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1. 08:09 AM - Re: Back-Up Battery (Jerry-TS-MkII)
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| Subject: | Re: Back-Up Battery |
Hey guys...
Just to add some generic info about li-ion batteries and their care and feeding.
Unlike lead-acid or ni-cad or ni-mh, lithium-ion cells are more critical of
under voltage, over-voltage, or over-current. Most cells have a protective device
that does not allow them to go below a certain cut-off voltage, or to be
charged too high. But for starting a engine, most of those high capacity cells
do not include that same protection device. If charged over 4.2v per cell,
they can be damaged. If allowed to discharge below 3.3 they can also be damaged.
Most consider 3.6-3.7 as a minimum cut-off. A pack can be assembled with
high discharge cells using 18650 size, with up to 20-30A max current per string.
IE 3 cells in series will net 11.1 - 12.6 volts. Multiple parallel sets
can boost the current. But.. you will want to add a circuit which protects them,
both in discharge and re-charge voltage, and to keep each group of cells
balanced, and the discharge current within limits.
The use of a diode, feeding a aux pack into the buss will keep it "out of the circuit"
until the main battery drops enough that the aux is .5-.6v above it..
where the diode will then begin to conduct and provide current. That DOES NOT
charge the cells however, as the diode is a one way device. And another factor,
is that if your primary circuit or buss has a high drain problem or failure,
THAT needs to be isolated first. Otherwise your back-up battery won't last
very long. Some will use 2 packs that are switched so that one is charged..
while the other is active in circuit. Both can be used in parallel with high
power diodes, for starting purposes.
Up-side to li-ion.. they do have a long life when not in use. Ni-cads and Ni-Mh
are both much worse at self discharge, and ni-cads bad about "memory" of not
cycled fully. But none of the new high density technologies are without their
particulars. Lithium polymer just tend to catch on fire if mistreated. Jerry
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=487568#487568
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