3.6 or 3.7 Volts: Why Lithium Ion Battery Cells Differ in Voltage?

1] VOLTAGE: 3.6V or 3.7V - 18650 Li Ion Batteries
Are all 18650 lithium ion battery cells 3.6 or 3.7 volts or do they differ? I want to replace my current SIGMA DP2 photocamera battery (3.7 1300mAh) with a 18650 2900 mAh battery, sometimes rated 3.6V sometimes 3.7V. (I am not sure whether information provided on eBay is accurate as I have seen conflicting data.)

2] Possible Voltage Shortage?
Does it matter for my project or can i use 3.6 without any Voltage shortage for the camera? In other words, do all 3.6/3.7V li ions use the same way therefore the given voltage is just the same or do they really differ?

3] Fundamental Reason for this Voltage Range
Why do we see only 3.6/3.7 V per Li Ion Cell, and never 3.0V or 4 V per cell? I am very curious...

4] Parallel Cell Charging - One BIG Li-Ion Battery Pack
I was thinking of putting two, or three of those Panasonic/Sanyo 18650 Li Ion cells in parallel, soldering together from the instant the are new, that way giving me about 5800 mAh (2 pcs x 46 grams = 92 grams) or about 8700 mAh (3 pcs x 46 = 1 grams) very light. Equaling to bypassing seven orso charges with my old li ion battery. Is this correct?

5] Charging... How?
I found a nice small cheap charger about 30~40\$ called Turnigy Accucel-6 (there is also an Accucel-8 for double price and double weight) . Could I attach the + to + of all the cells and the - to all the - poles of the cells without needing any extra in-between-wiring?

Thanks a lot for your opinion.

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In practice 3.6V and 3.7V are exactly the same. Just by looking at a circuit one value can change into the other one. –  stevenvh Aug 1 '11 at 10:28

1] VOLTAGE: 3.6V or 3.7V - 18650 Li Ion Batteries

All single cell lithium ion batteries are going to be 3.6-3.7v. There are applications where multiple cells will be tied together in series. This will result in voltages that are multiples of 3.6-3.7v. So as long as you match the number of cells and approximate mAH you should be fine.

2] Possible Voltage Shortage?

The voltages and battery life responses for all batteries are going to have slight difference. For the most part this won't matter. Most projects that use batteries are not terribly voltage dependent. They will either boost or regulate their voltage to get the voltage they want out, or they will be able to run at a wide range.

As a note, "Shortage" in this context usually means you are creating a short across your battery. Might want to be careful with that terminology.

3] Fundamental Reason for this Voltage Range

I am not an expert on this, but I know it deals with the chemistry of the battery itself.

4] Parallel Cell Charging - One BIG Li-Ion Battery Pack

This can be done. There are some issues that can come up when doing it. This might be worthy of a question by itself. If you do ask, might want to ask if the same can be done for packs in series.

5] Charging... How?

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Thanks Kellenjb for the answered parts! Good to get a positive push in my headings. the remaining 9more secialistic questions remain unanswered byt that Q nr. 3 needs an expert, indeed! –  Sam Jan 22 '11 at 21:23
In the item #1 of your answer you mention that capacity (mAh) should be simuilar, but the OP seems to be replacing a 1300mAh battery with a 2900 mAh, more than doubling capacity. Is there a problem with that? –  Ricardo Dec 12 '13 at 16:01

Well, actually I was also interested here.

1. Rated voltage might be rated different to make it serve more in expense of capacity.
2. LiIon batteries have lots of variants of chimestry - all are LiIon, but different voltages, prices, reliability.
LiCoO2    3.7 V   140 mA·h/g  0.518 kW·h/kg
LiMn2O4   4.0 V   100 mA·h/g  0.400 kW·h/kg
LiNiO2    3.5 V   180 mA·h/g  0.630 kW·h/kg
LiFePO4   3.3 V   150 mA·h/g  0.495 kW·h/kg
Li2FePO4F     3.6 V   115 mA·h/g  0.414 kW·h/kg
LiCo1/3Ni1/3Mn1/3O2   3.6 V   160 mA·h/g  0.576 kW·h/kg
Li(LiaNixMnyCoz)O2    4.2 V   220 mA·h/g  0.920 kW·h/kg


http://en.wikipedia.org/wiki/Lithium-ion_battery#Positive_electrodes

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Wauw, thats very interesing BarsMonster! So, if I understand correctly, then its a matter of give or take: you can have bit higher Volt 3.7, but give bit away in capacity of current. And vise versa the 3.6 V might hold bit more amp? I guesse the main question then would be: Can i rate/charge a 3.6V batt as 3.7 V batt and input a bit lower Amps in the smart charger? –  Sam Jan 23 '11 at 1:02
No, you sacrifice battery life, not current. (But in long term yes, max current will decline). If you charge 3.6 battery to 3.7v it will die sooner, but probably not instantly. Extra stored charge is too little. It's more important when undercharging 4.2v one :-) –  BarsMonster Jan 23 '11 at 10:27

LI-Ion batteries are left with a charge of about 40% when they leave their factory. The discharge under storage is best then. This results in a voltage of about 3.7 for most types. When fully loaded or 100% charge the voltage is about 4.1V to 4.2V Do not charge them higher or they will lose their lifetime. They can be discharged until 3.3V or even lower but again lifetime is shortened when discharged too much. I have much experience in charging an discharging as i am using solar cells to charge my li-ion batteries. I use for charging my mobile phone(s)

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It can in fact be very important!

The charging voltage of a 3.7V LiPo or LiIon is 4.2V, but for a 3.6V one it is only 4.1V! You could easily damage a 3.6V LiIon cell by charging it to 4.2V.

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A good charger doesn't use a single charging voltage. You want current limiting, voltage limiting, and the charge profile indicates when the battery is done charging. –  Ben Voigt Mar 20 at 22:45

The nominal voltage of Li-ion cells is 3.6V-3.7V, depending on the manufacturing technique. I doubt if you will see any difference between one delivering 3.6V and one delivering 3.7V, in practice, as there are many other factors involved.

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Thanks Leon! very clear. –  Sam Jan 22 '11 at 21:22

most of the devices are not that sensitive to voltage. but when they tied up in a series that results in a big voltage difference and this difference is directly proportional to number of cells you tied in series. If your series contains 2-3 cells thats fine if you go beyond 5-10 cells that results in around 1V difference that matters for sensitive devices such as loptops.

gave a good one

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protected by Kortuk♦Jun 26 '12 at 11:59

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