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Old 02-12-2013, 12:34 PM   #357
_cy_ OP
Beastly Adventurer
Joined: Sep 2011
Location: Tulsa, Oklahoma
Oddometer: 6,057
Originally Posted by awalker14 View Post
Any issues jumping a weak Shorai or other lithium battery from a regular 12 volt motorcycle battery with good cables?
an excellent question ... the short answer is no issues jump starting a low LiFePO4 battery with a PB battery.

think voltage/current in terms of water flowing through a pipe.
volts = pressure of water
current = flow of water ... larger the pipe, more water flows through

LiFePO4 = low internal resistance = Huge pipe
PB = high internal resistance = little pipe

a fully charged PB battery is 12.85v flooded PB to 13v AGM.

a fully charged LiFePO4 battery is 14.6v to 12.8v at 20% condition. so very little current will flow into discharged LiFePO4 battery from a fully charged PB battery.

lithium batteries has very low internal resistance and will accept a charge at about same rate as discharge. for instance if a LiFePO4 cell is rated for 28C momentary discharge... that cell will accept a charge at that rate too. which could be very bad for low LiFePO4 battery, if hooked up to another fully charged 14.6v LiFePO4 battery.

most LiFePO4 cell mfg spec charge rates at .5C to 1C charge rates to allow time for cell to absorb current.

this issue comes up in series/parallel battery pack configurations. when/if one cell goes bad, rest of cells in nearby parallel stack will attempt to charge up bad stack. resulting in a pack that will never reach full charge. worst case a melted down battery.

which brings up one of the main advantage of LiFePO4 cells ... it's very difficult to cause a LiFePO4 battery to catch on fire ... then resulting fire will not be any where close a catastrophic, thermal runaway like lithium cobalt battery used in Boeing 787.

_cy_ screwed with this post 02-12-2013 at 12:52 PM
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