Maximum Voltage Difference Between 18650 Cells

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hgg

Newly Enlightened
Joined
Nov 26, 2013
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Hi,

New to the forum and I would like to ask a quick question.

I am working on a little electronics project and I have decided to power it from rechargeable 18650
cells. I am going to use two protected cells in series for a total of 8.4V under no load.

Although the batteries are protected, I want to do a little check myself because these batteries
can be indeed dangerous. I will check for two things. 1st for low voltage and 2nd for voltage
difference.

I will light up a low battery indicator LED when either cell goes below 3.7V
I also want the batteries to be balanced because I will use them in series. I will be charging them
with an external charger. When the batteries are powering the device I will check the voltage
of each cell and if the difference is greater than a safe value I will light up another LED indicating
that a battery needs to be replaced.

Does anybody know what the maximum safe voltage difference is for 18650 cells when in series?
Cannot find any info.

Thank you.
 
My advice for maximum safety...

1. Use only new high quality (such as Panasonic) protected cells bought at the same time. (AW, Redilast, Eagletac, Keeppower, Orbtronic, Intl-Outdoor, Callies Customs to name a few)
2. Test the capacity of the cells using a hobby charger that has this capability...especially if you don't use brand new cells bought at the same time (which I highly advise that you do). You want the capacity to be pretty close....within 50mAh would be good, IMO.
3. 3.7v is not low voltage especially when under load. Depending on the load placed on the cell, this voltage may be reaching quickly. 3.7v is the "nominal" voltage. Low voltage is about 2.5-3v. Protected cells typically trip at 2.5-2.8v.
4. I would expect that you will see very little voltage difference between the cells if you follow what I said in #1 and #2.
5. #1 is the most important of these. :thumbsup:

Hope this helps.
 
Last edited:
Hi,

No1 Already done.
No2 I have no such charger... Any suggestions?
No3 Didn't know that! I will be drawing a small charge with a max value of 100mA. I thought that
3.6V was the safe low limit but if you say that the circuit in the battery trips at 2.8V then
I will light up the LED indicator just below 3V then. I don't want to kill the batteries with deep
discharge.

Thank you very much for your advice :thumbsup:
George.
 
I think Panasonic's safe 'low voltage' is 2.5v and LG, Samsung and Sanyo all use 2.75v for their cells. I don't know what Sony does. This is for the bare, naked cells.

Chris
 
Hello Chris,

I think I will use 3V as the lower limit for my indicator light.
I have already made the circuit and works pretty nice.

What about imbalanced batteries?

Do you know if I might have any problems if for example one battery runs lower than the
other for some reason? They will be in series.

Thank you.
 
hgg said:
Hello Chris,

I think I will use 3V as the lower limit for my indicator light.
I have already made the circuit and works pretty nice.

What about imbalanced batteries?

Do you know if I might have any problems if for example one battery runs lower than the
other for some reason? They will be in series.

Thank you.
Click to expand...

I'm no battery expert and I certainly don't play one on TV, so I'll defer to the Colonel on the matching of cells via a hobby charger, to be sure.

I would put my faith in buying new, fresh cells from somebody of repute, but even then, unless you're matching up date codes and you know for sure that they're BOTH healthy ones to begin with, we really don't know for sure unless we use that hobby charger to tell us.

Most of these consumer lithium-ion chargers don't go into depth like the NiMH chargers do, such as my LaCrosse BC-700 and Maha C-9000, which do give me/us more info about how closely matched and how healthy those NiMH batteries are.

You could always roll the dice without a hobby charger, but you're a 'scientist' and therefore, you'll want to be thorough in your testing protocol, I would imagine?

Chris
 
hgg said:
Does anybody know what the maximum safe voltage difference is for 18650 cells when in series?
Click to expand...

They are supposed to be the same voltage, but when a battery runs empty it will increase very quickly.

As long as you always turns off when one of the cells drops below minimum cell voltage (2.5, 2.75 or 3.0 depending on cell), there is no danger with differences.
 
Most lights that I know use 3.0v as the indicator for when it is time to charge the cell, so you are in good company with that level. It gives you some time to get the cell out to charge without it being an emergency.
 
I agree with the above. If you're using high quality cells that are the same type and age, they will be very close in capacity. A 100mA draw is very lite for these cells. A 3.0v cutoff will be very safe. Besides that, protected cells offer a backup cutoff should anything ever get out of wack for some reason. :thumbsup:

FWIW, I use an iCharger 3010b (about $200) to charge and test a variety of cell types but there are countless others far cheaper and far more expensive that will do well for your purposes.

Also, HKJ is the only battery guru in this thread so far as I know...certainly not me! :D
 
Hello Hgg,

Welcome to CPF.

When matching cells for a battery a good target is to keep the cells within 2% of each other. Since resting voltage is somewhat related to capacity with Li-Ion cells you may want to impose that on the voltage differences.

Tom
 
Guys, thank you very much for your suggestions! Very helpful.

I will buy two Panasonic protected cells from the same batch,
I will indicate a low voltage condition when under 3V and I will
switch off the circuit when a greater than 2% imbalance is detected.

Thanks again! :thumbsup:
 
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