LI-Ion battery usb double cell charging?

[Hannan.khan]

so i am using MAX1555 to make a usb charger for a single cell li-ion battery...any 1 knows how to convert it into somthing that could accommodate 2 cell (li-ion) charging

 

[Hannan.khan]

so i am using MAX1555 to make a usb charger for a single cell li-ion battery...any 1 knows how to convert it into somthing that could accommodate 2 cell (li-ion) charging

okay no one replied...so can any 1 confirm my theory that if i use 2 OR gates with the CHR(active low) pin followed by switching pnp transistors and use it to charge 2 single cell LI-ion cells one by one...will it work? and wht will be the issues aside from the fact that it will take alot of time

 

[bill66656]

there is no good to try to charge two Li-on batteries altogether. For no matter you connect them in series or in parallel, there is only 500mA available from the USB. More current will drag the port to be lower than 4.5V. You can not shrink the charging time without more current. Also for the batteries are not in the same discharge depth, charge balancing is needed, which make the solution costs more.

Hope this helps.

 

[mtwieg]

Depends on the battery. If it's a smart battery containing charge balancing circuitry, then you can likely treat it as a single cell with double the voltage. If you just have two "naked" cells in series then you will need a more sophisticated controller (or a separate IC) to handle charge balancing as well.

 

[bill66656]

Depends on the battery. If it's a smart battery containing charge balancing circuitry, then you can likely treat it as a single cell with double the voltage. If you just have two "naked" cells in series then you will need a more sophisticated controller (or a separate IC) to handle charge balancing as well.

Even in the case of smart battery with charge balance, if them are in series, the terminated voltage of each will be 4.2/2=2.1V, often the total capacity is very less than one normal Li-ion cell, unless maybe they are of another chemistry or with some additives. If they are in parallel, yeah the capacity will be doubled, while the charging operation time need also be doubled with that 500mA USB. right ? Normal USB is with 500mA, while some special USB in car or some handcrafted ones, more current maybe be available.

 

[Hannan.khan]

so the real problem here is balancing the charge so that drain is equal....so one last question guys will MAX1737 work in this situation i.e. charge via usb....time to charge is not the issue as long as the drainage<charging (which would depend on the number of cells i use)

thanks guys

 

[mtwieg]

Even in the case of smart battery with charge balance, if them are in series, the terminated voltage of each will be 4.2/2=2.1V

Obviously you won't set the controller to charge the battery to 4.2V, but twice that. Many controllers will allow for this (though you will need a charger that can step up voltage).

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so the real problem here is balancing the charge so that drain is equal....so one last question guys will MAX1737 work in this situation i.e. charge via usb....time to charge is not the issue as long as the drainage<charging (which would depend on the number of cells i use)

thanks guys
View attachment 106236

max1737 isn't suitable because it can only step down voltage. USB is 5V, which means you need to step up to ~8.4V.

The LT1513 is an example of a charge controller might suit your needs, but it does not handle charge balancing between cells.

 

[bill66656]

Yeah you need to stack the two and boost the USB output to about 8.4V,
while the output current will give only about 250mA, boost and balance control both consume extra energy. Another concern is the thermal management, for
smart batteries packs there is internal thermal sense and protection devices. Now when you stack them, the touching surface between them may generate very hot region, which is also very hard to sense and control .

 

[D.A.(Tony)Stewart]

2-cell Li-ion or 7.4V nom battery needs >8V for slow charge.
Boost voltage regulators will reduce current at output voltage even if 90% efficient (or even 100% although not possible.)

 

[Hannan.khan]

well i found ISL6291 & bq24030 that do exactly what i need... so thanks guys