Thread Starter

Carlosonline

Joined Jan 27, 2020
8
Hi,
I would appreciate if someone would be able to comment/advice on the following matter. I have been looking around for an IC that would charge and balance Li-Ion cells 2-3 of them in series. It seems that the balance feature is absent in most cases. MCP73841 appears to provide most of the feature need to safely charge 2 x Li-Ion cells, can this IC charge/manage cells rated at 6.4 – 3000mAh (2 x 3.2 - 3000mAh). The datasheet states “For the 500 mAh battery pack example, a standard value 220 m, 1% resistor provides a typical fast charge current of 500 mA and maximum fast charge current of 551 mA. Worst-case power dissipation in the sense resistor is:” Therefore, this leads me to believe that this is not a suitable solution for my setup. Please correct me if am wrong and if so, would you be able to advise an alternative IC?
http://ww1.microchip.com/downloads/en/DeviceDoc/21823D.pdf

Thanks in advance.
 

Audioguru again

Joined Oct 21, 2019
1,902
You asked about charging cells rated at 6.4 or 3.2 x 2. What? Lithium Iron Phosphate (LiFePO4) at 3.2V voltage per cell? They are fully charged at 3.65V per cell.
Look up a charger IC for them, not this one.
 

Thread Starter

Carlosonline

Joined Jan 27, 2020
8

Audioguru again

Joined Oct 21, 2019
1,902
One MCP73841 plus one external Mosfet will charge one Lithium-Ion-Coke or Lithium-Ion-Graphite cell at whatever charging current you set with a sensing resistor and at 4.1V or 4.2V selected by you. It will severely overcharge a 3.2V-3.65V Lithium-Fe-PO4 cell.

The IC that charges two cells in series is dangerous because it does not balance-charge them.
 

Thread Starter

Carlosonline

Joined Jan 27, 2020
8
One MCP73841 plus one external Mosfet will charge one Lithium-Ion-Coke or Lithium-Ion-Graphite cell at whatever charging current you set with a sensing resistor and at 4.1V or 4.2V selected by you. It will severely overcharge a 3.2V-3.65V Lithium-Fe-PO4 cell.

The IC that charges two cells in series is dangerous because it does not balance-charge them.
Hey thanks,
All of the IC I have come across for 2 cells do not a balance charging feature, any suggestions. You said "MCP73841 plus one external Mosfet will charge one Lithium-Ion-Coke or Lithium-Ion-Graphite cell at whatever charging current you set with a sensing resistor and at 4.1V or 4.2V" but what happens if you have them in series, it's no longer 4.2v but 8.4v? I am picking the charge current from the manufacturers' datasheet. Which highlights values that I now see the MCP73841 cannot provide. Any alternatives?
Thanks

1580162580478.png
http://dalincom.ru/datasheet/SAMSUNG INR18650-25R.pdf
 

Audioguru again

Joined Oct 21, 2019
1,902
The datasheet for the battery management IC has a part number ending with "1 or 3" to charge one cell and has a part number ending in "2 or 4" to charge two cells in series dangerously.

Your ebay link shows a powerful 15A battery but your datasheet shows a different weak 0.5A battery.
 

Thread Starter

Carlosonline

Joined Jan 27, 2020
8
The datasheet for the battery management IC has a part number ending with "1 or 3" to charge one cell and has a part number ending in "2 or 4" to charge two cells in series dangerously.

Your ebay link shows a powerful 15A battery but your datasheet shows a different weak 0.5A battery.
Hi,
Sorry about that. I did notice the proposed alternative voltage ICs, but i can't seem to find much about the current. Ultimately, I would like to charge 2 li-ion batteries rated at 3000mAh (https://www.ebay.co.uk/itm/254456399430?ul_noapp=true). The MCP73844 does not make it clear if it is suitable for such ratings, from what I can see it not, or am i wrong? Can this charge the batteries but at a slower rate? Unfortunately, ignorance is playing a big role her on my behalf, apologies.
1580166270939.png

Thanks.
 

Thread Starter

Carlosonline

Joined Jan 27, 2020
8
The MCP73841 and MCP73843 have a voltage set for single cells; the MCP73842 and MCP73844 have double the voltage for two cells in series.

View attachment 197702
Hi, Thanks for your reply.
My query is relating to the current of this ic, which is obscure. So, yes it can charge 2 x Li-Ion batteries but at a rating of 3000mAh, according to the datasheet (https://www.nkon.nl/sk/k/30q-specs.pdf) it needs a charging current of 1.50A?
Thanks.

1580166862517.png
 

jpanhalt

Joined Jan 18, 2008
10,203
You haven't said what charge rate you want to use for your 3000 mAH batteries. Some people demand a fast charge of 0.5 C. In the circuit I showed earlier, a smaller sense resistor will give that to you. That "pass" transistor is rated at about 6 A (with heat sink). I have been using an MCP73842 to keep a series pair of Li-ion batteries (charged for a few years. I use them to simulate actual battery power to some device I am building.

My main use for lipo's is to power electric model air[planes. Personally, I do not charge my lipo's at a high rate, unless I am at the flying field and will almost immediately be discharging them Generally, I have 3 packs and socialize during the time between flying and charging.
 

Thread Starter

Carlosonline

Joined Jan 27, 2020
8
You haven't said what charge rate you want to use for your 3000 mAH batteries. Some people demand a fast charge of 0.5 C. In the circuit I showed earlier, a smaller sense resistor will give that to you. That "pass" transistor is rated at about 6 A (with heat sink). I have been using an MCP73842 to keep a series pair of Li-ion batteries (charged for a few years. I use them to simulate actual battery power to some device I am building.

My main use for lipo's is to power electric model air[planes. Personally, I do not charge my lipo's at a high rate, unless I am at the flying field and will almost immediately be discharging them Generally, I have 3 packs and socialize during the time between flying and charging.
Hi,
To be honest, it is not something I have given too much thought about, but I would think I'd be okay with the standard charge rate. So to achieve a standard charge rate for these batteries in series (2xLi-Ion) what "pass" transistor would you recommend or would the NDS8434 be okay? Apart from the "pass" transistor is there anything else that needs to be changed to accommodate these batteries.
Thanks.
 

Audioguru again

Joined Oct 21, 2019
1,902
The standard charge rate is 1.5A so you need a Mosfet that is suitable and you need a sensing resistor value that is 0.22/3= 0.0733 ohms (who sells that very low value??). Bad math edited.
 
Last edited:

jpanhalt

Joined Jan 18, 2008
10,203
Look in the datasheet under "Thermal Considerations:"
Utilizing a Fairchild™ NDS8434 or an International Rectifier IRF7404 mounted on a 1in2 pad of 2 oz. copper, the junction temperature rise is 75°C, approximately. This would allow for a maximum operating ambient temperature of 75°C. By increasing the size of the copper pad, a higher ambient temperature can be realized, or a lower value sense resistor could be utilized.
If you double the current, the watts dissipated go up by the square. Of course, 1A at through 100 mΩ = 100 mV. To get 1.5 A, you would need 67 mΩ. Frankly, If I were not in a hurry,I would just use the design posted and a 100 mΩ resistor. The calculations for the MCP73842/4 may be slightly different (see datasheet) but similar. Hopefully you see that the current output is set by the voltage across that resistor. Voltage (E) = I x R

EDIT:
As to where to get them, 1% resistors are quite common today and available in fractional resistances too: https://www.digikey.com/products/en/resistors/through-hole-resistors/53?k=1%+resistors&k=&pkeyword=1%+resistors&sv=0&pv2085=u66+mOhms&pv2085=u68+mOhms&pv2085=u70+mOhms&pv2085=u75+mOhms&pv2085=u80+mOhms&pv2085=u85+mOhms&pv2085=u90+mOhms&pv2085=u91+mOhms&pv2085=u100+mOhms&pv3=1131&sf=1&FV=-8|53&quantity=&ColumnSort=0&page=1&pageSize=25

You do not need the most expensive. You can also put resistors parallel to get what you need: 1/R1 + 1/R2 + 1/R3... (etc) = 1/R(combination)
 
Last edited:
Top