Nimh vs. Nicd sizing, capacity & drop

[Locked Up]

From reading the stickies on here I am now confused.

The battery sticky says a Nimh battery will have more capacity for whatever given size, compared to a Nicd. I thought a subC was a sub C. Are Nimh actually smaller or do they just weigh slightly less or something? In all my shopping around it seems most 6-cell standard flat packs are pretty much the same size, regardless of Mah... but the higher Mah look like they weigh more.

It also says a Nimh will tend to drop off voltage more as it discharges vs. Nicd. If that's the case, does anyone have any charts or graphs comparing two comparable batteris (nicd vs nimh)? Is there a noticeable difference between the two here?


I'm thinking about picking up some longer runtime packs and am just wondering other than memory effect, why Nimh is better than Nicd. If I can get a longer runtime Nimh that is the same weight & size as my current 1500 Nicds it seems like a no brainer... unless the voltage drops off a lot more.

 

[JohnRobHolmes]

The voltage drop won't be noticable since you will have a longer runtime. The higher the runtime, the heavier the cell will be regardless of what type it is. Sub-C is basically sub-C size, and higher capacity cells just weigh more. Nicad just caps off around 1800mah in a sub-C, you can't effectively cram more runtime in.

 

[Locked Up]

The voltage drop won't be noticable since you will have a longer runtime. The higher the runtime, the heavier the cell will be regardless of what type it is. Sub-C is basically sub-C size, and higher capacity cells just weigh more. Nicad just caps off around 1800mah in a sub-C, you can't effectively cram more runtime in.

So would Nicd weigh more than Nimh if both packs were 1500 mah & same cell size?


So if I got a 2000 mah Nimh pack, would the voltage stay up longer than a 1500 mah Nicad under similar useage? or do I have to jump up to 2500+

 

[JohnRobHolmes]

I don't know how nicad and nimh compare, except that a 1500 sub-C stick pack of nicad weighs more than 6 cells 2/3a size 1500 nimh.

You won't notice the discharge difference between nicad and Nimh on a crawler, only the runtime and weight difference.

 

[Locked Up]

I don't know how nicad and nimh compare, except that a 1500 sub-C stick pack of nicad weighs more than 6 cells 2/3a size 1500 nimh.

You won't notice the discharge difference between nicad and Nimh on a crawler, only the runtime and weight difference.

Thanks.


So with the smaller 2/3A or even the 4/5SC cells ... how much punch do you lose compared to a full SubC at the same mah? I mean, 1500 mah is 1500 mah regardless fo cell size ... But a 4/5SC has to give up something to a SubC of the same Mah, right?

 

[K_B]

Usually amp draw capacity is gonna be the trade-off. But crawlers don't draw huge amperage. "Punch" will be dictated by voltage. So if you had a 7-cell (8.4V) 2/3A battery and a 6-cell (7.2V) SubC, you'll get more wheelspeed out of the 2/3A pack because it has a higher voltage. However, you can draw much more amperage from a SubC cell than you can a 2/3A cell.

 

[Locked Up]

Usually amp draw capacity is gonna be the trade-off. But crawlers don't draw huge amperage. "Punch" will be dictated by voltage. So if you had a 7-cell (8.4V) 2/3A battery and a 6-cell (7.2V) SubC, you'll get more wheelspeed out of the 2/3A pack because it has a higher voltage. However, you can draw much more amperage from a SubC cell than you can a 2/3A cell.

You mean amp draw will be more "bottle necked" with the smaller cells? Overall amp draw is dictated by the mah # ..... right? As in the smaller cells may only be able to draw 80% the amps at once compared to the regular size?

I'm thinking about going to a 7 or 8-cell 4/5sc pack. I'd assume that if guys are getting away with running 2/3a packs I'd be more than fine with a 4/5sc and not notice a lack of available amps.


I was playing around with some different cell sizes and came up with the following volumes in cubic inches:
Subc ~ 1.02
4/5SC ~ .79
A ~ .65
4/5A ~. 57
2/3A ~ .37

Now, I know cell volume won't be directly related to to the peak instant amps it can deliver but I wonder how close the correlation is?

 

[ClodMaxx]

on a side note, i can say that ni-mh cells have grown in size over time (the past 3-4 years). i had a chassis that had pre-set slots for ni-mh side by side cells...it was made about 4 years ago, and the slots were set to the size then of ni-mh cells. the current IB4200's won't fit in the slots properly. the current cells are too wide for them to fit properly.

 

[K_B]

on a side note, i can say that ni-mh cells have grown in size over time (the past 3-4 years). i had a chassis that had pre-set slots for ni-mh side by side cells...it was made about 4 years ago, and the slots were set to the size then of ni-mh cells. the current IB4200's won't fit in the slots properly. the current cells are too wide for them to fit properly.

Had that happen with my touring car. I could swear that the sizes are also specific to manufacturers; 2/3A or SubC for one manufacturer is different than that of another.

You mean amp draw will be more "bottle necked" with the smaller cells? Overall amp draw is dictated by the mah # ..... right? As in the smaller cells may only be able to draw 80% the amps at once compared to the regular size?

I'm thinking about going to a 7 or 8-cell 4/5sc pack. I'd assume that if guys are getting away with running 2/3a packs I'd be OK with a 4/5sc and not notice a lack of amps?

4/5 would be more than adequate but for space concerns, 2/3A is just as good. The trade off being only 1500mah as opposed to 4/5sc which has around 2200 I believe.