LiPo batteries should never be discharged below 3.0V/cell, or it may permanently damage them. Many chargers don't even allow you to charge a LiPo battery below 2.5V/cell. So, if you accidentally run your plane/car too long, you don't have your low voltage cutoff set properly in the ESC (Electronic Speed Controller), or you leave the power switch on, forget to unplug the LiPo, get your plane stuck overnight in a tree (the same tree, three separate times, for foolishly flying in areas too small because you are too excited to fly and it's almost dark), etc. etc., you may find yourself in a situation where you've discharged your LiPo down well below 3.0V/cell. What do you do?
Many people toss the LiPos in the trash. I don't. I restore them. Here's how.
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Step 1: Background, Cautions & Warnings
Before you begin this, you need to know that LiPos are traditionally considered to be somewhat "volatile" and "dangerous." This is because abused LiPo batteries are known to sometimes catch fire, and some have burned down houses or cars, and a fair number of Radio Controlled airplanes have caught on fire during crashes, due to damaged LiPos.
During a reasonable or slow discharge, however, LiPos will not catch on fire, even if discharged all the way down to 0V/cell. It is the *recharge* phase that would cause a fully discharged LiPo to catch on fire, not the discharge phase. The reason is that when a LiPo is brought below ~3.7V/cell, its internal resistance to taking on a charge begins to increase, some of which is permanent. Below ~3.0V/cell the damage becomes significant enough to care about. Below ~2.5V/cell, most manufacturers of LiPo chargers have said that the battery is too dangerous to be recharged. This is because the battery's internal resistance to charging has increased enough at this point that a standard recharge rate would be much too great for a LiPo at this low of a voltage level, since a standard 1C (1 x the battery's capacity) charge current could cause potentially unsafe heat build-up within the battery. Below ~2.0V/cell the LiPo's rate of permanent internal damage has accelerated, below ~1.5V/cell the rate of damage (again, permanent increase in internal resistance) has increased more still, and it only gets worse and worse. The rate at which this damage increases is not linear. It is perhaps a power function of, or exponentially related to the battery's voltage. In either event, it's bad, and special care must be taken.
I will now say that I have successfully restored dozens of batteries. Some of the worst ones which I have continued to use were as low as ~1.0V/cell. I have successfully recharged, however, batteries as low as a few mV/cell--perhaps 10mV/cell, or 0.010V/cell. These batteries were useless, however, and rapidly self-discharged back to ~0V/cell after removing them from the charger.
Before I go on, let me define what I mean when I say that I have "restored" these LiPos. I do NOT mean I have fixed them, or reversed their damage. I do NOT mean I have brought them back to good-as-new. Rather, I mean I have simply recharged them to a safe, usable level where they can continue to be used. That is all.
A word of caution:
What I describe below is how I've restored the batteries. Use caution. If your battery is at 0.5V/cell, its internal resistance is far higher than if it has only fallen to 1.0V/cell, and both of these cases have internal resistances far higher still than a LiPo at 1.5V/cell. Again, it seems to me that the relationship is *not* linear. And remember: high internal resistance is what causes heat buildup (and potentially fires if you are not careful), during recharge. So, if you attempt to "restore" your over-discharged LiPos, YOU take full responsibility of what happens next.
Having said that, I've never had a problem. The only battery that really concerned me was the one at ~0V/cell, so I really watched it carefully, and I charged it *especially* slowly.
Step 2: You'll Need a Smart Charger
I'm not going to go into the details of LiPo balancing chargers, but you'll definitely need a nice charger that can balance multi-cell packs and which has the ability to control the charge current.
Here are some links to get your started:
Note: if shipping speed and customer service are a high priority, just jump straight down to #4 in the list below to look at the Amazon prime LiPo charger options in the search results.
1) http://electricrcaircraftguy.com/2013/02/thunder-ac680-computer-data-logging.html - I highly recommend this charger; it works great and has an outstanding value. Comparable chargers to this at many other retailers cost at least 2x more.
2) Turnigy Accucel-6 50W 6A Balancer/Charger w/ accessories - also an outstanding, and dirt-cheap, yet highly functional smart charger. Excellent value; however, it requires an external power supply, such as this: Hobbyking 105W 15V/7A Switching DC Power Supply.
3) http://www.hobbyking.com/hobbyking/store/__216__408__Chargers_Accessories-Battery_Chargers.html - general list of chargers; be sure to READ THE REVIEWS!
4) And last but not least, don't forget Amazon! Here's the results for an Amazon search for "LiPo Charger". Check this list out for sure, as you get Amazon's excellent shipping speed and customer service too!
Step 3: Important Instructions Just Before You Begin Charging the Over-discharged LiPo
WARNING: during the initial restoration phase, while the LiPos are <3.0V/cell, do NOT leave them unattended. Constantly monitor them by touching them to ensure they do not get hot, and by sight/touch to ensure they do not puff up (puffing is an indication of released gases due to internal heat build-up). Once >3.0V/cell, you may place them in a fireproof charge container and continue the charge process as described in the following steps. If <3.0V/cell, I prefer to constantly feel the battery with my hand to monitor heat build-up, and I always keep a LiPo-safe charge bag nearby in case I need to throw the LiPo in the bag and run outside to let the LiPo burn in a safe area (again, never happened yet, but I don't want something bad to happen the first time there is a problem).
LiPo-safe charging bags can be purchased in many places, but Amazon always has a good selection and super fast shipping, so take a look at Amazon's search results for "LiPo charge bag" here.
Step 4: Begin the Charge (LiPo Is <3.0V/cell)
When <3.0V/cell, charge the LiPos at a significantly reduced rate of 1/20~1/10 C rate (1/20~1/10 x their capacity) until they are above 3.0V/cell.
Example: for the LiPo battery shown at the top of this instructable, a 1/20 C charge rate would be 1/20 x 1.3Ah = 0.065A. This is because the battery's capacity, as stated on the label, is 1300mAh (read as "mili-amp-hours"), or 1.3Ah (read as "amp-hours"). So, a 1/20 C charge rate is 1/20 of 1.3, or 0.065A. A 1/10 C charge rate is 1/10 x 1.3 = 0.13A. Note that although some smart chargers can charge at currents as low as 0.05A, many cannot charge at a rate lower than 0.1A. If you cannot set your charger to charge at a current as low as you'd like, simply choose its lowest setting possible, and carefully monitor the battery during the charge.
Additional Charge Setting Notes: recharging a LiPo below 3.0V/cell may require using a NiMh or NiCad charger setting on the LiPo batteries, as most smart chargers have safety features which prevent a user from attempting to charge a LiPo which is below 2.5V/cell, as this can be dangerous if a standard charge rate is used. Since all we are after is setting a low (and safe) constant charge current to get the LiPo back up to a safe charge level, using a NiMH/NiCad setting is fine until we get the battery >3.0V/cell. WHEN USING AN NIMH or NiCad SETTING TO GET THE LIPOS ABOVE 3.0V/CELL, ***NEVER*** LEAVE THEM UNATTENDED. You should not leave them unattended because the NiMh/NiCad end-of-charge detection method is not compatible with Lithium based batteries, and if left on the charger until full, the end-of-charge state will never be detected and the LiPo battery will be overcharged until it (likely) catches fire and destroys itself.
Step 5: Next Charging Steps
Once above 3.0V/cell, you may optionally increase the charge rate to 1/10~1/5 C rate until the LiPos are ~3.7V/cell or higher.
You may stop holding the battery/constantly feeling it at this time, and place the LiPo in a fireproof container or LiPo-safe charge bag at this point, if desired.
Once above approximately 3.7V/cell, you may optionally increase the charge rate again to 1/2 C rate until they are full (4.20V/cell).
Step 6: Back to Regular Use
Now, use the batteries as normal. The lower the battery was discharged, the more permanent damage it will have. If you use the battery (ex: to fly an RC airplane), and it works ok, then you can safely assume that subsequent charges at 1C are again acceptable. Watch it over the next few cycles, however, and ensure the battery does not puff during discharging or charging. This would be an indication that the internal resistance of the battery is still too high for normal use and standard 1C charge rates.
In any event, due to having over-discharged the LiPos, you may notice a permanent decrease in their capacity (mAh) or maximum discharge rate (ie: they will likely have a reduced discharge C-rating, as noted by lower power output & reduced performance), as the battery’s internal resistance will have been increased, and some permanent damage will exist. Additionally, the longevity of the over-discharged LiPo (ie: how many cycles you can get out of it) will have been reduced.
Let me know how this works out for you! Be safe!
Be sure to read my other articles here, especially this one:
Parallel Charging Your LiPo Batteries
I also highly recommend this one, called "The Power of Arduino."
Other Articles I've Written That You May Be Interested in Reading:
1) Parallel Charging Your LiPo Batteries
2) The Power of Arduino
3) Beginner RC Airplane Setup
4) Propeller Static & Dynamic Thrust Calculation
5) Getting into Scratch Building - 20+ Planes with ONE Motor & ONE Power Pod!
6) Thunder AC680/AC6 Charger & Computer Data-Logging Software