1st battery build (20s2p) - collective advice?

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Oct 19, 2014
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Lads, I'm hoping to gear up and start my first DIY battery build in the not too distant future. I do still need to collect the equipment needed but am keen to start soon. I am just tired of paying top dollar for what usually ends up still not being quite the right shape, capacity, cell type, and so forth...preaching to the choir I am sure haha. And building my own batteries is just something I've always wanted to be able to do.

I'd like to start out with either a 20s2p, or even smaller 20s1p , very compact rectangular brick, using Samsung 40T cells (I tend to do a lot of very short trips so range isn't a factor on these packs). I'd like to do bigger packs later, but I think smaller ones first.

In no particular order, if you have experience, is anyone able to share some thoughts?

- is the Kweld still considered a good choice?
- any long term feedback on using cell spacers vs. glue vs. other potting or mechanical stabilisation of the cells?
- major design flaws or regrets you have for batteries you have built to look out for?
- casing materials you been the most happy with, eg HD heatshrink, 3d printed enclosures, and so forth?

As always, appreciate everyone's hard won experience here, TIA.
 
i don't know your ride power level but if you plan 20S, it is probably not a 500w device?

Usually peoples choose these kind of cells for their peak power and sometimes forget that even if there is a very high continious amp rating, the cells are still heating as much as you load them and sag and wear a lot faster in that maximum region too.
For power tools that run just few seconds each time it isn't a problem but a vehicule could easily be loaded for manny minutes. The range come into play but the heat is also a reason why we more often see packs with many parralele cells to dispatch the work and heat into more cells with more volume to heat. Usually you also get more surface for cool down when proper spacing is used. (not glued but spaced)

Take a look at your cell detailed specs, you will see that life cycles can goes from 600 to 100cycles if you discharge them at 40C.
(1 cycle is a full charge and discharge session)
Voltage sag is also about 0.5v higher at 10C (40A per cell) VS 1-2C. For 20s this 0.5v means 10v drop right at the cells if everything else isnt adding resistance. Or about a potential 30% power loss for an average difference of 66v vs 76v...(V x V / R)
Selfheating start to get expotential above 3-4C also meaning you need cooling if you run above that for minutes and don't want to catch fire.

I could be over alarming a bit too since i didn't ran these specific cells and i usually design with good safety margin in mind but that is something to consider.

 
Whats your discharge requirements, better to put less stress on the battery with more parallel groupings.
 
Some thoughts on the cell / configuration choice:

This page
shows by testing that cell that it's probably best used under 15A, as even that will have significant voltage sag, by a couple tenths of a volt, wasting power as heat in the cells instead of getting it to the motor. (see testing chart below, magenta lines in the "middle" are 15A)

So a 1p pack is going to be either less useful or stressed, for higher current systems. So if this is for the bike in this thread
then depending on actual current you need, up to 50-60A, I'd recommend maybe 3 to 5p, to minimize the load on the cells and maximize the watts you'll get out of them. (more p for higher current). You can use less p cells, it'll just have more voltage sag under the higher loads (which means a lower top speed and less acceleration).

If you've gotten that 3p 40T pack you can test to see how it behaves, and then make your design choices based on that; if it is "adequate" then you probably want to stick with at least the 3p. If it's not adequate then you probably want more than that. If it's great, no noticeable voltage sag even under your highest loads, then you could stick with it or perhaps go one less p group.


1700170172904.png

Others with experience building these types of packs will have to chime in on the rest.
 
a 1p pack is going to be either less useful or stressed, for higher current systems
Thanks guys, as you've all mentioned it, yes I have to agree entirely, it is going to be a rough and probably shortened life, and have to admit this is a bit of a novelty or luxury. The 1p idea is intended for my around town commuter where 15a rate does go a long way. Still rough I know and definitely not the best economic decision, but having said that, I would reuse the BMS at a minimum if the battery fails.

Yes, the 20s3p pack I got is great, a big step up from the "58v" AEG packs that have become my defacto standard for all bikes (14s2p using LG 18650's, were actually a really good battery for the price, but annoyingly I've discovered the updated models have a smarter BMS that prevents bulk charging the pack with a generic charger). The 3p is for my cargo bike primarily as that needs a solid 40a.

Heat - potentially a big issue I agree. Again I'm not making any bones about this, it's inefficient AF with a lot of energy wasted by our standards but let's assume I'm ok with obtaining 2ah capacity from the 1P pack. Some thoughts I'm having on heat:

1. Smaller pack = greater surface area/volume ratio which should improve ability to shed heat
2. Unless spacers are actively ventilated I suspect glue will improve heat transfer to the outside surfaces via conduction. I feel that air gaps with spacers actually seem more likely to act as insulation as well as increase the physical dimensions?
 
The main contenders I have in mind are 30T, 40T, and P42a cells. To clarify, continual discharge around 10-15A, but it's going to need to handle 20-30A for one or two minutes at least
 
So I have run a 13S 1P pack of 21700 VTC6A and it works well. However it does get pretty warm and doesn’t last long until the low voltage protection kicks in. Suggest trying for 2p if at all possible.
 
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