Saturday, September 20, 2008 5:16 AM
A little bit about batteries
I tired to write about programming, code and similar nerd stuff. So today I’ll write about … batteries. How is it? :)
Today, batteries are in use all over our life. We have a battery in our mobile phone, computer, camera, mp3 player, even microwave, alarm and hand clock. So, the main problem with batteries, scientists all over the world work about is how to extend the working and life time. Let’s try to understand how to know whether the battery is good for us.
How to measure batteries
The number, can tell us whether the battery will work longer for us is charge (or actual). We measure charge in Ampere/hours (Ah). One ampere-hour is equals to 3600 coulombs (ampere-seconds) and represents an amount of charge, transferred by a steady current of one ampere for one hour. As higher this number is, your battery will work longer for the same consumer.
What does “steady current” means?
Steady current is the power in watts, associated with an amount of electricity, named voltage (V). So, constant voltage, associated with one Ah produces the power of the battery in Watts per hour (Wh). So, why we cannot measure batteries in Wh? The problem is, that the power vary during charge and discharge process. So, the exact energy is the integral over time of the instantaneous voltage time and the current. Calculation of those three parameters is simple:
W = V * A
Today’s battery contains large number of elements (cells) with different fillings. As higher the number of elements, this your battery will work longer. Total work time may vary because of different parameters: charging methods, temperature, the way we’re charging it, number of charge-discharge cycles, etc.
Types of batteries
There are various batteries, however the most famous are following:
Li-ION (lithium-ion): Number of charge-discharge cycles is between 500 and 700. The number of it depends on the depth of discharge. As more the battery discharged, this less number of cycles the battery will provide. It necessary to make a number of cycles for 14-16 hours until the battery will provide its nominal capacity. Each cycle the battery’s current will be increased until the nominal.
Pros: Good energy to weight ration – the battery rather small in compare to the current because of their high energy density.
No memory effect (no loss of maximum energy capacity on repeatedly recharge after partial discharge)
Slow loss of charge when not in use
Cons: Those batteries might explode under certain conditions.
Energy loss starts directly after first charge, thus don’t buy this battery if you need spare battery and will not use it directly after purchase.
NiCd (Nickel-Cadmium): Number or charge-discharge cycles is between 1000-1500. This number might increase if you’re using the battery properly. However, you need to “train” this battery to assure maximum performance.
Pros: Tolerate to deep discharge for long period.
High energy density
Low self-discharge rate – about 20%/month
Cons: Cadmium is toxic material
Memory effect – wrong usage pattern may cause to “false bottom” effect. The battery will stop charging, before the total capacity gathered.
Negative temperature coefficient – As the cell temperature rose, the internal resistance fell.
NiMH (Nickel-metal hydride): Number of charge-discharge cycles is under 1000 and depends on depth of discharging. Those batteries are very similar to NiCd, however those batteries can have two or three times the capacity of an equivalent size NiCd, but discharge rate is also higher.
Pros: Less toxic, price effective and have higher capacity then NiCd
Cons: High self discharge rate
High application discharge rate
Voltage drop near as it nears full discharge
Li-Pol (Lithium-polymer): Number of charge-discharge cycles is very low 100-150 and depends on depth of discharging. Newer Li-Pol batteries has higher cycle durability, however they are still expensive. This is successor of Li-ION batteries.
Pros: Energy density is over 20% higher, then that of Li-ION.
High charge rate, about 1-3 minutes for cell
Greater life cycle degradation rate in comparison to Li-ION
Very efficient current per size ration
Cons: High cost
Low charge-discharge rate
Today, most of batteries are Li-ION, in spite of the fact, that it has high life cycle degradation rate. This is about two years by now for general user. Also, those batteries degrades, even when not in use inside devices. You cannot leave uncharged battery unattended, because of the fact, that recharge may become impossible if the current drops under certain level. Also, those batteries are sensitive to temperature changes. On very low or high temperature the current degrades.
Ni-Cad batteries provides the most optimal life cycle degradation rate, however it very sensitive to the way, you’re using it. The ideal pattern for such batteries is “full charge - full discharge – full charge”, else you’ll suffer from the “memory effect”, I spoke earlier.
As you already understand, there is a wide range of battery types, so chargers are also different for those types of batteries. So, how to know if the charger we have is good for me and what to choose.
The best charger for your battery is the one, you got with the device. It tuned for the battery you have. But can we use 3rd party chargers? The answer is: yes, we can, however it’s very important to understand, that if you have Li-ION battery and slow charger, you might be unable to charge it, even if you’ll put it in forever.
Slow chargers works with current equals to about 1/10 of nominal battery current, thus it will take about 10-12 hours to full recharge cycle. Quick chargers uses 1/2-1 of nominal battery current, so recharge cycle can take between 1 to 3 hours.
In both cases, do not leave NiCd and NiMH batteries in charger for a long time after the end of charging process. Even after the end of charge, those batteries keep charging, thus the quality will degrade. The story is different for Li-ION and Li-Pol batteries, those types of batteries are indifferent for overcharging. They usually have controllers to stop charging process after full recharge.
Car chargers are not very healthy devices to charge batteries. Each time you’re turn your car on, it initiate new charge cycle, thus the quality of battery will degrade.
How to prolong battery life time?
Let’s assume, that most of devices have Li-ION battery. Once, you got a new device do not start using it with minimal capacity, also do not want to full discharge. Recharge it number of times until the capacity will be equal to almost equal to the nominal power.
Also, switch your device to turn into idle mode after reasonable amount of time. It’s better, if it possible to switch or hibernate the device, rather then turn it into idle. Turn off all unused modules (such as GPS, Wi-Fi, Blootooth for mobile phones). Large number of concurrently running processes are also degrade the power quickly, so you can use Vista Battery Saver to decrease this number in Windows Vista. In PDAs, almost all plugins for battery level and processes performance measurement usually only use the battery, rather then provide usable information. If you can, turn GPRS in your mobile phones and use only GSM, this might save about 30% of energy without QoS degradation. Also, in places without coverage mobile phones increase the level of signal, so decrease the time, you can use the device.
If during the charging process, the temperature of battery exceeds 60C (140F), stop charging immediately and recycle the charger. If the battery become swollen, recycle the battery. If Ni-MH battery discharges very quick, it’s possible to restore it, however restore is impossible for Li-ION batteries. If you’re feeling, that the capacity of Ni-MH battery degrades, you can calibrate it. Never train Li-ION batteries, the quality will degrade.
Do not store your battery empty. Charge it unto 40%-50% of nominal capacity and store in 15C (60F) in fridge. Also it worth to recharge unused batteries once a half-year. However, the best you can do is to use battery. This what it designed for.
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