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I have two groups of cells, Group A and Group B.

Group A are 14 cells connected in series, with each having 1,5 V = 21 V.

Group B are 7 cells connected in parallel, with each capable of 3 A = 21 A.

I wish to connect Group A and B to get one big battery with both a higher voltage and current; 21 V x 21 A = 441 W.

How do I charge all of these cells when arranged as described? Keep in mind the way the cells in series/parallel are being charged at the same time.

The battery type is NiMH. Battery size is AA (5 cm/2" by 1.5 cm/0.5"). These are rechargeable cells.

And yes, I do need all these cells as I need around 400 W of power and a rechargeable battery that is small.

Battery usage is 1 hour.

It's the same type of cell (3000 mAh, 1.5 V cell).

Some cell examples: https://www.amazon.com/BONAI-Rechargeable-Batteries-2800mAh-Discharge/dp/B07PJZKB8X/ref=sr_1_5

https://www.amazon.com/Deleepow-Rechargeable-Batteries-Pre-Charged-Discharge/dp/B09222NNDV/ref=sxin_15_pa_sp_search_thematic_sspa

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    \$\begingroup\$ But that makes no sense. You can't combine 14 series batteries with 7 parallel batteries. And NiMH batteries are not 1.5V but 1.2V. \$\endgroup\$
    – Justme
    Commented Dec 18, 2022 at 23:25
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    \$\begingroup\$ That's not how batteries work. \$\endgroup\$
    – Hearth
    Commented Dec 18, 2022 at 23:26
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    \$\begingroup\$ (Draw a sketch of A and B connected and see what characteristics you think you get.) \$\endgroup\$
    – greybeard
    Commented Dec 18, 2022 at 23:53
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    \$\begingroup\$ (Actually, a single cell is called, well, a cell. A bunch of like cells in close proximity, even interconnected, is is what originally has been called a battery in this context.) \$\endgroup\$
    – greybeard
    Commented Dec 18, 2022 at 23:56
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    \$\begingroup\$ Forget NiMH cells. Forget making your own battery. Just buy an off-the shelf 24 V power bank capable of 400 W. \$\endgroup\$ Commented Dec 19, 2022 at 0:00

3 Answers 3

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  1. Make 14 each of "Group B"
  2. Connect these 14 groups in series

Total number of cells = 7 x 14 = 98.

That's the only way to do it. You can't cheat physics laws.

Then, you can charge it and discharge it as you would any 21 V battery.

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    \$\begingroup\$ Once he has managed to connect that pack together, he would be dissappointed to see it doesn't deliver 400W. With 1C load, the cells will drop to 1V, 1,1V at best, so he'll get 14,1V and assuming ohmic load 14,1A so about 200W. At least 200Wh is a more realistic expectable capacity of such a pack without any bms/cell balancing. \$\endgroup\$ Commented Jan 27, 2023 at 13:04
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This comes to essentially the same conclusion as Davide Andrea's answer.
I've used a slightly different method which may possibly help with understanding.

Assuming your batteries are rated at 3000 mAh x 1.5V as you say (and, they aren't) then
energy per cell is Ah x V = 3Ah x 1.5V = 4.5 Wh.

You want 400W x 1H = 400 Wh.

Cells needed = Wh_total/Wh per cell
= 400Wh / 4.5 Wh ~= 90 cells.
There is mo means of avoiding this :-( .

In fact you need more.
A 3Ah NiMH cell will deliver 1.2V at modest load.
At 3A Vavg will be more like 1V.
So you get 3A x 1V = 3 Wh per cell. Maybe less.

To get 400 W for 1h you need 400/3 ~= 133 cells.
There is no means of avoiding this.
Sadly.

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  • \$\begingroup\$ Damn, I guess i´ll just use cable instead. appreciate the answer :) \$\endgroup\$ Commented Dec 19, 2022 at 20:44
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Please don't do this. Your unclear language and your other question suggest you don't really know what your doing. For short-duration-applications you really want to look into lithium-based solutions. 2 12V lifepo4 packs would be simple and safe with a BMS but might be to bulky if you don't want to stress them. Lipoly/lithium is much more capable of fast discharge applications per weight/size. Buy a ready-made pack and learn how to charge and discharge it safely. That could safe you around ~67% of volume and be way more reliable than a pack made up of 98 unselected NiMh cells. Factor in connectors for the pack and construction time, it is likely also cheaper.

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