[time-nuts] Li-ion Battreries

MLewis mlewis000 at rogers.com
Mon Jan 23 01:14:40 EST 2017


Those hobby packs are typically LiPO, chosen for the energy density, and 
they often (usually?) run them with minimal protection circuits to 
reduce weight. Not recommended practice for building devices.

BMS
When I built a 12 cell series 10Ah pack, I went with LiFePO4 and a 
custom-configured Battery Management System (BMS). The BMS doesn't 
balance the charge after charging (which consumes time & energy), but 
runs a separate lead to each junction between cells so it can monitor 
each cell to vary each cell's charging to bring each to cell Vmax 
without going over. Those leads also allow each cell to be monitored 
against under voltage (cell Vmin). The cheap alternate is to rely upon 
monitoring pack Vmax and pack Vmin protection, but this is a recipe for 
killing a cell, as a cell may go over or under voltage while the pack 
voltage appears within the safe range.

The LiFePO4 cell I chose is interesting for its self-containment. 
Headway 38120, which was recommended as the cell is built encased within 
a containment shell with a threaded terminal on each end. The negative 
end's terminal is entirely sealed. The positive end's terminal is also 
sealed, but there are air holes spaced around the cap-to-container join 
and the true seal is an inner cap under that terminal cap (visible 
through those holes). When the cell fails, rather than pressure causing 
a leak, the inner sealing cap can expand into the space under the 
terminal cap thereby relieving a critical level of pressure. Or so it's 
claimed; I've never had an incident to test that.

Michael


On 22/01/2017 9:25 PM, Mark Sims wrote:
> Hobby RC packs are one of the most dangerous type of rechargeable lithium cells out there...

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