gyre wrote:Toyota says lithiums pattern, as do others.
That's why they use the charging/discharge cycle they do, mostly 40-60% if I remember correctly.
You'll toast your lithium cell if you under-voltage it. Different cells have different voltage specs.
Having a lithium pack of cells isn't safe enough. Each pack should have a Battery Management System (BMS) built into the pack. They usually include a temperature sensor. Should protect for over-voltage, under-voltage, max charge current, max discharge current. Better ones will protect from an outright short-circuit.
The BMS also affects charging. Some packs charge and then have to sit and rebalance the charge. Others have voltage detection of each cell and adjust the charge voltage per cell in a series so they charge equally - no waiting for a rebalance after charging.
Note that although you may be safe on the total max/min voltage for a series of cells, due to manufacturing variance, an individual cell may over or under voltage and fail the pack. Some pack builders will test cells and (like high-end audio components) will match cells of like performance. This avoids the cost of a per-cell BMS and they use/monitor a total pack voltage for under/over voltage, be it internal or external to the physical pack. This has more risk as a cell within a pack may change over time to out of spec with the other cells in the pack. Common for a failed lithium pack to be due to a single cell; replace it, and you're good to go. The per-cell voltage monitoring is safest, doesn't require matched cells to be safe (although for optimum efficiency, matched cells is technically the best). Currently, a per-cell BMS for up to 13 cells in series can be had for around $20. The LiFePO pack I built has a per-cell BMS that protects to the limits I chose, but I also monitor the pack's total voltage with an external voltage monitor that will alarm on whatever voltage I select (I pick up slightly above under-voltage so I know when the charge is getting low).
Different lithium battery chemistries have different properties, voltage limits, flammability of the internal fluid if it leaks, etc.. Research what works for your needs.
Note that if you're building your own pack (or buying one ready made from who knows who), that lithium cells need to be physically restrained to prevent their physical expansion and rupture. Some cells can be purchased as the pure cell without a containment package, as you're supposed to link them and then contain the pack. Others come individually packaged, like the blue headway cells, which also includes a leak-containing expansion area (reminiscent of smokeless/flashless shotgun shells); should the internal cell have an expansion failure, the leak is contained within the external shell. (photo lower right shows the expansion end: the sealed expansion cap is behind the visible metal end - note the holes for the air to escape from between the visible end and the sealed expansion cap)
Headway had 10Ah cells, then 12Ah and now 15Ah. Which means some deals can be had for 10Ah and 20Ah packs made with the older 10Ah cells.
If you have a project that will require a lot of cells (some people power cars with the headway cells), I hope you can wait a while. Looks like we may have a game changer just around the corner.
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