Best value 18650 cell for 60a cont 100a peak (or 26650)

[smeagol222]

Potentially looking to build my first DIY battery 72v 40ah. Would need about 280 cells- wanted to get battery specific info on this thread, but will be
for my project here (4000w/7000w peak QS motor) https://endless-sphere.com/forums/viewtopic.php?f=10&t=85164

Seen quite a few different cells from Aliexpress Samsung 30q, 25r and Panasonic NCR, but they all seem to be sold in packs of 2 averaging about $7.50CAD each
works out to $2100CAD in bare cells alone which seems crazy.

Am I better off creating two 20ah packs and "hotswapping" them, since most of my trips are going to be under say 10km and since I have battery paranoia I would get twice the charge cycles (or am I thinking about it wrong?)

Other option is 26650 cell or nissan leaf as others have been saying (where would I buy those?)

the space I have to work with

38cm depth
23cm width
36cm length

 

[kdog]

youre better off making one large pack than two smaller and swapping, each discharge is shallower resulting in better cycling life. also charge to 4.1v for a rough doubling of cycles and don't store fully charged or in a hot plce. this is going to treat them about as best you can.
don't buy in packs of 2 that is madness! seeing as you need several hundred contact a whole saler and buy direct. MOQ is ususally only 100-200.

 

[smeagol222]

youre better off making one large pack than two smaller and swapping, each discharge is shallower resulting in better cycling life. also charge to 4.1v for a rough doubling of cycles and don't store fully charged or in a hot plce. this is going to treat them about as best you can.
don't buy in packs of 2 that is madness! seeing as you need several hundred contact a whole saler and buy direct. MOQ is ususally only 100-200.

So you're saying if I had a large 40ah pack, discharging to say 50% and recharging doesn't count as a full "charge cycle" (ie. this is better than fully discharging a 20ah pack- to get a full charge cycle)

I know that you can increase the cycles by only charging to 80% etc

 

[kdog]

Full cycle is defined in the mfrs data but is usually full to fully flat (4.2v-2.5)at a low rate. Deep discharging will reduce cycle life more than a shallow one so a bigger battery copes better for a given discharge. Your 50% dod in the 40ah is 100% in the 20ah. even though you can swap the 20s out it doesn't add up. You'll also have better performance with less sag and heat. Check out some of the data logging done at various discharge rates it'll give you an idea.
Other benefits are not having to make/break high power connections, moving batteries around all the time, hitting your controller with inrush current or precharge circuit, one BMS, some sort of easy access lockable waterproof housing for the battery that you're going be accessing frequently.
Make one good one-done.
Jmo- others might have other good ideas.

 

[smeagol222]

Full cycle is defined in the mfrs data but is usually full to fully flat (4.2v-2.5)at a low rate. Deep discharging will reduce cycle life more than a shallow one so a bigger battery copes better for a given discharge. Your 50% dod in the 40ah is 100% in the 20ah. even though you can swap the 20s out it doesn't add up. You'll also have better performance with less sag and heat. Check out some of the data logging done at various discharge rates it'll give you an idea.
Other benefits are not having to make/break high power connections, moving batteries around all the time, hitting your controller with inrush current or precharge circuit, one BMS, some sort of easy access lockable waterproof housing for the battery that you're going be accessing frequently.
Make one good one-done.
Jmo- others might have other good ideas.

All good ideas- I think I will take your advice when building my own 18650 pack. On a side note in my impatience on wanting to get the project started I ended up grabbing a 72v 20ah lifepo4 battery on a good deal. Did some visualizations on future batteries. Luckily I seem to have quite a bit of room in my battery box. I'm just not quite set up yet to make my own 18650 battery pack.

Hopefully the BMS in this pre-made lifepo4 is okay (claims to be 65a continuous, 85a peak- needed 90a peak, but hoping this wont make a huge difference)

Will continue to update my project thread, thanks for the battery specific advice from everyone so far!
https://endless-sphere.com/forums/viewtopic.php?f=10&t=85164&start=25