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Old Oct 16, 2005, 06:59 PM
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what is mAh? can i use a higher mAh?

hey
another total newb question from me I have a 7.2V 1700mAh battery for my electric r/c car. If i were to buy a battery, would it have to be a 7.2V 1700mAh battery, or can I go for a 7.2V 3000mAh battery!? tahnks again guys
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Old Oct 16, 2005, 07:01 PM
Jim
North Alabama
Joined Mar 2004
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Yep. Go for the 3000mah. Just means a longer run.
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Old Oct 16, 2005, 07:02 PM
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"mAh" is the capacity of the battery.

Silent
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Old Oct 16, 2005, 07:04 PM
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thanks guys really do appreciate it
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Old Oct 16, 2005, 07:04 PM
Power in the hands of few
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The issues are size & weight. Go as big as you can, but not so big or heavy as to not fit in the car.
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Old Oct 16, 2005, 08:28 PM
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so if a car comes with 7.2V 1200mAh, it wont do any damage to the car or motor because a newer battery is 7.2V 3000mAh?
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Old Oct 16, 2005, 08:47 PM
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Georgia
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I had the same question before, my freind explained it to me like having a bigger gas tank in your car.
Lexi
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Old Oct 16, 2005, 11:55 PM
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Omaha, Nebraska, United States
Joined Aug 2002
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The mah rating on the battery is like the size of the gas tank on a car like Lexi mentioned.

Here is one way to look at it. Someone please correct me if I'm wrong.

A battery's capacity is rated in mah or milliamp hours. A 1700 mah battery is rated to deliver 1700 mah (or 1.7 amps) per minute for one hour. So this battery will deliver 102,000 mah (or 102amps) in one hour. 1700 mah x 60 minutes = 102,000 mah.

Now take the amp draw of your car motor. Let's just say it pulls 20 amps or 20,000 mah.
Take the capacity of the battery which is 102,000 mah or 102 amps. Divide by 20,000 mah or 20 amps, which the motor is pulling and you get a total motor runtime of 5.1 min. i.e. (102,000 / 20,000)

So a basic formula could be;

total motor runtime = (battery mah capacity x 60) / motor amp draw

for your 1700 mah battery;
total motor runtime = (1700 mah x 60) / 20,000
total motor runtime = 102,000 / 20,000
total motor runtime = 5.10 min.

for a 3000 mah battery;
total motor runtime = (3000 mah x 60) / 20,000
total motor runtime = 180,000 / 20,000
total motor runtime = 9 min.

So bigger battery = more gas in the tank to run your motor.

There are other ways to calculate this as well.

I hope this makes sense and helps a bit.

Where you can get into trouble is if you have a motor that pulls a higher amp draw than the battery is designed to deliver. Pull 30 amps on a GP2200 battery and it will be no problem. This cell is designed for high amp draw applications.

But pull 30 amps on a 2300 mah AA cell that is designed for digital cameras etc. and that cell will be toast.
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Last edited by kwilliby; Oct 17, 2005 at 12:02 AM.
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Old Oct 17, 2005, 12:15 AM
CamLight Systems
New York City, USA
Joined Oct 2003
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Another way to do it...

Assume 20A motor load.

1700mAH cell = 1.7AH cell
(1.7 ampere-hours) / (20 amperes) = .085 hours
(0.085 hours) * 60 = 5.1 minutes

3000mAH cell = 3.0AH cell
(3.0 ampere-hours) / (20 amperes) = 0.15 hours
(0.15) hours * 60 = 9 minutes

Notice that when you divide ampere-hours by hours that you're left with hours, i.e., your run time. Just convert the run time to whatever units you need (hours, minutes, seconds, etc.).
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Old Oct 17, 2005, 02:43 AM
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Letchworth, Great Britain (UK)
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Quote:
Originally Posted by kwilliby
Here is one way to look at it. Someone please correct me if I'm wrong.

A battery's capacity is rated in mah or milliamp hours. A 1700 mah battery is rated to deliver 1700 mah (or 1.7 amps) per minute for one hour. So this battery will deliver 102,000 mah (or 102amps) in one hour. 1700 mah x 60 minutes = 102,000 mah.
Your conclusions are spot-on, but just one correction A 1700mAh battery is rated to deliver 1700mA (or 1.7 amps) for one hour: Not mah per minute for one hour

Once you get the units right, it all begins to make sense: 1700mAh (or 1.7Ah) divided by 20A leaves you with time (Ah/A = hours).
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Old Oct 17, 2005, 10:07 AM
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Omaha, Nebraska, United States
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Quote:
Originally Posted by abenn
Your conclusions are spot-on, but just one correction A 1700mAh battery is rated to deliver 1700mA (or 1.7 amps) for one hour: Not mah per minute for one hour

Once you get the units right, it all begins to make sense: 1700mAh (or 1.7Ah) divided by 20A leaves you with time (Ah/A = hours).
thanks for the correction. guess i was up too late.
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Old Oct 17, 2005, 10:26 AM
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Letchworth, Great Britain (UK)
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Quote:
Originally Posted by kwilliby
thanks for the correction. guess i was up too late.
No problem Now you're up too early, aren't you?
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Old Oct 17, 2005, 10:47 AM
Registered User
Omaha, Nebraska, United States
Joined Aug 2002
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Quote:
Originally Posted by abenn
No problem Now you're up too early, aren't you?
no kidding. need to find more sleeping time. just can't get everything done. i understand the battery mah stuff, just don't do a good job of transfering it from my brain to words.

thanks again.
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Old Oct 17, 2005, 10:55 AM
Registered User
Omaha, Nebraska, United States
Joined Aug 2002
656 Posts
I know this may seem backwards but it is easier for me to calculate the motor amp draw per minute then divide battery capacity by the per minute draw.

20 amp draw / 60 min. = 333 mah per min.
1700 mah battery / 333 = 5.1 min.

just another way to look at it i guess.
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