Agreed. The navy used lithium batteries in buoys anly because when a
catastrophic failure results they are 500 feet under the sea.
Bill Jacobs
------ Original message------From: Tony King via KRnet Date: Thu, Oct 15, 2015
11:13 PMTo: KRnet;Cc: Tony King;Subject:KR> Lithium BatteriesLithium batteries
make great starter batteries because they are capable ofmuch higher discharge
rates than lead acid batteries for a given amount ofenergy stored. This one of
the factors that enables users to install amuch lighter lithium battery than
the lead acid equivalent - a 10AH lithiumbattery might have the same cranking
performance as a 20AH lead acidbattery.Lithium batteries also have other
characteristics that need to beunderstood and accounted for in the electrical
design. For example, wherea lead acid battery has a fairly smooth discharge
curve (in that the outputvoltage decreases somewhat linearly as the battery
discharges), thedischarge curve for the typical lithium battery has a very
sharp drop offonce it discharges beyond a certain point. One minute it'll be
deliveringenough voltage and the next it won't.This, combined with the smaller
amp hours rating for a given crankingperformance (a lithium battery capable of
cranking say an O-200 will haveless amp hours than a lead acid battery capable
of cranking the sameengine), makes a lithium battery quite a different
proposition as a backupbattery.A further consideration is that lithium
batteries are damaged by bothover-voltage and under-voltage. A couple of
incidents will illustrate.A friend of mine installed a lithium battery in his
Zenith CH200. One daywhen he went to fly he found the master had been left on
and the batterywas flat. He checked the aircraft throroughly and then hand
propped it.He confirmed everything was normal, the alternator was charging and
thentook off. 15 minutes later (at 4,500ft) the cabin filled with acridfumes.
He was able to get the canopy open and perform a forced landing ina field.
Analysis showed the lithium battery had been damaged by thecomplete discharge
and reacted badly to being recharged. He was lucky.Less fortunate was the
pilot of a LSA seaplane where a lithium batterycaught fire in flight (less than
an hour after it was installed) and meltedthe epoxy/carbon fibre tailboom.
Research into this incident indicates anissue with lithium batteries in
electrical systems that use PWM (pulsewidth modulation) voltage regulators
(e.g. most Rotax andautomotive installations).When exposed to voltages above
about 14.6 volts lithium batteries start todevelop little crystals inside that
can short circuit the cathode andanode. When enough of these build up the
battery will experience acatastrophic failure.Alternators produce something
like 22 volts peak. This is rectified andthen 'regulated' (in a PWM system) by
switching the voltage on and off toproduce a waveform that averages the
required voltage - usually 13.8V, asuitable voltage for charging a lead acid
battery that's nominally 12V.The problem is that while the waveform coming out
of the regulator isnominally 13.8V (and averaged over time actually is 13.8V),
at any instantin time it could be any voltage in the range from zero to the
peak voltageof the alternator (around 22V). Whenever it's over 14.6V it'll be
doingdamage if there's a lithium battery in the system. Lead acid
batteriestolerate this method of regulation. Lithium batteries much less so.
Themore damage (those little crystals) that accumulates the greater
thelikelihood of a catastrophic failure.In many cases people have used these
batteries with no dramas - I hadone in my (Rotax powered) aircraft for 70 hours
of incident free flying.But that was before I knew what I know now.Although
these batteries are marketed as a drop in replacement for the leadacid
batteries that are standard in many recreational vehicles, thedifferences can
be significant - especially in an aircraft. I'd now onlyuse a lithium battery
if the charging system can never produce avoltage higher than 14.6V. And if
the battery was ever fully discharged(e.g. by leaving the master on) it'd be
going in the disposal
bin.Cheers,Tony_______________________________________________Search the KRnet
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