Question about bulk Capacitor placement and uneven AAA discharge

[alexg]

Hello all, I have two questions that I cannot figure out myself even after searching about net for answer I cannot get clear idea about these two simple things:

1. I am running Atmega328p on breadboard powered by 4 AAA batteries via switching regulator, I want to add bulk 220uF tantalum polymer (for smaller size) capacitor across batteries terminals and my question is: when I power device on (I apologize for lack of proper terminology) I think capacitor will be sucking alot of current form batteries for some time, almost shorting batteries for little while, which per my understanding is not very healthy for battery, right? Should I place a resistor or something else to limit current from battery while capacitor is charging?

2. For about second time in last few months when I power atmega328p on breadboard by 4 AAA batteries, when eventually after working for few days it won't power up if I measure batteries voltage individually 3 of them will be showing about 1V in them and fourth battery will be showing 300mV, so apparently this 300mV battery is pooping everything else, what may have caused this battery operate normal and then suddenly drop so low (even lower than normal discharged battery voltage)?

Thank you in advance for any info.

[MosherIV]

I want to add bulk 220uF tantalum polymer (for smaller size) capacitor across batteries terminals 

Why? What possible reason would you need a capacitor in parallel with the batteries?

my question is: when I power device on (I apologize for lack of proper terminology) I think capacitor will be sucking alot of current form batteries for some time, almost shorting batteries for little while, which per my understanding is not very healthy for battery, right?

The cap will indeed suck as much charge as it needs when first connected to the battries. If the batteries are in good shape, they should easily be able to supply the current without any dip in voltage.
No, you do not need a current limiting resistor.
The real question as I have already asked is why you want this capacitor?

apparently this 300mV battery is pooping everything else, what may have caused this battery operate normal and then suddenly drop so low

Did you mix old and new batteries?
There is no reason I can think of that 1 battery will suddenly drop in volts unless one battery is already partly drained or came faulty in the pack.

[The Soulman]

Was beaten by mosher anyways, but here goes:

1. The internal resistance of a aaa battery is already pretty high four in series is four times that,
I'd say enough resistance to limit the charging current.
How many times is this done?

2. Not all battery's are made equal because of production tolerances, so one will have more or less capacity than the
other, 1 V would also count as empty.

Look at Dave's batterizer debunking video's, he has some nice alkaline discharge curves.
(assuming you use alkaline battery's)

[The Soulman]

I want to add bulk 220uF tantalum polymer (for smaller size) capacitor across batteries terminals 

Why? What possible reason would you need a capacitor in parallel with the batteries?

Buck converter.

[alexg]

to answer why add capacitor:
I have buck-boost switching regulator that regulates voltage to 3.3V, I also have some loads coming on/off, beeper, LEDs, causing voltage drop on the battery so in my tests placing 220uF cap reduced voltage drop, it does not affect it much when batteries are fresh but somewhere along the way bulk capacitor helped, so I decided to stick to it.

to answer about mixing batteries:
first time when it happened, device was working steady discharging predictable for few days, then suddenly stopped and that's when I measured voltage and all 3 out of 4 batts were 1ish volt and one was waaay below that.
second time pretty much same story, device prematurely quit working and same measurement, again, one battery out of 4 quit

I will get  different brand and will see how it perform, if you saying that inrush current won't be too large to damage batteries and  and it is possible that one battery is faulty to do trick like that I suspect and hope that was the case.

I want to add bulk 220uF tantalum polymer (for smaller size) capacitor across batteries terminals 

Why? What possible reason would you need a capacitor in parallel with the batteries?

my question is: when I power device on (I apologize for lack of proper terminology) I think capacitor will be sucking alot of current form batteries for some time, almost shorting batteries for little while, which per my understanding is not very healthy for battery, right?

The cap will indeed suck as much charge as it needs when first connected to the battries. If the batteries are in good shape, they should easily be able to supply the current without any dip in voltage.
No, you do not need a current limiting resistor.
The real question as I have already asked is why you want this capacitor?

apparently this 300mV battery is pooping everything else, what may have caused this battery operate normal and then suddenly drop so low

Did you mix old and new batteries?
There is no reason I can think of that 1 battery will suddenly drop in volts unless one battery is already partly drained or came faulty in the pack.

[Ian.M]

1.0V terminal voltage indicates a nearly dead Alkaline cell or ZnC dry cell, with maybe 10% of its capacity left till its nominal fully discharged voltage of 0.8V.  Its not surprising that there can be sufficient manufacturing variability for there to be 10% variation in actual capacity, leading to one cell totally failing while the others have a little charge left.

[viperidae]

I have the same problem with the cheap varta batteries from Bunnings in the kids toys. Worst I've had is 3aa's at 1.4v and the 4th one was reversed charged.

[rsjsouza]

Just adding to other's comments, I usually see uneven discharging in most battery powered electronic devices. The reason was already nicely described above: variations in manufacturing will cause the weaker battery to discharge much faster than the rest.

One of the most unusual situations I have seen were batteries with negative voltage! No, it was neither a problem with the device nor with my measurement method, but mostly a manufacturing issue with the battery.

[mariush]

Check the quiescent current of the voltage regulator ICs .. some use more than 1mA when nothing is connected to them. That could explain discharging while you're not using the device, though at 1mA the batteries should still discharge very slowly.

Unless the voltage regulator requires a minimum capacitance, you may be able to get by with just 2-3 ceramic capacitors in parallel (for example something like 10uF 25-50v ceramics stacked on top of each other or in plain parallel). Even 22uF 25v x5r are cheap, under 20 cents each.

also .. it may be worth investigating if it's not cheaper and more efficient to use a BOOST only voltage regulator instead of buck boost.

If you're going to use only rechargeable batteries you're going to have 1v..1.25v x 4 = 4v .. 5v in, and you can just boost it to 5.1v-5.2v
It's a bit trickier with alkaline batteries which could have up to 1.65v ... but you could solve this by just changing from 4 aaa to 2 x AA (aaa will have 700-900mAh vs AA which can have up to 2800mAh or more in the case of lithium batteries)

[mikerj]

I want to add bulk 220uF tantalum polymer (for smaller size) capacitor across batteries terminals 

Why? What possible reason would you need a capacitor in parallel with the batteries?

  Err...decoupling perhaps?