DIY Battery Calc

Manc44

Pedelecer
Jun 21, 2021
32
3
I made a simple DIY battery calc in Excel to work out voltage, Ah, the amount of cells needed, cost, weight etc. Thanks to Woosh for correcting it.

Link to calc:

Screenshot:

 
Last edited:

Woosh

Trade Member
May 19, 2012
16,046
13,722
Southend on Sea
wooshbikes.co.uk
How come that says 12Ah but my calc says it comes out at 15.6Ah? Which one is wrong?

Is some Ah lost to the nickel strips?
cells in battery packs are organised in an array, in your case, 13S4P.
Each string has 13 cells in series, 4 strings are paralleled together to form the pack.
So for each string, you have 13 x 3.7V =48.1V, 3000mAH.
For the pack, you have 48.1V, 4x3000mAH = 12AH.
 
Last edited:

Nealh

Esteemed Pedelecer
Aug 7, 2014
13,487
5,239
58
West Sx RH
4 cells in parallel (aka a string or group) is the capacity 4 x 3000 is 12000mah/12ah.
The max capacity one can get with 4 x 18650 cells is 13600/14000mah.

Each parallel string of cells in series connection make up the pack voltage required.
Each parallel string can only be the voltage of the max charge rate of one cell, so each string voltage added is accumulative.

In parallel only the capacity/mah rating and the current rating is accumulative, in series only the voltage is accumulative.
 
Last edited:

Manc44

Pedelecer
Jun 21, 2021
32
3
Cheers Woosh & Nealh.
Calc fixed :)


Woosh, do you not sell battery packs on their own?

I am thinking about something like a shrink wrapped 21700 based pack @ 48V/25Ah.

I know someone on eBay sells exactly that thing... but you're Woosh, he's not :p

Something like Samsung 21700 (50E) with a claimed 4900 mAh per cell, about £6 per cell at nkon.

Such a pack would only measure 28cm x 11cm x 7.5cm which is a big reduction in volume compared to the usual Hailong with 18650 cells.

It doesn't help that they are called all different things: "shrink wrapped", "PVC", "soft pack" etc.

I was looking at what goes into building them and sort of lost interest when it got to the BMS part lol, I'd just mess up somewhere and short it.

There's "Jimmy" with the high quality copper strip packs but those are out of my price range.
 
Last edited:

Woosh

Trade Member
May 19, 2012
16,046
13,722
Southend on Sea
wooshbikes.co.uk
Whoosh, do you not sell battery packs on their own?
Not at the moment unless it's for fixing our bikes or kits.
There is a worldwide shortage of Samsung and Panasonic cells which we use for our batteries.
I have terrible problems with sourcing batteries for the last 6 months.
 

Nealh

Esteemed Pedelecer
Aug 7, 2014
13,487
5,239
58
West Sx RH
One can do a lot of damage before the BMS stage if t]parallel/series connections errors are made.

A 25ah pack will be quite heavy at about 6 - 6.5kg.
 

Manc44

Pedelecer
Jun 21, 2021
32
3
Updated the calc... added cost per cell and weight per cell to calculate totals for both. It's setup based on Samsung 21700 (50E) cells.

Let me know if anything's wrong with it and I'll remove the link. ;)

It could have BMS, nickel strips and case weight added, but then it's becoming a bit much lol

EDIT: Urgh I forgot to add Wh to it!
 
Last edited:

Woosh

Trade Member
May 19, 2012
16,046
13,722
Southend on Sea
wooshbikes.co.uk
Let me know if anything's wrong with it and I'll remove the link
you need to factor in about 15%-20% additional weight for the case, connecting strips, BMS, wires and insulation.
 

vfr400

Esteemed Pedelecer
Jun 12, 2011
9,253
3,675
Basildon
Just in case you didn't realise it, amp-hours don't have a constant value. An amp hour when the battery is flat is only worth 75% of one when the battery is full. The 3.7v is only thenominal cell voltage. They're 4.2v when full and 3.1v when empty, so at a constant current, power is only 75% when empty.
 

Manc44

Pedelecer
Jun 21, 2021
32
3
Cheers vfr400.

First post updated with latest calc. Added Wh to it and ability to add BMS and case weight (for wires and nickel just add to those weights). It's already got out of hand and was meant to be simple lol
 

Advertisers