lithium battery issues

Hello,
I am working on a portable battery backup. I needed a help about battries. I am using lipo battery(18650). But i want more mah how do i do this i have 2 18659 rated at 3.7v 2600mah so how do i soilder them either parallel or series i want to keep the voltage 3.7v only increse the mah. Please suggest something.

Thanks in advance
Unmesh

Go to www.batteryuniversity.com to learn about the details of a rechargeable Lithium battery cell.
Look at all the FAKE 18650 Lithium battery cells on You Tube videos like this one: https://www.youtube.com/watch?v=eOshOXcSkDA

If you connect two cells in parallel then they might explode since one will probably have a higher charge than the other and cause a very high violent current flow between them.
If you connect them in series then they might explode when charging since your charger might over-charge one cell.

A cell is 4.20V when fully charged so two in series make 8.4V.

You need a circuit that disconnects the load on a battery cell when its voltage drops below 3.2V or the cell is ruined. You also need a charger that limits the charging voltage to 4.20V then disconnects when the charging current drops low.

IF YOU WANNA keep 3.7V , you should choose "A "
OR you choose the way of "B"

A half-decent 18650 cell might be able to produce a current of 40A if it has no protection circuit inside. If you connect two or three cells in parallel and one has a charge a little higher than the other then the one with the higher charge will charge the other with a current of up to 40A and cause an explosion or fire. Don't Doo it!

Maybe if you connect them together for a few hours with a current-limiting resistor of about 4.7ohms then their charges can slowly become the same.

Thank you for all your replies benshaw reply is opposite to the two of replies. I want a lithium battery charger circuit for 18650 i have checked the batteries they are not fake so how do i charge them. Also let me know the dc to dc boost circuit for them . Thanks in advance

Benshaw shows how to connect weak old carbon-zinc battery cells together. Lithium is MUCH different.

Go to www.batteryuniversity.com to read about the details of a Lithium battery cell charger. There are charger ICs made to do it properly.

A good charger circuit will measure the voltage of the battery cell. If the voltage is less than 3.0V then it will try to charge a cell at a low current for a pre-determined time. If the voltage rises above 3.2V then it charges the cell at a normal current.
If the cell voltage does not rise to 3.2V then it stops charging the defective cell to avoid a fire.

The battery manufacturer should state the recommended charging current which might be 2A. The charger must limit the charging current to the recommended amount and limit the charging voltage to 4.20V. It monitors the charging current and when it drops to 1/40th of the normal current then it disconnects the charger.

I charged some 18650 battery cells with a changer I made from a 6VDC/500mA wall-wart that limits the current and an LM317 voltage regulator IC set to 4.20V. I disconnected it after about 4 or 5 hours. I never had a cell with a voltage that was too low.
The cells came from an old laptop computer.

Guys, At last i have found my fault and with the help of your replies i was able to solve the issue, so thanks everyone.
Now i am working on the circuit part i have orderd a lithium battery charging module and a dc to dc step up charging module for the power bank. What i habe done is i have put a on-off-on switch to the charging module,so that i can charge 2 cells but i am thiking of making a small circuit to switch automatically without switch please suggest something. I thought og putting a microcontroller but couldn't get an idea about it. Please help me out of this.
Thanks in advance.
Unmesh

- - - Updated - - -

Guys, At last i have found my fault and with the help of your replies i was able to solve the issue, so thanks everyone.
Now i am working on the circuit part i have orderd a lithium battery charging module and a dc to dc step up charging module for the power bank. What i habe done is i have put a on-off-on switch to the charging module,so that i can charge 2 cells but i am thiking of making a small circuit to switch automatically without switch please suggest something. I thought og putting a microcontroller but couldn't get an idea about it. Please help me out of this.
Thanks in advance.
Unmesh

- - - Updated - - -

Guys, At last i have found my fault and with the help of your replies i was able to solve the issue, so thanks everyone.
Now i am working on the circuit part i have orderd a lithium battery charging module and a dc to dc step up charging module for the power bank. What i habe done is i have put a on-off-on switch to the charging module,so that i can charge 2 cells but i am thiking of making a small circuit to switch automatically without switch please suggest something. I thought og putting a microcontroller but couldn't get an idea about it. Please help me out of this.
Thanks in advance.
Unmesh

Unmesh Sawant said:

Guys, At last i have found my fault and with the help of your replies i was able to solve the issue, so thanks everyone.

Click to expand...

You never said you had a fault. What was the fault?

Now i am working on the circuit part i have orderd a lithium battery charging module and a dc to dc step up charging module for the power bank.

Click to expand...

You never said you are making a power bank. A power bank with one 18650 lithium cell or two cells in parallel needs a charging circuit that has a 5V input. The charged cell powers a voltage stepup circuit so that the output is 5.0V.

What i habe done is i have put a on-off-on switch to the charging module,so that i can charge 2 cells but i am thiking of making a small circuit to switch automatically without switch

Click to expand...

Why do you need a switch?

Sorry, if i have not said about the powerbank and anything else. So, the thing is i am making a power bank with 2 cells, but i am using a single charging module and i have put a switch to charge a single battery at a time but thats not the problem. Problem is i want to make both the batteries charge automatically without a switch. Please let me know about the circuit. And please serious replies it's urgent so ..
thanks in advance
Unmesh

Unmesh Sawant said:

Sorry, if i have not said about the powerbank and anything else. So, the thing is i am making a power bank with 2 cells, but i am using a single charging module and i have put a switch to charge a single battery at a time but thats not the problem. Problem is i want to make both the batteries charge automatically without a switch. Please let me know about the circuit.

Click to expand...

Are you making two powerbanks, each with its own battery?
Or are you making one powerbank with two paralleled cells?
Or are you making a power bank with two cells in series and your charger can charge only one cell at a time?

A power bank usually has one cell, not two cells. If you want to charge two cells in parallel then do what I told you in post #4 so that both cells have exactly the same amount of charge and can safely be connected in parallel: "Maybe if you connect them together for a few hours with a current-limiting resistor of about 4.7ohms then their charges can slowly become the same."

I have a Li-Ion battery from my daughter's old cell phone. It has two cells inside welded in parallel and are charged together at the same time by a charger that thinks it is charging one larger cell.

I am working with Li batteries for 2 years.
Use battery in parallel or series is very dangerous
You need pay attention for the definitions about your project for avoid problems in future.
How many cells you will use?
Which is max consumption of the circuit?
The battery will be removed of the charger or battery only stay in charger during charge process?
After you know all about system you can search for IC to charge the battery.
No is easy make the circuit, some takes months, keep that in mind.
You can use sites how TI, Microchip, Linear e etc to choose the IC that you will be use.
Always the datasheet of IC teaches you how to be the peripherals around them.