How do you charge batteries while still powering device?

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#1
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Hi, hope someone can give me a pointer or a search term to find what I'm looking for.

I am using a Rechargeable lithium 4.2V 50 mAh battery coin sized batter (CR2032, (LIR)). The battery powers a micro controller and bunch of other low voltage stuff.

I get about 5 hours of life out of my current design iteration, I'd like to be able to plug my device into the wall transformer some how and have an onboard IC charge my battery (I've found several Constant Voltage Constant Current charger ICs that put out 40mA which is what I want).

I also need the design to stay on when it is plugged in to be charged. Is there any way I can design this so that the 40mA from the charger IC goes into the battery while the battery keeps powering the rest of the system all at the same time?

My cell phone for example stays on while its plugged in and the battery still charges properly... is there a right way to do this in my application?

Thanks!

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Looks like 5V thru a 160 ohm resistor would charge it at 5ma... about .1C. Would that work ok?

Imagecraft compiler user

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Not with an LiPo Uncle Bob.

The circuit below, unapologetically stolen from Jim's DSO thread, shows how the LiPo can be charged from the USB port while still powering the circuit.

Cheers,

Ross

Attachment(s): 

Ross McKenzie ValuSoft Melbourne Australia

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OK, I read about charging lipos. Charge at .4 to .7C up to 4.2V, then charge at .1c until the current drops to 1/10th of the starting current. Or something like that. Anyway it just involves measuring and adjusting voltage and current. An easy job for a computer with a built in a/d converter.

Imagecraft compiler user

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Quote:
I am using a Rechargeable lithium 4.2V 50 mAh battery coin sized batter (CR2032, (LIR)).

THe CR2032 IS NOT rechargeable from the looking I did online. I am assuming you really mean the LIR2032 3.6v 40mAH.

@ROSS,
What makes you think I would have accepted an apology to begin with? ;) :)

Jim

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Jim,

1. The OP was/is referring to the size of his LiPo being the same as the commonly recognised CR2032 ... at least that is how I read it first and subsequent times.

2. You are too gracious to accept an apology ... I know!

Happy New Year.

Ross

Ross McKenzie ValuSoft Melbourne Australia

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Shunt regulator Li-ion charger: LTC4071; use LTC4070 if don't need the cell discharge undervoltage protection. Wall-wart/photovoltaic/supply current will be split between cell charging/shunt and the load. Will have to add a current limiter to protect the LTC4071 and the cell from over-current during charging/shunting (50mA max. recommended; 60mA max. absolute) which will make USB compatability potentially semi-difficult.
Series pass regulator Li-ion charger: LTC4059A is a low-dropout Li-ion cell charger. The supply current is self-limited; shows 100mA at the low end in the datasheet but may be able to reduce this. USB compatible.

"Dare to be naïve." - Buckminster Fuller

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If you can reduce the discharge voltage by about one volt, consider moving to the ML-series manganese-lithium secondary (rechargeable) coin cell from Panasonic Industrial (ML2020 (45mAh) or smaller). Panasonic Industrial shows a simple charging/power circuit (zener diode for charging voltage limiting, resistors for current limiting, diodes for switching load from external to internal power). A lot of analog and digital can now run on about 2 volts. If use the VL-series (vanadium-lithium) secondary coin cell then lose about 0.8 volts but the cell's price is greater.
Attached is another possible way to charge the cell and switch power. Caution: Does not prevent charging when cell temperature is outside the charging temperature range; do not charge if cell's temperature is below freezing.
Note: Added caution.

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"Dare to be naïve." - Buckminster Fuller

Last Edited: Sat. Dec 31, 2011 - 06:55 PM
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I was working on a Li-ion charger when I stepped away many moons ago. I want to reference this thread later, is there a way to favorite a thread? :)

Just some guy

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Quote:
is there a way to favorite a thread?

Not that I know of.

Why not just create a shortcut to the thread? Or send the link to yourself by email?

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Thanks, I will do that in the future! That's why you make the big money Johann;)

Just some guy

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@Johnrk
There is the watch icon at the top of the thread.
It will email you when new replies are posted.
Somewhere you can find a list of watched threads but I can't find it now...