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easy battery question

O

Omegachi

Diamond Member
how do you calculate how long batteries will last by its mah? say a battery is rated at 2000mah... how long will it last?
 
If you use it in a device that draws 1000 mA, then it will last 2 hours. It it draws 500 mA, it will last 4 hours, etc. Just divide 2000 by the current draw in mA.
 
Originally posted by: Heisenberg
So if it draws 0.5 mA, then 2000 mAh/0.5mA = 4000 hours. The mAh rating is the product of the current draw and time.
Click to expand...

Well...it looks like heisenberg has this well under contol.

<wanders out casually>
 
Originally posted by: cavemanmoron
mah?

Cold Cranking Amps is better.
Click to expand...
Uh, no.. it isn't.

CCA doesen't tell you anything about the capacity of the battery, other than the amperage it can deliver at 0ºF in like 30 seconds... Useless.

Amp hours, or in the case of small batteries, milliamp hours, is the standard measure of battery capacity.

In the case of car batteries, it doesen't apply since the battery shouldn't ever be discharged fully anyway.

The higher the draw, the lower the capacity. For example, if you have a 2000mAh(2.0Ah) battery, it will probably only support a 1 amp load for an hour and a half, and a 2 amp load for 30-45 minutes. On the other hand, it might support a 20mA load for ~120 hours.

Dividing capacity by load will give you a rough estimate, though.
 
Originally posted by: Heisenberg
If you use it in a device that draws 1000 mA, then it will last 2 hours. It it draws 500 mA, it will last 4 hours, etc. Just divide 2000 by the current draw in mA.
Click to expand...

Not quite. The battery manufacturer claims the battery can output 2000mA down to a certain per cell voltage. They have different curves published for varying discharge rates. The requirements of the OP may be such that if he put a constant load of 1000mA for two hours, then the per cell voltage would drop so low that the device would not stay powered up. It would be best to consult with the battery manufacturer to find out the discharge rate to 80-85% of the nominal cell voltage.

The rating of 2000mAh is probably based on a cell voltage drop down to the 60-70% range which is not sufficient for powering most equipment; especially for electronics with a significant inductive makeup (small motors, transformers, etc.).
 
Originally posted by: Heisenberg
So if it draws 0.5 mA, then 2000 mAh/0.5mA = 4000 hours. The mAh rating is the product of the current draw and time.
Click to expand...

Which is mostly correct except that voltage starts sagging as the battery ages, I imagine that the mAh doesn't take into account the fact that at the end of it's life the voltage is probably quite low. So the short answer is that higher is better but there's no easy way to tell exactly how long a battery will last in your device. For example, if you're driving something that has can't turn on below 1.2V you might only be able to get 2000 of those 4000 hours in your device.

edit: I guess I should have read through the thread before posting. KMurphy explained what I was trying to say better than I did.
 
Originally posted by: KMurphy
Originally posted by: Heisenberg
If you use it in a device that draws 1000 mA, then it will last 2 hours. It it draws 500 mA, it will last 4 hours, etc. Just divide 2000 by the current draw in mA.
Click to expand...

Not quite. The battery manufacturer claims the battery can output 2000mA down to a certain per cell voltage. They have different curves published for varying discharge rates. The requirements of the OP may be such that if he put a constant load of 1000mA for two hours, then the per cell voltage would drop so low that the device would not stay powered up. It would be best to consult with the battery manufacturer to find out the discharge rate to 80-85% of the nominal cell voltage.

The rating of 2000mAh is probably based on a cell voltage drop down to the 60-70% range which is not sufficient for powering most equipment; especially for electronics with a significant inductive makeup (small motors, transformers, etc.).
Click to expand...
Yeah, I realize that the produced voltage falls as the battery discharges and so will effect how long you can actually run something at a certain current draw. I was just trying to give him a simple answer. 🙂
 
Originally posted by: KMurphy
Originally posted by: Heisenberg
If you use it in a device that draws 1000 mA, then it will last 2 hours. It it draws 500 mA, it will last 4 hours, etc. Just divide 2000 by the current draw in mA.
Click to expand...

Not quite. The battery manufacturer claims the battery can output 2000mA down to a certain per cell voltage. They have different curves published for varying discharge rates. The requirements of the OP may be such that if he put a constant load of 1000mA for two hours, then the per cell voltage would drop so low that the device would not stay powered up. It would be best to consult with the battery manufacturer to find out the discharge rate to 80-85% of the nominal cell voltage.

The rating of 2000mAh is probably based on a cell voltage drop down to the 60-70% range which is not sufficient for powering most equipment; especially for electronics with a significant inductive makeup (small motors, transformers, etc.).
Click to expand...

Sssshhhh...

<waves hands> "Nothing to see here folks"

Aside to Kmurphy: Don't tell them things about exponential dropoffs. They'll just STARE at you.
 
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