Charging Lipos: Voltage too low
John Board
Posts: 371
G'day,
I have just depleted for the first time a battery I got from HobbyKing. I just plugged it into my charger (PowerTech MB-3632), and I selected the right settings (1.8amps, Lipo Charge, 4C (14.4v)), and I set it to charging. The only thing about it is, I got a "voltage too low" error. The problem is, that when I use my multimeter to measure the voltage it says 7.x volts, whereas, the charger is expecting 14.x.
How did the voltage get so low? I had left the robot to go have a nap, after getting frustrated with it, and left it on for about 2 hours. When I came back, battery depleted, 7v. The question is, how do I rectify this problem.
One idea I had, which I'm not sure if it's a good idea or not, is setting the battery to the 2 or 3 option, which will hopefully boost it's voltage to 7.4/11.1 volts, then from there do another charge to bring it up to 14.4. However, 2 issues I have with this is,
1) Will this cause internal bad stuff to happen in the battery, aka is it a good idea (will the battery burst into flames).
2) Will this solve the problem? After reading through the manual of this charger, it seems that it can detect the voltage of each cell, so if one cell has a lower voltage than the rest of them, then it'll sound the alarm and stop charging.
As a side note, I'm currently stuck out in the bush, 2 hours from the nearest electronics store. So any solutions you might know of will have to be done with the equipment I have on me.
Thanks for your help!
I have just depleted for the first time a battery I got from HobbyKing. I just plugged it into my charger (PowerTech MB-3632), and I selected the right settings (1.8amps, Lipo Charge, 4C (14.4v)), and I set it to charging. The only thing about it is, I got a "voltage too low" error. The problem is, that when I use my multimeter to measure the voltage it says 7.x volts, whereas, the charger is expecting 14.x.
How did the voltage get so low? I had left the robot to go have a nap, after getting frustrated with it, and left it on for about 2 hours. When I came back, battery depleted, 7v. The question is, how do I rectify this problem.
One idea I had, which I'm not sure if it's a good idea or not, is setting the battery to the 2 or 3 option, which will hopefully boost it's voltage to 7.4/11.1 volts, then from there do another charge to bring it up to 14.4. However, 2 issues I have with this is,
1) Will this cause internal bad stuff to happen in the battery, aka is it a good idea (will the battery burst into flames).
2) Will this solve the problem? After reading through the manual of this charger, it seems that it can detect the voltage of each cell, so if one cell has a lower voltage than the rest of them, then it'll sound the alarm and stop charging.
As a side note, I'm currently stuck out in the bush, 2 hours from the nearest electronics store. So any solutions you might know of will have to be done with the equipment I have on me.
Thanks for your help!
Comments
They say that it can be recovered, although can cause battery unstability, and shortened life span, or I could have killed it. Just depends on age and severity, etc...
http://www.rcgroups.com/forums/showthread.php?t=1072324
I don't think this is a good idea. You'll want to find the cell that's too low and charge it independently.
Do you have an adjustable power supply? If so you can use to charge a single cell. You may have several cells with voltages too low for your charger.
Here's a thread I started about this sort of problem.
Make sure and get some battery monitors. They plug into the balancing port of the battery and display the cell's voltage. They also make alarms to warn you when a cell is getting too low.
Erco posted a link to an alarm/volt meter combo. I purchased a bunch of these myself and use them all the time. Don't leave these in the battery all the time. They will slowly drain a battery.
But unfortunatly I don't have a lab power supply! It's on the to-get-when-I-get-money list...
Just did some more reading, different sources say that it's not a bright idea to try to resurect the battery once any cell is below 3.0v, or try to mix and match the other cells...I checked all the cells, they seem to be all above 3v... But when I squeeze (carefully) the battery it makes a kind of funny "squlchy" noise, kind of like if you had mud in your hand, and you squeezed your fist tight... I *think* it might have swolen slightly... Or it could be just me being paranoid...
I've put it in a fireproof bag, going to leave it under observation overnight, and decide on a course of action (if it hasn't exploded in the night), tomorrow...
Eagerly awating your edit :P
I don't think you need to get rid of the battery. I often use LiPos that are a little swollen.
I generally only use these questionable batteries in ground robots and devices (not RC aircraft) but if you have an alarm on the battery, it would probably be okay even in an aircraft.
I'm pretty sure the most dangerous time for a LiPo is during the charge cycle. It's a really good idea to charge a battery in a location where a fire won't cause damage. Apparently LiPos let out a lot really nasty smoke when they burn so the charge area should be both fire proof and ventilated to the outside air. You don't want that smoke inside a house or apartment. I think the garage is a good location for a charge station.
Don't charge LiPos while you're not nearby to monitor them. I don't charge mine overnight since I don't want to be asleep if there's a problem with the charge.
As long as you're very safe about charging the questionable pack, I don't think you have much to worry about. (I hope someone corrects me if I'm wrong about this.)
The problem is simply that once you go below a certain voltage, the chemical structure of the cell changes and it will no longer charge safely AND may not charge at all - there is a short in the cell. I have already had one experience with two brand new fully charged 18650 cells completely useless after their first use because I let a child run the robot until the cells were too low.
The 'smart charger' of today is another problem. It may have a threshold for low voltage that is conservative. It does NOT diagnose the ability of the battery to charge. It just refused to charge beyond a certain low voltage to avoid potential explosions.
If you want to fool around with bulging lithium cells and cells failed due excess depletion, be very careful as a fire or explosive burst are possible. The best thing to do is to either have lithium cells that prevent themselves from reaching this condition (more expense) or switch to a chemistry that isn't so exotic to manage.
Voltmeters alone are not enough to measure the amount of charge with lithium as they have extremely flat charge and discharge curves. Also, big sudden loads, like a stalled motor can easily pull the cell below the damage level before electronic protection can shut down the device.
If you want to measure how much charge, you need to have a rather sophisticated microcontroller measuring the rate of discharge and logging it. So far, only laptops and automobiles are doing this.
If you want to run motors that are demanding, maybe a different chemistry is better. Most of my lithium cells sit around unused as I have not found the right projects for them.
There are many applications where switching chemistry isn't practical. This is particular true with radio controlled aircraft.
Voltmeters won't tell you how much charge you have left but they are a huge help in determining when one should shut off a motor (land a quadcopter). I generally try to land my quadcopter before the battery alarm sounds but it sure is nice to have the alarm to warn me if I've mis calculated the remaining flight time.
It's my intention to add quadcopters to this list by adding current sensing to my quadcopters. Even if one can't measure the discharge rate of a battery back while in use, many LiPo chargers include a discharge mode which allows one to accurately measure the capacity of a battery pack.
I contend (hopefully in a non-contentious way) there isn't a better chemistry for running many demanding motors. Most of my robotic projects use LiPo cells as the power supply. LiPo batteries have become very affordable (I generally purchase my cells from HobbyKing). I've replaced my NiMH cells in my transmitters with LiPos since they have more capacity and I don't have to charge them as frequently. By adding an alarm to a LiPo pack you can greatly reduce the danger of draining the cells too low.
I think I'd classify the motor in this video as "demanding". (Jump ahead to 0:15 for flight.)
It wouldn't be practical to use any other chemistry in the above application.
Some Li-Ion cells have some sort of automatic fuse which disables a battery if it's discharged or charged too quickly. I know some of the cells I purchased from Parallax wouldn't accept a charge until this fuse was reset. I'm not sure what I did to reset the fuse (I think I discharged it a bit) but once the fuse was reset it behaved well. I had this problems with several of my new Li-Ion cells. You may want to make sure you're bad Li-Ion cells just don't need to be reset.
Lithium cells are very handy for electric R/C flight.. I have no doubt about that. It certainly helps that newer ESC include a low voltage cutoff system to protect the cells with early warning to allow one to land.
Problems arise when you attempt one of two things.
A. Build your own robot with an h-bridge and have no low voltage circutry included (That is what happened to me with the 18650 cells).
B. You use larger multiple cell lithium batteries in series at a high rate of discharge. It may just be one cell in series that depletes first.. it is not a balanced depletion and the results are that cell shorts and is no longer useful.
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One has to get a good system and stay with it to protect your lithium cells.
Years ago, I purchased an r/c electric glider package with everything ready to fly. I purchased on replacement battery from another outlet and used it one tme. When I put it on the charger that has always worked with the original battery, it swelled up like a pregnant guppy. So the $40 USD spare was a complete loss after one use. Not sure why, these one shot experiences happen all too often with lithium cells. Especially if you are buying from new or unknown sources. The same problems have been seen in electric bicycles and eletric motor scooters.
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Yes, an alarm is a very good thing. All lithium (and also the NIMH) have a low voltage cutoff to prevent damage.
The cut off is a must know item. Don't buy a battery without getting that info. The problem is there are a lot of different Lithium chemistries that have different low voltage cutoffs. 3.0 volt may be right for one cell and too low for another.
If you are stuck without being sure what your Lithium's low voltage cutoff is, you might be able to work it out from what its peak normal voltage is. 3.7v, 3.6v, 3.3v, or 3.2v. But nothing beats a responsible manufacturer that provides good information and values loyal customers.
My helicopter flew like that too, just not so long, and only once. Never did find all the peices.