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[Shop-talk] Milwaukee vs Dewalt cordless Hex Impact Driver

Subject: [Shop-talk] Milwaukee vs Dewalt cordless Hex Impact Driver
From: TR3driver at (Randall)
Date: Thu, 24 Nov 2011 16:25:47 -0800
References: <> <>
> Two of the packs lasted two charges each.  I wonder if maybe my usage 
> was an issue--

Lots of people fixate on the "memory effect" and try to run their battery
packs all the way down.  This is the worst possible usage for a NiCd battery
pack!  It is far better to recharge the pack sooner rather than later.

The problem is that the individual cells don't have exactly matched
capacities and so there is always one cell that runs down a bit sooner than
the others.  If you continue using the pack at that point, that one cell
will have the voltage across it reversed, which causes a NiCd cell to bridge
and short internally.  And at only 1.2v per cell, it can be hard to judge
when the pack voltage is down enough to reverse one cell.

The much publicized "memory effect" only shows up if you charge and
discharge the cell by _exactly_ the same amount _every_ time; and also only
if you have an over-sensitive voltage monitor (trying to prevent one cell
from getting reversed).  That is why NiCd batteries had been in use for many
decades before the problem was discovered, in a satellite where the charge
and discharge times were always identical.

NiCads also self-discharge to some extent, so it is best to refresh the
charge occasionally even if the pack hasn't been used.  My Ryobi charger
also refuses to charge packs that have self-discharged (even though it
shouldn't harm the pack) so I sometimes have to charge them a bit externally
before it will start charging.

-- Randall  

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