A a ICless charging off circuit @4.2V for lithium

[dave_j_fan]

 STory :
 Wanted to make a reliable power bank
 
 modified a 1$power bank , swapping reliable cells
 
 Issue :
  The cheapy somehow dont cut @4.2V ...
  need to add a tiny thingy preferably BJt+mosfet circuit so that
  the cut happens @4.2V


comment on  the  cut circuit .. i want to swap he mosfet with IRLML2502TRPBF



  [I help many to make useful circuits and they cant  procure special chips
   so bjt mosfet as they are available all d places
   will be suitable for every one, more over even small loss while charging is no issue
   ]

[mariush]

I'd suggest just using a MCP73812 : http://ww1.microchip.com/downloads/en/DeviceDoc/22036b.pdf  (or a similar IC)

It's easy to find at stores like Digikey, Farnell / Newark or even directly from Microchip's store : http://www.microchip.com/wwwproducts/Devices.aspx?product=MCP73812#pricindAndSamples  (may even be able to request one as sample but it may take a long time or may be denied)

[dave_j_fan]

we have a slight problem 
to get a 0.44$ chip .. need to pay digikey 40-50$

suggested chip just goes 0.5Amps or less it will take ages to fill 10400mAH

[JoeO]

If you turn on your country flag, someone may be able to help you by directing you to a place you can buy the parts from.

[amyk]

What IC does it use and what voltage does it actually output? Note that the typical "absolute maximum" voltage for lion cell is actually 4.25V and typical charging ICs have a tolerance of 1% which fits within this range.

There are also some 4.30V and 4.35V cells around.

[mariush]

dave_j_fan , i suggested several online distributors so that you could pick which ever works better for you, since I didn't know your country.

Farnell has an online store customized for your country, in.farnell.com , and here's the IC you want on that site: http://in.element14.com/microchip/mcp73812t-420i-ot/ic-li-ion-li-poly-charger-sot23/dp/1627187

Shipping will probably be a few dollars (in your Rs currency), just make an account (it's free, no obligations) and check how much they charge for shipping. They have a warehouse in Singapore and one in UK, if the products are only in the UK warehouse, they may charge an additional fee per package to ship it to you. But it says so before you complete the order.

If you don't like that chip because it's limited to 500mA, just search for another one that can do more, there's 459 ICs listed on the website: http://in.element14.com/battery-management_battery-chargers

Just filter to see only ICs that charge lithium batteries, then if you want filter by the amount of current they can do. I suggested that one because it was stupid easy to use and solder, and you needed just that chip and a couple of ceramic capacitors and a resistor.

Here's two that can do 1A : http://in.element14.com/microchip/mcp73833-ami-un/ic-battery-charger-1a-msop-10/dp/2146603   and http://in.element14.com/microchip/mcp73834-fci-un/li-ion-li-poly-charge-controller/dp/1332162
and this one can do up to 2A: http://in.element14.com/texas-instruments/bq2057sn/prog-battery-charger-2057-soic8/dp/1101403

[dave_j_fan]

USES hotchip HT4921 http://www.hotchip.com.cn/en/Product.asp?tid=2&SortID=33

acc specs it sd cut at 4.22V but accuracy 3% .. that may be risky ?

mariush,
that web links look they ship but here in INDIA they are hassle
dont take orders w/o VAT TIN (only business holders can have ) numbers etc

plz suggest a MOsfet bjt solution
Please rule out ALL IC solutions


coz people living in small towns also sdhould be  able build  at low cost !
(i some how get element 14 delivery butnot for all)
 

[dave_j_fan]

A mosfet SI2302  Has been ordered ....   
Until it comes 2-3days to go .

I could  guess the  cut off 3% tolerance  making the china  power bank go till 4.35V
mean while i took few other cheapy power banks to try .

Some of these will have perfect cut at 4.2V

For others my simple circuit will do that !
This arrange ment will give reliable power bank @16$ cost  to all ..
Many of the branded ones are like 30-39$..
People from small towns loosing  money in fake ones for 8-20$

[dave_j_fan]

mosfets mounted ..will test soon

[T3sl4co1l]

I wonder if you could add a small bias voltage to the voltage sense terminal, so that it reads 4.35V when the cell is actually at 4.20V.  It could be from a trimpot, so the awful 3% range can be calibrated correctly...

Tim