# Question on Lithium battery charger

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#### hxa7630

##### New Member
Hello, i'm new here and was hoping someone could help me with my school project. I want to be able to charger and power my board with lithium-ion batteries. I was thinking of using this battery: LG Chem Li-Ion 18650 Cylindrical 3.7V 2600mAh Rechargeable Battery ( Optional Tabs). I calculated the estimated watt/hour i would need is 28.8. Using the mock up design i drew-up and attached below, i figure i'll take 4 batteries and put them 2 in parallel then in series. This would give an estimated output of 7.4V and 5.2aH which would give me a total of ~38.5 watt/hour. I have a few questions about this design.

- Would it matter if the cells match? I'm guessing so cause if i had one at 3.7 and the other at 3.6 it would drain the 3.7V, correct?
- Would it be more cost efficient to buy 4 batteries like this or is there some type of pack with this design already made that someone can link me to?
- Like I stated this would power my board when it is not plugged into the wall charger. But if it is, it would charge the batter pack. How can I add something (i'm guessing a switch) to cut the charge going to this battery pack if it starts to heat up (to prevent explosions and/or a fire)

If I am unclear on anything please let me know, thanks a lot!

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#### Externet

##### Well-Known Member
For that arrangement series-parallel of cells, you need two isolated voltage regulated chargers set to 4.100 V applied each to each couple of paralleled cells.

#### hxa7630

##### New Member
For that arrangement series-parallel of cells, you need two isolated voltage regulated chargers set to 4.100 V applied each to each couple of paralleled cells.
Can you elaborate on the purpose of this? Would that still give me my desired output of watt/hour? Where would I find these regulated chargers? I am new to all this so I am not sure. I also found this website where it is in a package, but i'm not sure if it is the same thing I am asking: Rechargeable Batteries & Battery Chargers - Flat Li-Ion 18650 7.4V 5200mAh Rechargeable Battery PCB module with 20AWG Bare Leads and Hitec Connectors if someone could confirm, that'd be great.

#### Externet

##### Well-Known Member
The purpose is to apply to each pair of paralleled cells its own charging regulated voltage independently; the proper way to charge a LiIon battery.

Make the chargers yourself with adjustable voltage regulators

+------------cell1pair-------------C---------------cell2pair----------------gnd

The + of charger 1 connects to cell 1pair +
The - of charger 1 connects to central point C

The + of charger 2 connects to central point C
The - of charger 2 connects to gnd

There is many circuits in the net about adjustable chargers, set both to 4.100 V and not more. Their (-) terminals must not be common grounded nor joined. The supply for the 2 regulators can be provided by 2 wall warts.

The paralleled cells should match their voltages after 24H of no use before soldering together. Their age and amount of use better if similar.

! The wrong way is to apply 8.2V to the whole battery arrangement, does not guarantee each series will not exceed 4.1V. One may get 3.9V and the other 4.3V; leaving one undercharged and the other overcharged and damaged. If the charge rate is too high, the second can even explode.

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#### Damo666

##### Member
Realistically, you'll need a " balanced charger ". As somebody said previously, some LiPos will be holding a different charge to others. This question is regularly asked on many RC model forums. All the best. Damian.

#### hxa7630

##### New Member
thanks for the replies! But it seems like this pre-made battery pack will do the job. Rechargeable Batteries & Battery Chargers - Flat Li-Ion 18650 7.4V 5200mAh Rechargeable Battery PCB module with 20AWG Bare Leads and Hitec Connectors Now all I need is a decent charger for it. I was looking online and if someone can confirm that any of these battery chargers will work with the battery pack above, that would be great! QuickView - MAX1908, MAX8724, MAX8765, MAX8765A Low-Cost Multichemistry Battery Chargers thanks again for the help!

#### audioguru

##### Well-Known Member
Most Lithium-Ion cells are charged to 4.2V each. They are not anywhere near fully charged when at only 4.1V. The Smart-Charger shown limits its voltage to 4.2V for each cell.

I charge my Lithium-Ion 18650 cells, two in series to 8.4V with a current-limited wall-wart feeding an LM317 voltage regulator set to exactly 8.40V and its works perfectly without balancing. No polyswitch and no protection pcb.

#### hxa7630

##### New Member
oh, i would not be using the smart charger, Just the battery pack with a charger from maxim-ic.com (the second link). I am just wondering if I purchased the batteries from the first link, would it be compatible with the charger in the second link? please let me know, thank you.

#### audioguru

##### Well-Known Member
Instead of blindly buying the battery, find and read its detailed datasheet.

#### hxa7630

##### New Member
hm, the charger seems compatible since it takes li-ion, includes my voltage, and is 2-4 cells. I'm sorry I'm new to all this and am still learning. Is there something more specific I should be looking for? thanks