Maximum Voltage Difference Between 18650 Cells

[hgg]

Hi,

I have almost finished the first version of the little project I am working on 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 voltage LED when either cell goes below 3.7V

I also want the batteries to be balanced because I will use them in series.  With a comparator
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 changed.

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

Thank you.

[Zepnat]

Hi im not a battery expert but seeing as no one else replied I will try and offer a little advice.
Info seems to suggest it is bad to discharge below 2.8/3.0v under load and charge more than 4.2 will shorten the battery life. 4.25v being the absolute safe maximum.
So if you charge to 4.2v the maximum difference would be 50mV, although that doesn't sound much I tried discharging a pair of cells in series a few times and the voltage difference between the cells stayed so close I could hardly measure it, around 1mv I think it was.
To balance, charge them up individually then leave connected in parallel for a few hours to settle, then build into series circuit.

It is good that you have used protected cells, second I hope they are quality from well known manufacturer like sanyo, LG etc, I have used these cheap ultrafire/trustfire batteries and although I haven't had any problems, the capacity was much lower than stated and internal resistance quite high, also there are safety mechanisms under the positive end and cheap cells might not have those, cheap cells are a disappointment eventually.
If you start with a balanced pair of quality same age cells in series in normal use they will stay balanced for a long time with no attention, high discharge rates might make them become unbalanced faster though.

For curiosity what is your discharge current, some protection circuits can cut out as low as ~3.5 amps apparently,

18650 batteries are amazing in their capacity and light weight but not as robust as the old ni-cd cells which I still have an strange affection for.. Also I don't think they suffer from Peukert's law,

Peukert's law

Maybe someone else can offer more advice than this novice.

They are very safe batteries when used with respect. Have fun.

[hgg]

I will be charging the batteries with an external charger so the balancing will be handled by the
charger.  What I want to check is if there is a battery imbalance while they are discharging.
The circuit will draw 100mA maximum and 6mA at rest.

Most of the 18650 batteries I have (same batch) are very well balanced but what I want to know
is what would be considered a maximum safe voltage difference when discharged.  If the difference
is greater than this value the I will light up an LED.

[amyk]

Most of the 18650 batteries I have (same batch) are very well balanced but what I want to know
is what would be considered a maximum safe voltage difference when discharged.

As much as the difference in capacity you're willing to tolerate. It's already safe if you monitor both cells independently and make sure neither of them gets overcharged or overdischarged. If they become unbalanced you just get less capacity.

[hgg]

Since I will be using an external charger which does not let any battery go more than 4.2V, maybe its
simpler to only measure the total voltage and if its less than 7.2V (3.6V*2) 6.0V (3.0V*2) then I will
light up the low battery warning.  (which might also indicate an imbalance.)

[Whales]

If you are individually charging the cells then any voltage imbalance issues will be sorted out by the charger.  Provided your cells are similar (ie same batch + model) and are not left for long periods of time without re-balancing you will not have problems.

Some 18650 li-ion cells are designed to be used down to 2.6 volts or lower, and some charge higher than 4.2 .  The exact voltage range you use is up to you: regardless of the various differences in cell quality you can opt to use a smaller charge range and extend the life of the batteries.  This works both ways -- only discharging to 3v per cell and only charging up to 4v per cell will both improve cell life (at the cost of mAh/WAh).

None the less, don't ever exceed the general guidelines of 2.7v to 4.2v without knowing the exact specs of your cells!