charging 3 batteries...

[TucoRamirez]

hi

I'm a little confused about all the stuff about battery chargers

I see for example that microchip offers a smart ic quite pretty to charge batteries  ... with max output 4.5 V... this means i can put 3 AAA  compatible with the ic type  in series to acchieve 4.5 to be charged and monitored as a single batt    

i'm wanting to charge standard AAA rechargeable nimh commercial batts with at least 1000mAh ...


thanks in advance

[webdog]

Charging batteries can be real complicated! Depending on what type you are using NMHi, NiCd or LiIon they all needs different type of charging cycle. Most type do handle slow charging well, i.e. 0.1xrated capacity for 10-15 hours. It's when you want do speed-charge you have to very carful, monitor temperature, voltage and current! This is especially important with battery packs! Temperature can be a real killer when speed charging packs!!

Charging a battery pack as one cell often works, but you have to be aware that deep cycling may force one cell to reverse polarity and then refuse to be charged. If you cannot remove the faulty cell, you have to replace the whole package.  Newer Li-based battery packs do have intelligent charging electronics built in so each cell is monitored.

OK, if I was to charge a 3xNMHi pack i.e. 3x1,25volt, and not in a hurry, I would use a charge current of 100 mA, and use a large resistor in series with pack. Playing with numbers; if an open  circuit have 40 volt, and shorting with a resistor should give you 0,1A you need 400 ohm. Now put in the 3,7 volt battery with very low ri (interna resistance) you will see 100 mA flowing through the batteries.

Bottom line: yes you can charge the pack as one battery, but make sure you never run totally flat before charging. Lot easier to charge 10-15 hours than to speed charge.

[Gallymimu]

You can charge NiMH batteries with current control and nothing else.  I concur that if you want to charge at 1A you need to monitor the temp of each cell.

[TucoRamirez]

so, the classical opamp solution will be enough???    like http://www.stefanv.com/electronics/usb_charger_photos/schematic.gif   ??

(off course an increase on the rail voltages is needed :p )

i'm thinking to automat a little bit that circuit recovering the led status as an mcu input and using the same usb input but with a  step up converter to 6.5V (assuming the TIP transistor as a 2V Vce)  , in that way when the circuit will be charged i can disable the step up with a signal and the charger stage will be disabled.   is that a good idea??

otherwise, i'm looking for the  simple version of http://cds.linear.com/docs/en/application-note/an64f.pdf

anyway this i2c one is quite fine ...

But i was expecting one like mcp73832 ....

[thunderer]

But i was expecting one like mcp73832 ....

This is for a 3.7V battery, so it would not be good for 3xAAA.

I would adapt Stefan's scheme, it works and it is reliable, my 2 cents.

[robotai]

For Li-Ion cells, it can be simply use CC/CV to charge.

For Ni-Mh cells, normal way is utilizing -deltaV detection. Special IC like MC33340 is designed for it. It's more simple to implement the charger.

http://www.onsemi.com/pub_link/Collateral/MC33340-D.PDF

If MCU is used, you can control it by ourself. Use MCU to keep feeding constant current and watching the voltage of cells, once it voltage won't increase and start to drop, that's the point to stop charging. To prevent over charge on bad battery, a maximum voltage protection should be implemented.

[thunderer]

Look at this project http://tangentsoft.net/audio/ppa/bb/

[DreamCat]

There is a unit named in "C", it is capcity of a battery. generally, NiMH or NiCd should charging under 1/10C rate, unless it can charging in fast mode. It means we need 10 hours to charge full it.
For example, a 1200mAH battery, its capcity is 1200mAH, so we had best charge it in 120mA.

many battery charging IC provide a excellent document, contains process for charing, So if you want to make a intelligent automated charger by employ microcontroller, I suggest you read them.

[Mentor]

There is a unit named in "C", it is capcity of a battery. generally, NiMH or NiCd should charging under 1/10C rate, unless it can charging in fast mode. It means we need 10 hours to charge full it.
For example, a 1200mAH battery, its capcity is 1200mAH, so we had best charge it in 120mA.

I do not know how it works for other battery types, but for the NiCd ones, with the C/10 charge, you'll have to charge then for about 14 to 16 hours. You can see this in most datasheets from this cells.  What happens is that we "loose efficiency" with these slow charges, by the heating of the cell, gas generation inside the cell and so on.

[Parmin]

Most NiMh can handle trickle charge at 1/40C, if you are lazy simply limit the current with a resistor, set and forget..
will keep on charging but cut down the battery life, then again who cares.. LOL

[MTong]

@TucoRamirez: do you know already what type of battery do you want to charge?
Is there a specific reason why you wanted to charge them while in series? Or was it just because that IC gets up to 4.5V?

[TucoRamirez]

hi, we chose a 4 battery LiPo Pack , to use with a quadcopter, but, motors are killing the batt, now the problem is...
does the batteries discharge uniformly when settled in series??

and ...  the 2/3Vtotal alarm criteria is still valid to recharge???


(i think ill open a new topic to ask for help for the quad ...)

[thunderer]

If all have the same condition and rating, all should discharge equally. Ideally!

The 2/3 rule is too low, for a better lifespan is more like you never go under 80%. A good site (even if are not a beginner - my feeling) is this one http://www.rchelicopterfun.com/rc-lipo-batteries.html. Good reading, as it touched the balancing subject too.

[movf_jmpz]

Sorry for digging a bit outdated thread, but i'd like to add my few cents on this.

It all depends on internal resistance of battery, off course. And it solely depends on battery wear, so fabrication condition and current chemistry status.
Normally it doesn't really matter how battery pack discharges. You can monitor each cell voltage in flying model/robot/etc, but it's rather uncommon. More important is to do balance charging.
Most commercially available chargers implements some sort of balance charging (passive or active). Passive balancing means discharging cells that are charged over battery mean cell voltage. Active balancing mean charging undercharged cells individually.
Of course, passive balancing is cheaper, more common, and takes longer to balance whole pack.

To prevent Lithium-XXX cells you must have motor controller with proper cut-off.
It depends on battery type, but most common values are 3.2V for Li-Ion/Li-Poly batteries. Discharging below this threshold would bloat the cell and can cause serious overheating.

[nordiceng]

Try to use any convenience commercial battery chargers