Building DIY "Powerwall" using 18650 cells
By: +David Herron; Date: Sun Feb 10 2019 18:26:06 GMT-0800 (Pacific Standard Time)
Tags:Tesla Motors makes it look easy, just throw a bunch of 18650 cells in a box and call it a battery pack. Some, therefore, decide to build their own at home. These videos demonstrate the ideas, but I would not follow all their recommendations. For example, a BMS is extremely important.
- No thermal management -- what about cold or hot weather, both of which can limit energy capacity
- No attempt to stop or mitigate thermal runaway -- What if something goes wrong and the cells start popping, lithium battery fires are extremely hot and cannot be put out with normal fire extinguishers
- No BMS, no automated monitoring system --- There are many in the DIY EV community that say as this guy does, that the pack stays balanced with no BMS so why build a BMS? Thing is they're looking at the unloaded cell voltage which doesn't tell you a damn thing. See discussion below
- Low pack voltage -- they could get away with thinner wires at a higher voltage, of course
I built a DIY electric car with a Lithium-iron-phosphate pack (see http://ekarmann.com). The BMS included a display showing cell voltage while driving. Hence, I have many hours of experience watching the per-cell voltage of a lithium pack under load.
The cell voltage while driving is a result of the battery pack being under load. The pack voltage under load versus the unloaded voltage, or resting voltage, is very different. The amount of "voltage sag" under load depends on the safe discharge rate of the cells in the pack and on the current state of charge. The key observation is that the pack voltage when it doesn't have a load doesn't tell you anything. It's only when the pack is under load that you see the real voltage (state of charge) of the pack.
The voltage sag of each cell will differ as well, depending on the actual capacity of the cell. Cells from the same manufacturer will have slightly different characteristics and may not age identically. A cell with less total capacity will show more voltage sag, and will get to the discharged state more quickly than cells with greater total capacity.