polymer li-ion battery monitoring and under-voltage protection

[M. Noone]

Hi - I recently found out that li-ions can be overly discharged to the
point where they get damaged. Does anybody know at what point this
normally happens at? I was told 2.7V, but then when I've been looking
for a 3 cell under-voltage protection chip I stumbled upon the max1665
(http://pdfserv.maxim-ic.com/en/ds/MAX1665S-MAX1665X.pdf) that has a
cutoff voltage of 2.5V. So what is the proper cut off voltage? Also,
does anybody know of any chips similar to the 1665 but that have a
higher cutoff voltage (if that voltage is too low). It looked perfect
for my needs besides the low cutoff. If it matters, I plan on using
three of these:
http://advantagecircuits.com/ProductDatasheet/ED425585datasheet.pdf

On a related note - is there a good way to monitor the voltage of a set
of 3 li-ion cells in series? They will be powering a dc/dc converter
that will be powering a ARM microcontroller. I thought about just using
a resistive voltage divider, but is there anything better out there?

Thanks,

-M. Noone

 

[budgie]

Hi - I recently found out that li-ions can be overly discharged to the
point where they get damaged. Does anybody know at what point this
normally happens at? I was told 2.7V, but then when I've been looking
for a 3 cell under-voltage protection chip I stumbled upon the max1665
(http://pdfserv.maxim-ic.com/en/ds/MAX1665S-MAX1665X.pdf) that has a
cutoff voltage of 2.5V. So what is the proper cut off voltage? Also,
does anybody know of any chips similar to the 1665 but that have a
higher cutoff voltage (if that voltage is too low). It looked perfect
for my needs besides the low cutoff. If it matters, I plan on using
three of these:
http://advantagecircuits.com/ProductDatasheet/ED425585datasheet.pdf

On a related note - is there a good way to monitor the voltage of a set
of 3 li-ion cells in series? They will be powering a dc/dc converter
that will be powering a ARM microcontroller. I thought about just using
a resistive voltage divider, but is there anything better out there?

There are a number of manufacturers out there producing porotection chips for 1
through 4 cell stacks, but thye were VERY hard for us to locate two years back.

One of the key roles of the protection module in multi-cell configurations is to
guard against differences in cell voltage. When these differences appear, the
result of a voltage divider approach is that the lower voltage cells discharge
further than the "average" represented by the divider. Similarly, on charge the
CC->CV transition and termination conditions will be met with (at least) one
cell at a higher than desired potential. Both are potentially dangerous
situations. Use a 3-cell protection chip for a 3-cell string.