[SOLVED] Battery drop problem

[amayilsamy]

Battery voltage drop problem

I'm using Li-Ion battery 7.4 V , 2600mAh. Its full charging condition voltage is 8.10.

Load current is motor 1000mA and controller circuit 200mA., totally load current 1200mA.

If I start to run the motor battery voltage automatically drop from 8.10 V to 7.60 V.

and I stop the motor battery voltage automatically goes to 8.10 V.

There was problem is I can't monitor the battery properly.

 

[erikl]

Re: Battery voltage drop problem

If I start to run the motor battery voltage automatically drop from 8.10 V to 7.60 V.

That's the typical operating voltage for 2 of those Li-Ion batteries in series. Totally normal at such load current.

... and I stop the motor battery voltage automatically goes to 8.10 V.

No-load condition. Normal behaviour for such batteries at no-load.

 

[amayilsamy]

Re: Battery voltage drop problem

Its ok.. But There was problem is I can't monitor the battery properly.

How I do it...

 

[chuckey]

"I can't monitor the battery properly." Why, whats wrong with a meter, do you want a "bleeper" at low voltage? You have not said what you want to do.
Frank

 

[amayilsamy]

I want measure battery level using ADC and display it on lcd display . In this case battery full discharge voltage is 6.0 v. I displayed battery level bar graph .

 

[Audioguru]

A 2600mAh Lithium battery should be able to supply at least "25C" which is 25 times its rated capacity. 2.6A x 25= 65A.

I have a 325mAh Li-Po battery rated at 65C. Its maximum allowed current is 0.325A x 65= 21.125A. 70C batteries are available now.

The motor on my radio control electric airplane draws a maximum current of 5A. With a 150mAh/45C Li-Po battery the motor does not slow down (the voltage does not drop) until the battery charge is almost finished. Maybe your battery needs replacement.

 

[amayilsamy]

It can support the load but the battery voltage range is 6.0 V to 8.10 V . If battery voltage in 6.5 V , I start the motor battery voltage automatically goes down to 6.0 or 5.50

I display 2 levels in battery
1. Low voltage at 6.7 V
2. Connect power at 6.2 V

I plan draw the current up to 6.0 V . If start motor at the battery voltage at 6.0 it goes to 5.50 V.

I think it is damage the battery or reduce battery life..

 

[Audioguru]

It can support the load but the battery voltage range is 6.0 V to 8.10 V.

Your Lithium battery has two cells in series. It is fully charged at 8.4V, it is half charged at 7.4V and it is dead at 6.4V.

If battery voltage in 6.5 V , I start the motor battery voltage automatically goes down to 6.0 or 5.50

That is because your battery is not charged and it is almost dead.

I display 2 levels in battery
1. Low voltage at 6.7 V
2. Connect power at 6.2 V

I plan draw the current up to 6.0 V . If start motor at the battery voltage at 6.0 it goes to 5.50 V.

I think it is damage the battery or reduce battery life..

Why don't you buy a new battery, fully charge it with a Lithium battery charger to 8.4V then test it with your motor?

My worn out Lithium batteries cannot provide high current anymore and they cannot drive less current for long.

 

[amayilsamy]

In new battery also I faced same problem..

 

[Audioguru]

In new battery also I faced same problem..

Did you fully charge the battery? A Lithium battery is only about 70% charged when its voltage reaches 4.2V per cell (8.4V for your battery that has two cells). Then the charging must continue until the charging current becomes very small.
Some cheap batteries produce horrible performance.

 

[amayilsamy]

I fully charged the battery using given schematic .

I tested with 3 batteries. I'm using samsung batteries with protection circuits...

I don't know what is the problem...

Any another way to find a problem

And also one battery dead...

 

[Audioguru]

A Lithium battery usually loses its capacity and maximum output current when it is 2 years old. Maybe your batteries are older.
A failed Lithium battery frequently swells up like a balloon.

 

[amayilsamy]

Its.. Ok I have two questions..

1. The given charging circuit is correct or not. In my circuit LM317 input is 10.5 V

2. What is the maximum discharge current from the battery.....

 

[Audioguru]

The charging circuit does not shut off when the charging current drops very low so the battery is trickle-charging continuously that is bad for a Lithium battery.
The charger should be adjusted for an output of 8.40V.

Many Lithium batteries can supply a very high output current that causes them or the wiring to catch on fire. You said your batteries have built-in protection circuits that would limit the current, ask the manufacturer.

 

[amayilsamy]

I need better charging circuit for 2 cell Li-Ion battery could you give any suggestions.

 

[Audioguru]

Over here we use a battery charger IC made to charge a Lithium battery.

 

[amayilsamy]

I discussed with battery with manufacturer, the reason for battery failure temperature rising in battery during at the time of discharge. It rises up to 70 deg. C.

And also they don't have solution for battery voltage drop..

 

[YEAR_]

amayilsamy, you said you are supplying 10.5v to LM317. if you need 8.4v at the input to the battery you should consider the dropout voltage on the LM317, which is well over 2.5v, the design uses a 12v supply, so better use that voltage otherwise the battery won't charge enough.

 

[amayilsamy]

amayilsamy, you said you are supplying 10.5v to LM317. if you need 8.4v at the input to the battery you should consider the dropout voltage on the LM317, which is well over 2.5v, the design uses a 12v supply, so better use that voltage otherwise the battery won't charge enough.

I can charge the battery @ 360mA. It's enough for my application.

If I increase the battery voltage at 12 V . Can charge the battery up to 650mA..

 

[Audioguru]

A 7.4 V, 2600mAh Lithium battery should easily provide 1.2A continuously without overheating and dropping its voltage.
Your battery is extremely poor quality. Use a good battery that can provide at least 2.6A x 25= 65A continuously.

Here are the spec's for an American ThunderPower 7.4V, 2700mAh Li-Po battery that is made to power an electric model airplane: