DECEMBER/JANUARY 2018 33
While we have the generating and
control piece solved (more on this in a
future article), the batteries are a work
in progress. Such a system requires a
battery that can store substantial
amounts of energy in a relatively com-
pact and lightweight format (a high
energy density) and is capable of
absorbing extremely high charge rates
to high states of charge. It must be
able to tolerate deep discharges so the
full capacity can be used at each cycle.
It’s a more extreme pro le of a battery
type that has been required on recre-
ational boats for decades. Various
lithium-ion o erings meet these new
requirements, but in general they are
shockingly expensive. On paper they
may be up to 10 times as expensive for
the same nominal capacity as lead-
acid (although, as we will see shortly,
this is not a fair comparison), so ini-
tially we have been concentrating on
lead-acid.
Lead-Acid Options
All forms of lead-acid batteries have
several major drawbacks in a cruising
environment. ey don’t like deep dis-
charges. To minimize the depth of dis-
charge at each cycle, you need at least
double the capacity you will use,
which translates into a lot of volume,
weight, and cost. en, when it comes
time to recharge, once you get up to
around 50% to 60% state of charge, the
batteries’ ability to absorb charging
current steadily tapers o and can’t be
forced without doing damage. If the
operator gets fed up with the extended
engine or generator run time resulting
from the low charge acceptance rates
High-rate charging
and overcharging can
produce suf cient heat
and hydrogen to cause
battery meltdowns and
explosions.
Top left—The case was
blown open when this
battery got too hot. In the
lab the battery was tested
for leaks with a soap
solution. The damage to
the battery's plates (top
right) created a new
mystery. When the batter-
ies were taken apart at
the factory, no one could
determine what caused
the many holes.
Above left—These batter-
ies melted and would
have blown up if there
had been a spark.
Left—This overcharged
battery resulted in
thermal runaway, which
eventually caused the
battery to blow up.
Pushing Batteries to the Limit
Batteries170-ADFinal.indd 33 10/31/17 12:19 PM
