Question: Measure the status (capacity) of a battery 12V 7000mAh

[acinonix14]

Hi all,

In a circuit I would like to measure the capacity of the power supply (12V 7000mAh) with an avr or arduino?
I prefer to measure it online (without remove it and disconnect it from circuit).

Any idea /links to kick off?

Thanks

[Magnox]

There are two ways this is generally done: Coulomb counting and voltage monitoring. Both have issues and there is no 'easy' way to get a really accurate number. You can get an estimation though.

Coulomb counting means accumulating the current (along with voltage) that flows through the battery to determine how much energy has been transferred, starting from a known capacity level. You also need to allow for continual differing efficiencies due to current drawn, battery temperature, current charge level and battery age.

The other way is to measure the battery voltage. This is dependent on the level of charge but unfortunately is not linear, varies with age and temperature and is not really accurate while the battery is being used. In fact, it really isn't a good guide until the battery has rested for a few hours.

So, those are the methods and pitfalls. Choose whichever suits. If you just need a rough guide, the voltage measurement is probably simplest. Circuit Cellar have a small ATtiny project for the latter method, issue #226 May 2009. Might be worth a look.

There are plenty of 'gas gauge' ic's available from the usual suspects: TI, Maxim, Linear etc. For instance, TI's bq34z110 claims to be able to measure capacity of an SLA battery to around 1% accuracy, using their method of 'impedance tracking'. If I needed to do this for real, I would probably be looking at such a solution rather than rolling my own in code.

[idesign]

for the best estimation you will need to measure both the current and the voltage. you can use a low resistance shunt wire and an instrumentation amplifier or a hall sensor for the current. measuring the voltage is trivial. then you can discharge the battery from full to below threshold capacity and record current and voltage with a nominal load.

[crunx]

You didn't mention what kind of battery you want to monitor. For example, lead-acid battery is a completely different animal than a lithium battery or a NiMh -based battery. Also, the operating temperature is one important parameter - for instance the capacity and voltage are usually quite temperature-dependent.

As Magnox points out, Coulomb counting is an excellent way to monitor the charge. However, it is not as simple to implement than just monitoring the battery voltage - which, in general, is not as accurate.

Know your battery before you decide what is the optimum solution. You can learn a lot about the different battery types and their charge/discharge characteristics from http://batteryuniversity.com/

[solutions]

You also did not mention the load current levels and its dynamics.

You cannot get a good answer from anyone without giving up that little secret of yours.

[PICker]

Few suggestions as starting point...
http://www.diva-portal.org/smash/get/diva2:348810/fulltext01.pdf
http://wp.josh.com/2014/11/06/battery-fuel-guage-with-zero-parts-and-zero-pins-on-avr/
http://electronics.stackexchange.com/questions/64490/low-current-battery-monitoring
http://picprojects.org.uk/projects/vmon/
http://www.atmel.com/images/doc4083_mega406.pdf