It Seems the More I Learn about LiPos...

[rebelyellNC]

...the more questions I have. So here's the deal. I have a 4s 8000 mAh 10C battery with only 10 charge cycles on it. I flew the F550 yesterday and after about 17 minutes, I decided to land early, due to the aggravating mosquito population (yep, already out). When I landed, I still had 14.0v according to telemetry, and when doing a balance charge with my Imax charger, it ended early before it was full with a CAPA warning. According to the readout on the screen, I had put 8000 milliamps back into the battery and it would have still been charging if I didn't have the capacity override warning set at 8000.

So naturally, I'm puzzled as to how you can put more milliamps back into a battery than what it's total capacity is anyway. Can anyone explain this phenomenon to an old dude who just wants to take care of his battery and fly?

BTW, this is the second time in a row this has happened exactly the same way. As always, thanks in advance...

 

[ryantrax]

Are the cells balanced? what are they each reading? I assume you are plugging in the balance lead too and using the Imax's LiPo Balance setting?

 

[rebelyellNC]

Are the cells balanced?

Reasonably. Time before last when this happened, I read the individual cells with a buzzer. I can't remember exactly what they read now, but from the lowest to the highest, they were within three hundredths of a volt, except for cell four, which was six hundredths lower than the highest cell. When I first got the battery, about a month ago, I noticed after the first full charge, that cell four was a little lower than the others, and wrote Hobby King's support and asked if I needed to return it. They said to go ahead and return, and the techs would examine it, and IF it were defective, they would send out a new one. Well, it took forever to get this on a slow boat from China, so there's three to four weeks to return ship, maybe a week or more to test and determine if it's faulty, and regardless of whether it is or not, another three to four weeks to get one back, and it might be the same one. I told them "no thanks, I'll just keep it". I didn't want over two months down time and possibly have them find nothing wrong.

I assume you are plugging in the balance lead too and using the Imax's LiPo Balance setting?

Yep and yep. Always use the balance lead and the balance charge setting.

Thanks for your response...

 

[alokbhargava]

There are practically 4 sections : charger, balancer, protection and battery. If nothing is limited, battery can absorb more than it's rated energy capacity but will develop over voltage and can get damaged. It's the protection circuit that senses voltage levels. Balancer tries to equalize the voltages on each cell. Charger just pushes the energy into battery through balancer and protective circuits.

If protective circuits are not functioning correctly, charger can push more charge in to the battery.

If balancer is not working correctly, all cells will not be charged at equal level, a risk when one cell reaches the capacity.

 

[rebelyellNC]

Hey Alok, thanks for the detailed info. I never knew a battery could absorb more than its rated capacity. With that in mind, I think I should keep the capacity warning cut-off as is, which is set at 8000 mA. Would you agree? Also, in reference to balancing, is it practical, and more importantly safe for battery life, if the individual cells are within a few hundredths, as mentioned in my original post?

Again, thanks for the details...

 

[dwallersv]

A lipo always consumes more energy charging than it stores. This is because there is internal resistance, resulting in some I2R losses as heat. So charging strictly to 8000mAh of input energy will leave a batter charged somewhat less. Heat loss efficiency is a function to some degree of how fast you charge the battery.

Measuring input energy as a means to protect against overcharging is a good, but conservative approach. Much better and just as safe is following the classic li-ion charge profile (CC first up to a threshold voltage, then 4.2 CV after that as current falls off; when current falls below a "done" threshold, its fully charged).

Second, a better test of the overall "health" of the cell balance is to check them when partially discharged (say, 50%) in use, from a full charge. Fully charged, a balancer is going to do its job and make sure every cell gets fully charged, so there will be little difference in cell voltages unless there is a truly bad cell.

What the balancer does that's bad is it masks any significant differences in internal impedance between the cells. This can cause a cell to discharge faster than another, and if the difference is big enough, the balancer will be doing a lot of work when you're charging. Part of how a balancer works involves dissipating excess charge through resistive loads -- IOW, heat. That's more energy going in from your charger that's simply heating the air around the balancer, rather than charging the battery.

The battery gets warm, and the balancer gets warm. That's energy manifesting as heat, rather than stored charge, that your charger thinks it's delivered. There you go

 

[rebelyellNC]

More good info, thanks a lot. So should I increase the capacity cut-off beyond 8000 mA?

 

[Helihover]

More good info, thanks a lot. So should I increase the capacity cut-off beyond 8000 mA?

NO!

You have an issue somewhere.

If your battery is rated at 8000 mah (that's the size of the "gas tank") and your putting anything significantly more than that back in, there is an issue. You either have a bad charger or an underrated battery.

Here's what you need to do borrow or buy another battery or charger. You need to find out how many mah you burn per minute and this is real easy to do. Now you have a base line to go off of. The general rule is never go past 80% of your capacity which is the "C". C = capacity. So, you should never be putting back more than 6,400 mah, if your using your battery correctly. The HK batteries I've used over the past ten years have been overall great. I think you could have an issue with your charger which could get very dangerous very fast.

Remember, voltage on lipos is pretty much irrelevant. You want to be more concerned with the miliamps. What your burning and what your putting back in.

 

[rebelyellNC]

Hey Helihover, thanks for the info.

If your battery is rated at 8000 mah (that's the size of the "gas tank") and your putting anything significantly more than that back in, there is an issue.

Well that's not good news. My HK batteries have always been reliable as well, so I would lean toward the charger. But I don't see how I can make a warranty claim if I can't prove what's wrong.

Here's what you need to do, (besides changing your advitar ), borrow or buy another battery or charger.

I don't see any of those three things happening. I had to save up to get the battery, can't just go out and drop a chunk of change on a charger.

You need to find out how many mah you burn per minute and this is real easy to do.

You say it's real easy to do, but don't mention how to do it. I'm guessing I start out with the capacity rating of the battery, 8000 mA. Then fly for a minute and charge to see how much goes back in, is that right? So if I'm using "X" amount of milliamps per minute, and I don't want to exhaust more than 80% of capacity, that would tell me how many minutes I can fly, is that right?

voltage on lipos is pretty much irrelevant.

If voltage is irrelevant, why is it used in all the telemetry devices? It's really all I had to go on up to this point. Sorry for all the questions, but like I said, I'm still learning...

 

[Helihover]

Yes you are correct on finding how many milliamperes you burn. Just fly it longer, like 5 mins. And do that 3-5 times and you'll have a good base to go off of for the life of the craft.

Everyone likes to see voltage. It's power right? Wrong in a way, but kind of true...... More miliamps will have less voltage drop with the same load, so go figure. It's kind of the "C" rating marketing scam that happened a few years back...... it's what the customer wants so that's what he gets. It really means nothing as far as usage goes. It's nice to check voltage however if you are checking to see if a battery is charged.

If you can't borrow a battery or charger, take your depleted battery to a LHS and have them charge it and see how much they put back in.

 

[BigAl07]

Come on now... let's not let such a valuable topic get derailed especially on such a sensitive topic as this one was headed.

If anyone has any problem with the changes I've made please contact me via PM to discuss it.