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Battery selection, Li+ vs NiMH

D

Deefoo

Jan 1, 1970
0
Hi,

I have to choose a battery for a new portable application. The device will
consume about 2W (worst case) continuously, with a rather flat load. The
device has to be autonomous for at least 12 hours and will operate outdoors.
The device has to be very reliable, maintenance is available but should not
be too expensive. Unfortunately, I do not know how the device will be used
in practice so I do not know how the user will be charging and/or
discharging it.

Looking at the pro's & con's of NiMH and Li-Ion batteries I find it
difficult to choose one or the other.
Is NiMH obsoleted by Li-ion or is it still a viable choice? Any suggestions?

Thanks,
--DF
 
P

PeteS

Jan 1, 1970
0
Deefoo said:
Hi,

I have to choose a battery for a new portable application. The device will
consume about 2W (worst case) continuously, with a rather flat load. The
device has to be autonomous for at least 12 hours and will operate outdoors.
The device has to be very reliable, maintenance is available but should not
be too expensive. Unfortunately, I do not know how the device will be used
in practice so I do not know how the user will be charging and/or
discharging it.

Looking at the pro's & con's of NiMH and Li-Ion batteries I find it
difficult to choose one or the other.

Is NiMH obsoleted by Li-ion or is it still a viable choice? Any suggestions?

Thanks,
--DF

Both NiMH and Li+/LiPoly aeer viable choices, but if you use Li+, you
(not the user) will be required to set the charging as there are
precautions that must be observed.

Given your requirements, you have 24WH of energy required (so assuming
90% efficiency, which is perhaps marginal), you need 26.7 WH of energy.

I don't know what space you have available for batteries, but the
highest energy density between these two is definitely Li+ (about 3x
the energy density of NiMH). LiPoly has higher energy density, but
costs quite considerably more.

A single slightly oversize A cell in Li+ has up to 2.2AH at 3.7V
nominal (terminal voltage ranges from 3.0V at nom. discharge to 4.2V at
nominal full charge), giving 8.14WH [typical] of available power. A
typical NiMH in the same size is 1.2V @ 2.2AH, yielding 2.64WH (perhaps
a little less, depending on the cell), so if you don't want to have a
pretty heavy unit, you need to look closely at Li+.

Charging Li+ batteries is not rocket science, but you do need to take
care. I use Li+ in some current products, and I use a single chip
charge controller (needs some external components) from TI (BQ24103),
but a number of mfrs make such things. Li+ and LiPoly share the same
charging requirements.

Not knowing what system voltage you need internally doesn't help the
analysis either. If the parts you are using can all run from 2.7V to
4.2V, then running them direct from a battery makes sense, otherwise
you will obviously need power supplies (and I strongly suggest
switchers for their efficiency).

So the call really comes down to how many batteries you can fit in your
box and what weight you're prepared to put in it.

Cheers

PeteS
 
D

Deefoo

Jan 1, 1970
0
PeteS said:
Deefoo said:
Hi,

I have to choose a battery for a new portable application. The device will
consume about 2W (worst case) continuously, with a rather flat load. The
device has to be autonomous for at least 12 hours and will operate outdoors.
The device has to be very reliable, maintenance is available but should not
be too expensive. Unfortunately, I do not know how the device will be used
in practice so I do not know how the user will be charging and/or
discharging it.

Looking at the pro's & con's of NiMH and Li-Ion batteries I find it
difficult to choose one or the other.

Is NiMH obsoleted by Li-ion or is it still a viable choice? Any suggestions?

Thanks,
--DF

Both NiMH and Li+/LiPoly aeer viable choices, but if you use Li+, you
(not the user) will be required to set the charging as there are
precautions that must be observed.

Given your requirements, you have 24WH of energy required (so assuming
90% efficiency, which is perhaps marginal), you need 26.7 WH of energy.

I don't know what space you have available for batteries, but the
highest energy density between these two is definitely Li+ (about 3x
the energy density of NiMH). LiPoly has higher energy density, but
costs quite considerably more.

A single slightly oversize A cell in Li+ has up to 2.2AH at 3.7V
nominal (terminal voltage ranges from 3.0V at nom. discharge to 4.2V at
nominal full charge), giving 8.14WH [typical] of available power. A
typical NiMH in the same size is 1.2V @ 2.2AH, yielding 2.64WH (perhaps
a little less, depending on the cell), so if you don't want to have a
pretty heavy unit, you need to look closely at Li+.

Charging Li+ batteries is not rocket science, but you do need to take
care. I use Li+ in some current products, and I use a single chip
charge controller (needs some external components) from TI (BQ24103),
but a number of mfrs make such things. Li+ and LiPoly share the same
charging requirements.

Not knowing what system voltage you need internally doesn't help the
analysis either. If the parts you are using can all run from 2.7V to
4.2V, then running them direct from a battery makes sense, otherwise
you will obviously need power supplies (and I strongly suggest
switchers for their efficiency).

So the call really comes down to how many batteries you can fit in your
box and what weight you're prepared to put in it.

Cheers

PeteS

Thanks a lot for your answer.

Space is an issue, but not as in wrist watch mp3 player. Weight is an issue
too. I looked at some Li+ batteries (from Yoku (www.yokuenergy.com, the
first supplier I found) and those would fit easily. I can get away f.i. with
three 2.7Ah in parallel (4.5x64x105mm). This would give me almost 30Wh.

The device will run mainly on 3V3, maybe a bit on 5V and a small analog part
needs 8V.

The charger will be integrated in the device so I have full control over
that.

I guess I'll have to use Li+.

Thanks,
--DF
 
F

Fritz Schlunder

Jan 1, 1970
0
A single slightly oversize A cell in Li+ has up to 2.2AH at 3.7V
nominal (terminal voltage ranges from 3.0V at nom. discharge to 4.2V at
nominal full charge), giving 8.14WH [typical] of available power. A
typical NiMH in the same size is 1.2V @ 2.2AH, yielding 2.64WH (perhaps
a little less, depending on the cell), so if you don't want to have a
pretty heavy unit, you need to look closely at Li+.


1.2V, 2.2Ah NiMH in the same size as 2.2Ah 3.7V Li-Ion? That doesn't sound
like an apples to apples comparison. Presumably the cell you refer to as a
2.2Ah slightly oversize A cell Li-Ion cell is a "18650" sized cell (18mm
diameter, 65mm height) with an internal volume of roughly 16.5 cubic
centimeters. This works out to a volumetric energy density of roughly
493Wh/liter. Slightly higher energy density is available on the market for
lithium ion, but we will use this for comparison.

If you go to your local Wal-Mart or similar store and look at their
batteries on the shelf... You should find at least one brand of NiMH "AA"
sized cells of at least around 2500mAh 1.2V rating. Somewhat higher rated
cells are also available in "AA" size if you shop around. Ebay seems to
have at least a couple of brands that claim NiMH 2950mAh in "AA" size
format, though who knows if such ratings are trustworthy. Anyway, "AA"
sized cells have an internal volume of about 8.3 cubic centimeters, very
nearly half that of your "18650" sized lithium ion cell. If we use the
2.5Ah 1.2V nominally rated cells for comparison, they should store roughly
3Wh each for a volumetric energy density of 361Wh/liter.

Lithium ion technology is great, but NiMH technology is hardly obsolete.
They both have their uses and relative strengths and weaknesses.
 
Fritz said:
A single slightly oversize A cell in Li+ has up to 2.2AH at 3.7V
nominal (terminal voltage ranges from 3.0V at nom. discharge to 4.2V at
nominal full charge), giving 8.14WH [typical] of available power. A
typical NiMH in the same size is 1.2V @ 2.2AH, yielding 2.64WH (perhaps
a little less, depending on the cell), so if you don't want to have a
pretty heavy unit, you need to look closely at Li+.


1.2V, 2.2Ah NiMH in the same size as 2.2Ah 3.7V Li-Ion? That doesn't sound
like an apples to apples comparison. Presumably the cell you refer to as a
2.2Ah slightly oversize A cell Li-Ion cell is a "18650" sized cell (18mm
diameter, 65mm height) with an internal volume of roughly 16.5 cubic
centimeters. This works out to a volumetric energy density of roughly
493Wh/liter. Slightly higher energy density is available on the market for
lithium ion, but we will use this for comparison.

If you go to your local Wal-Mart or similar store and look at their
batteries on the shelf... You should find at least one brand of NiMH "AA"
sized cells of at least around 2500mAh 1.2V rating. Somewhat higher rated
cells are also available in "AA" size if you shop around. Ebay seems to
have at least a couple of brands that claim NiMH 2950mAh in "AA" size
format, though who knows if such ratings are trustworthy.


Speaking of trustworthy... anyone have experience with the higher
capacity NiMH Ds? The Energizer ones seem to just be an AA with a size
adapter - still only 2500 mAh.

Check these out - 12000 mAh Ds.
http://thomas-distributing.com/cta-d-rechargeable-batteries.php
 
Having looked at a lot of Li-Ion battery chargers they all have a
thermistor input which I assume is attached to the battery to check for
overheat. Is this really needed?
This is what has put me off using Li-Ion as I want my customers to fit
there own batteries, sourced locally.
Thanks
 
B

budgie

Jan 1, 1970
0
Having looked at a lot of Li-Ion battery chargers they all have a
thermistor input which I assume is attached to the battery to check for
overheat. Is this really needed?

The thermistor is (or should be) used to verify that the cells are in a safe
thermal region so that charging can proceed. Too hot or too cold are issues.
 
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