findin out .


thanks

RE: Battery ampage
 

Use a multimeter. Being kind of new myself, why do you want to know the amps? From what I understand volts are more important as far as opperating your aircraft and amps affect the duration of flight...correct me if I'm wrong.

RE: Battery ampage
 

You'll want to check the volts. If the voltage drops beyond a certain level, things go south in a hurry. Not knowing what kind of battery you're talking about makes it difficult to give a specific answer. For receiver batteries, you'll want to use a loaded tester, such as an expanded scale voltmeter. This will check the voltage under a small current load (say, 300 mA). This gives you a more precise measurement of what's left in the battery as compared to just checking the latent voltage. Don't know much about lipo's--which you may very well be talking about with that kind of amperage.

RE: Battery ampage
 

mAh refers to the capacity of the battery. Your 3000mAh battery can theoretically deliver a current of 3000mA for one hour, 300mA for 10 hours or any combination where mA X Hours = mAh capacity. 1000mA = 1AMP.

I say theoretically because there is a limit to the amount of current a given battery can deliver. Some types are capable of pushing out more current than others.

It's kind of like a tank of water with a pipe coming out.

mAh is the size of the tank
mA is the size of the pipe
The bigger the pipe the faster you drain the tank.

To answer your question, the only way you can check the capacity of a battery is to drain it whilst measuring. Some smart chargers will do just that.

In normal use however the voltage (under load) is a reasonable indicator of how much capacity is left in a battery.

Hope this doesn't confuse:eek:


Terry

RE: Battery ampage
 

thanks guys for your help -- really apreciated

RE: Battery ampage
 

Falcon - just as an idea of what the battery capacity is like in relation to flying time ... a new fully charged 4.8V 600 Mah battery will easily give you 4-5 flights (maybe 6 even) of 7 mins each at sportman level (not your heavy 3D flying) with standard servos. I run 6V 1,100 Mah NiMh batteries, I get a lot of flights and I still have a very safe margin.

RE: Battery ampage
 

As Redback pointed out the mah is the capacity of the battery and the only way to measure it is to put a known load on the battery and measure how long it takes to drain. The easiest way is to get a computer charger/discharger that will keep track of this for you, but another way is to get a resistor and a meter and do the math yourself.

If you have a 4.8 volt pack and put a meter on it and see that it has a 5v charge then you can calculate the size of the resistor you need to draw any specific current (amps). The formula known as ohms law is current = volts / resistance. So if you take a 10 ohm resistor and put it across the battery you would draw 500ma per hour (.5 amps). The math is 5V / 10Ohms = .5 amps. Make sure your resistor is up to the task of taking the heat (resistors convert energy to heat) so you need to calculate how many watts you are going to produce. That formula is volts * amps. So if you take .5 and multiply it by 5 you get 2.5 watts. A 10 watt resistor will do the job nicely.

Once you have your resistor attached to your battery pack you can make some measurements with your meter. You can put the meter inline with the resistor and set it to measure amps (as long as it is able to measure a current draw of .5) or you can measure the volts on the battery. The volts will start going down as the battery drains so you will need to know what the cut-off voltage is. I typically use 1.1v on nimh and .9V on nicad batteries. Take the number of cells and multiply it by the cut off voltage, 3.6 volts in the case of my 4.8V nicad pack.

Once your pack is at the cut off voltage disconnect the battery. If you drain your cells all of the way it may damage them, so you need to be watching this on your meter. If your pack only lasts 1 hour with a 500mah draw on it then it's only able to store 500mah of energy. If it lasts 197 minutes then you can take 197/60 to get 3.28 hours or 3.28 * 500ma = 1641.6mah.

Keep in mind that the volts will change as the pack goes dead so it's not going to be exactly 500mah draw all of the time so your numbers won't be perfectly accurate, but it will give you a good idea how this all works and is a good method to observe how the batteries hold up to any given load.

Hope that helps,
schu

RE: Battery ampage
 

is it possible to bridge a resistor across the meter probes to put a load on the batteries?? this way you can see what the true voltage is.

RE: Battery ampage
 

Yes, your basically putting the meter and load in parallel with each other.

RE: Battery ampage
 

thanks schu!! do you have an equation to get the right ohm value??


thanks for the help:D


tom

RE: Battery ampage
 

Its in my post. Ohms law....

schu