3.7 lithium battery charger: an alternative. No buck.

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bubugian
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Posted by bubugian: Tue. Sep 25, 2007 - 04:34 PM
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Hi all,
I need to develop a circuit to charge a single cell 3.7 lithium 2600mAh battery.
Due to EMI needings, I'd like to not use buck or something else so, I've thought to a circuit like te one attached.
Diode D could be (for example) LM431.
R1 and R2 could be calculated in order to have 4.2V (4.2V=battery's charging voltage)...
Box at the center of circuit it's obviously a uP: maybe a tiny or atmega88 if I choose to implement some other features...

uP's tasks are:
- activate/deactivate OA (charge ON/OFF)
- read current (while charge ON)
- read battery's voltage (while charge ON)
- read battery's temperature (charge ON)

Questions:
- Could someone suggest me a cheap high current opamp with 'enable' command (farnell, rs-components, distrelec...) ?
- What do you think about this solution?
- Do you have cheap alternatives or suggestions?

I could also evaluate to not use a uP!

Best regards, thanks for your help!
Bubugian.

Attachment(s): 

Image icon Battery charger proposal

Tags:

Learning and Information, General Electronics, ATmega8, ATmega88
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John_A_Brown
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Posted by John_A_Brown: Tue. Sep 25, 2007 - 05:01 PM
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Hi, what do you mean by "I need to develop a circuit"? Is this for an assignment, for a commercial product or simply for fun?
I have "developed" several circuits controlled by microcomputers for nickel cadmium and nickel metal hydride batteries for commercial products, but I would think twice before doing the same for lithium cells. The way I understand it, it's too risky, and there are dedicated chips available.
Just my opinion...

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bubugian
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Posted by bubugian: Tue. Sep 25, 2007 - 05:33 PM
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Quote:
Hi, what do you mean by "I need to develop a circuit"? Is this for an assignment, for a commercial product or simply for fun?

It is for a commercial product.

Quote:
The way I understand it, it's too risky, and there are dedicated chips available.

If I've well understood, risks are related to overcharging and heating... there's something else?
I evaluate some dedicated chip (like Texas instruments BQ24200DGN) but charge time is very long. With a uP I could also do more intelligent things (display battery level ecc.).

Moreover, (farnell, rs, distrelec ecc.) doesn't offer interesting things. Any suggestion?

Thanks to all.
Bubugian.

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John_A_Brown
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Posted by John_A_Brown: Tue. Sep 25, 2007 - 06:16 PM
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Quote:
If I've well understood, risks are related to overcharging and heating... there's something else?

You need more?
Seriously, AFAIK the charging voltage needs to be very accurately controlled. I would guess that the series resistor you've added, for example, (I know you've not specified the value) could affect the voltage control, since it comes after the op-amp feedback point. I reiterate that I wouldn't want to attempt it. However, you may be a great circuit designer.

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

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jgmdesign
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Posted by jgmdesign: Tue. Sep 25, 2007 - 07:08 PM
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Pardon my naivete' but if I recall lithium batteries are use once only, like the coin cells. since the capacity on this one the OP lists as 2600mah is it safe to say it is Li-ion? If it is not and purely Lithium, then Mr. Brown is right.

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Guillem Planisi
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Posted by Guillem Planisi: Tue. Sep 25, 2007 - 07:19 PM
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I would suggest to read about Maxim, Texas Instruments, Microchip and other manufacturers Li-Ion charging solutions. As Mr. Brown stated before, the accuracy of the voltages should be really good, less thant 25mV. Otherwise, you can have fireworks. Specially if you want to charge it fast (about 4 hours). And this is not a joke. There had been few videos about laptops on fire, and whole batchs of Li-ion battery packs retired from the market.

Guillem.

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Plons
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Posted by Plons: Tue. Sep 25, 2007 - 08:47 PM
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Nod-nod

I agree with the previous posters: Lithium cells can behave very nasty when not treated with the greatest respect. These baby's burn. And since you say it's for a commercial product, you need to be even more carefull: if it goes wrong, you get sewed.

Yes, it can be done. But don't start from scratch. Use Guillem's suggestions.

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Posted by Jepael: Tue. Sep 25, 2007 - 09:25 PM
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The dedicated Li-Ion battery chargers also are EMI compliant enough, but extra filtering might do some good too. I suggest reading the Wikipedia article on Lithium-ion batteries why it is so hard to charge them by yourself.

Linear charger would dissipate enormous amounts of heat if using high charge rates. And yes the battery charging voltage is not 4.2, it's more like 4.200 to be more exact, and tolerances of like 0.5% is required. Besides you need to do current limited constant voltage charging until the charge current drops to 10% from the initial constant current limit, and then stop the charging.

- Jani

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daqq
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Posted by daqq: Tue. Sep 25, 2007 - 09:34 PM
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http://www.maxim-ic.com/quick_vi...
http://www.maxim-ic.com/appnotes...

The wisdom of Maxim is at your disposal. And so are the parts. The chip is 3.25$. Which is the price of an ATMega88.

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bubugian
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Posted by bubugian: Wed. Sep 26, 2007 - 10:09 AM
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A read some documents... seems that charge voltage must not exeed 50mV from 4200mV.
After some calculation, i found that my circuit is not suitable due to min-max Vref value (Vout will vary from 4.127V to 4.272V) a and resistor thermal derive.

So, I'd like to switch to an integrated solution...

Quote:
The wisdom of Maxim is at your disposal. And so are the parts. The chip is 3.25$. Which is the price of an ATMega88.

Unfortunately MAX486A is not commercialized by Farnell, Rs, Distrelec.
I need to buy just 5 for prototyping...
Someone could help?

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daqq
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Posted by daqq: Wed. Sep 26, 2007 - 10:19 AM
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Well, maxim does sell directly. AFAIK even in small numbers. And you'll get 2 pieces for free, as free samples.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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jayjay1974
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Posted by jayjay1974: Wed. Sep 26, 2007 - 10:32 AM
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When I needed to design such charger for a client I choose a Linear Technology chip. The LT distributor was very helpful in supplying free LTC4002 charger and LTC4150 gas gauge demo boards 8)

Also, Farnell does carry a selection of dedicated charger ICs from TI and Intersil.

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jgmdesign
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Posted by jgmdesign: Wed. Sep 26, 2007 - 12:06 PM
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I have sampled $30.00 parts from Maxim at 10pc requests and they have sent them for FREE. I was in total shock that they granted my request and apologized they would take a week as they needed to 2nd day them from the phillipines! You should have no problem getting 5 of them from Maxim. Just fill out the online request and they will let you know when they ship

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

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davidpr
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Posted by davidpr: Tue. Oct 2, 2007 - 02:50 AM
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This might be helpful (it's a chip from maxim):
http://www.sparkfun.com/commerce...
and with a breakout board, for testing:
http://www.sparkfun.com/commerce...

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ka7ehk
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Posted by ka7ehk: Tue. Oct 2, 2007 - 04:20 PM
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I've built LiIon chargers for hand held units. I have used both linear and switching chargers from TI. We abandoned the linear because power dissipation was too high and the IC got too hot.

A LiIon battery wants to b charged at a rate between 0.5C and 1C. For a 2.2AHr cell, that means 1.1 to 2.2Amps. Yes, AMPS! Further, they want to be charged at a constant current until the battery voltage reaches a certain level, then at a constant voltage until the current drops to a certain level OR for a certain maximum time.

I highly recommend NOT using that circuit that was proposed in the original post. The batteries will die before they need to.

Jim

 

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