Slightly OT: Choosing battery technology

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Hi All -

I'm trying to decide on a battery technology to use for a project - it needs 2.7V min, though it can take up to 5.5V. The case it's going in will fit two AA batteries (about 2 1/4" x 1 1/4"), though I'm not stuck on that form-factor - rectangular or whatever will work, so long as it will fit in there.

I also want to include a charging circuit for it if possible. So one can run it off the batteries, or plug in a wall-wart to run/charge. Technology-wise, it looks like the choice is between :

1) NiCD
2) NiMH
3) Li-Ion

4) Lithium - non-rechargable
5) Standard alkaline - non-rechargable

Max current draw will be around 80mA, usually somewhat lower. (Lots of LEDs). Preference would be for easy availability - as in in-stock at RadioShack :)

One non-rechargeable possibility that looks pretty good is using two 3.6V 2.1Ah AA batteries in parallel for 4.2Ah. That would be good for around 52 hours of use I believe at 80mA draw. Did I do that right ?

There's absolutely no way to recharge lithium cells is there ?

NiMH AA cells have large capacities, but are only 1.2V per cell. I would need either four 1/2 AA cells for 4.8V, or a 3-cell pack for 3.6V that fits in the space of two AA ...

NiCD doesn't seem to have particularly good capacity, plus there's the memory effect to have to deal with. I suppose the charging circuit could be "smart" and either not actually charge it until the battery is almost completely depleted. Or it could cycle the battery itself, first discharging it all the way, then charging it back up again.

That requires a load though - hrm, could turn on all the LEDs full bore, but still, not the best of solutions.

So, what are people using these days ? How would you go about this ?

I've had mention in another forum to use something like a MAX756 step-up converter, which will allow pulling the last dregs/ergs from the batteries. Sounds good. I wonder if there's a combination chip that does also does charging ? Maxim has a lot of charging chips - have to look through them.

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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Another battery type you may want to look at is rechargable alkaline. This has most of the advantages of the primary alkaline but also has very low self discharge. This biggest thing with these is to recharge often, the less capacity taken between re-charge, the more energy you will actuall get long term. They don't work well at high or very low discharge rates, but your 80mA is an ideal rate for the battery chemistry. Rechage is easy, just use a constant volatge with current limited to less than 1A. You do need to control the voltage on each battery (1.65-1.68VDC) though, the chemistry is VERY sensitive to overcharge.

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My choice would be the NiMH power source. Maxim makes a few chargers, and don't quote me but, I beleive that they have a converter/charger specifically for the NiMH devices. One thing I have noticed about NiMH cells is that their charge storage retention is far better then NiCAD cells. NiCAD cells loose about 10% of their stored charge per month. NiMH is about 1 to 2 percent per month.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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remember to study the discharge cycle of the batteries. Generally, depending on battery type and load, the volage will decay as the battery discharges.

example: alkaline button cells suffer a slow voltage drop all the way through their life cycle. Silver oxide button cells on the other hand suffer hardly any voltage decay over 99% of their lifecycle....i.e. they die suddenly .

so if for example you use 2*AA alkaline cells, you might only be able to utilise a very small proportion of their capacity......i.e. until the voltage drops to 2.7 volts, which could be fairly early during the lifecycle (depending on the load).

be carefull with lithium batteries (particularly button cells) as sometimes they are only able to provide a few milliamps (depending on battery size).

On one occasion, when faced with a similar dilema, I tested several types of battery by discharging each one with an appropriate load. After obtaining a plot of battery voltage over life cycle, choosing a particular battery was then very easy.......i.e. the choice was obvious. If you can spare the time to do such an experiment now, it may save you a lot of trouble later........particularlly if the battery choice is critical.....i.e.. affects either the design of the circuit and/or case.

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GP Batteries makes 950 mAh NiMh AAA batterises. Can you fit three of those in you device? If that's not enough, they make prismatic 1450 mAh NiMh batteries. I don't know the exact size. Length and width is ~ the same as a AA battery, but they are flat, so I think three of those would take about the same space as two AA (or maybe even less). They are a bit expensive though.

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how about a single battery and then use a maxim IC to give you a regulated 3 volt output.

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AVR450 from the Atmel site gives a schematic and code for a smart battery charger for several battery types.

Ralph Hilton

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This is the space I have available. The battery connectors are for AA batteries, and it will fit two side-by-side like that, perfectly. Almost as if the case battery compartment was designed like that :P U1 and U3 are Mega8 footprints - just seeing how much room they take up. I would prefer DIP - easier to work with and I have lots - but will go with tqfp32 if I have to.

The enclosure is a Hammond 1593Q. As you can see, I've extended the pcb under the battery area, since I just know I'll need the space ...

http://www.hammondmfg.com/1593ptbl.htm
http://www.hammondmfg.com/pdf/1593Q.pdf

I've been looking at the MAX712 charger - this looks like it will do the trick. It can charge NiMH and NiCD, and will power the load while it's charging.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666/ln/

The only trouble is that it takes a fair few external components, and I really don't have much room. In fact, I would have to fit the whole charging circuit on the bottom layer in surface-mount, under the batteries. There's not much room - the components only have 177mils (4.5mm) of height available on the bottom layer. The charging jack is at the other end of the box.

Also, I can't easily use AA NiMH, unless there are more than one cell per AA. They're only 1.2V per cell, and 2.4V is not enough - need at least 2.7V, preferably 3V or 3.3V. So I would need to find a NiMH pack that will fit in that 2 1/4" x 1 1/4" space that puts out 3.6V. Oh, and be commonly available.

One option for AA NiMH is also include a MAX756 stepup converter, to boost the 2.4V up to 3.3V or 3.6V. More parts though, in an already tightly constrained space.

Hrm, just fitting AAA cells in there. It's really really tight, but I might be able to swing it. The cells would have to be staggered, high-low-high on the pcb, to let the wings of the connectors expand when the batteries are inserted. Except then there's no room for the switches. Hrm again, could just snip off the legs of the battery connectors, solder them to the board directly a la surface-mount ... Not very stable, would probably rip up the copper foil when changing batteries.

Yeesh, code is sooo much easier than all the other crap. If the Lithium 3.6V AA cells weren't so expensive ($12.50ea !) I would just go with those, and to blazes with recharging.

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Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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Before looking at the lithium batteries have a look at the pulse discharge characteristics Also what temperature range do you want to operate at?

Keep it simple it will not bite as hard

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Pulse-discharge characteristics ? I'm not sure what you mean ...

This is for a small hand-held toy, so temps would be normal room-temps.

I've been digging around the Analog/National/Maxim/Linear sites for battery charger chips, and there's quite a selection out there. Once thing has me a little confused though - Most of the Li-ion chargers indicate that a single cell is 4.1V or 4.2V. Yet I've seen mention elsewhere that a single cell is typically 3.6V. Which is it ?

Ah, wait, I see. Normal voltage is 3.6V, while charging voltage is 4.1/4.2V

Analog looks a winner here. The ADP3820 Li-ion charger chip is a SOT-23-6, and only needs 2 caps, a resistor and a p-ch fet. No inductors or anything.

http://www.analog.com/UploadedFiles/Data_Sheets/490002128ADP3820_a.pdf

The National LM3620 looks even better. You can select the anode type with a jumper, and it looks like it only needs 1 cap, 1 resistor and 1 NPN. The datasheet is a little thin on values though ...

http://cache.national.com/ds/LM/LM3620.pdf

The MAX1551/1555 look great - just needs 1 or 2 caps, but I don't see them available from any of the regular distributers via http://www.findchips.com. That seems to be typical for Maxim - you find exactly the right part, but can't source it anywhere.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4002/ln/

It looks like one of these would do the trick nicely. To ensure getting a full output from the cell(s) I can also add in the MAX756 or similar step-up if there's room.

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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One other issue with batteries in parallel is that if they aren't well matched, you will have one discharge faster and then the other charge it which may cause problems depending on the battery type.

If you need 2.7 volts minimum, it sounds like you are having to use a boost converter of some type if you want to run most of the battery types in parallel.

Why not run in series? I just did a project using two AA batteries in series with a boost converter to 3.3 volts. The current draw was such (an LED load) that converter efficiency was around 90+% and it will run all the way down to 1.6V (0.8 volts per cell) before the power circuit gets too noisy to run.

And as the battery voltage goes lower, the converter's impedance drops so the actual endpoint of the cells isn't too important time-wise. Just doesn't make a lot of difference. I used alkalines in my project only for convenience for the people having to keep them in batteries.

Please note - this post may not present all information available on a subject.

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Hrm. Just found what looks like an ideal Li-ion battery.

http://www.allelectronics.com/cgi-bin/category.cgi?category=160&item=LBAT-43&type=store

It's the right size (just) and has plenty of capacity. Well, 1200mAH instead of 2100mAH like the AA plain-litium type, but it's rechargeable. Given the voltage curve of Li-ion, particularly graphite anode types (instead of the Sony coke anode type) I wouldn't even need the step-up converter. Just have to pick a charger chip now.

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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TI has some nice Li-Ion chargers. I am using bq24103 for a pair of series-connected cells.Its a switcher so internal power dissipation is a lot lower. Used a linear in an earlier design and heat was a real problem.

One difficulty with TI chips is that they have a solder-down "power pad" in the middle of the chip. Its difficult to hand solder and nearly impossible to remove intact.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Hrm. Let's see, the Li-ION batteries are about 3.6-3.7V per cell, yet they want 4.1/4.2V for charging. Could this lead to some difficulties if you want to run the device while it's charging ?

Dang, in my reading up on charger chips I recall one that specifically said it would charge and also provide additional current for the app as required. But I can't find that datasheet again. I think it was a Maxim chip.

The others (LM3820 and the Ti ones I've looked at so far) don't make any mention of that.

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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For new design i plan to use MAX1811 charger

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FWIW, Maxim's chargers are pretty bad with NIMH.
Too much trickle current for most NIMH cells.
NIMH is probably the most complicated chemistry to really charge right.
I did a NIMH charger that filled a tiny-26 in assembler, and deals with pretty much everything.
Got a spare six man-months? You can't rush charger testing.

Li-Polymer is pretty simple to deal with, and TI makes nice simple charge chips.
There are two flavours of cells, make sure the cell and charger match.

Buy cells from reputable sources.

KC6ETE Dave's Engineering Page: http://www.dvanhorn.org
Microcontroller Consultant specializing in Atmel AVR.
Analog and Digital hardware design.
Custom Palm Applications.

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That one looks great. It's available from Digikey, but only at 100 count min. I don't need 100 :) It's available direct from Maxim in small quantities, but with a 6 week lead time :(

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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Li-ion batteries have a nominal voltage of 3.6/3.7V. when fully charged this voltage rise to 4.1/4.2V per cell.
The exact voltage depends on the type of electrode used.
The charge voltage is extremely critical, and should under no circumstances exceed 4,25V (I think).
Above that there is a serious risk of the battery exploding or catching fire, so you should use high-pricision components where needed.

Now, don't let those risks set you back. most consumer battery packs, such as those for cell phones,
have built in protection circuitry, and accidents with li-ion batteries seem to be quite rare.

I have seen different figures as when to stop discharging, but most sources say about 2.5 to 3V per cell.

I found some discharge curves for li-polymer cells used in model aircraft. I assume you won't draw that much current, but it gives you an idea.
http://www.mamut.com/homepages/S...

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The various charger chips look like they handle the precision bit themselves very well. I was wondering how they would handle having the additional load of the app at the same time.

For example, I will be pulling around 80mA at full load. Some of the USB chargers can/will limit the charge current to 100mA, so I'm assuming that if the app is on, 20mA will go to the batt, 80mA to the app. Wonder what happens to the voltage - that's what they all look at to determine when to stop charging.

For discharging, I have several Mega8 pins available, including ADC inputs. I'll use one to watch the battery voltage, and error out when it drops down to say 2.5V.

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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I'm using one of the TI chips in a project.
It handles load current reasonably well. Like you say, Iload + Ibatt = 100mA (or more if you can get the request approved by the hub for more current)
Oddly enough, my load is also about 80mA

KC6ETE Dave's Engineering Page: http://www.dvanhorn.org
Microcontroller Consultant specializing in Atmel AVR.
Analog and Digital hardware design.
Custom Palm Applications.

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Quote:

For example, I will be pulling around 80mA at full load.

There is a lot of battery and charging discussion on the MSP430 Yahoo group, as many candidate apps for that microcontroller are designed to run off the fumes from a urine sample. There is a very knowledgable poster Al/Onestone that put this out as a summary in thread
http://groups.yahoo.com/group/ms...

Quote:

Matthias, don't go digging into this sort of battery stuff yourself.
battery life prediction is a bit of a black art in its own right. thats
why there are many battery manager chips out there that people have
spent a lot of money developing. PIC's rather excellent 14000 part died
because it was primarily pushed as a smart battery manager, but was
firstly too expensive, and secondly the 'managment' part was left up to
the software designer, most of whom lack the requisite skills.

Take a look at some of the newer battery maanger parts, and read their
data sheets in combination with extensive data sheets available from
some manufacturers. panasonic are particularly good at providing data
sheets on juts about everyhting, from an overview of the technology to
charging discharging long temr effects, safe handling etc. I've tested
many battery technologies over the years, conducting rough life and
temperature related tests. Typically with a current regulated load I'd
measure battery temp, lifetime, voltage droop and voltage recovery over
time. At the end of all this I effectively use 4 battery types as follows:-

1. Lithium coin cells. When current is very low, peak current is < 15mA
for a CR20xx and < 5mA for a CR12/CR16xx, size is important, and mean
power consumption lets me get away with this. ie they have adequate life
(expected life x 1.5)

2. Lithium Thionyl Chloride where very low temperature operations are
required. Or there is a comparatively high mean current and no
recharging is possible, ie longer operating time than a coin cell can handle

3. Li-poly, where mean current exceeds the capacity of a coin cell, but
recharging is possible, and/or a very long operational life is required,
and operating temperatures will not fall below -20C. This actually
covers about 75% at least of my applications that use batteries.

4. High pulse current silver oxides. Where operational life is not as
important as small size, comparatively high pulse currents are required,
and widish temperature range, then these fit the bill.

I've also used zinc air in VERY low current applications, where peak
currents are below 2.5mA and mean below 1mA. These can give an excellent
performance, if you need several weeks continuous operation only. Once
the seal is removed they have a limited life. I drifted away from these
towards silver oxides because many of my applications needed to be sealed.

In the case of silver oxide there are low current and high current
types, depending upon the electrolyte used. KOH gives better low temp
operation, and higher pulse currents than NaOH. When I use these, or
Zinc Air I ALWAYS use them with a low quiescent current dc switcher such
as the MAX1724. This allows me to operate down to 0.8V for a full 3V3 or
5V0 output, and consumes just 1.5uA.

If anybody knows of a switcher with lower Iq and similer operation down
to 0.9V I'd be keen to hear about it.

Cheers

Al

The whole thread is rather wide-ranging, but there is a LOT of discussion, even in this one thread, about the suitability of various battery chemistries under differing load and charging conditions. Al's current (pun intended) favourite seems to be LiPoly for the capacity/weight ratio, but I don't know if it would be suitable for your [relatively heavy] 80mA load.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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refields wrote:
One other issue with batteries in parallel is that if they aren't well matched, you will have one discharge faster and then the other charge it which may cause problems depending on the battery type.

I think a similar thing can happen with batteries in series. I'm talking big aircraft Ni-cads (not sure if it whether it happens with smaller battery packs). As the battery pack gets old, it's possible for one or two cells to obtain a reverse polarity. But the cells can be rebalanced during maintenance to give them a new lease of life.

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True. When going for high discharge rate or deep cycling, you have to watch for the weak cells going to zero charge and then continued discharge reverse-charging the cells. Kills them pretty quick. Each time you reverse a cell, it gets weaker and more likely to get reverse charged on the next cycle. Not really an issue with primary batteries though as by the time one goes dead, the pack is pretty dead and needs to be replaced.

Please note - this post may not present all information available on a subject.

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Good information in that msp430 thread. Probably more that I really wanted to know :P

I have my National LM3620m5's now, just waiting on the batteries from Electronic Goldmine. These are the prismatic ones at

http://www.goldmine-elec-products.com/prodinfo.asp?number=G14948&variation=&aitem=2&mitem=2

The LM3620 looks nice and simple. A tiny SOT-23-5 package, only needs a cap and a PNP pass-transistor (with a base pullup). The datasheet is a little thin, so I'm not sure what PNP to use, or whether it actually needs current regulation.

http://www.national.com/ds/LM/LM3620.pdf

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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Nothing in the LM3620 circuit limits the current flowing into the battery. You must limit the incoming current to avoid providing too much.

PNP transistor? It needs to be able to pass the current that you intend to use charging the battery, and since this is a linear regulator, it also needs to dissipate the power. For example if you plan to charge one cell (4.1V) with 500 mA, and the power supply is 8V, the transistor needs to be able to dissipate 2W.

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Yeah, a little more digging and I figured that. Hrm, I think I'll switch to one of the TI chips. I just requested samples of 4 types to play with.

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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You guys who've used the TI parts ... Have you used the bq24010-14 series yet ? The ones in QFN-SON-10 package. This sucker is only 3mm x 3mm, with 10 pins, and a thermal pad ain the center under it. I'm shuddering to think how to solder it ...

One idea was to put a small via in the thermal pad. That way I can solder the chip on the top using the 10 pins first, then flip the pcb and apply heat and solder to the via. The solder should wick up and attach to the thermal pad and the copper pad ... I hope.

The footprint looks like this ... The top layer is a rectangular pad, while the bottom layer is a 1.3mm round pad with an 0.8mm hole.

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Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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Yick. That's some tight spacing. This is the first shot at placement/routing ...

The track-track spacing is only 7.5mils. Solding this puppy should be fun ... Hrm, pcb-fab house will have to be able to do 7.5mil spacing ...

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Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

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Slightly OT, but you might be interested in this:
http://www.newscientist.com/article.ns?id=dn7081
for your next project.

Four legs good, two legs bad, three legs stable.

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Hah - very cool article. Can you imagine the currents when you can charge in just 6 minutes ?

Well, I got my TI samples ... Dang, that QFN package is small, and the connectors are just colors on the casing. I don't think it will be possible to solder these by hand with an iron - it will have to be with hot-air and paste.

I don't think my original idea of having a via in the center will work. It's so dang small that unless it's lying absolutely flat on the pcb it will be very easy for a connection to fail. I think. The pads are too close together to allow soldermask between them, so I think there will be a distinct possibility of solder bridges. And to make matters worse, they would be under the package where you can't see them, not fix them without removing the chip.

I think this package is essentially the same as the MLF one ... How much luck have people had with that ?

Oh, btw - the images above are bad. I had the pin order reversed. I've fixed the footprint now :) Still noodling the via in the center bit though. What do you all think ?

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)