Device time from a battery

[Overclocked]

Is it possible to calculate how long a device will last on a battery? For example, I have a device that consumes 20mA. The battery supplies 1500mAH (mA per hour). how can you calculate how long the battery will last before you have to recharge it (ie, time, how many hours)?

Im guessing you divide 1500/20ma (1500mAH would be 1.5Amps per hour right?) to get 75 hours?
(I used google
https://www.google.com/search?sourc...F-8&rls=GGGL,GGGL:2006-19,GGGL:en&q=1.5/20e-3)

Note: the 20mA Is just an example figure, its not something I was planning on using. Then again, a Blue LED takes about that much current (excluding the resistor)

 

[dknguyen]

What you say is theoretically correct. However, be aware that real batteries have internal resistance, therefore the more current you draw from them, the more energy is lost. This means that if you double the current draw from a battery it will last LESS than half as long. This effect gets exponentially worse the faster you draw the current from the battery.

The mAh rating theoretically means that the battery can supply that much current for 1 hour (and theoretically can be scaled up and down proportionally with run-time and current draw).

So then...if the mAh rating varies then how do they come up with the rating? Well the rating displayed is for a certain time period. For lead-acid batteries, the rating is for 1 hour run-time (the battery will provide this many mAh if drained at a current such that it becomes empty in 1 hour). For most other batteries the it is the 10 hour rating that is displayed. (Although I believe either NiCds or NiMh have a 1 hour rating displayed, I don't remember but my guess is NiMh). So, yeah, that just means that if the run time you get is greater than 10 hours (or 1 hour) then the run time will actually be longer, and if the time you get is less than 10 hours (or 1 hour) the run time will be less...and the greater the time difference then the exponentially larger (or smaller) the run time.

 

[audioguru]

Why not look at the graph of output current vs how long the battery lasts on the battery's datasheet from the site of a battery manufacturer.
I go to www.energizer.com and see their alkaline, Ni-MH and their new Lithium battery datasheets.

 

[Overclocked]

Which type of battery holds the most amount of current?

 

[dknguyen]

Which type of battery holds the most amount of current?

It depends on how fast you draw current from the battery. A battery with huge capacity but high internal resistance may not last as long as a battery with less capacity and low internal resistance when you draw high currents from it.

I would say Lithium-Polymer batteries are really good- high density, light weight, low internal resistance, very finicky to charge and can cause lithium fires if mischarged in the slightest way. Did I mention that they cost a TON? Very expensive...but oh so light and high density!

Lithium Ions are expensive, and have high capacities for low current draws.

NiCds or NiMh are usually the choice because of cost. NiMh= more capacity, slightly higher internal resistance,environmentall friendly, slightly more strict rules for charging than for Nicds. NiCds, a bit less capacity, lower internal resistance, not environmentall friendly.

THen you have the huge lead-acid batteries which can come in very VERY high capacities and are very heavy.

 

[philba]

on a related note, maxim carries a lot of battery meter chips, some of which appear to do a very good job of telling you how much capacity has been consumed. this is a decent overview of their offerings: https://www.electro-tech-online.com/custompdfs/2006/08/BATTERY_MANAGEMENT_8.pdf