# How to tell if battery is fully charged?

I have 12-volt, 200-ampere lead acid battery. I want to know how much capacity is left in the battery, expressed as a percentage.

What is the exact mechanism to be used to determine find that, and what exactly is the logic behind it?

I am so curious about how mobile cell phones show exact percentages of charge remaining; how do they do this?

I found some articles which say that if your battery voltage exceeds 12.06 volts, it means it has charge, I mean it's not clear.

I would like a professional answer to this question.

Thank You

Edit:

My question could also be read as:

What tools and devices should I need to check if my 12-volt, 200-ampere lead acid battery is fully charged?

• Is "Amphere" supposed to mean "Ampere hours"? Commented Jun 19, 2017 at 20:22
• Exactly this one. I am not sure just they used to write: 12V, 200A. I don't know. But let describe case for both ;) Commented Jun 19, 2017 at 20:27
• Measure the specific gravity of the electrolyte with a hydrometer. batteryuniversity.com/learn/article/… Commented Jun 19, 2017 at 20:27
• You can't mix different physical quantities. But to answer your question, hydrometer or refractometer is the way to go. Commented Jun 19, 2017 at 20:35
• I agree to this: @BruceAbbott and @winny. Appreciate. But I wonder in market we have Uninterrupted Power Supply (UPS) which have different scales for charge like: Low, 20%, 40%, 60%, 80%, Full. My question is they don't put any hydro checking device with battery how can they tell? Same is the case with Cell Phone Batteries? If it's fake, are they playing with our minds? Commented Jun 19, 2017 at 20:41

The most accurate way to measure lead-acid battery SOC (State Of Charge) is read the specific gravity with a hydrometer. When the battery is fully charged the electrolyte has the maximum amount of sulfuric acid so the specific gravity is highest. As the battery discharges the acid is converted into lead sulfate plus water so the specific gravity drops. The manufacturer should provide specific gravity numbers for full charge and discharge.

Of course this method can only be used with 'wet' batteries which have access to the liquid electrolyte. SLA batteries can only be measured electronically.

It is possible to estimate SOC simply by measuring battery voltage. However to get an accurate result you must factor in charge/discharge current, temperature, and length of time under those conditions. Also the curve is not linear, and capacity falls at high discharge rates.

On the example graph below you can see that at C/100 (100 hour discharge rate) there is virtually no difference in voltage between 100% and 70%, while the full charge voltage drop at C/20 (20 hour rate) is equivalent to losing 30% capacity.

At lower SOC the voltage change is greater, but still severely affected by discharge current. Furthermore, when current changes the voltage takes some time (seconds to minutes) to reach a stable value. Therefore it may not be practical to use voltage to measure SOC with more than about 20% accuracy, even if the discharge current is known.

Another method that can be used is to measure the current continuously and integrate over time to determine how much charge has been put into or taken out of the battery. If the starting SOC and battery capacity is known then this 'Coulomb counting' can tell you the current SOC. However it doesn't take into account leakage, capacity loss at high current or low temperature, or battery capacity loss due to aging. therefore the SOC calculation tends to drift and needs to be recalibrated occasionally.

Accuracy can be improved by combining Coulomb counting with voltage and current measurement, and calibrating to the capacity at full charge and discharge. best accuracy is obtained by tailoring the calculations to a specific battery whose characteristics are known. Therefore it is more effective when used in a device which has a dedicated internal battery, rather than as a generic SOC indicator.

Lead acid batteries are kind of a special case among battery chemistries that the open circuit voltage is a reasonable measure of the charge of the battery. For this you just need a voltmeter. See this question for what voltage levels correspond to what charge level of a lead acid battery.

Lithium-based batteries (LiPo, Lithium Ion, etc.) generally have too flat of a discharge voltage to be able to measure accurately from voltage directly. Instead, the charge level is measured by a Coulomb counter. These devices measure the current leaving/entering the battery (just like an Ammeter would), and integrate this to get an estimate of how much charge has left/entered the battery. This in turn can be used to calculate how much charge is left in the battery. Battery packs sometimes have these built-in, otherwise just follow the application notes for the particular Coulomb counter chip you have.

There are other more intrusive methods such as using a hydrometer of refractometer for lead acid batteries, though these are kind of specialized and require extra work to check the charge level.