Off of another list:
Subject: Trickle or not
From: Rick Dill <rdill@cyburban.com>
Date: Wed, 23 Aug 2000 12:50:51 -0400
A trickle charger goes back to the 'old days' when electronics was crude
and the rectifiers in most battery chargers were selenium and not
silicon. The idea was that you could leave the battery on the charger
with a small current flowing into it to keep the battery topped up.
Great concept, but it works well only if the particular battery has a
self-discharge current that is roughly equal to the trickle charge.
Since self-discharge depends upon temperature, it can't work well summer
(high self-discharge current) and winter (low self-discharge current).
In this era, the batteries had relatively high self discharge currents
due to the alloys they used to 'harden' the lead plates.
In the 'old days' getting a battery charger to shut off when the battery
was fully charged was near impossible although they did 'taper' the
charging current a little.
Today with modern electronics it is simple (and cheap) to make a charger
which sharply cuts off any charging at a fixed voltage. While there is
some argument about whether this should 2.3 or 2.4 volts per cell,
either will do an excellent job of keeping the battery charged. 2.3
volts per cell (6.9 or 13.8) is reputed to give longer life but 2.4
volts per cell (7.2 or 14.4) maximizes the capacity and cranking ability
so the higher voltage is generally used.
Battery tenders are simply chargers with a precise cut-off voltage.
They will maintain a battery fully charged and only feed current in to
match the self-discharge.
Many modern batteries (particularly the Optima) have very low self
discharge rates when new and this can be very low when cold. These
batteries can be seriously damaged by a trickle charger which will
significantly overcharge them and 'boil' away the electrolyte. These
same batteries are best maintained with a battery tender (i.e. a battery
charger with a precise cut-off voltage). Trickle chargers should be
relegated to your museum of antique auto stuff.
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