Battery selection, Li+ vs NiMH

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

Reply to
Deefoo
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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

Reply to
PeteS

will

outdoors.

not

used

suggestions?

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

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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

Reply to
Deefoo

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.

Reply to
Fritz Schlunder

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.

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Reply to
onehappymadman

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

Reply to
sjones

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.

Reply to
budgie

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