Is Charging of Li Ion battery with series resistor Safe ? Is there any safety concern ?

Thread Starter

mishra87

Joined Jan 17, 2016
742
Hello Guys,

My charger specification 9.5V@600mA and battery is 2000mAh.
I want to charge two Li Ion series battery with 400mA in some case so i want to limit the current using series resistor.

My theoretical calculation is as per below.
I assume battery is closer to empty state:

R = 9.5V/0.4A = 23.75ohm/3.8W
Now i am not sure what happen if battery voltage increases the charging current deceases. I assume charging time will be increases.

Is there any safety concern ?
Opinion will be highly appreciable.
 

Thread Starter

mishra87

Joined Jan 17, 2016
742
Yes. The batteries will explode/catch fire if sufficiently abused. You need to charge them properly. batteryuniversity.com has a lot of information.
Thanks for your reply !
I found efuse from texsas which is having programmable current limit. can i use this chip to charge li ion battery with limited current.
http://www.ti.com/lit/ds/symlink/tps2596.pdf

why am i looking for it because battery manufacture suggest below charging profile.
1590512362805.png
 

Thread Starter

mishra87

Joined Jan 17, 2016
742
Could anyone help me charge the Li Ion battery as per below temperature profile ?
My charger is 9.5V@600mA
1590512474851.png

Thanks !!!
 

Thread Starter

mishra87

Joined Jan 17, 2016
742
For safety's sake, buy a dedicated lithium battery charger for a 2S battery.
Many thanks !
Now i have some confidence not to design circuit with any discrete components.

I have searched some manufactures to get the chip which charge the battery as per below temperature profile. There are so many dedicated ICs are available but not meet this temperature profile.

So at the end could you please help me to find chip for same. I am looking chip which meet below temperature profile and having overcharge cutoff.
1590514171245.png
 

Audioguru again

Joined Oct 21, 2019
1,503
An ordinary Li-Ion cell charges to 4.20V absolute maximum and two cells in series MUST be charged with a "balanced" charger circuit that regulates the voltage of each cell and prevents the weakest cell from over-charging.
Two cells in series are charged to 8.40V absolute maximum so your charger with 9.5V is not made to charge a Li-Ion battery and will make a spectacular explosion by over-charging your battery.

Additionally, a Li-Ion battery must NEVER be trickle-charged. The charger circuit is supposed to measure the charging current and when the current drips to a low amount then the charger is supposed to turn off the charging.

You assume that your battery is 0V, then it is destroyed and might explode if you try to charge it. A lithium battery cell must NEVER be discharged below about 3V, the circuit that your battery is powering must disconnect the battery when its voltage drops to 3.0V per cell.
 

jpanhalt

Joined Jan 18, 2008
9,426

Thread Starter

mishra87

Joined Jan 17, 2016
742
An ordinary Li-Ion cell charges to 4.20V absolute maximum and two cells in series MUST be charged with a "balanced" charger circuit that regulates the voltage of each cell and prevents the weakest cell from over-charging.
Two cells in series are charged to 8.40V absolute maximum so your charger with 9.5V is not made to charge a Li-Ion battery and will make a spectacular explosion by over-charging your battery.

Additionally, a Li-Ion battery must NEVER be trickle-charged. The charger circuit is supposed to measure the charging current and when the current drips to a low amount then the charger is supposed to turn off the charging.

You assume that your battery is 0V, then it is destroyed and might explode if you try to charge it. A lithium battery cell must NEVER be discharged below about 3V, the circuit that your battery is powering must disconnect the battery when its voltage drops to 3.0V per cell.
Thanks for your useful information !

Some additionally i wanna add here.
1. Temperature measurement of battery pack will done and based on temperature we disconnect the charging/discharging circuit
2. Battery pack voltage measurement will be done and based on that disconnect the overcharging charging(8.4V) and discharging(6V)
3. Battery current measurement will be done and based on that disconnect the charging.
4. Second protection for battery overcharging will be done (e.g. protector circuit)
5. Some agency e.g. UL allow the cell dis-balancing upto 150mV.

Now i was looking for the chip which meet below temperature profile of battery.
1590515954954.png
 

Thread Starter

mishra87

Joined Jan 17, 2016
742

Thread Starter

mishra87

Joined Jan 17, 2016
742
Most of ICs i looked so far are standalone chip and only charging current can be programmed at a fixed charging current. The charging current can not be altered based on battery manufactures temperature profile.

Since i am measuring the temperature using uC. I am looking for chip whose charging current can be changed based on temperature either by uC (microcontroller) or chip itself measure the temperature and meet temperature charge profile.

Regards,
 

jpanhalt

Joined Jan 18, 2008
9,426
Most of ICs i looked so far are standalone chip and only charging current can be programmed at a fixed charging current. The charging current can not be altered based on battery manufactures temperature profile.

Since i am measuring the temperature using uC. I am looking for chip whose charging current can be changed based on temperature either by uC (microcontroller) or chip itself measure the temperature and meet temperature charge profile.

Regards,
Did you actually look at the links I provided?
This is from TI for BQ24000x chargers:
1590519888133.png

I would not call that constant current. Most important, on what basis do you think you have a better profile to follow? Your question about using a simple resister certainly implies that you do not Whether for single cell or multiple cells in series, the companies I reviewed all use a similar profile.

Moreover, you can control the temperature cutoffs by selection of a voltage divider values.
 

Thread Starter

mishra87

Joined Jan 17, 2016
742
Did you actually look at the links I provided?
This is from TI for BQ24000x chargers:
View attachment 208222

I would not call that constant current. Most important, on what basis do you think you have a better profile to follow? Your question about using a simple resister certainly implies that you do not Whether for single cell or multiple cells in series, the companies I reviewed all use a similar profile.

Moreover, you can control the temperature cutoffs by selection of a voltage divider values.
Many thanks to your support, time & interest on thread.
I just went through the datasheet of BQ24004 for two cell. So basically this IC will provide charging at programmed temperature level.
lets say user want battery to stop charging if battery temperature reached 55degC. This chip will not adjust any charging current recommended by chip manufacturer

As per my understanding but i am not sure.
This can only be achieved with the chip who is having something called adaptive charging. Lets say the chip is communicating with microcontroller using I2C and uC send data to chip to programme the charging current based on temperature .
eg. 1590523109377.png
 
Last edited:

Thread Starter

mishra87

Joined Jan 17, 2016
742
All of the chargers I reviewed last year, start with an initial fast charge then convert to a particular profile. My interest was for single cells, but maybe these links will help you:

Microchip:
https://cdn.sparkfun.com/assets/learn_tutorials/6/9/5/MCP738312.pdf
http://ww1.microchip.com/downloads/en/DeviceDoc/22005b.pdf

Texas Instruments:
http://www.ti.com/power-management/battery-management/charger-ics/products.html#p1152=1;1&p1153max=0;1
Hi,

Datasheet says " If the PROG pin is open or floating, the MCP73831/2 are disabled and the battery reverse discharge current is less than 2 µA "

If this PROG pin is short with ground what will happen then ?

1590737129679.png
Thanks
 

jpanhalt

Joined Jan 18, 2008
9,426
1) Why are you concerned about the charger? It is for single cells and you plan on using at least 2.
2) Set to ground is not described, but I wouldn't recommend it based on the function of the Prog pin:

1590740568910.png
 

Thread Starter

mishra87

Joined Jan 17, 2016
742
1) Why are you concerned about the charger? It is for single cells and you plan on using at least 2.
2) Set to ground is not described, but I wouldn't recommend it based on the function of the Prog pin:

View attachment 208412
Thanks for reply & time .

Yes, my application is for two cell.
so i use below chip. now if PROG pin shorted with GND what will happen ?
FMEA needed when you go for some certification eg. UL. So theoretical simulation is needed.
Now i wanted to understand what will happen to chip, battery and charger if Rprog pin is shorted to VSS(GND).
1590741397541.png

Regards
 
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