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Li-Ion battery charger voltage monitor. MOSFET advise?

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Emil Gospodinov

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Hi, friends.
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 4x 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.

Screenshot_6.png

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.
 

Audioguru

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Why do you want to know the voltage of the charging Lithium battery? The voltage rises to 4.2V fairly quickly but the battery is no where near being fully charged, it is about 70% fully charged. The battery voltage stays at 4.2V and the remaining 30% charging is slower and the charging current slowly drops to about 4% of the battery mAh rating when the IC detects it and stops charging. Read all about it at www.batteryuniversity.com .

Your P-channel Mosfet has its drain and source connected backwards. The source pin should be the most positive (the charger). When connected backwards the internal diode charges a dead battery even when the Mosfet is turned off.

I have charged my Lithium batteries thousands of times and never looked at their charging voltage. The products I use the batteries in, disconnect them to prevent battery damage when their discharging voltage gets too low.
Sometimes I look at a discharging battery voltage to see if an older battery can still provide enough voltage when heavily loaded.
 

Emil Gospodinov

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Thank you for your reply and the useful link.
I was thinking of monitoring the battery charge level, but as I understand this would not be the way.
 

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A laptop computer or smart phone KNOWS the spec's for its battery so it measures the amount of discharge current with time and calculates and displays the charge level remaining. It also might calculate how much charge time is needed because it also KNOWS the spec's for its charger.
 

Emil Gospodinov

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As someone in another forum said PROG pin voltage drops according to current flow to battery. I'll try to understand the datasheet for LTC4054 better. This must be much easier to monitor.
 

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