Date:         Sat, 3 Aug 2013 12:39:06 -0700
Reply-To:     Steve Williams <sbw@SBW.ORG>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Steve Williams <sbw@SBW.ORG>
Subject:      Re: Battery Charger Sizing (Amperage) Woe and Warning (?)
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              mail.com>
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neil n wrote:
>... I had to review Ohms Law again.  If I look at voltage as the
>"push" ... a charger capable of a higher voltage output to push Amps
>into the battery, would be more beneficial?

OK, well, not exactly a higher voltage rating, but rather a higher
current rating (amps).  But, yes, there is an optimal voltage for
each charging mode, so we'd like the charger to maintain that
voltage, if possible.

However, a given charger can provide only so much current.  A small
charger may not be able to provide enough current to bring a
deeply-discharged battery up to the optimum voltage.

A discharged battery is a resistive load.  The more discharged it is,
the lower its resistance.  As it is charged, its resistance gradually goes up.

 From ohms law, you know lower resistance means higher current.  So a
deeply discharged battery requires more amps to reach the optimum voltage.

Maybe more amps than the charger can provide.  That was Karl's point, I think.

My little 8-amp ProSport charger will try to bring the voltage up to,
say, 14.6 volts for its lead/acid bulk-charging mode.  But my
100-amp-hour battery, if deeply discharged, may present such low
resistance that the ProSport reaches its 8-amp limit at a voltage
lower than 14.6 volts.

When that happens, the ProSport keeps the current at 8 amps, charging
the battery more slowly than if it could provide enough current to
get up to 14.6 volts.

As the battery (slowly) charges, its resistance increases, so the
ProSport, sticking to its 8 amp limit, can gradually increase the
voltage until it reaches 14.6 volts.  After that, the ProSport will
reduce the current to maintain 14.6 volts.

There's a chart in this ProMariner PDF: http://urlzr.mp/hcb

>"an appliance cycling on/off (like a fridge) may confuse or mess up
>the automatic or intelligent aspect of an intelligent charger"

Sure.  Even a continuous load must confuse the algorithm.

Some charger brands talk about a "dockside" or "power supply"
mode.  I'd be curious to learn more about that.  Do any chargers
isolate and power the loads while charging that battery?  That'd be
the only way to completely eliminate this effect, and I have to
believe it'd increase the complexity of the charger a
lot.  (Increased complexity implies higher cost and/or lower
reliability.  Hard to imagine it's worth it.)

=====
A note on charger ratings:

Strictly speaking, it might make more sense to rate a charger by its
POWER limit, rather than its current limit.  Power is measured in
watts, that is, amps times volts.  We probably rate chargers by
current rather than power because the voltage output is variable by
design, so it would be more confusing to talk about power limits.

Also, wire size is determined by current, not power, so knowing the
maximum current is more useful in choosing the right wire and fuses
for the charging circuit.