Battery capacity measurement and life cycle using microcontroller

[jean12]

Hello,I would like to design a circuit which should help the user to determine the capacity of a battery by measuring some of its parameters mainly the voltage an current so it could determine the discharge time and then provide information using an LCD about battery life.
Some link here on the net say that there is a need of measuring the discharge current in terms of time and then calculate the Amp-hours basing on discharging time and display on the LCD.
Could you please suggest the way of reading the current with battery load and how the time elapsed is determined so that the amp-hour is determined?

Please help me to think and find good solution and design using PIC16F877A which is interfaced to an LCD fr displaying purpose.

I am able to measure the battery voltage but I am not coming to the point of measuring the current and discharge time which helps in calculating the battery amp-hours.

help please to deal with this

 

[Easyrider83]

You trying to redesign Imax B6?
Take a look at this document:
http://cds.linear.com/docs/en/application-note/an105fa.pdf
Application Note 105 from Linear Technologies.
Look for HIGH SIDE CURRENT SENSING.

 

[Warpspeed]

There is no way to really know what any battery is capable of without actually running it down to the discharge end point at the expected load current.

Only when you have actually done that can you confidently display a meaningful time and amp hours capacity figure.

The obvious problem with that is that the battery will then be flat, and completely unusable for a long time until it recharges. Nobody wants to do that, but there is no other way.

Its like wanting to know what time you can run the mile, without getting out of your chair. Only way to find out for sure is to actually do it.

 

[jean12]

is the measurement of voltage enough for concluding saying that the battery is in good condition of operation?

 

[Warpspeed]

No, because the battery may be weak or damaged, and that does not show up until you try to draw a heavy current for a length of time.

Think of it this way.
A small battery may have the same voltage as a much larger car batter, but it is not going to be able to start your car.

 

[wizpic]

You could try this one, I know it uses arduino may be easy enough to covert to work with your pic micro or even give you some good pointers. I've played with on bench test seems to work well, keep meaning to fully build it and test more indepth gives a good reading
https://www.4x4community.co.za/forum/showthread.php/190968-Cheap-DIY-Semi-Intelligent-battery-monitor

 

[schmitt trigger]

You may want to take a look at so-called "battery fuel gages":

https://www.ti.com/lsds/ti/power-management/battery-fuel-gauge-overview.page

 

[jean12]

I would like to mae a circuit which can handle this function of testing the battery capacity,in details I would like to use PIC16F1937 and external components like resistors,capacitors,etc in such a way that I can test a battery before charging it and this helps in knowing also battery status whether its good or bad.

I was thinking on measuring its voltage but I saw that sum people here used to load the battery(by applying a load on the battery) and then measure the the voltage at different instant and also the current flowing so they determine the Q (Ah) of the battery which they base on for knowing whether the battery is good or bad.
Please help

 

[wizpic]

There are various methods of testing batteries all depends on what type. I've seen all types like measuring the internal resistance of the battery and are quite expensive and not recommended for deep cycle batteries. The common one again for lead acid battery is voltage and SG reading. The best way I've found and carry out testing our batteries I've used mosfets and Pwm to create a constant load starting the test recording the time and monitoring the battery voltage while under load but the battery must be fully charged to start with after all a flat battery while but under load will always fail. We only carry out this testing if we think the battery matpy be suspected of been faulty and its proven to be very reliable and saved many batteries over the years and there are still some that are still in service today and that's another year to 2years.
Here is an example a Trojan 6v T125 with a rated capacity of 240AH 20hr rate if it's good it should be able to supply 25amps of constant current for 488 minutes from this you can work out how good the battery is the test is terminated by either time or voltage which ever one comes first so if the battery reaches 5.75volts before 488 minutes the test is ended and this means the battery has reached ihe end of its useful life and scraped.
What type of batteries are you testing ?
What language are you using to write the code for the pic ?
Another good method is in the link I posted this gives good results where you can record the amp hours in and out of the battery like others stated there are some ready built stuff out there for this but depends on how much you want to spend

 

[jean12]

I see,you are giving helpful idea on the methodology,for language I would like to use Microc but I am not expert with this language and want to test the battery before starting charging it which means it is not fully charged anymore,could you advise with this?

Thx

 

[wizpic]

Ok you said what language you want (which I've never used), but you still not said what battery you are testing. You can't really test a flat battery to say either if it's good or bad. It's like trying to get that extra few miles out of your car when it's ran out of fuel it's got nothing in the tank so it can't go. You can measure the battery voltage for quick test so if it's 11.75 volts then it's flat also the SG reading all cells must be uniform if you have one that read zero then this will indicate the battery has a dead cell and it would not last after a charge. The link I posted is what I think your looking for to say how much battery life is left and it also works when you charge the battery it counts the amp hours in and out. With batteries the heavier the load shorter the current it will provide

 

[jean12]

I am testing a Lead Acid Battery.

Thx