Is it a matter of how strong the current is coming out of the battery? Is less power in the battery meaning “free space” in the battery and that’s what is measured?
In: Technology
There is a chip, nicknamed the fuel gauge, which looks at what’s coming out of the battery and comparing it to historical performance, as well as the expected performance. It takes care of variance due to heat, age, etc. automatically via algorithms to predict the remaining charge accurately
https://www.ti.com/product/BQ27200#features
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A battery has a normal voltage when charged, say 1.5 volts. But as it is depleted, it goes down a little bit. 1.49, 1.48, 1.47, etc.
It can still power things at the lower voltages, up to a point, and it’s somewhat of a race between the total energy left in the battery and the voltage needs of the thing that it is powering.
But most things that need batteries have some power regulation circuitry that can handle small voltage drops, and in that circuitry is a voltage meter that measures how much voltage the battery is supplying. That voltage is turned into the battery percentage after some math.
Depends on the amount of fucks the manufacturer gives. Some just measure voltage, since a fully charged battery will have a specific voltage and will gradually decrease to another well known “empty” value. The scale is not linear though, while this works okay on old batteries, li-ion chemistry has the middle half of its capacity at almost the same voltage. Going by voltage also brings current into the equation, a battery actively supplying more current at the moment will experience a sag in the voltage.
The proper way is a coulomb counter/fuel gauge chip. It does what it says on the tin, measures the total amount of energy going in and out of the battery by performing continuous current measurements. The input to output energy ratio will be something around 90% for lithium ion, and not subject to nearly as many annoying inaccuracies as voltage measurement.
You’re indirectly measuring how much of the energy storing chemicals inside the battery are left unreacted.
The most reliable method is usually counting how much charge goes in and out of the battery. Compare the two on an ongoing basis.
You can measure the voltage.
You can measure the voltage with a load applied.
You can measure how much voltage changes with a change in current.
You can measure how much heat is generated with a given charging current(I believe this was used to tell when NiCd cells were done charging).
These different methods can be more or less effective depending on the battery chemistry in question.
The battery is empty when any of the following conditions is met:
The battery falls below the voltage, current, or power delivery requirements of the device it’s powering. It may still be full enough to power some other device with different requirements, or if the temperature rises.
If it’s a rechargeable battery, when further discharge will cause damage to any of the cells, this is usually determined by a cutoff voltage.
Some can’t.
Depending on the battery technology it’s not always easy.
A great example is the rechargeable batteries I use at home for almost everything.
They are NiZn and I like that they have a stable voltage all the time. But when they are completely empty it drops instantly.
Stable voltage is cool but you get no warning when it’s about to run out.
On my quest 3 controllers or xbox one controller it says they are completely full, completely full, BOOM turn off, no warning. Kinda annoying and I wish I knew that before replacing all my NiMh.
Electronics gauge battery life by measuring the voltage drop of the battery’s output. As the chemical reactions inside the battery deplete, the voltage output decreases, which the device interprets as a reduction in available power.
What marvellous timing
Tldw, the voltage from the battery under load will decrease as it loses power
It’s hard. With modern rechargeables the voltage depletes with lower charge, so like a fully charged battery is 4.2v. But it’s not really proportional – a 50% charged 4.2v battery isn’t 2.1v. And to make matters worse, the voltage at 75% is really close to the voltage at 25% – it’s kind of a ‘hockey stick’ curve where from 100% to 75% will drop the battery from 4.2v to 3.7v, but then it stays at 3.6-3.7v until it’s below 25%, getting down to maybe 3v at 0%*
So if you’re using your device a lot and often partially discharging it so it’s between 25% and 75%, you can’t rely on voltage measurement to tell you directly how much the battery has left. Instead you have to measure how much energy you put into the battery and how much energy you take out of it. This works pretty well but if you do it for a really long time, any errors can add up.
That’s why a lot of electronics have an automatic battery calibration routine – when you fully charge it, it knows that the battery is at 100% (4.2v) and then resets the estimate of how much you charged/discharged. You don’t need to fully charge it for the battery to be healthy, you just need to fully charge it so the device knows how much energy is in the battery.
*Technically fully discharging a lithium ion battery to 0v can be done, but doing so will damage the battery permanently. So electronics will cut off the battery below 2.5-3v depending on how aggressive the designers were.
Since this is ELI5; they measure the number of electrons flowing in and out of a battery. Analogous to a pump counting the gallons of liquid going in to a an empty tank and then counting the gallons of liquid coming out of it. Difference gives you what remains. Design capacity is programmed and set. Zero is set when nothing comes out of it (full discharge). Current (as in present) capacity is when nothing goes in (full charge).
As to how the electrons going in and out are measured, thats done by placing a resistor in line and measuring the voltage drop across that known resistor. Ohms law then gives you the current. Same principle as an orifice type flow meter.
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