# Choose right transistor for Battery protection IC

I would like to use the BQ2970 as a protection cuircuit for my litium battery (Varta CP1254 A3). It has a max. continuous discharge current of 120mA and also max. charge current of 120mA. How do I have to choose the external transistiors? I read that I need to take the overcurrent detection voltage (Uodv = 100mV) and devide it by the maximal current (I): Ron= 100mV/2*120mA = 0.41 Ohm. This restistance has to be the Ron restistance of the transistor. Is this correct?

I could not find any transistor which has such a high on-resistance. Will I get problem regarding heat, if the transistor has such a high resistance? Usually the resistance is in the mOhm range.

Did I make a mistake?

I think that I am not the only one who is using a battery with such low current limit. Is there a way to choose a transistor with low On-resistance by for example using a voltage devider? Thanks a lot for your answers. So could I do it like on this picture?  Is it better to choose a high resistance and low Ron or like I did in the picture?

Will I lose a lot of battery capacity for my application when choosing high resistances because of low efficiencies? Is there a problem regarding heat?

One easy way to get more on resistance is to select a higher voltage part that has the same package .>

What do you mean by that? Can you give an example?

• It is the total resistance FET + resistor + FET that matters. You can arrive at a desired value using low Rdson and high resistance or higher Rdson and and lower or no resistance. For a given total resistance the result is the same. May 14 '20 at 7:53
• Power = I^2 x R. Or Vsense-max x Imax . Both give the same result. You can calculate power loss and required resistor dissipation. At Vsense = 1V and Imax = 120 mA dissipation is about 120 mW = minimal. May 14 '20 at 7:55