Written: 5 March 2014
Posted Here: 13 Oct. 2014
Last Updated: 7 Feb. 2015
-added Q&A section & link - 7 Feb. 2015
- Parallel Charging Your LiPo Batteries
- [NEW!!] Question About Over-discharged LiPo--How do I quantify the amount of damage done to the battery pack?
- [NEW!] Recommended Soldering Kit & Tutorials (for Arduino, Electronics, & Radio Control)
- Beginner RC Airplane Setup
- Thunder AC680/AC6 Charger & Computer Data-Logging Software
- Using the Arduino Uno’s built-in 10-bit to 21-bit ADC (Analog to Digital Converter) (via eRCaGuy_NewAnalogRead library)
- Arduino micros() function with 0.5us precision - using my Timer2_Counter Library
- eRCaGuy_ButtonReader Library for Arduino - Debounce and read the current button or switch state, & most recent action ("just pressed" or "just released") easily!
- The Power of Arduino <-- be sure to check out the various really useful Arduino-related links at the bottom of this post! They have helped me a ton.
- Arduino Power, Current, and Voltage Limitations
- Quick Tip: 4 Ways to Power an Arduino
Though I first posted this article on my Instructables page, I decided to update it and repost it on my main website here in order to keep my articles more consolidated and easily maintained by myself. From this point on, I will keep the most up-to-date version of this article right here, instead of on my Instructables page, so if you want the latest info, read here. Don't forget to subscribe to receive an email whenever I post something new by clicking the subscribe link at the top-right of this page! I will never use your email addresses for spam.
|A LiPo that self-discharged, while in my plane, hanging overnight in a tree. :(|
Intro:LiPo batteries should never be discharged below 3.0V/cell, or they may be permanently damaged. Many chargers don't even allow you to charge a LiPo battery that is 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.
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 normal discharge, however (even if running in your equipment), LiPos will not spontaneously catch on fire, even if discharged all the way down to 0V/cell. It is the *recharge* phase that would normally 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. It did get hot during recharge, but I charged it at such a low current that the heat did not become excessive (too hot to comfortably touch).
You'll Need a Smart Charger
|My Thunder AC680 Smart Charger, charging a 4S NiMh pack|
Here are some links to get your started:
1) Thunder AC680 80W balance charger w/accessories - 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. I have this charger and I love it!--and I should note I have used many chargers, including high-end ones that are several hundred bucks a piece.
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) General list of chargers from HobbyKing - be sure to READ THE REVIEWS!
4) Here are some relevant search results from Amazon for "LiPo charger":
Important Instructions Just Before You Begin Charging the Over-discharged LiPo
|LiPo that caught on fire during charging (source)|
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 (such as the LiPo-safe fireproof charging bags shown below) 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).
|A couple of my LiPo-Safe Charging bags.|
LiPo-safe charging bags can be purchased in many places, but here are a few of my favorites:
- Lithium Polymer Charge Pack 25x33cm JUMBO Sack
- Lithium Polymer Charge Pack 18x22cm Sack
- Amazon search results for "LiPo charge bag":
Begin the Charge
1) LiPo is <3.0V/cell:
When <3.0V/cell, charge the LiPo at a significantly reduced rate of 1/20~1/10 C rate (1/20~1/10 [0.05~0.10] x its capacity) until it is 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. My charger could only go down to 0.1A minimum charge rate, so I set my charger to 0.1A at this point to charge this 1300mAh 3S LiPo. This equates to a 0.077C charge rate (0.1/1.3 = 0.077).
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 (see the burning LiPo picture above; you don't want that to happen).
Continuing the Charge Process
2) LiPo is 3.0~3.7V/cell
Once above 3.0V/cell, you may optionally increase the charge rate to 1/10~1/5 C rate [0.10~0.2 x its capacity] until the LiPo is ~3.7V/cell or higher. At this point, I set the charge rate for my 1300mAh 3S LiPo to 0.2A. This equates to a 0.2/1.3 = 0.154C charge rate.
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.
Finishing the Charge
3) LiPo is 3.7~4.2V/cell
Back to Regular Use
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.
|Watch me fly this plane in my YouTube video here|
Let me know how this works out for you! Be safe!
If you found this article interesting, please subscribe to email updates whenever I post new content, via the links at the top-right of this site!
Additionally, you may be interested in some of the other articles linked at the top and bottom of this article, especially this one! - Parallel Charging Your LiPo Batteries.
- Question About Over-discharged LiPo--How do I quantify the amount of damage done to the battery pack?
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
Responses to comments below:
2 Nov. 2014: links for Lawishard for a small 1S LiPo.
Lawishard needs a tiny (~2mm x 20mm x 20mm ~50mAh single-cell LiPo), so I recommend considering the following as replacements for only a few $$ + shipping:
- ZIPPY 50mAh 20C single cell (19x14x5mm)
- ZIPPY 100mAh 20C single cell (30x15x4mm)
- ZIPPY 70mAh 20C single cell (18x14x6mm)
- ZIPPY 240mAh 20C single cell (24x19x5mm)
- Whole list of 1-cell (1S) options
I have personally never needed to solder single cells yet, but my understanding is this:
- if the metal tabs have solder already on them, you can just solder to them with no problem.
- the metal tabs are aluminum, so you may need aluminum solder paste to get solder to bond to them otherwise
- if you don't feel comfortable trying to solder to bare 1S Lipo tabs directly, look for a packaged 1S LiPo with battery lead and everything already attached, and see if you can clip the wire and solder to the clipped wires instead.