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Laptop LiCo are mostly cylindrical cells in the 18650 format. 18mm is the diameter and 65mm is the height of the cylinder. As the name implies, laptop cells are found mostly in laptop computers.
Most laptop batteries consist of 6 individual cells like the Dell battery shown above. When new, those 6 cells provide 53Wh of energy and weigh less than 0.7lbs. 10 of those packs would make a 0.53Kwh pack weighing less than 7lbs. The pack in the above picture is at least 5 years old. Today's latest laptop cell (Panasonic 3.4Ah) has 50% more energy.
Laptop batteries are NOT for you: Different people need different battery.
- If you use up (deplete) or plan to use up all the battery energy in less than 60 minutes, move on. Laptop batteries are not for you. You need batteries that can sustain 4C or higher discharge rate like RC LiCo or A123 LiFePO4.
- If your runtime is 1 to 2 hours, laptop batteries MIGHT work for you. You are within the cell's operating parameter but closer to its limit. Your battery will be warm (almost hot) most of the time. It will most likely have a shorter life (300 cycles? as opposed to 500+). Common LiFePO4 with an actual minimum 1C continuous discharge rate is probably a better fit for your need.
- Great, you are still here. So you plan to have a battery runtime of more than 2 hours. In other words, your average discharge rate is 0.5C or less. Laptop cell is ideal for this operating mode because it allows one to build the lightest (weight) and the most compact (volume) battery. Laptop 18650 LiCo cell has been and still is the cell with the highest energy density for at least 5 years now, and by a respectable margin. And thanks to the small size of each individual cell, the battery pack can be built into almost any shape, making it easier to fit the irregular empty spaces on a bicycle.
Discharge rates
No, it does not mean that the discharge rate is limited to 0.5C. It can be higher, much higher but for only a short period of time. Let's say we have a 51.8V, 1,000Wh battery [made by stringing (connected in series) 14 groups of cells, 12lbs if made with new 2.4Ah cells, 8lbs if made with the latest 3.4Ah cells.]
0.5C means 500W (1,000/2) power. You can draw 500W indefinitely (actually 2 hrs) and the cells will not get hot.
You can draw 1,000W (1C) without any worry for about 10 minutes. After that, you need to back down to 500W and stay there for at least 15 mins.
How about 2C or 2,000W? Not a problem for a short period of time, say up to 60 seconds. This is usually needed for the quickest acceleration from stop. Usually this demanding power burst only lasts a few seconds (5?) to get you up to cruising speed. Once there, power drops significantly.
On the bench at home, I have repeatedly tested 4C draw many times, 10 secs at a time followed by about 2 minutes rest. I have no intention to ever use the battery that way on my ebikes but it nice to know that I can if needed.
Maximum and minimum voltages
The highest voltage recommended by manufacturers is 4.20V +/- 0.05V. Common wisdom says that 4.10V should be the max for extended life. I have no way to prove or disprove that wisdom. What I know is that about 4% to 7% of the total energy resides in that 4.10 to 4.20V range. What I also know is that my laptop computers stop charging when the cells reach 4.12V, my cell phones stop charging when the battery reaches 4.10V. Computer and cellphone engineers presumably have done their homeworks before deciding on those thresholds. I just follow their leads. 4.10V for me.
"At rest" means that the battery is doing nothing, not being discharged, not being charged. At rest laptop LiCo cells are (for all practical purposes) empty at 3.40V. But at rest voltage is almost useless in ebike application. What's meaningful is the cutoff voltage when the battery is under load. Some people fret over the exact threshold. 3.00, 2.95, 2.90, 2.85, 2.75, etc... The reality is that it does not make much difference. Under a 1C load, it takes only about 3 minutes to drop from 3.00V to 2.50V, and about 20 seconds to drop from 2.60V to 2.50V. Panasonic discharge graphs show both 2.50V and 3.00V cutoffs. My absolute cutoff point is 2.50V. The voltage bounces right back to 3.20 to 3.50V within 30 seconds after the load is removed.
Again common wisdom says that longer life is achieved by shallower depth of discharges and again I don't have the means to prove and or disprove it. You're ok if your cutoff voltage (LVC) is in the [2.50 - 3.00V] range. Closer to 2.50 if you want to extract every last bit of available energy (and to get better performance). Closer to 3.00V if you are in agreement with the common wisdom. If you still insist on a specific value recommended by an "expert", then this "expert" says 3.00V, just to be on the safe side.
Voltage sag
Let's say we have 3 different battery packs with the same nominal voltage and same capacity:
- 51.8V, 1,000Wh, 19.3Ah, Laptop LiCo
- 51.8V, 1,000Wh, 19.3Ah, RC LiCo
- 51.8V, 1,000Wh, 19.3Ah, A123 LiFePO4 (it's not possible to build a 51.8V nominal A123 battery, but let's assume one exists for simplification)
All 3 batteries are used on one same ebike (one battery at a time of course) and ridden the same way by one same rider. The controller is limited to 38.6A or a maximum of 2C discharge. Keep in mind that our average discharge rate is still and always 0.5C (500W) or lower.
Which battery performs best? Well, it's NOT the laptop LiCo battery. This is most noticeable when accelerating. The laptop LiCo battery accelerates slower than the other two batteries. Cruising speed is slightly lower too although the same current is drawn from each battery. Why?
No battery can maintain a constant voltage under load. All batteries sag to various degrees. RC LiCo and A123 have a much lower internal resistance (IR) and thus sag less. Laptop LiCo has a higher IR and sags more, much more. So when wot (wide open throttle) acceleration is called for, the maximum current is delivered, 38.6A or 2C. At 2C, laptop LiCo sags about 20% (41.4V {0.8 * 51.8}) while RC LiCo and A123 only sag about 5% (49.2V). Power is the product of voltage and current. The current is the same for all 3 batteries, but the voltage is not. So RC LiCo and A123 deliver 1,900W (49.2 * 38.6) and laptop LiCo delivers only 1,600W or 16% less. Less power results in slower acceleration. A few seconds later, cruising speed is attained and the current drops down to 0.5C. At 0.5C laptop LiCo sags about 8% and delivers 460W while RC and A123 deliver 490W at 2% sag. A smaller difference of 6% but enough to result in a slightly slower cruising speed.
Note: In the above scenario the laptop LiCo battery pack voltage was at 41.4V during the brief wot acceleration. Since the pack is a string of 14 cells (groups of cells), the average cell voltage was 2.96V (41.4/14). Remember the LVC (cutoff voltage) discussed in the last section? If it was set at the most conservative value of 3.00V, then that LVC threshold was reached. The controller would momentarily stop providing power to the motor until the cell voltage recovered (a fraction of a second at a time, but repeatedly). The end result is that acceleration is even more slower. Everything is a compromise. A conservative LVC supposedly prolongs the battery life but affects performance negatively.
Safety
Any device designed to store energy is a potential hazard. It can catch fire and/or explode. A piece of wood, a lump of coal, a tiny AAA battery, gasoline, natural gas, a lead acid starter battery, etc... On youtube you can find countless sensational videos showing all sorts of battery catching fire or exploding violently. I suppose one can hit a piece of paper with a hammer hard and long enough to ignite it. So much for sensationalism, let's get back to reality.
Laptop 18650 LiCo cells are built with an internal safety vent with thermal cutoff protection. I only have a vague idea how it works so I won't attempt to explain it. I share my actual tests with you instead. I too worry about these cells catching fire or exploding in my home. So I deliberately push them beyond their limits expecting catastrophic results. I have failed miserably so far in creating a catastrophe. Each of the following test is done at least 2 times using a good cell. The cell is permanently damaged and discarded after each test.
Over discharge: Discharge at both 2C and 0.5C until 0.00V is reached. The cell gets hot at 0.5C and really hot at 2C. No fire, no explosion.
Over discharge in series, forcing voltage reversal: 3 cells connected in series. The two outer cells are fully charged, 4.20V. The middle cell is about 20% charged, 3.65V. The middle cell gets hot as it descends to 0V and then reverses its voltage. No fire, no explosion.
Hard short: A fully charged cell shorted by clamping it in a metal vice. Really hot. No fire, no explosion
Over charge: Using a 0.5A, 5.30V charger. The cell gets hot at about 4.7V and failed permanently at around 4.9V. No fire, no explosion.
Extreme over charge: This test was done only once and was reported in another thread in Dec 2012
SamTexas said:
I just finished another overcharge test using two 2.2Ah laptop LiCo Sony cells in series. Charge rate is 8.5A, 7.7 times higher than the maximum recommended 0.5C rate. The cells are at 4.0V before charging.
Within 10 seconds, the combined voltage reaches 13.5V, hovers around that voltage for a 5 seconds and starts dropping to 13.2V. Remains at 13.2V while the cells begins to heat up fast. After another 60 seconds they lose the ability to accept charge and the current drops to 0.5A. Voltage remains at 13.2V (6.6V each). The cells are at about 60C hot (ambient is 20C). After 5 more minutes they stop accepting charge, 0A. Voltage is at 13.0V and dropping, temperature is about 70C and dropping. No fire, no explosion.
Am I saying that laptop 18650 LiCo cells are 100% safe? Not a chance, not in this life. But safe enough for me to not fret over common unintentional mistakes like over charging or over discharging or accidental shorts or unnoticed out of balance pack (voltage reversal). If you really want to know how safe or unsafe laptop 18650 LiCo is relative to other chemistries/formats, I invite you to repeat the above tests with HobbyKing RC LiCo. Do be sure to protect yourself adequately while performing those tests.
Charging
Maximum charging current is 0.5C. 1.10A for 2.20Ah cell, 1.70A for 3.40Ah cell, etc... At 0.5C charging rate the cell does get warm. I always charge at 0.3C or lower. Like other lithium chemistries a CC/CV (constant current, constant voltage) charger is recommended (although TRUE CC/CV chargers are hard to find.)
Quality cells and bad cells
Cells manufactured by reputable companies like Panasonic, Sanyo, LG, Samsung and Sony are good.
But there are quite a few no name or bad name brands out there. The most notorious one is UltraFire. Next are "red" cells that look almost identical to Sanyo cells, but they don't have the same markings, "FDMPT2" or "FB5PT2" are printed on these. These cells are mainly found in clone battery packs. They are worth nothing regardless of their price.
"Bad" cells are easily recognizable. They get warm even charging at low 0.2C rates. They get even warmer at discharge and their voltage sags 10 to 20% more than good cells. They cannot hold full 4.20V voltage for more than an hour.
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