Battery charging, lost energy? - Page 1
Forum section: electronics / beginners

author: MikeK
author_rank: Super Contributor
timestamp: December 24, 2012, 07:35:54 pm
content: So, a basic question here...Does the current charging the battery times the amount of time closely approximate the amount of charge in the battery?The reason I ask is, I picked up a 2200mAh Li-Ion cell from Hobby King and had it hooked up to my charger yesterday for way more time than it should have taken and it's still not at full capacity.It's a simple charging circuit using the MAX1555. Supplied with 5V, the datasheet says it should output approximately 280mA. The cell started out at 3.7V and after 12 hours it was only at 4.0V. It's back on the charger right now for the past hour and the voltage still reads 4.0V.Do I have a bum cell? I want to do a load test on it, but first I need to fully charge it.

author: Skimask
author_rank: Super Contributor
timestamp: December 24, 2012, 08:01:35 pm
content: http://en.wikipedia.org/wiki/Peukert's_lawApplies to every type of battery out there.

author: MikeK
author_rank: Super Contributor
timestamp: December 24, 2012, 08:27:05 pm
content: That's for discharge.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 24, 2012, 08:50:55 pm
content: Quote from: MikeK on December 24, 2012, 07:35:54 pmMight be a good time to measure it?

author: rollatorwieltje
author_rank: Unknown
timestamp: December 24, 2012, 08:59:37 pm
content: For li-ion and lipo it should be roughly input = output. Almost no energy is lost during charging (in the order of 1 or 2%).If the cell is bad and the charger still supplies 280mA at 4 volts I would expect the cell to be quite hot. That energy must go somewhere. Lipo and li-ion cells shouldn't heat up significantly during charging.

author: MikeK
author_rank: Super Contributor
timestamp: December 24, 2012, 09:04:39 pm
content: Well, it finally finished charging. Never got even warm. Cell voltage is 4.09V.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: December 24, 2012, 09:14:24 pm
content: For charging you can't say that if you charge a 4Ah Battery with 4A you will need 1h to fully charge it. Because charging is different. First of all the charger will charge the LiPO with constant current. If the voltage on the battery terminals gets closer to the end of charge voltage the charger will enter constant voltage mode and the charging current will slowly decrease. Most chargers shut off, if the charging current falls below 1/10th of the original charging current and the battery is considered fully charged.Only one sidenote: Energy can't be lost, it only can be transformed.So if there was much more energy pumped in that battery, than it has stored in it's chemistry, something would have gotten really hot.F.e. look at that graph of my Battery Management System (using TI's BQ78PL114S12) while a 2800mAh (actually it only has approx. 2400mAh) capacity battery is charged with a 4A charger. One would presume that a 2400mAh battery charged with 4A will only take 36min to charge to full. Instead as you can see on the screenshot it takes over 1h.

author: Skimask
author_rank: Super Contributor
timestamp: December 24, 2012, 09:38:07 pm
content: Quote from: MikeK on December 24, 2012, 08:27:05 pmDOH! My bad...Either way, the way I understand charging is that some of the energy going in gets dissipated as heat, e.g. overcoming internal resistance, and a hundred other factors.So short of an actual joule counter type thing, I give it a 25% loss. That's always seemed to work for me anyways. (uh...very unscientific methodology involved...e.g. none...)

author: MikeK
author_rank: Super Contributor
timestamp: December 24, 2012, 10:00:43 pm
content: For charging, I was going by the datasheet, which says it should have been pumping near that 280mA almost to the end.I'll run it through a load test and see.

author: IanB
author_rank: Super Contributor
timestamp: December 25, 2012, 07:32:05 am
content: Something doesn't seem quite right either with your cell or with your charging arrangements. If you have a brand new 2200 mAh cell and you charge it at 280 mA using a proper CC/CV algorithm the voltage should get a lot closer to 4.20 V at the end, probably something like 4.17 V or more.How accurate is your voltmeter? If the display only reads to two decimals (i.e 4.09 rather than 4.093, say), then your voltage reading may be off by 0.05 V or more. Depends on the meter.

author: MikeK
author_rank: Super Contributor
timestamp: December 25, 2012, 05:54:39 pm
content: Yeah, that's what I thought too. I was disappointed that it didn't get closer to 4.2V. Meter has 0.5% accuracy. I still have to load test it and the others I got from Hobby King.

author: IanB
author_rank: Super Contributor
timestamp: December 26, 2012, 12:11:13 am
content: Quote from: MikeK on December 25, 2012, 05:54:39 pmLook at the meter specs carefully though, and check the meter against a known reference if you have not already done so.Meters are usually specified as +/- X% +/- Y counts. So if you had a 4000 count meter that was +/- 0.5% +/- 5 counts, say, then the maximum deviation on a reading of 4.2 V would be as follows:Target reading 4.20.0.5% of this: +/- 0.025 counts: +/- 0.05So your actual reading could be from 4.13 to 4.27 and still be within tolerance.It's tricky reading lithium ion cells on 4000 count meters as the range rolls over and you only get 3 digits of precision.

author: MikeK
author_rank: Super Contributor
timestamp: December 26, 2012, 03:02:14 am
content: It was my UT-61E. 22000 count, I think. I don't have a known reference, but my other Li-Ion batts read closer to 4.2V even after sitting for a while. Still have the load test to do.