Lithium ion battery charge state measurement

[powersolar]

Hi,
I’m currently working on a project which charges a 1000mAh lithium ion battery from a solar array using a maximum power point tracker and a simple charge controller (MAX1555). This allows a system load to be attached to the battery whilst charging takes place. I’m trying to find a way to measure the remaining charge level of the battery whist the battery is connected to both the charger and the system load. I’ve looked into various approaches such as coulomb counting but cannot seem to find a suitable solution. I’m primarily looking for a monolithic IC to perform this function communicating back to a central power management microcontroller using I2C for example however I’m open to suggestions.
Cheers...

 

[Ubergeek63]

Hi,
I’m currently working on a project which charges a 1000mAh lithium ion battery from a solar array using a maximum power point tracker and a simple charge controller (MAX1555). This allows a system load to be attached to the battery whilst charging takes place. I’m trying to find a way to measure the remaining charge level of the battery whist the battery is connected to both the charger and the system load. I’ve looked into various approaches such as coulomb counting but cannot seem to find a suitable solution. I’m primarily looking for a monolithic IC to perform this function communicating back to a central power management microcontroller using I2C for example however I’m open to suggestions.
Cheers...

Battery Management - Battery Fuel Gauges - BQ20Z95 - TI.com

 

[Oznog]

Well, that's a pickle. Li-ion charge is something like 70% when it first hits 4.2v. The finishing algorithm is to regulate at 4.2v and wait until the current drops below a threshold.

Solar can't do this. For one, it's typically closer to a trickle-charge and the finishing algorithm requires more current than this. There is no established algorithm for trickle-charging. If you leave a regulated 4.2v on it indefinitely, even though the current drops off at full charge, it may continue to slowly overcharge which is dangerous.
Second, it's inconsistent. The shut-off algorithm does not address back on the current available going up and down.
Coulombetry (amp-hr counting) may help, but the answer's not necessarily accurate. We don't even know how much capacity a cell really has. Nameplate's often inaccurate and capacity changes with age.

Honestly? Bigger batt, if the cell HAS been lower than 4.2v and hits 4.2v, either cut it off or limit how much longer it's allowed to charge, and error on the side of caution. Then cut off the charging until a lower voltage is exhibited for a significant time (not just a turn-on surge that dropped the voltage). So if we see it go to 4.1v for awhile, sure, we know it's below 70%. Let the charging current bring it up to 4.2v then "give it a little more" like 10% of the nameplate capacity, regulated to 4.2v, before shutting off until the voltage drops down again. Or, if you can see that the batt voltage is 4.2v, and the batt is not drawing significant current EVEN THOUGH the panel has more current available, then you know the charge must be terminated.

Note that a switching power supply like an MPPT may create voltage and current ripple which can complicate the reading. There's a huge difference in a li-ion batt at 4.1v, 4.2v, and 4.3v.

 

[roltex_rohit123]

hey friend i think one thing could solve your problem... its the dead laptop battery. the battery contains 6 cells charged by a circuit. the circuit is obviously good. guess what you could charge 6 cells of this sort from one charger circuit. I am not good at electronics but my idea can help you. carefully open a dead laptop battery from edges. dont bang or break it. li ion cells are real bombs. heat them and shot them can kill a person per one cell. so you just get the circuit from a dead laptop battery. your problem would be solved. the circuit has everything in it. try it out. no efforts!! just open the battery frm edges without breaking the circuit board. if you would live by house i could give you one i hav

 

[Vizier87]

Although the post looked a bit silly with all the typo and textspeak, I think it sounds like a good idea.

 

[skjafar]

Same subject different situation

hi guys,

so i am asked to make this Lithium-Ion battery charger, i guess the charger circuit is not that of a problem, as there are pre-manufactured charger ICs, but my problem is in detecting the charge level of the battery at all times, i am intending to display it on an LCD so i want real time tracking for the charge level of the battery, as it was explained in previous replies it seems like detecting the voltage on it's own is not enough, and i need to disconnect the charger and let the battery settle, which would take time as well as making the displayed level go up and down, which is a very bad user experience in my opinion, any suggestions?

thanks in advance.

 

[Mike odom]

Li-ion batteries are dangerous, but they do hold the max charge/weight of any type of battery. Though the "take the circuit board from a used laptop battery" initially sounds like a good idea, BE CAREFUL. There are actually two types of Li-ion batteries, 4.1V types and 4.2V types. Linear Tech makes a line of Li-ion chargers. Basically, you charge at a constant (max) current until the voltage reaches the max (either 4.1 or 4.2), then supply constant voltage until the charge trickles down to 20% of max current. At this point the battery is considered fully charged. Also note, a Li-ion battery is considered fully discharged at 2.5V... if you fully drain one below this level, charging it again could cause it to EXPLODE (our first R&D fire)... the battery manufacturers test them in a bunker type arrangement. We applied for and received a patent for the way we monitored our 4 stack battery charger, and shut the battery down if any cell went below 2.5V, not just the stack going below 10V... so, if the dead Laptop battery has six cells in it, don't use the board to try and charge 2, 4, or even 6 if they aren't the same type and flavor as what the original batteries were.

Now, for battery capacity... very difficult problem. Voltage in no way reflects remaining capacity. It changes with: temperature, age, load, battery type, battery size. The only way to know how much charge remains is by accumulating what went in, and subtracting what came out... hence the line of fuel gauge chips. There just is no easier way, especially if you're using it while you're charging it.