Unequal size Li-Ion batteries in series?

T

Thomas David Kehoe

Guest
Is it possible to use two rechargeable Li-Ion batteries of unequal sizes in
series?

I'm designing a device that will run at 4V, 25-30mA. The device is pager
size so a 9-volt battery is too big. I thought I'd found the solution in RC
car batteries. These are flat so they can lie on top of the circuitboard.
They are polymer li-ion, 3.7V. They are available in two sizes: 750mA and
170mA. Two 170mA batteries lasts only 7.5 hours. I need at least 15 hours.
The case can fit one large battery and one small batteries, or two small
batteries. Possibly I could fit one large and two small, or three small
batteries.

I set up one large battery and two small batteries in series. The voltmeter
read 11.9V. I set up the device and came back ten hours later. The large
battery was at 4.1V - fresh - and the small batteries were at -0.1 volts.
They had puffed up. I charged them but they never went over 0.1 volts. How
could the large battery be fresh and the two small batteries be destroyed? I
can't figure out a way to measure current draining from each battery.

Now I have two test devices running. Each has one large and one small
battery, in series. The difference is that one device has the large battery
connected to the positive terminal and the small battery to the negative
terminal; the other device has the small battery connected to the positive
terminal and the large battery connected to the negative terminal.

Any other suggestions?
--
Electronic Anti-Stuttering Devices http://www.fluencydevices.com
Thomas David Kehoe Casa Futura Technologies (303) 417-9752
 
"Thomas David Kehoe" <kehoe@casafuturatech.com> wrote in message
news:BEAD3C5C.9958%kehoe@casafuturatech.com...
Is it possible to use two rechargeable Li-Ion batteries of unequal
sizes in
series?

I'm designing a device that will run at 4V, 25-30mA. The device is
pager
size so a 9-volt battery is too big. I thought I'd found the solution
in RC
car batteries. These are flat so they can lie on top of the
circuitboard.
They are polymer li-ion, 3.7V. They are available in two sizes: 750mA
and
170mA. Two 170mA batteries lasts only 7.5 hours. I need at least 15
hours.
The case can fit one large battery and one small batteries, or two
small
batteries. Possibly I could fit one large and two small, or three
small
batteries.

I set up one large battery and two small batteries in series. The
voltmeter
read 11.9V. I set up the device and came back ten hours later. The
large
battery was at 4.1V - fresh - and the small batteries were at -0.1
volts.
They had puffed up. I charged them but they never went over 0.1 volts.
How
could the large battery be fresh and the two small batteries be
destroyed? I
can't figure out a way to measure current draining from each battery.

Now I have two test devices running. Each has one large and one small
battery, in series. The difference is that one device has the large
battery
connected to the positive terminal and the small battery to the
negative
terminal; the other device has the small battery connected to the
positive
terminal and the large battery connected to the negative terminal.
Click to expand...
You've already found out the hard way that the small battery won't work.
If you must have a certain voltage, you should use a V converter to
boost the single large cell up to what you need, and skip the multiple
oddball cells.



Any other suggestions?
--
Electronic Anti-Stuttering Devices http://www.fluencydevices.com
Thomas David Kehoe Casa Futura Technologies (303) 417-9752
Click to expand...
 
Thomas David Kehoe wrote:
Is it possible to use two rechargeable Li-Ion batteries of unequal sizes in
series?

I'm designing a device that will run at 4V, 25-30mA. The device is pager
size so a 9-volt battery is too big. I thought I'd found the solution in RC
car batteries. These are flat so they can lie on top of the circuitboard.
They are polymer li-ion, 3.7V. They are available in two sizes: 750mA and
170mA. Two 170mA batteries lasts only 7.5 hours. I need at least 15 hours.
The case can fit one large battery and one small batteries, or two small
batteries. Possibly I could fit one large and two small, or three small
batteries.

I set up one large battery and two small batteries in series. The voltmeter
read 11.9V. I set up the device and came back ten hours later. The large
battery was at 4.1V - fresh - and the small batteries were at -0.1 volts.
They had puffed up. I charged them but they never went over 0.1 volts. How
could the large battery be fresh and the two small batteries be destroyed? I
can't figure out a way to measure current draining from each battery.

Now I have two test devices running. Each has one large and one small
battery, in series. The difference is that one device has the large battery
connected to the positive terminal and the small battery to the negative
terminal; the other device has the small battery connected to the positive
terminal and the large battery connected to the negative terminal.

Any other suggestions?
Click to expand...
Do NOT charge or discharge equal lithium ion batteries in series without
some monitoring circuitry to keep 'em from doing what yours did.
NEVER, EVER charge or discharge unequal lithium ion batteries in series.
Equal means same part number, same history, same batch.

You don't want to do that with ANY battery technology, but at least
other technologies are LESS likely to explode when you do. I said LESS
LIKELY...

If you intend using lithium cells, you need to do a LOT of reading on
the subject. The Cadex site is a place to start.
mike
--
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
..
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Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
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On Sun, 15 May 2005 17:50:20 -0600, Thomas David Kehoe
<kehoe@casafuturatech.com> wrote:

Is it possible to use two rechargeable Li-Ion batteries of unequal sizes in
series?
Click to expand...
No - in general, it is a Very Bad Idea to use batteries of different
capacities in series, as the lower-capacitiy battery will become fully
discharged before the higher capacity one, then the higher capacity
battery will start to reverse charge the low capacity one..

I'm designing a device that will run at 4V, 25-30mA. The device is pager
size so a 9-volt battery is too big. I thought I'd found the solution in RC
car batteries. These are flat so they can lie on top of the circuitboard.
They are polymer li-ion, 3.7V. They are available in two sizes: 750mA and
170mA. Two 170mA batteries lasts only 7.5 hours. I need at least 15 hours.
The case can fit one large battery and one small batteries, or two small
batteries. Possibly I could fit one large and two small, or three small
batteries.

I set up one large battery and two small batteries in series. The voltmeter
read 11.9V. I set up the device and came back ten hours later. The large
battery was at 4.1V - fresh - and the small batteries were at -0.1 volts.
They had puffed up. I charged them but they never went over 0.1 volts. How
could the large battery be fresh and the two small batteries be destroyed? I
can't figure out a way to measure current draining from each battery.
Click to expand...
What was the current drain? If it was 30 mA, your 170 mAH batteries
would be fully discharged in about 5.6 hours, while the 750 mAH cell
would be good for about 25 hours.
Now I have two test devices running. Each has one large and one small
battery, in series. The difference is that one device has the large battery
connected to the positive terminal and the small battery to the negative
terminal; the other device has the small battery connected to the positive
terminal and the large battery connected to the negative terminal.
Click to expand...
The order of the batteries doesn't matter - the 170 mAH cell is only
good for 5 hours, regardless of where you put it.

Any other suggestions?
Click to expand...
--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
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