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battery question

Old 12-07-2008, 05:59 PM
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y7deluxe
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Default battery question

i can't figure out what the mah vs voltage on a battery pack means. I've googled and found all this info on amps, watts, ohms and voltage, but i still can't seem to grasp it.

what does the voltage rating mean? The higher the volts of the pack = the more power pushed to the motor? and what does the mah rating mean? the higher the mah = the longer the battery will last?

if there is another post with this information or a webpage somewhere that explains this, please link me. i was not able to find one. thanks!
Old 12-07-2008, 06:07 PM
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Default RE: battery question

Yep, you got it right.
Old 12-08-2008, 12:23 AM
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y7deluxe
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Default RE: battery question

ok, then why is it when we charge batteries, we can charge at a certain mah rating and the voltage increases as we charge them? i'm picturing a battery as a cup and the voltage is the volume of the cup and the mah is the rate at which the cup is filled up. if this was the case, then voltage would be the capacity and mah would be the output of the battery... do we charge them with amps and then discharge them with volts? there's gotta be a thread on this somewhere on this site??? Thanks.
Old 12-08-2008, 07:29 AM
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jmcvicker
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Default RE: battery question

You only charge at a amperage rating - mAh is "milliamp hours" - Amps (current) over time. Now, when charging it charges with Amps without regard to time - it's just a flow of amps. You first need to lookup amps and volts in wikipedia, howstuffworks.com or somewhere electronic (even at a Radio shack store). You may notice that on chargers, you cannot set the 'volts' of output, only amps.

Voltage rise during charging is due to the chemistry of the battery and part of it is internal resistance (IR). Voltage is also known as electromotive force.

The output of a motor is in Watts - voltage times amperage. If you have a draw of 10 Amps of 10 Volts that's 100 watts. On house-current, a 100w lightbulb is 120v * .84 amps. This is a unit of Power or Work. Power is potential times flow (volts * amps).

Let's look at a 6-cell "pack". That is 6-cells of 1.2v in series. Or 6S. 6 in a series is voltage added up. 1.2 * 6 = 7.2volts (or so). You may find a 7.2v pack having 7.8 or 8.1 volts when fully charged. It'll be about 7.2volts when it's used, however. Anyway - voltage rises on a voltmeter when the pack is charged for a number of reasons (I'm not entirely sure) - but partly due to the chemistry, part due to the cells warming up and other stuff. The usable range of a cell that is 1.2 volts is about 1.5v down to .9v. It stops delivering good amperage about 1.1volts. So, the voltage is never perfectly constant but is about 1.2v more or less in a pack.

Point is - consider electricity more like a flow of electrons (similar to water in a garden hose). Amperage is the rate of flow - so if you use the garden hose analagy, it's gallons per minute. I think volts would be kind of like water pressure wihle the flow of water is also limited by hose size (amps). 10psi through a 1" hose is say 2gpm while 2" hose would be 6gpm. A higher amp battery output which runs at 1.2v would let more electrons flow out of it per minute than a lower amp battery - giving you the ability to utilze that flow for higher work (effort over time). A motor can work harder with a higher amp flow.
Old 12-08-2008, 10:33 AM
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Rodney
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Default RE: battery question

A better analogy is to think of a tank of water. The volume is equal to the mah (how much water you can take out of a full tank). The voltage is equal to the water pressure (how high it is above the tap if gravity feed). The C rating is the size of the nozzle, i.e. how fast you can empty the tank. Amperes or current is the rate at which the water flows out of the tank. Hope this helps your analogy.
Old 12-08-2008, 11:13 AM
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y7deluxe
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Default RE: battery question

thanks guys, i think the issue i was having is that i thought 1000 mah = 1amp, but it seems that mah is a unit of time, where an amp is not a unit of time. i guess it'd be 1000mah = 1amp hour and 1000 milliamps = 1amp. anyhow thanks for your input. i'm going to be purchasing a book on this to see if i can get myself to a point where i can understand it like you guys
Old 12-08-2008, 04:11 PM
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Default RE: battery question

I think you got it really.

mah is a total (like miles) milliamphours OR lets make it simple and say 1 Ah
amps is a rate (like miles per hr)

So the capacity is rate times the time. (like 110 miles if travelling at 55 mph for 2 hrs)

1.1 Ah if chargeing at .55 amps for 2 hrs.

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