I'm trying to understand LiPo charging, and batteries. Now, I understand that they must be charge at full voltage, and not to exceed a specified current. But certain models have so-called protection boards integrated into the battery. An example would be this:
It claims to have a protection circuit. When I look at the datasheet for that type of battery here:
In the datasheet it has a specified charging current of '1CmA (250ma).' However on page two the protection circuit states a over current protection of '3 +- 1A'.
Now, I am assuming that as long as the voltage supplied to this battery is equal or less than 4.2v, and the current doesn't doesn't exceed 1amp at the given voltage, then all is good, and the battery will manage itself. However, as I understand it, LiPo battery's can be dangerous. And I would like to live a long and pleasant life.
Also, I don't understand what this 'C' type current is. I've heard it referred to as 1C. So I can only assume that C refers to current so basically it is referring to 1(C) = 250mA in this particular example. Or am I way off here?
You're close ... If I have a 1000mAH Lipo, that is the Capacity (or "C"). This means you can draw 100mA for 10 hours, etc. though I'm not sure what the specified voltage of the cell is after 10 hours.
As for charging, a LiPo can generally be charged at at least "C", often higher. This will be a parameter in the battery's specification. It has to do with heating of the cell when charging at high currents.
You have to watch the battery at BOTH ends of the discharge/charge cycle. You dare not charge it past 4.2V, and you don't want to let it fall below 3.0V, else irreversible damage occurs.
Lipos have an inherent characteristic that allows a discharge current far in excess of the charging current.
This is why when you buy one from a hobby store it will have a spec of 25C , 35C, 45C etc… If the battery is a single cell, for example, rated for 850 mA, and is a 25C battery then 25 * 0.85 A =21.25 A discharge current, but you cannot discharge it below 3.3V per cell, so ESCs, (Electronic Speed Controllers) have a built in “Shutdown” circuit that allows (through programming) you to set the shutdown to SOFT, or HARD which shuts down the speed control when the battery reaches a certain voltage. If you are NOT using an ESC, then you need to regulate the current /voltage by some other means (like limiting the load to a maximum current) or by adding some protection circuitry that accomplishes the same function as the ESC shutdown circuitry. This can be accomplished by measuring the battery voltage using an analogRead and using conditional statements to turn off the output when it reaches a certain voltage. If there is a series resistor in your circuit , you can measure both sides of the resistor using two analog inputs , do a subtraction and obtain the voltage drop across the resistor , divide that value by the resistor value and obtain the current through the resistor. If the resistor has one end connected to the system GND, then you only need one analog input to measure the voltage drop. Comparators and op amps and other circuitry can be used to accomplish similar functions.
I'm trying to understand LiPo charging, and batteries. Now, I understand that they must be charge at full voltage, and not to exceed a specified current.
Its somewhat more complicated than that. There are two phases to a proper LiPo charge cycle:
- Charge at a constant current until the cell reaches 4.2V. This current is usually in the range of 0.7-1C.
- Charge at a constant 4.2V until the current drops to 0.1C
You typically want to use a dedicated lipo charge controller that automates this all for you.
Now, I am assuming that as long as the voltage supplied to this battery is equal or less than 4.2v, and the current doesn't doesn't exceed 1amp at the given voltage, then all is good, and the battery will manage itself.
If the battery is near fully charged then you can hold it at 4.2V, but if you try throwing a full 4.2V at a discharged battery you may go over the desired charge rate. And as has been said "1C" is a current rate that will charge or discharge a battery in 1 hour ("2C" is half an hour, etc.).
If you look around you'll find lots of chips in SOT-23-5 packages that handle single cell lithium battery charging. It's a small chip size but you can't beat the simplicity.
"C" is actually a measure of how fast you can take current from the battery.
If you have 2 batteries, and they both have a capacity of 1000 mAh, then they should be able to provide 1000 mA for 1 hour, or 500 mA for 2 hours, or 100 mA for 10 hours.
Where C comes into it, is you want to discharge the battery very quickly in a high-powered device like a helicopter, which might only run for 3 minutes.
If you have 2 batteries, and they both have a capacity of 1000 mAh, and one is a "10C" battery, and the other is a "20C" battery, this is what it means: The 10C battery can be discharged at 10 x the current implied by the battery's capacity, taken over 1 hour. If the 1000 mAh battery has a nominal ability to supply 1000 mA for a hour, it can actually supply 10 times that current, 10000 mA, for 1/10 of an hour ( six minutes ).
The 20C battery, can supply 20 x the nominal 1 hour current. So, 20000 mA, for 1/20 of an hour ( 3 minutes ).
The C rating you will see batteries advertised with, is for discharging them. None of them can be charged that fast. Unless you do a lot of testing and know what you are doing, 1C is a reasonable rule of thumb for a maximum charging current. So in the example, 1000 mA.
Keep in mind that the capacity written on the battery is a nominal capacity under certain conditions. If you draw a large current, you will not get that capacity out of your battery.