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I know that the simple way to measure the voltage stored in a lead-acid battery is to simply measure the positive and negative using a voltmeter.

In my case, I think that my battery has a builtin charge controller and it is sealed like the picture below. How can I effectively measure it without opening?

asure

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If there is no way to measure it, how do I open this thing? I tried to open it using flat screwdriver and it leaves a dented mark.

enter image description here

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The DC 12v output may or may not be direct from the battery, if it is current limited then probably not - you will need to check the spec sheet.

The top is probably screwed down as the pv controller is under there. The screws to get access are hidden under the orange graphic with all the labels, removing it without damage depends on how strong the adhesive is.

One way is to rub your thumb over the surface to find the screw holes then just uncover those... but if they fitted plastic hole covers you may not be lucky.

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The question you want to ask is "how do I measure the amount of charge stored in a battery", and the answer, for most sealed batteries is "with great difficulty".

Most battery "fuel gauge" applications track the battery charge state by watching the battery voltage, the current, and by integrating the current during charge and discharge. And they still often get it wrong. "Battery charge monitoring" is the subject you want to search for.

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With a sealed battery or one with a gelled electrolyte, you are stuck using the techniques listed by Tim above. In nonsealed lead-acid batteries with liquid electrolyte, the charge state of a cell can be determined by measuring the specific gravity of the acid electrolyte, which varies according to how much lead is dissolved in it. Lots of dissolved lead means the battery has been in use and the electrolyte density goes up, very little lead means the battery is carrying a full charge and the density of the electrolyte is lower.

If you are clever, you can insert into the electrolyte a little plastic ball with a density slightly less than that of the electrolyte in a fully-discharged cell. The ball will sink when the battery is fully charged and float when it is almost fully discharged. If you put a clear plastic window in the top of the cell, you can deduce the charge state of the battery by looking for the ball through it.

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It is almost impossible to accurately know the value of charge stored in a battery. The amount of charge a battery can hold varies with battery life, battery use, charging method and environmental issues.

Niel's explains nicely how to know if the battery is fully charged (with liquid electrolyte filled batteries) and Tim's tracking of the battery charge state by watching the battery voltage, and the current, likewise.

However, both are only indicators of how much charge is in the battery. Not of "How much charge the battery can hold" and, more to the point how long it will take to discharge.

Battery Lifetime Characteristics of Lead Acid Batteries

Over time, battery capacity degrades due to sulfation of the battery and shedding of active material. The degradation of battery capacity depends most strongly on the interrelationship between the following parameters:

the charging/discharging regime which the battery has experienced the DOD of the battery over its life its exposure to prolonged periods of low discharge the average temperature of the battery over its lifetime

Measuring voltage is pointless as this will decrease as soon as load is applied. Even a nearly useless lead acid battery can, under certain circumstances, indicate a high voltage.

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