I put this in the comments of my last question but I realised that might not get many eyeballs so I will ask it here.
I have have been thinking of making a battery using 18650's.
the battery arrangement would be 12 parallel 18650's making the 'block' and 13 'blocks' in series, totalling 156 batteries.
Each Battery has it's own PCM and the 12 parallel 'blocks'are each attached to a 48V 30A BMS device.
I will have the batteries seperated (not touching) and a thermal sensor fan that activates when tempratures go beyond a certian point, the sensor will be near the middle.
This makes one battery.
I then take 24 of these batteries and parallel them together to make the final array.
I attach the final large battery array to a CHarge Controller that can charge them from my solar panels thus a PV charger and it is a 48V 45A controller
MorningStar TS-MPPT-45
The plan is to make a SAFE battery that can store around 30Kw of energy so the house can run purely on batteries for a few days.
I need advice on the design of this operation and when I wire the 24 batteries to a single + terminal and – terminal is it the correct way to do this?
Have I taken enough procautions?
Should I abandon the idea?
In total there will be 3744 18650 Batteries.
Best Answer
First things first. In your previous post, you stated that your source would not provide any certification about actual battery capacity. Unless you can get some sort of guarantee, drop the whole idea. At claimed capacity and price, this http://www.ebay.com/gds/18650-Battery-Buying-Guide-test-on-all-from-eBay-below-3-/10000000178020340/g.html is likely what you will get. If you insist on dealing with these bozos, arrange for your total order to be shipped in smaller (say, 100 cells ea) with sufficient time between lots to allow for testing. Structure the order so that if a certain percentage (a few per cent or less) fails test, the entire order is cancelled. If they won't do it, don't buy from them. And good luck getting your money back if they ship you crap.
Second. If you are going to build up blocks, you're going about it backwards. Start with strings of 13 in series, with a PCM. Due to the variations between cells, you MUST not connect many cells in parallel. 2 is generally OK, and some manufacturers are OK with 4, but connecting 12 is a fabulous way to destroy your cells. The problem is that the weakest cell determines the cutoff voltage for the entire block, and it also hogs the entire charge current at the start of charge, resulting in high charge currents and battery temperature problems.
Third. You need a BMS for each string.
Fourth. For your application, you need a different package than 18650s. If you're willing to take chances, you can buy or build a spot welder and put tabs on the cells, then solder your strings. If you're thinking of using standard cell holders, forget it. Over long periods of disuse, corrosion at the contact points will be a killer.
Fifth. You need to rethink your cooling setup. The cell assemblies must be physically separated to allow airflow, and you'll need a powerful fan. If you don't know how much fan you'll need, you're in trouble. And you need a mechanism to allow the temperature sensor to disable charging as appropriate if the fan is not adequate.'
Sixth. Your calculations are off somewhere. Even allowing for the fact that you don't seem to know the difference between power and energy, the calculations for your battery array are straightforward. 3.7 volts x 4.9 Ah x 3744 equals about 86 kWh. If you want this to last "several days" (let's say two), then the allowable power is about 2800 kW, not 30kW. 30 kW / 3744 is 8 watt per cell, or about 2 amps. While this is not, by itself, an unreasonable number, it makes no sense given your stated battery capacity. You'd expect a lithium ion cell to handle at least a 1C rate, or 5 amps. If 2 amps is the number given by your prospective supplier, it indicates that he's not being honest (although the price/capacity/documentation issues pretty well establish that).
7th. Oh, feh. Enough of this. Try another approach. Do a lot more research, and start experimenting. Start small, and get experience with progressively larger arrays. Trying to jump in at the scale you're proposing is just not a good idea.
First the bucket, then the pail,
Then the laboratory scale.
Ever bigger, ever faster;
Faster, faster, then - disaster.