NiMH battery pack mystery

[Harald Kapp]

I was opting to go from 6 cells to 8 cells in order to have longer operating times.

Makes little sense. Assuming the holder could carry the higher current, the higher voltage from a stack of 8 cells would lead to a higher current and actually faster draining of the batteries.
If you need to increase the operating time, you need to add batteries in parallel. In your case 2 parallel stacks of 6 batteries in series:

Each series stack will carry only 1/2 of the total current, thus the batteries will last twice as long. Plus the losses within the holder are reduced due to lower current.

 

[HANKMARS]

Makes little sense. Assuming the holder could carry the higher current, the higher voltage from a stack of 8 cells would lead to a higher current and actually faster draining of the batteries.
If you need to increase the operating time, you need to add batteries in parallel. In your case 2 parallel stacks of 6 batteries in series:
View attachment 54422
Each series stack will carry only 1/2 of the total current, thus the batteries will last twice as long. Plus the losses within the holder are reduced due to lower current.

Current space constraints do not allow for 2 - 6 cell packs. An 8 cell holder fits great.

 

[HANKMARS]

Makes little sense. Assuming the holder could carry the higher current, the higher voltage from a stack of 8 cells would lead to a higher current and actually faster draining of the batteries.
If you need to increase the operating time, you need to add batteries in parallel. In your case 2 parallel stacks of 6 batteries in series:
View attachment 54422
Each series stack will carry only 1/2 of the total current, thus the batteries will last twice as long. Plus the losses within the holder are reduced due to lower current.

 

[HANKMARS]

I apologize for leaving you stranded, or rather "in the dark" in post #42. This circuit operates by producing a square wave out with a very long time period. Minimum time period of square wave is 3 to 4 seconds. This duration is determined by R4 and adjusted "at the factory" and is constant. During that section of the square wave, heater element is not energized and considered a rest period for the battery. The second part of the square wave is determined by R5 and user adjusted. This time duration varies from virtually 0 seconds to 20 seconds. During that portion of the wave, the heater element is energized and current is limited by only internal resistances of the battery pack, the FET, and the load The load value is nominally 4Ωs. So the overall time period of the square wave varies as user demand for heat changes. At the 555 timer output, pin 3, the set time is high and is the charge period of C2. Again, at the timer output pin, the adjustable time duration, the output to the heater is active and pin 3 is low, the discharge time of C2 which is affected by the low current value flowing thru R4, so the formula T = RC is a bit skewed. During the C2 charge period, discharge element internal to the timer is virtual infinite resistance, theoretically, and negligible. Aout to the heater element is sufficiently stable during the usable portion of the NiMH battery current output curve. Overall user time of the unit varies from 20 minutes, extreme demand, to a nominal valve of 90 to 120 minutes, to a value of 4 hours, minimal demand. Hopefully I haven't skipped over any portions of theory of operation.
Another device which will use 2 - 6 cell batteries in parallel, uses an off the shelf PWM which produces a square wave at a frequency of 25KHz, a time period of 40μs, ( that frequency seems too fast so I will verify it before I post again). The conventional use of this PWM gives a finer control of power out but in actuality Aout is limited only by battery pack internal resistance and FET resistance. I need this high current to achieve fast response from load, in this case, not a heater element.

 

[HANKMARS]

Got two 6 cell holders? Try two in series and see what you get.

Occams Razor - the 8 cell holder....

Occam's Razor it is. I have a clamp on type meter which includes an Ωs function which, much to my surprise, is quite accurate, apparently. The interconnects of the 6 cell holders consistently read 0.1Ω. The interconnects of the 8 cell holder read 0.5 somewhat consistently. An obvious difference in construction between the 6 and 8 cell holders is the material used for the rivet type fastener that is the contact for the positive terminal of the cell and also the connection/fastener of the spring wire which connects adjacent cells. Rivets on the 6 cell unit appear to be aluminum based on its color. The rivets on the 8 cell unit are a brass color and I am assuming they are. Also, the spring wire on the 8 cell unit seems to have a glossier, shinier appearance. Ballpark calcs give the compounded resistance of interconnects in 6 cell unit as (5 X 0.1Ω) 0.5Ωs. In the case of the 8 cell unit (7 X 0.5Ω) interconnects contribute 3.5Ω to the series string of cells. Wowzer. I guess I'll search for different battery holder manufacturer.

 

[HANKMARS]

That was not the point.
Output capacity decreases as one increases the discharge rate.

This photo may clarify my statement regarding battery capacity. You are correct in the statement, as a battery is being used to power a circuit, the total amount of available energy is decreasing, the characteristic of battery discharge. Actually this graph shows amount of charge used over time, not charge remaining.

 

[HANKMARS]

Imagine a series chain of resistors. The current flow is a function of the sum of the series resistances. Current flow can only increase if one (or more) of those series resistances is lowered but will always be a function of the HIGHEST resistance even if all the other resistances drop to zero. You can't 'force' the resistance lower....

Replace those resistors with batteries (and their equivalent internal series resistance) and you have the very same issue. the battery internal resistance will drop as a function of its charge status but only to a defined minimum resistance according to their chemistry and that 'minimum' determines the maximum flow when in a series circuit.

The current flow will always be proportional to the sum total series (internal) resistance which is why, if you want higher current, you connect batteries in parallel (as per paralleling resistors allows a greater current flow as a function of the reduced equivalent resistance).

Agreed. I think it was in Battery University, a statement regarding the charging of parallel NiMH cells as risky because the algorithm of the charging unit can misinterpret the values or more likely won't realize a faulty cell or a cell with a significant charge level difference and will not stop charge current resulting in an overcharge situation.

 

[HANKMARS]

Not at all. As the resistance increases, so does voltage. And by Ohm's law I = V/R, the max. current is the same for any number of cells in series. Limited by the cell in worst condition.

Right. A determining factor of how many cells I need in series is a function of required current and load resistance. Ballpark values here are a 4Ω load and desired current of 1.5 amps. Therefore my battery pack needs to present no less than 6 volts across the load. In one of the applications of these NiMH batteries, current supplied to the 4Ω load has a large tolerance band. The 6 cell, 7.2V battery pack is a nice fit for this app. However, if I choose to use an 8 cell battery with 9.6V out I can do so with no circuit modifications because the end goal here is to regulate a heater coil dedicated to maintaining temperature of a remote device. Here again the tolerance band of the desired temperature is relatively wide and the physical characteristics of material heated offers a nice buffer. Desired temperature is user adjusted so if current out equals 2 amps, then on time of the power FET is reduced.

 

[HANKMARS]

I ordered and received more 6 slot battery holders. They ohm out at 0Ω and I am assuming that 0Ω across the interconnecting spring wire is pretty close to actual resistance. I find the fact that the 8 slot battery holders I purchased have such a high resistance value on each internal connector. Unless somebody can present me with a logical reason why a substantial impedance is needed in those battery holders, I will be writing an unfavorable product review in the Amazon review section. Thanks for your tips and suggestions on this matter.

 

[HANKMARS]

The problems I experienced that inspired the initial thread here, were definitely caused by the physical construction of the cell holder used in my project. My conclusion is, the determining factor of the poor performance of the set of cell holders that are problematic, is the material of which the rivet type fastener, that secures the connecting wire/spring component to the holder frame, is made from. For whatever reason these brass color rivets (presumed to be brass) have a significantly higher resistance than the aluminum colored (and presumed to be aluminum) rivets. I don't have the values in front of me, but I am sure that brass does indeed exhibit a higher resistance per linear unit than aluminum. Initially I did not entertain the idea that the material resistance differential through such a short run (1/64" - 1/100") would have such detrimental effect upon my circuit. Just thot I should finalize this thread.