I am currently doing a project where I’m developing a LiFePO4 battery charger to charge 4 of these cells in series. I am using the bq2954 IC and the circuit that I have implemented is found in the “Using BQ2954” application notes as follows:
The calculations for the above circuit are done using the formulas in the “bq2954 Datasheet” as follows:
As can be seen from the ‘datasheet’ of the bq2954 IC, the Charge Current (Icharge), to charge the battery pack (4 cells in series, in this case), is determined by the Rsns (Current Sense Resistor). Thus, using the formula “Rsns = Vsns/Icharge” which comes to “Rsns = 0.25V/Icharge”, I set my charging current (Icharge) as 5A, so the Rsns value comes to 50 mohms.
However, the problem that I am facing is that even with this value of Rsns, I cannot get my Icharge as 5A. It is only showing as arnd 200mA, which causes the charging to take place, but very slowly.
Could someone please help me out? How can I solve this problem and increase my Charge Current (Icharge) ?
Thanks a lot! :]
I need more information and maybe some waveforms of each pin of the IC.
Is the input voltage at least 18VDC?
Is the battery at least 12.4V? If below ~12V the charger will be in precharge and one gets 1/10 the current.
I would suggest using a newer IC, like the bq24600 or '610.
Thanks a lot for your reply. As you have requested, the following is the info to add on to what I provided earlier.
1) We are connecting 4 cells in series, total battery pack voltage for the 4 cells is arnd 13.2V (arnd 3.3V per cell).
2) The Vin we are providing is 17V. We have observed that providing anything more than this 17V for 4 cells in series, causes the bq2954 IC to show fault condition.
3) The reason we can't use a new IC as you suggested is because, this is our Final Year Project, and it ends this month by the end of April. Thus we do not have enough time on our hand to implement a new circuit. So, since we have been researching and working on this bq2954 IC, we have almost got everything correct except this charging current. Thus,we hope to identify and rectify the problem with this IC itself, given the lack of time.
Some more info on our circuit when its charging the cells:
LED 1: ON
LED 2: OFF
CHG pin LED: OFF
BTST pin LED: OFF
LED 1: OFF
CHG pin LED: ON
BTST pin LED: ON
I shall be posting the waveforms at the different pins of our IC by tmr morning. Till then, thanks a lot for your help and advice. Looking forward to hearing from you.
PS: Do let me know if you would like any other info abt our circuit.
the LEDs seem normal except for the BTST..does ON mean high or low?
Battery test output: Driven high in the absence of a battery in order to provide a potential at the battery terminal when no battery is present.
You indicate initially 4 cells and then 7 cells, which is it and what chemistry?
If you are getting 120 t 130mA and expecting 5A I think you have something seriously wrong. What is the voltage across the sense resistor?
is your battery return floating with respect to ground.
Do you have a schematic?
For the LEDS,
1) BTST led on means: BTST pin from the IC is high.
CHG led on means: CHG pin from the IC is low.
2) I'm sorry, that was a mistake, it is 4 LiFePO4 cells in series.
3) The voltage across our current sense resistor (0.05ohms) is 12.5mV. The datasheet of our current sense resistor is as follows:
4) the battery return means the feedback from the voltage divider circuit right? the one that connects to BAT pin on the IC? It has been connected, it is not floating.
5) The following is the schematic of our circuit:
Thanks a lot. Do let me know if u need any other info.
The following are the waveforms at the various IC pins of bq2954 and the power mosfet Q5 in our circuit (All waveforms taken CH2 on oscilloscope):
MOD waveform at pin 14 of IC:
TS waveform at pin 8 of IC:
SNS waveform at pin 7 of IC:
BAT waveform at pin 3 of IC:
Waveform at Drain pin of Q5 power mosfet:
Waveform at Gate pin of Q5 power mosfet:
Thanks. Do let me know if u need waveforms at other pins of the IC bq2954.
Any updates based on my circuit schematic and the waveforms I posted?
PS: We are using a breadboard to test our charger circuit, specifications for which are as follows:
breadboard project gl 12
Will that cause any problems to the charge current? (current limitation? etc...)
Thanks for the waveforms they are informative.
1) The TS pin looks like it is close to the cold fault threshold (3V) if you are using a 5V Vcc on the IC. Why is this not more in the center of the TS range.
2) The SNS pin photo is taken a 2V/div and does not show good resolution...should use 0.2V/div.
3) The BAT pin also has poor resolution and looks like it is close to regulation at 2.05V...if the cell is at 13V then the pin should be 13/14.76*2.05 = 1.8V.
4) don't understand why there is not ~250mV at the sense pin.
5) I found the problem (just thought of it)......search the data sheet for "80". You will find several places where the maximum duty cycle is 80%, so to get 14.76V out with 80% maximum duty cycle one would have to have 14.76/0.8 = 18.45V input plus about 1V of head room, so I would recomment 20V minimum input.
If you can't specify a 20V or greater supply, then I would switch to another IC.
I see, is using the breadboard causing any problems?
We have fabricated our entire circuit, and completed it connecting all the components.
1) The TS voltage is around 2.3-2.4V, which is well between the operating threshold.
2) The SNS pin is showing 13.6mV, not 250mV.
3) The BAT pin has voltage of 1.86V.
The input current at the power supply shows arnd 260mA. We are also supplying arnd 20V+ to the input..
So do you know why the charge current problem is still persisting?
What is the voltage at the switching node and duty cycle.
What is the battery voltage?
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