Bringing a LiPo Back to Life
Duane Degn
Posts: 10,588
I've been using some three cell LiPo batteries to power my radio control transmitter. I accidentally left the transmitter on most of the day (I think it was 3AM to 8PM) it final squealed at me that the voltage of the battery was getting too low (around 9.2V). I know it's not good for a LiPo cells to be drained lower than 3.0V so I was concerned I had damaged the pack. Using the battery tester erco sent me (thanks erco), I found the cells where 3.3V, 3.4V and 2.6V. The 3.3V and 3.4V cells should be fine but I was worried about the 2.6V cell.
When I went to charge the pack the battery charger complained "Open Circuit" or something like that and wouldn't start charging.
I then used my bench top power supply. I set the voltage to 4.0V and set the current to 100mA. (Thanks Spiral_72 for teaching me how to do this.) I connected the power supply to cell #3 and the voltage immediately dropped to 2.6V while maintaining 100mA of current. I guess this is what is referred to as "constant current" charging.
I waited until the voltage rose to 3.0V and tried the regular charger again. This time the charger didn't complain and charged up the 5.0Ah pack. I'd imagine the pack might have a bit less capacity now but it's not bulging at all and it appears to work fine.
I thought I'd share this in case anyone else has a pack their charger wont charge. I'm pretty sure I could have used a higher current than 100mA since it was a 5,000mAh pack but I wanted to error on the side of caution. The charger charges the pack at 5 Amps so 1 Amp would have still been a cautious charge rate.
When I went to charge the pack the battery charger complained "Open Circuit" or something like that and wouldn't start charging.
I then used my bench top power supply. I set the voltage to 4.0V and set the current to 100mA. (Thanks Spiral_72 for teaching me how to do this.) I connected the power supply to cell #3 and the voltage immediately dropped to 2.6V while maintaining 100mA of current. I guess this is what is referred to as "constant current" charging.
I waited until the voltage rose to 3.0V and tried the regular charger again. This time the charger didn't complain and charged up the 5.0Ah pack. I'd imagine the pack might have a bit less capacity now but it's not bulging at all and it appears to work fine.
I thought I'd share this in case anyone else has a pack their charger wont charge. I'm pretty sure I could have used a higher current than 100mA since it was a 5,000mAh pack but I wanted to error on the side of caution. The charger charges the pack at 5 Amps so 1 Amp would have still been a cautious charge rate.
Comments
Thanks for the pointers though. Lithium stuff definitely is not so easy to just slap it in and use in your project, but man is it nice for lightweight power!
Fortunately it's not mine, I found that pic on the web.
The charger has a balaning system. It just didn't like one cell being so low.
I haven't seen any of the large high current packs that have balancing systems for discharge (i.e. while powering a RC helicopter).
The LiPo in this case were relatively inexpensive ones from HobbyKing (ya got to love that place).
If it's around the 5000 mah then you know you really didnt have damage occur or you can find out how much capacity was lost if any.
If its less the around 90% of the 5000 I would say the pack is done. The cell might dip to low in the future and the problem will occur again with more capcity lost once more.
But keep a closer eye on the pack as you doing this in case something happens.
The charge cycle was over 4900 mAh. You're right, I should measure the capacity as I discharge it. I'll do that.
I check new batteries like this to see what there actually rating is when I get them. I pick up mine from HK as well.
Some are plus alittle some are minus.
http://www.google.com/products/catalog?q=lipo+balancer&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&tbm=shop&cid=14669176353090903898&sa=X&ei=Sl5hT8unE86CtgfwzaypBQ&ved=0CEQQ8gIwAA
The most expensive packs might have per cell discharge monitoring but I've never seen one (never looked either). Generally that's a function of your fancy ESC..... which is also rare in my low budget world. I can't imagine a TX having this feature.
It sounds like you know your Lipos. In my experience you get a couple freebies for a multicell pack before it gives up. When it does, that's it, the cell never charges. If you're flying with the pack I'd cycle it 3-4 times and take note of the capacity each time before using it. If your just controlling a robot, boat or ground vehicle the info might be interesting, but I wouldn't sweat it. In my experience the cell either died, lasted only a very few cycles and swelled or lived on forever more..... which I suppose are the only three options.
Another trick depending on the charger (Triton) if you're in the field is select a two cell pack on the charger (7.4V) and charge it without the balancer. As soon as the voltage comes up to 3V * 3 cells (generally less than a minute), cancel the charge and switch to 11.1V
Lipos are funny animals. They're very durable (air combat) but very fragile at the same time. Heck, I have one bent into the shape of a banana from repeated impacts.... it still works fine.
I see the intial charge after the problem was 4977mAh. That was from a very low pack.
As Paul suggested, I discharged the pack. The discharge ended a bit early with an "Open Circuit" error. I'm not sure why the charger does that sometimes but it does.
I had set the cut off voltage to 3.0V (I usually use 3.1V but I thought 3.0 would give a more accurate measurement of its capacity). Since the discharge ended a bit early the voltages on the cells were 3.44V, 3.18V and 3.22V. 4,814mAh was discharged. I think that's pretty good.
The second charge cycle output to the pack 4,927mAh, with the pack's total voltage being 12.59V.
It appears the pack survived the over discharge.
I like Spiral_72's idea of using a two cell unbalanced charge in the field. I usually bring several packs with me so I doubt I'll try this anytime soon. Plus, I usually just fly my helicopter in my (and several neighbors') backyard(s) so I'm not in the "field" very often.
Short answer: Yes!
The longer answer: Yes, absolutely positively, for sure, LiPos are worth the extra circuitry (I don't think they really cost much more than quality NiMH).
They are really what makes electric RC helicopters possible. Quadcopters wouldn't be much fun without them.
LiPos used to be really expensive but now you can get them pretty cheap online (HobbyKing). They're probably cheaper (I'm not sure about this) than purchasing quality NiMH batteries of the same capacity (I am sure the NiMH would weight a lot more).
As you may have seen with my popsicle stick hexapod, the weight of a robot can have a big influence on its performance. LiPos let walking robot walk around longer.
I have several dozen LiPo packs of various sizes, I use a special bag when charging in case they blow up (they don't really blow up, the just catch fire with enthusiasm). I've never had one catch fire.
The multi-cell packs have very thin sheets of metal to connect the cells together. These connections act as fuses when short circuited. The metal quickly burns away disconnecting the cells from each other and the short (I've had this happen too many times (if you're cutting off a battery connector to replace it, only cut one lead at a time)). Sometimes there will be enough left of the lead that you can still use the cell.
I don't ever leave LiPo or NiMH batteries charging without myself being a few rooms away.
Besides the times when weight is imperative, I use LiPos for all sorts of other projects. Once you have a charger, the LiPos, aren't very expensive. I think the chargers cost less than they used to (I haven't priced them recently).
It's tough to beat LiPos as a power supply for robotic projects.
They are more trouble than NiMH. But both (all?) chemistries you need to be careful not to let the packs drain too far.
A LiPo charger does cost more than a simple NiMH charger. And LiPos should be balanced as they are charged.
So I suppose they can be a bit of a "headache". And maybe a little more so than other battery chemisties.
But LiPos can solve problems that would be a "migraine" when using other chemistries. (Try to make an electric helicopter with lead acid, NiCd or NiMH batteries.)
I must say, in this day and age, alkaline batteries just don't make any sense at all. If they weren't grandfathered in, if someone tried to introduce them today, they would simply fail. They are ridiculously expensive (in many cases, a small AA cell costs as much as a large D cell) and they poop out quickly in modern electronics, where they often don't work once their voltage drops to 1.3 or 1.4V. There may be plenty of useful life left in them for flashlights or motors, but many people just toss them, hopefully into the recycle bin. Moms HATE buying toys that use batteries and I don't blame them.
Duracell rates their D cell at 12-18 AMP HOURS, not calling the cell "dead" until it's down to 0.8 volts.
These discharge curves are abysmal: http://www.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1300_US_CT.pdf
Back before my transplant, I wore some monitor that ran off a couple of AA alkaline cells. Those cells were dead as could be after a day (it might have been a week) of monitoring. The device had some sort of boost circuit that drained the batteries dry.
I haven't seen consumer products that do that.
BTW, A fully charged LiPo is 4.2V. You'll want to make sure your electronics can handle the voltage of freshly charged cells.
No expense spared on medical devices. Actually I think Pololu's 3pi robot has boost circuitry too. Quite rare in this world of cost-cut consumer products.
And my cheap purple NiMH cells get up over 1.4V when fully charged. They make poor voltage regulators!