battery tester (help needed thanks!!!!!!)

[bryant83]

hi all:
i am currently doing a project on a battery tester. To test the capacity of a battery, i will need to discharge it. One of the requirements of my project is that i need to discharge the battery at a constant current. Normally, to discharge a batt, we just have to connect the batt to a discharge load (ie. a resistor) but in order to discharge at a constant current, i would have to change the resistance of the discharge load. Although i could use a variable resistor to vary the resistance, but i have to discharge at a current of about 300 mA to 400mA and also i will have to vary the resistance electronically. I found that a power mosfet could handle current 300mA and higher. hence, i would like to ask that by using a power mosfet (eg. mtp3055) with a heat sink , could i vary the resistance of the mosfet's channel? Thanks in advance !!! help needed

 

[ante]

Hi.
Why not use a LM317 regulator for the constant current discharge?
I think it is perfect for this job.

Ante :roll:

 

[k7elp60]

Constant current discharge

A power mosfet will work fine. Put a low value resistor from the source to ground. Connect the junction of the source and the resistor thru a 1k resistor to the - input of an opamp. Apply an variable + voltage thru a 1k resistor to the + input of the opamp. Connect the output of the opamp to the gate of the mosfet. Connect the + of the battery to the drain of the mosfet. Connect the - of the battery to the ground of your circuit. The op amp will keep the current constant to discharge the battery. If you you an adjustable regulated voltage as the reference to the + input of the opamp you can adjust the discharge current. If you use o to 2.5 V as the reference voltage, calculate the R in the source lead to have 2.5 volts drop across it with the maximum current.
Good luck

 

[ante]

This is the easy way, two parts and self powered.

Ante :roll:

 

[ante]

Sorry, here is the picture

Ante :roll:

 

[bryant83]

thanks ante and k7elp60 for all your kind advice!!!! i really appreciate it

 

[bryant83]

k7elp60:
i would like to ask you what does the op amp in the circuit u provide does? also what is the discharging load in this case? the channel resistance of the mosfet?

 

[ante]

The LM317 uphold 1.2 V trop over R so if you want 300mA (1.2 / 0.3 = 4) R is 4 ohms and the power rating is (1.2 * 0.3 = 0.36) use 0.5W or more to be on the safe side. With lower heat in the resistor you get a more stable current.

Ante :roll:

 

[k7elp60]

i would like to ask you what does the op amp in the circuit u provide does? also what is the discharging load in this case? the channel resistance of the mosfet?

The op amp compares the reference voltage to the voltage drop across the resistor in the source lead and keeps the drain current of the mosfet
constant. The mosfet acts like a variable resistor to provide the constant discharge current. For most currents the mosfet must be mounted on a heat sink.

 

[bryant83]

Hi ante and k7elp60
Thank you both for helping me out or else i will be in deep ****!!!! HEEEEE i really really appreciate for all your guidances! i will try out both circuits which u both have given me and analyse them then will decide on which 1 i could use. By the way, k7elp60
, about the low resistance connected to the source, how could i calculate the value of the resistance? Also for the variable '+' voltage, what is the range for it? n what do u mean for the range of 0 to 2.5 v which u had said earlier?

 

[bryant83]

hi ante:
could you describe to me how does ur circuit work? bcos i do nt quite understand what does the IC does? sorri for my ignorance!!! :roll:

 

[bryant83]

hi K7elp60:
about the low resistance connected to the source, could u teach me how to calculate the value of the resistance? Also for the variable '+' voltage, what is the range for it? n what do u mean for the range of 0 to 2.5 v which u had said earlier? thanks in advance!!!!

 

[bryant83]

hi K7elp60:
btw i also would like to ask for the op amp in ur circuit design, is 741c a suitable op amp for the circuit?

 

[jem]

Just adding my bit to this discussion... What would you the end voltage of the battery when it is considered fully discharged? In other words, what kind of battery do you want to test?

The reason for this is that while LM317 type of constant current loads are fine, they stop working as CC sinks at somewhere around 3.6V. If your battery still has any juice at this voltage (for example a single Li Ion cell has between 10% - 20% remaining energy at this voltage, assuming rm. temp.), then the 317-type CC sink might not be ideal.

 

[bryant83]

hi K7elp60:
regarding just now i asked u abt the op-amp (741c), sorry i've got the part name wrong. the correct part name is ua741. also i have added an attachment of the data sheet of the op-amp. can i use this op-amp? then the ua741 have the vcc+ and vcc- inputs i want to ask what are those?is it the power supply to the chip?or is it the offset null input? pls reply asap cos i need to do testing thanks you so much!

 

[bryant83]

sorri K7elp60 i cannot add the attachment . but can u go to this url n download the datasheet? the url is https://www.st.com/stonline/books/ascii/docs/2161.htm
thanks!

 

[k7elp60]

battery testor

A 741 op amp will work fine as the controlling op amp. The resistor in the source lead is calulated with ohms law. R=E/I. E=2.5 and I is the maximum current you want the mosfet to draw. For example if you want 5A of drain current: R=2.5/5 or 0.5 ohms. The reference voltage must be regulated. I used a 78L05 regulator, a 10k fixed resistor and a 10k ten turn potentiometer in series, with the 10k fixed connected to the +5 volts. The pot then give 0-2.5 volts, this makes the current adjustable from 0 to 5A with a 0.5 ohm resistor in the source lead.

 

[ante]

You have to use Ohm’s law. The LM317 maintains 1.2 Volts drop over the resistor and its R=U/I. And avoid draining the batterys to a lower voltage than 1.0V/cell unless the manufacturer states this for capacity measuring. This circuit is not suitable for less than 4 cell-packs.

Good Luck

Ante :roll:

 

[bryant83]

hi K7elp60:
thanks for ur reply! sorri again i told u the wrong chip ,is ua741cn there are offset null 1 & 2 , vcc+ , vcc- , NC inputs. what are these inputs for? do i need them? and also u've just mentioned about the regulator , a fixed resistor(connected to +5 v) , and a potentiometer in series, what are they for?(also if my max current is not 5 A, do i have to change the values?)

 

[bryant83]

hi K7elp60:
i am quite confused by the regulator all these. btw last time u said apply the regulated voltage to the + input of the op amp, i would like to ask if i could use a D to A convertor to control the voltage applied to the + input of the op amp rather then using the regulator,res.,potentiometer? beacause i need to control the voltage applied to the + i/p of the op amp automatically using a 8051 (programmed) thru a D to A convertor. hence, by controlling the voltage automatically, it means that the discharge current also varies automatically depending on the applied voltage to the + i/p of the op amp.