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Battery Not Charging

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Advanced Member level 2
Mar 21, 2015
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Hi All.
I am having a battery charging issue. i have connected 2 li-ion 3.7v 4000mah battery cells in series to get 7.4v. can anybody please tell me what voltage Chargers will help me to charge the battery full. i am currently using 5v 500mah charger to charge the battery... but the battery does not get charge fully.

your post is not clear.
are you connecting the two cells in series and connecting them to the 5V charger ?
or separately one by one?

yes.... have connected them in series.. so i am getting an output if 7.4v from battery. and now i am connecting this 7.4v battery to 5v charger

you cannot charge the two series battery setup you have connected with the charger available with you.

What should i do in such cases...
what is the way where i can charge the cells in series....

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is is possible if i make a circuit which connects the battery in series for my circuit and at a same time give a parallel connection to my cells for charging....

1) requires a higher voltage charger for the series connection.
2)you cannot have series conn for the circuit AND parallel conn for the charger "at the SAMETIME"

for charging takeout the battery and do it in parallel.
for circuit , put the batteries inside in series form.

apart from that ,
be careful when charging Liion cells separately when you experiment.
unless the battery is just two terminals you cannot charge it like normal method.

am sorry... but its not possible to take out the battery for charging.... so i have to plan it to charge. in series...
is it dangerous to charge the batteries in series....??. what precautions should i take...??
so now what should i do to charge the battery in series....will 9v 1000mah charger work in this case...

DO NOT connect the two cells in parallel because each cell will have a different amount of charge and cause an extremely high current to flow between them.

A Lithium battery is EXTREMELY DANGEROUS if you charge it wrong. It can explode and splatter molten or burning Lithium metal all over the place or it can catch on fire with a VERY hot flame, and water makes it burn hotter.
Go to and read about how to charge a Lithium battery. The voltage is measured and if it is too low then the charger refuses to charge the battery or it tries charging with a low current and if the voltage does not slowly rise then it shuts off. Cells in series are charged separately with a "balanced charger" so that each cell has its current limited and its voltage is never allowed to exceed 4.20V. The charging current is monitored and when it drops to a low amount then the charger is disconnected.

Cell phones have a lithium battery and they have a "charger" that is simply a power supply. The charger circuit is inside the phone.

Audioguru is right. You need to be careful when charging lithium batteries.

Gaura, to charge two lithium ion (Li+) batteries in series, you need a specific battery charger that is designed to charge two series Li+ batteries. In addition, each battery cell needs to be balance. Balancing batteries is actually harder than charging it. To understand cell balancing, think of this way, battery cell characteristics vary from one cell to the next. When charge two cells in series, one cell can be over-charge while the other is under-charge. Over-charge battery generally will reduce its life time or worst fire hazard.

If you don't want cell balancing, you have to under-charge both cells to avoid any risk of damage or fire.

Here is a compiled list of IC manufacturers as well as some common battery charger circuits. Once you have designed your charger, you can test it with a battery simulator.

As u said not to charge this batteries in parallel.... then how is battery banks made which are basically used to charge mobiles.... even they are connected in parallel.... how do they work....
If lu-ion is quite dangerous then can i switch to Lipo batteries....?? but they are very expensive which increases my production cost.

Mobile phones (cell phones) can be charged in parallel because each phone has a battery charger circuit inside it. Each battery charger circuit takes only as much current as it needs.
A Battery Bank is a Li-Ion battery plus a voltage stepup circuit and a charger circuit for its battery. I have never seen them connected in parallel.
I have a battery from a cell phone that is 15 years old. It has two Li-Ion cells welded together in parallel. The cells must be identical and were joined together only one time in the factory.

A Li-Po battery has some of the same specifications as a Li-ion battery. The two-cells Li-Po batteries for my radio controlled model airplanes have 3 wires and are charged from a balanced charger.

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