Battery Indicator: How to Measure Remaining Energy

[Jiggy-Ninja]

Pretty much every single electronic device in the world with an internal, rechargeable battery (MP3 player, phones, handheld games, etc) has some way of measuring how much energy is left in the battery.

How do you measure the remaining energy in real time like that? How accurate are those methods?

 

[berkeman]

Pretty much every single electronic device in the world with an internal, rechargeable battery (MP3 player, phones, handheld games, etc) has some way of measuring how much energy is left in the battery.

How do you measure the remaining energy in real time like that? How accurate are those methods?

There are two main methods that I'm aware of. What is the context of your question? Is your question related to schoolwork? If it is, you need to tell us your thoughts on the subject before we can guide you to the answers. Even if it is not directly schoolwork, you should be able to post the results of your research so far.

Hints -- (1) pull up some datasheets on the batteries you are asking about. (2) How could a charging circuit measure and remember energy transfers...?

 

[Jiggy-Ninja]

There are two main methods that I'm aware of. What is the context of your question? Is your question related to schoolwork? If it is, you need to tell us your thoughts on the subject before we can guide you to the answers. Even if it is not directly schoolwork, you should be able to post the results of your research so far.

Hints -- (1) pull up some datasheets on the batteries you are asking about. (2) How could a charging circuit measure and remember energy transfers...?

Not schoolwork related, just one of those "I wonder how that works?" situations. So I didn't have any specific batteries in mind, just wondering about some ways that it's done in general.

I did some googling, but the results I got were stuff like timing how long the battery takes to drain to figure out the full charge, which obviously doesn't happen for the indicators I'm thinking of. I don't quite know the right magic words to search for to pull out the answer.

My first thought was that there might be some V-Ah (Volt -- Ampere-hours) characteristic curve for to battery, ie the voltage changes in a slight, predictable way depending on the remaining charge in the battery. But then I remembered that the voltage would also vary with the load so that's no good.

I know that when the battery gets drained too far the voltage can drop pretty significantly, but that's not precise enough for something to measure, for example, that the battery is 75% charged.

 

[berkeman]

Not schoolwork related, just one of those "I wonder how that works?" situations. So I didn't have any specific batteries in mind, just wondering about some ways that it's done in general.

I did some googling, but the results I got were stuff like timing how long the battery takes to drain to figure out the full charge, which obviously doesn't happen for the indicators I'm thinking of. I don't quite know the right magic words to search for to pull out the answer.

My first thought was that there might be some V-Ah (Volt -- Ampere-hours) characteristic curve for to battery, ie the voltage changes in a slight, predictable way depending on the remaining charge in the battery. But then I remembered that the voltage would also vary with the load so that's no good.

I know that when the battery gets drained too far the voltage can drop pretty significantly, but that's not precise enough for something to measure, for example, that the battery is 75% charged.

The two I was thinking of are the keeping track of current and time, and measuring the battery voltage. Depending on the battery chemistry, you may or may not be able to tell the % remaining from the output voltage. The battery datasheet should show the output voltage as a function of time for various output current loadings (multiple curves).

BTW, I just googled Battery Discharge Calculator, and got some interesting hits. Check out the hit list to see if it helps:

http://www.google.com/search?source...L_enUS301US302&q=battery+discharge+calculator

.

 

[vk6kro]

The ones I have seen just give an expanded voltage scale.

So, for a NiCd battery, they give 1 volt as the minimum reading and 1.3 volts as the maximum reading but calibrate it as 0 to 100 % capacity.

Better than nothing, but you can't take the % reading too seriously.