I have been reading a lot about 18650 laptop battery packs, and I would like to ask about a few questions, that I couldn't find anywhere...
1. How many cells can be "SAFELY" get in parallel? I was thinking in more than 50 to get something like 12-14S and maybe 50-60P to get 3 KW-h in a stationary environment. I'm thinking about an isolated house with no power connection and photovoltaic solar panels.
1st, decide on what working voltage you want to run.
12V, 24V, 48V through an 120V AC inverter?
120V direct DC?
But, in regard to how many can safely be run in parallel ...
I don't foresee any "danger" in an unlimited quantity run in parallel, the more the better.
segalion wrote:Hello.2. Any recommendation of BMS? What about balancing so big packages? I suppose standard 12-16S BMSs are all 3-5P oriented..., not 50P
"Balancing" is not as important as an initial build to banks of equal capacity.
Li-ions will "naturally" discharge, then recharge to the same "balanced" voltages per bank.
If care is taken in the original construction, optimal charge and discharge "balance" will maintain automatically.
If equalized at full charge, but one bank discharges deeper at max discharge voltage, than add one, or more cells to the weak bank .
You do not need an equal number of cells in parallel banks. - You do want equal capacity in each parallel banks.
It is, however important to limit charge voltage and limit DOD (Depth of Discharge)to optimal cell voltages.
AC inverters of fair size can usually be set to shut off if a lower voltage is reached.
But any BMS is not affected by any limit on quantity of cells in parallel.
segalion wrote:Hello.3. I always see welding cells. Could be possible build a pack with some clipping mechanism (no welding) to connect cells. I have reading something about "Neodymium magnets" instead weld... and it could be better for build and maintain the pack (removing and replacing bad cells).
A good connection to each cell is important!
Spot welding is best, soldering is good, soldering spot welded tabs seems the best for most applications.
"Clips" and probably magnets are subject to tarnish, corrosion, arcing etc ... not durable or reliable.
segalion wrote:Hello.4. Any recommendation to get even more KW-h. I.e. reach 15 KW-h with 5 of this packs in parallel?
"Packs" in parallel is a great idea.
You can even run multiple packs through the same BMS, you just need a modular plug with a sufficient number of connecting wires, I have used 20 and 24 pin computer power supply connectors on my early builds.
Modular packs are a good idea, you can detach and maintenance-repair one module while retaining functionality at a slightly reduced capacity.
segalion wrote:Hello.5. Any recommendation for the sun power charge profile (It can take big fluctuations with clouds...etc.) and the need to maintain the supply power to the house, that's means some different as typical LI-ION batteries charge-discharge mobile phones, ebikes. etc.
With laptop type 18650 cells ...
Limit charge and discharge voltage to below .5C, to limit wasted heat production.
Li-ion are extremely efficient at storing and delivering electricity, at modest rates.
Charge and discharge efficiencies are both in the 98% efficient area.
Compare this to SLA which can run at 75%, or worse, each way!
! You might need 1/2 the number of solar panels to produce-store-provide the same amount of electricity? !
You certainly will need a smaller "rated capacity" of battery to store the same amount of usable electricity.
Limiting charge and discharge voltages will greatly increase the longevity of a Li-ion battery pack.
Of course this will require a larger pack ... but might increase daily cycles into the 1000's???
Many "Greeners" add wind power to their home power systems.
Winter solar production is reduced while natural defoliation increases wind production,.
Perhaps more importantly, nighttime production is maintained!