# how to know the capacitance of a battery ?

hello guys, i need to know please how to know how much capacity a lead acid battery or any battery still have ? is there any way to build a device or something for that ? i need something like the battery indicator in the cell phone for example my target is to build a battery indicator with leds for example or display for a specific circuit that i am building

the battery that i have is a lead acid 12V - 3 Ah

how to know the capacitance of a battery

Your query is a bit vague. Give us a paragraph describing what you want to do please?

The best website for that: http://batteryuniversity.com/

If you have a known battery and you know how the discharge curve is, you could guess the remaining capacity. That is how it is done in tablets and laptops.

However, if you have an unknown battery, the only way to know it is to discharge the remaining capacity an measure how much was discharged.

ADDED: as dc42 writes: A tablet and laptop measure the time and the current ofcourse to make a guess.

i have a 12V - 3Ah so it is a programming topic and hardware ?

The problem is that the discharge curve if a SLA battery is quite flat until it is nearly discharged, for example see http://www.steveduncan.net/assets/images/mfg_curves.jpg. The usual approach, if you know the battery capacity, is to start with the battery fully charged, measure the discharge current continuously, and from that calculate the remaining capacity when required.

dc42: The problem is that the discharge curve if a SLA battery is quite flat until it is nearly discharged, for example see http://www.steveduncan.net/assets/images/mfg_curves.jpg. The usual approach, if you know the battery capacity, is to start with the battery fully charged, measure the discharge current continuously, and from that calculate the remaining capacity when required.

i'm sorry but i dont have any idea about what you said i mean i am a beginner in this so i need to know what to do step by step please :S ?

1) Fully charge battery 2) Let it rest for at least 2 hours 3) use amp meter set to say 0.25 amp range and voltmeter set to 20 volt range 4) Obtain 82ohm / 3 watt resistor 5) Connect voltmeter across battery 6) Connect amp meter and resistor in series across battery 7) Note time, voltage and current readings 8) Repeat step 7 for next 15 hours or so, but do not let battery voltage fall below 10.7volts 9) Integrate ampere reading with time and this will show ampere-hours of discharge at 20hour rating 10) Assuming battery started out at 3AH then subtract result of (9) from 3AH and what you have left is battery capacity remaining 11) Immediately recharge battery

jackrae: 1) Fully charge battery 2) Let it rest for at least 2 hours 3) use amp meter set to say 0.25 amp range and voltmeter set to 20 volt range 4) Obtain 82ohm / 3 watt resistor 5) Connect voltmeter across battery 6) Connect amp meter and resistor in series across battery 7) Note time, voltage and current readings 8) Repeat step 7 for next 15 hours or so, but do not let battery voltage fall below 10.7volts 9) Integrate ampere reading with time and this will show ampere-hours of discharge at 20hour rating 10) Assuming battery started out at 3AH then subtract result of (9) from 3AH and what you have left is battery capacity remaining 11) Immediately recharge battery

ok i have some questions that need to be explained please 1- how this will work while the battery is in use by a circuit ? 2- in 7) what do you mean by note time ? you mean note the time where the battery started to discharge like 10:00 am ? 3- in 8) you said repeat for next 15 hours, you mean i keep the battery and resistor on for 15 hours then i check the time ? 4- in 8) how do i prevent the battery voltage from dropping under 10.7 v ? 5- in 9) how do i integrate :D ? 6- in 11) you said immediately charge battery, is that right to do if the battery is not fully empty :S ?

my target is to build a battery indicator with leds for example or display

All those steps, 1-11 inclusive, are meant to be done -before- you start using the battery. What you are doing is measuring the actual capacity (not capacitance) of the battery in Ahrs. Then, you must have a circuit that keeps track of how much current is being consumed in use.

With a sealed lead acid battery, the only way to know how much capacity is left is to have already measured the total capacity, then kept track of how much has been used. With a liquid electrolyte, you can measure the specific gravity of the electrolyte, but that will only give you some idea of the percentage of charge, not the actual number of Ahrs left.

That was why step (1) said "Fully charge battery" :) :)

A far simpler system, assuming an average current drawn, would be to measure typical load current, let us say it works out at 45mA. Battery, when full, is rated at 3AH, therefore with 45mA load one should get at least 70 hours of use before the battery is dead. It'll actually be greater because we are loading at C70 and battery specs are usually given at C20. Now let's assume we are nice to our battery and never discharge it beyond 50% depth, so use time should be limited to 35 hours (50% of 70 hours). Now run a simple on-board clock routine which starts at 100% capacity at time =0 and counts down in percentage terms to 0% available capacity at time = 35 hours. ;) ;)

firashelou: hello guys, i need to know please how to know how much capacity a lead acid battery or any battery still have ? is there any way to build a device or something for that ?

No, not in general, capacity depends on future current drain and temperature, and you can't predict the future. The best capacity estimates are done by battery monitoring systems that record data on every charge/discharge cycle and thus learn common usage patterns.

The most reliable estimate for an old wet-type lead-acid battery used to be to measure the electrolyte density with a hydrometer.

If lead-acid batteries are abused the capacity drops like a stone - never over-discharge a lead-acid battery or leave one in a low-charge state, or store without regular top-up charging. Manufacturer's data is always the best source of information for a particular battery model (capacity v. discharge rate, capacity v. cycles).

guys i appreciate all the help you're giving but still i am confused and i need please a simple way to start :S

With a resistor divider a zener (as reference) and an opamp you can have a circuit buzz or light up to let you know when the battery is drained, and or have a pin out to your arduino etc.

A simple cheap circuit to avoid a micro, and then place a constant load from fully charged a decent watt resistor will do once you know the current discharge time how long before it's flat / opamp signals 10.5v etc disconnect and do the math..

That's the only real way, voltage alone is not suffice to determine capacity unless the micro compensates for voltage drops to recalculate it's time left .

cjdelphi: With a resistor divider a zener (as reference) and an opamp you can have a circuit buzz or light up to let you know when the battery is drained, and or have a pin out to your arduino etc.

A simple cheap circuit to avoid a micro, and then place a constant load from fully charged a decent watt resistor will do once you know the current discharge time how long before it's flat / opamp signals 10.5v etc disconnect and do the math..

That's the only real way, voltage alone is not suffice to determine capacity unless the micro compensates for voltage drops to recalculate it's time left .

can i have schematics please for this ?

guys i appreciate all the help you're giving but still i am confused and i need please a simple way to start :S

There is no simple way to do what you want, that is what we are trying to tell you.

You can detect when the voltage drops below 2V per cell as a method to tell when to stop discharging, but -that- is only accurate if you disconnect the load and let the battery sit for 10 or 20 minutes first. The curve is to complex for you to just take a voltage reading and tell the charge state, add to that the problem of temperature and recent history. Voltage will be different even a few minutes after removing a load, depending on how much current was being drawn and for how long.

Anyone who tells you it is simple is not well-informed.

ok then i need please some schematics for the circuit to be build to tell when a voltage drop

and after i mesure how can i make the indicator of LEDs ?!

Gosh, you certainly are not taking "NO" for an answer.

On that basis you could use an LM3914 chip which is specifically designed as a multi-LED voltage metering chip. It is "old" technology but it works !

The following website will give you an example of a system design for measurement of a 12 volt lead acid battery so could perhaps meet your needs. Be aware however that the circuit and LEDs will probably take more power from your battery than your actual load device so I suggest you only power it on when you actually want to take a voltage reading.

http://www.enide.net/webcms/index.php?page=lm3914-batmon

jackrae: Gosh, you certainly are not taking "NO" for an answer.

On that basis you could use an LM3914 chip which is specifically designed as a multi-LED voltage metering chip. It is "old" technology but it works !

The following website will give you an example of a system design for measurement of a 12 volt lead acid battery so could perhaps meet your needs. Be aware however that the circuit and LEDs will probably take more power from your battery than your actual load device so I suggest you only power it on when you actually want to take a voltage reading.

http://www.enide.net/webcms/index.php?page=lm3914-batmon

aha great thanks this is some useful link so all i have to do after building the circuit is wiring the battery to this small circuit and of course tha main circuit 2 ?

Yes, that's it, but as I said, the LED circuit will place a burden load on your rather small 3AH battery. Using the designer's resistor values, this is about 30mA. This current must be added to your actual system circuit load to determine total load and hence probable battery life. You could of course add an ON/OFF switch to the metering circuit so that it only draws current when a reading is actually required.