will some one tell me about 0.25C8 charging scheme. what does it mean???
But you can't discharge a battery with 0.25C for 8 hours because 0.25C means a quarter of the rated capacity is emptied every hour and so the battery will be empty long before the eight hours are over. But when charging, you need to charge for additional time to take into account the losses. So for "dumb" charging that isn't measuring delta-V or cell temperature, it's common to charge a NiMH or NiCd cell at 0.1C for 14 hours. At higher charge currents, the temperature increase when the cells are fully filled with be high enough that it's important to use an intelligent charger that reacts when the cell is full and then reduces or totally cuts the charge current. Few battery technologies will age well at higher temperatures, and when the cell is full the electrical energy can't be continue to be converted into chemical energy but instead all the energy will become heat.
But discharge hours are important when mentioning how much capacity a battery has - because a battery has larger capacity when discharged at a low current than when discharged at a high current. A lower current gives lower voltage drop from internal resistance, which allows you to continue the discharge for a longer time before reaching the end voltage.
So a battery might have the capacity specified at a discharge current of 0.1C. You will get less energy out of it at 1C and much less energy out of it at 10C.
As an addendum to the previous post about the batteries having different capacity at different discharge currents a manufacturer may specify the capacity as 22Ah at C8, i.e the total energy you get out of the battery at a load that will empty the battery in 8 hours. That digit is sometimes written as a subscript.
0.25C is specified for charging. it has no relation with discharging. actually my question was what is the meaning of 0.25C8 for battery charging or charger? but still i'm confused.
Every reference to C is a reference to discharging - it doesn't matter if the sentence discusses charge current or discharge current.
Because C is the total capacity of the battery, as the integration of the energy you can discharge from the battery at a specific discharge pace.
So even if you see 0.25C for charge, it's still a charge current relating to the total discharge capacity. So for battery with capacity C = 1.2Ah, a charge current of 0.25C means 300mA.
And the battery might specify the capacity as C<subscript>8</subscript> = 1.2Ah meaning the usable capacity for an 8-hour discharge cycle. While it might describe C<subscript>0.5</subscript> = 0.8Ah meaning the usable capacity for a 30-minute discharge cycle.
But specifying the charge current isn't enough. They also need to specify any terminating conditions. So if you should look for + dV/dt Or if you should look for - dV/dt Or if you should look for Vmax Or if you should look for + dT/dt Or if you should look for Tmax Or if you should just use a fixed time. Or if the charge current should be dropped to a lower value after a specific condition is reached.
If the manufacturer tries to use the expression 0.25C8 as meaning "charge with 0.25C current for 8 hours", then they are not nice. First because it's not a standard expression. And secondly because it means the charge current is quite high while the total energy sent into the battery is twice the battery capacity. So a significant overcharge. And with no other end condition, it also means you may start from a half-charged battery and still try to send in twice the maximum capacity at a quite high charge current.
Have you noticed that you still haven't posted a link to the battery datasheet where you find this expression?
If you look at this battery, you can see why there is a reason to not just specify one battery capacity, but instead specifying multiple capacities depending on discharge rates: " for the battery in the linked datasheet would be C=2.30Ah, i.e. the capacity at a 20-hour discharge which in this case would be a 115mA discharge current.
please study the charging section of the following manual
www.gaston.com.hk/.../GT-FA & GT-F range manual.pdf
Perhaps this indicates you're the wrong guy to work on this project?
Maybe you should be chatting with the guys at Gaston, rather than those a Keil?
Perhaps this indicates you're the wrong guy to work on this project
BOOM
batteries tend to explode if the charger is made by "the wrong guy"
Not sure why you want me to study that document - you want me to write a battery charger for you?
An as you can see, the document does not write C8 - which for normal formulas would be expected to mean the same as C*8 and so be a meaningless figure since it's irrelevant do discuss "eight times the capacity of the battery". The text does write on quite a number of places C<sub>8</sub> - so the digit 8 is a clarification of exactly which capacity they mean. In this case the capacity the battery can supply for an 8-hour discharge speed. I saw one place where they missed to subscript but it was very clear this was a typographical mishap.
From what it looks like, the full sentence (that you should have supplied verbatim) that your first post seems to refer to is: "The recommended recharge method of GT-F batteries to maximize battery life is to use a constant voltage equal to the float charge voltage (2.27 VPC at 20°C) with a maximum charge current of 0.25 C<sub>8</sub> amperes."
So you get a clear indication what C means. And then it's business as usual with the meaning of 0.25 C as maximum charge current. If C<sub>8</sub> is 100 Ah, then you must never get above the current that would charge the battery in four hours, i.e. 100 [Ah]*0.25[1/h] = 25 A (Alternatively expressed as 100Ah/4h = 25A) as charge current (to avoid cooking the battery cells). Since we are talking about constant voltage charging, we are talking about the charger using a current limiter for a while at the start of the charge cycle. The battery cells will later reach a state where the cells themselves decides how much current to accept based on the fixed charging voltage of 2.27 V/cell.
A problem when asking questions, is that the questions must contain enough context. Missing, important, context makes them meaningless. Supplying incorrect information is even worse when expecting a good answer. That's why links, paragraph numbers etc are so valuable when asking questions, allowing people reading the question to pick up additional context if needed.
The datasheet in question is for quite large batteries. Large batteries also means large accidents when things go wrong. Which means this isn't the kind of batteries to play with for people who need to ask basic questions about the meaning of the individual formulas. For these battery sizes, ventilation is also important unless you want the neighbors to inform you that your roof has left the building.