AA cells versus c and d cells

[captainslogParticipant @captainslog]

Hello, not being very good on electrics I have a querie that you might know. 3 8 cell packs of 1800 mah but different cell sizes. ie AA/C/D now if all these packs were fully charged would they be able to run a 360 or 480 motor for roughly the same time? in other words do they all hold the same ooomph as each other or will the D cells be better than C or AA? i hope you understand the Q just curious as to what to install

cheers keith

[captainslogParticipant @captainslog]

[Colin BishopModerator @colinbishop34627]

The physical size is irrelevant. It is the quoted capacity in mAh that matters.

Sizes differ as the cells have to fit in different appliances. Generally speaking, the really high capacities only come in D size as more power has to be packed into the casing.

Colin

[captainslogParticipant @captainslog]

thanks colin thats what i thought so I can now put the aa pack into the mtb which is what i wanted as the weight will be perfect as opposed to the c cells.

keith

[Dave MilbournParticipant @davemilbourn48782]

Not quite as simple as that. The useable capacity also depends on the rate of discharge. Some AA cells can be discharged at a high rate while others can't. I know of no D-size cells which are high-discharge whereas many Sub-C sized cells are. You need to check on the full specifications and if none are available for the exact type you're looking at then move on.

Have a read of this if you need further information **LINK**

Dave M

[Malcolm FraryParticipant @malcolmfrary95515]

It has been rumoured in the past that many D cells are in fact C cells inside a larger package. No idea about the truth of that.

All cells, when charging or discharging, have a chemical reaction going on inside them, which is always exothermic, i.e.. heat is given off. As a general rule, the more current, the more heat. To avoid overheating, the cells have to pass this heat to the outside world. The more surface area, the better the transfer. C cells have more surface area, they handle it better. High capacity AA cells have a very large surface area of electrode packed into a very small volume, when they operate at high currents, the heat is even harder to get rid of. If they can't keep their temperature within limits, they vent gas. This is usually water vapour from their store of electrolyte. When that reduces, it stops being a battery and starts being ballast.

If the use to which they are being put is within what they can stand, they will work and survive and be OK. Otherwise, to put a lot of ability into a small volume and at low weight, the answer is to go with Lithium.

[Colin BishopModerator @colinbishop34627]

Dave and Malcolm are quite correct, if you have a power hungry application you need to be sure the cells can deliver the required current. Your original post didn't mention an MTB!

Colin

[captainslogParticipant @captainslog]

oops sorry colin, judging by the answers it seems (a) amount of space and (b) final weight of the boat. as I understand it also how much amps you will be pulling out and avoiding overheating the cells. anyway I have gone for c type cells. years ago i ran it on 4-6 amp gellcells and had many hours sailing from that 1 charge.As for lithium, yikes technology frightens me, I have the charger (from overlander) to charge them but just cannot for the life of me understand the instructions. I was taught that V=Ir and everything was worked out with that equation. any way c cells nimh does the job.thanks for the replies guys

keith

[Author]

Posts