At 05:42 AM 26/12/2009, you wrote:
>Does anyone know how important it is to have a Battery Management
>System on a bank of LiFe PO cells. It adds considerably to the cost.
>I can understand taking the risk on a bicycle or motorcycle bank but
>in a boat, is it prudent?
>Ned
G'day Ned
Over on the EVDL (Electric vehicle discussion list) this is a current
topic, and all the words-from-the-
without a proper management system) tell it like this:
1) Your BMS must not allow any cell above a certain maximum voltage
(typically 4.0 volts, depends on the particular manufacturer)
2) Your BMS must not allow any cell below a certain voltage (typically 2.2V)
3) Your BMS must "tell you" if it can't "see" a cell monitor.
If it lacks '3' then you'll end up with a '1' or '2' when a cell
monitor fails. If it lacks '1' or '2' then after a few
charge/discharge cycles (unless you have a perfectly matched set of
cells, in which case it'll be longer, but will still happen) you'll
have to start replacing cells.
With a good BMS that correctly interfaces to the charger, the charger
can be pretty basic. Without a BMS, the charger can be the best ever
and will still go killing cells. Worst case (depends on the
particular Lithium chemistry) you can have a fire, but more likely
you'd just have the cell capacity go way down, or internal resistance
go way up - either effect can cause a fire if undetected.
Not having a BMS will add considerably to the cost, as soon as you
have to replace cells in the pack.
If you can't afford the BMS, then in my opinion don't get lithium. If
you are buying in installments, get the BMS and charger first.
Hope this helps
Regards
[Technik] James
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