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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 Helpful 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
Most Helpful 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
 

audioguru

Well-Known Member
Most Helpful 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?
The "smart" charger also does not have a detailed datasheet. it says it stops charging when the battery voltage reaches 4.2V per cell which is wrong because then the cell is only about 70% fully charged. The voltage must be limited to 4.2V per cell but the charging should continue until the current drops after a while to about 3% of the battery's rating.
 
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