Older smart chargers vs. new high-capacity cells

This is in relation to information gleaned from posts such as this one and the others to which it links.

I learnt that you shouldn't charge a C-cell in a AA charger because:

The larger cells have lower internal resistance, which allows it to draw a larger charge current, which the charger may not be able to support, which causes the charger to overheat.

Smart chargers use deltaV to determine end-of-charge. This is the slight depression in the cell voltage when it has been charged to full capacity. This phenomenon can only be seen by the charger when the cell is subjected to a certain rate of charge. If the rate of charge is too low for that particular cell capacity, the deltaV will not be evident and the charger will not know when to stop the charge.

Is my understanding of the situation correct?

If so, how does the improvements to capacity in each cell size affect our current chargers? At what point will it be unsafe to use the new, larger-capacity cells in our current chargers?

For example, I use a smart charger which looks, in all respects, like the Vanson charger (but is labeled Powertech). It has deltaV cutoff as well as timer cutoff. The charging characteristics for each cell size is listed below. This charger was probably designed when AAA cells were 600mAh, AA were 1200mAh.

Size Charge Trickle Discharge Cutoff
==== ====== ======= ========= ======
AAA 200mA 20mA 150mA 5hours
AA 500mA 50mA 400mA 5hours
C 700mA 70mA 400mA 6hours
D 700mA 70mA 400mA 11.5hours
9v 20mA - - 11.5hours approx.
Note: 9V battery cannot be discharged, deep cycled or Pulse charged


I have a bunch of new C and D cells. The C-cells are 4500mAh, the D-cells are 9000mAh. As you will note, even charging until the timer cutoff, the charger would not have delivered a charge equal to the full capacity of the cells.

I figured that this would be fine as I would very seldom run my rechargeable cells flat, and even if I did, I could run it through 2 charge cycles where the first one would fill up the cell most of the way, and the second would top it off with the deltaV detection providing cutoff point.

However, with the increased cell capacity, would the cells attempt to draw more power than what the charger can supply, causing it to over heat? And would the stated rate-of-charge be able to induce a deltaV sufficient to signal end-of-charge? Will there come a time when the cell capacity is so large that this charger will not be able to handle it?

Hello Steelwolf,

I believe that if a charger is designed to be used with C and D cells, it will probably work fine. As you have pointed out, the main problem with the high capacity cells is the length of time for a complete charge. You would have to stop and re-start the charge cycle to completely charge a dead 9000 mAh D cell.

I have a Vanson charger as well and have no problems keeping my 4500 mAh C cells and 9000 mAh D cells charged. I have experienced no problems with the charger heating up.

Tom

Thanks SilverFox. I too have yet to notice any problems with my charger, but I thought I'd check.

Does the increased capacity allow the cell to draw more current, or has internal resistance stayed pretty much constant even with the continued development of NiMH cells?

I was under the impression that rate-of-charge must be a certain percentage of the cell capacity to be able to induce deltaV. Is that true? If so, what are the percentages and when, or under what conditions, will the charger be unable to induce deltaV?

Hello Steelwolf,

I believe the charger circuit limits the charge current. There may be some changes in internal resistance with the higher capacity cells, but I don't believe it is drastic enough to change charge characteristics, so far...

I don't know what the lowest level of charge rate is to induce the delta V. I am doing quite fine with 9000 mAh D cells. When I get some higher capacity cells, I will have to keep an eye on them.

Tom

Hi there,

Yes there is a sort of min charge current that will make
the minus delta V show up and i think the recommended
value is C/2. Lower than that and the delta V gets very
low. If the charger is detecting max V then it might
still see it because i think the detection level is
only 2mv for that method.
Also, if there isnt enough power available (not enough
current) to change the temperature much then delta Temp
methods might not catch it either, so it's best to
stick with C/2. Once you get too low it might not show
up at all.

Take care,
Al

Hello Al,

Just to make sure everyone is on the same page here, a question...

The C/2 charge rate for a 9000 mAh D cell is what?

Tom

Hi there Tom,

Sorry i forgot to reply to this thread earlier :-(

Yes, C/2 for a 9000mAh cell is 4500ma (see PM also).

Take care,
Al

Well, with C/2 at 4500mA, I'm sure my charger isn't providing anywhere near that level. The specs say 700mA, which is under C/10, and worse yet, the transformer is rated at only 1A. So if I'm charging 4 cells at a time, it is highly doubtful that I'm even getting the rated amount.

But it probably works with detecting max V and then the 2mV deltaV that was mentioned. If that is the case, does anyone know what is the minimum charge current to induce deltaV in this scenario?

Steelwolf, the specs say 700ma at what voltage? If you have something like a 700ma, 12v wall transformer (like my Vanson, IIRC), you could easily have 700ma per channel whether it charges in pairs or single cell per channel. The output voltage is much lower so the current can be higher.

Hehehehe.... my bad /ubbthreads/images/graemlins/blush.gif

I've gotten everything jumbled together.

The charging spec does say 700mA per channel. This is probably at 1.45V which is about nominal charging voltage. 0.7 x 4 x 1.45 = 4.06W

The transformer is naturally well able to provide that amount of power since it is rated 12V 1Amp. 12 x 1 = 12W

But still 700mA charge is not C/2. Which obviously means that detecting deltaV by itself will not work. So it must be detecting maxV and deltaV together. And so the question still stands. What is the minimum charge rate for this method of detecting end-of-charge?