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### Topic: Li-Ion battery charger voltage monitor. MOSFET advise? (Read 2273 times)previous topic - next topic

#### hell

##### Feb 13, 2015, 01:00 pmLast Edit: Feb 13, 2015, 02:35 pm by hell
Hi, I've been reading the forum from more than an year now, but this is the first time I'm writing. I've learned so much.
Now I need advise. I want to know if I'm missing something in my project. It will be a Li-Ion charger for four cells separately. Controlled from LTC4054-4.2V. I have doubts about measuring battery voltage while charging. Here is the simple schematic. Don't mind atmega connections, it's just an example. If needed I'll post the whole charger schematic.

So, this is what i want. Let's say every minute or so atmega checks the voltage of the battery and display it somewhere (lcd, serial, etc.). So I'm getting the battery voltage but if the charger IC is charging it at that given moment I'll measure the ICs voltage. I'm gonna use two MOSFETs (P-ch and N-ch) for switching the circuits on-off so i can get the battery voltage but not the LTC4054 - schematic in attachment.

Questions:
1) Are those MOSFETS (P-ch: IRLML2502, N-ch: IRLML6402) good for my project?
2) if 1) == YES { what am I missing there? Limiting resistors, filters, ... ? }
3) Is there an easier way to measure voltage? Opinions?

P.P. I'm hardly understanding MOSFETs and I just want them to switch the circuits with no voltage drop across it.
P.P.1 LTC4054 will stop charging when the battery is full. I'll be watching it (there's a CHRG PIN) but I want the exact voltage every moment.

Edit: R1 = 220R
Atmega controls the FETs: if PIN 17 is HIGH Q2 is off and Q1 is ON and vice versa.

#### MorganS

#1
##### Feb 13, 2015, 01:42 pm
If you switch off the charger and then immediately (like a millisecond) read the battery voltage then you will get the charging voltage. The surface charge will take some time (and some current) to dissipate and give the true battery voltage.

I would try to measure the current while charging and don't switch the charger off. The LTC4054 could have problems if you switch it off and on rapidly.
"The problem is in the code you didn't post."

#### Peter_n

#2
##### Feb 13, 2015, 01:46 pm
Those LTC4054 is supposed to be connected directly to the Li-ion cell. I don't know what the mosfet in between will do. Can you use four LTC4054 ? They are 2 dollars each ?

To measure a voltage, connect the battery (+) via 1k protection resistor to Arduino A0.
Perhaps you need a good voltage reference for the Arduino for precise voltage measurement.

I don't undestand the 10k, p-channel and 220k. The 220k can be 1k to the gate. The p-channel should change source and drain.

#### MarkT

#3
##### Feb 13, 2015, 01:51 pm
You should be reading the cell voltage under charge in order to protect it - once
the LTC chip has entered the trickle-charge regime the difference in voltage you'll
see by disconnecting the charger will be absolutely tiny as these cells have extremely
low internal resistance.

So don't bother switching the charger at all.

If you want to multplex reading several cells then an analog switch chip is simpler
than using separate MOSFETs.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### hell

#4
##### Feb 13, 2015, 02:30 pm
If you switch off the charger and then immediately (like a millisecond) read the battery voltage then you will get the charging voltage. The surface charge will take some time (and some current) to dissipate and give the true battery voltage.
This is what I'll do - switch off the charger, measure voltage and switch it back. What if i do it every minute? Will there be a problem?

I would try to measure the current while charging and don't switch the charger off. The LTC4054 could have problems if you switch it off and on rapidly.
I know this will sounds stupid, but how do i calculate the battery voltage like this? Can you be more specific? I'm a newbie in this.

Those LTC4054 is supposed to be connected directly to the Li-ion cell. I don't know what the mosfet in between will do. Can you use four LTC4054 ? They are 2 dollars each ?
When atmega triggers the PFET it'll will cut off the charger VCC, while the NFET connects the atmega adc pin to battery  and measure the voltage. This way it is the exact battery voltage.
For 50pcs LTC4054-4.2V SOT-23  I've paid \$3. Extremely cheap.

To measure a voltage, connect the battery (+) via 1k protection resistor to Arduino A0.
Perhaps you need a good voltage reference for the Arduino for precise voltage measurement.

I don't undestand the 10k, p-channel and 220k. The 220k can be 1k to the gate. The p-channel should change source and drain.
I wasn't sure about the resistors as I said, so I don't need the 10k? 220k was meant to be 220R, any difference between 220R and 1k for protection? Both will do the job, right?

You should be reading the cell voltage under charge in order to protect it - once
the LTC chip has entered the trickle-charge regime the difference in voltage you'll
see by disconnecting the charger will be absolutely tiny as these cells have extremely
low internal resistance.

So don't bother switching the charger at all.

If you want to multplex reading several cells then an analog switch chip is simpler
than using separate MOSFETs.
It isn't exactly true. I have a simple LTC4054 to battery charger with nothing to measure, just charge. I've been testing the voltage and there's big difference when i turn off the charger and when it is on?
I was thinking about analog switch chip but these are not so cheap, are they?

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