Difference in battery capacity between Alkaline and NiMH cells

Ronnie_Space

Jun 19, 2014
35
Hello all!

I am looking to spec D cells for a point of sale display unit which will power LED lights and a media screen. Ideally I would like to use rechargeable cells.

Looking at off the shelf Alkaline D cells the typical battery capacity is approx 12000mAh. In comparison NiMH rechargeable cells have a capacity of only 2500-4000mAh.

I am wondering why there is such a drop in stated capacity? Counter to this I have read that these rechargeable cells can actually last longer!

If anyone could shed some light and explain, it would be much appreciated.

Thanks

Ronnie

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Secondary cells are typically larger and heavier than primary cells of an equivalent capacity.

For one thing they don't have to be built to last longer than a single discharge.

For another thing they don't have to include vents or other safety features to prevent explosive damage if overcharged.

Because a secondary cells often have a lower internal resistance, under heavy discharge their greater efficiency can yield a higher voltage and a longer effective life.

Ronnie_Space

Jun 19, 2014
35
Thanks for the quick reply Steve. That all makes sense.

Is it possible to predict/calculate a secondary cell's run time life within a circuit? I understand with primary cells I can use the mAh stated, but with secondary cells everything I read is that whilst they are of a lower capacity they can often run longer than primaries.

Therefore, I'm somewhat bemused in how to spec the correct secondary cells and workout how often they will require recharging.

Any pointers well rec'd.

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Just use the mAh values.

If you have an unusual situation where discharge is way above or below a 20 hour rate, check the datasheets.

Practically you'll also need to know the minimum voltage requirement for the load too.

A simple option is to run some tests and find out the actual life.

Jan 5, 2010
7,682

Ronnie_Space

Jun 19, 2014
35
Just use the mAh values.

If you have an unusual situation where discharge is way above or below a 20 hour rate, check the datasheets.

Practically you'll also need to know the minimum voltage requirement for the load too.

A simple option is to run some tests and find out the actual life.

Thanks Steve. I am in the process of specifying a design and creating a prototype and hoping to calculate run time options before I make it. You are of course right though and test to find actual life may be the way to go.

Ronnie_Space

Jun 19, 2014
35
According to the chart:

https://en.wikipedia.org/wiki/D_battery

An NiMH D battery can be up to 12000 mAH

And take note of this sentence in the same article:

Bob

That's very useful information, particularly regarding some being sub-C cells in a D-sized holder. Thank you Bob.

Audioguru

Sep 24, 2016
3,656
Some D size Ni-MH cells have a little AA cell inside, because nobody complains and nobody uses a D size battery anymore..

Ronnie_Space

Jun 19, 2014
35
Thanks Audioguru,

Appreciate they are rather old now.

I was looking for a way to add a 6V battery source with a large capacity to a point of sale display, and was therefore thinking of using x8/x16/x24 D cells. Wired in series and parallel to give 6V and x2/x4/x6 overall capacity.

What could be an alternative rechargeable battery source if "nobody uses a D size battery anymore"?

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Lipo are more common these days. NiMH are harder to charge (you really need a dedicated charger)

But if you've got them and have a charger...

Sep 24, 2016
3,656

Ronnie_Space

Jun 19, 2014
35
Thanks for the input.

The key for me here is to find a relatively low cost battery solution, that has a good lifespan and that is easily rechargeable. By easily rechargeable I mean any store assistant could simply remove and plug into a charger to recharge, without any specialist know-how/training.

Through hobby I have used LiPo batteries in radio controlled cars and found the chargers and method requires a reasonable technical understanding.

Putting NiMHsecondary cells into a consumer battery charger is quite simple. But the cost of high capacity NiMH D cells would likely make this out of reach.

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