Date: Fri, 11 Oct 2002 23:28:32 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Battery Reserve Capacity Calculations - 2nd Posting
In-Reply-To: <01e001c2719c$12d23c80$f2aa6620@laptop>
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At 11:04 PM 10/11/2002, Larry Chase wrote:
>I'm getting ready to do some Battery requirement calculations.
>
>Assuming a OPTIMA D750 Yellow Top Deep Cycle Battery
>
>Rated at 120 minutes of Reserve Capacity at a 25 Amp draw.
>
>Specifications.
>
>Reserve Capacity - Minutes: 124
>Capacity (C/2 rate) - Amp Hours: 52
>Capacity (C20 rate) - Amp Hours: 65
>
>Am I correct in assuming that a 10 Amp Per Hour Power requirement
>would mean this battery would last 5 hours before needing a recharge?
Ten amps draw for one hour equals ten amp-hours, so five hours would be
fifty amp-hours. That part's easy. Now comes the fun stuff.
First thing is that your alternator with the installed regulator will never
bring the battery to full charge. For thorough discussion of this visit
http://www.amplepower.com/ and read through the discussions of charging
available there (not always under completely obvious headings). For your
reference, the stock regulator has a fixed setpoint of (I believe) 14.1
volts, measured inside the alternator and not at the battery.
Second, a rule of thumb for most economical overall operation (taking into
account battery life, initial cost and charging cost, but not necessarily
the cost of hauling the weight around) is to assume that only 35% of the
nominal capacity of the battery will be used. This depends on two factors:
the first is that putting the last 15% of charge into a battery takes
hours, even assuming that the regulator will do it at all. Second, the
lifetime (in charge-discharge cycles) of a lead-acid battery is greatly
increased (factor up to five) by not discharging it below 50%. Hence 85%
max - 50% min = 35%. In that case you might only be able to draw ten amps
for a bit over two hours.
However, you may well decide to discharge farther than that, down to a
minimum of maybe 80%. That would give you a usable capacity of [up to] 65%
of nominal, at the expense of having many fewer charge/discharge cycles
(for argument, say 200 instead of 1000). Doing it that way, you might be
able to run ten amps for about 3 1/2 hours.
Note that the Reserve Capacity rating assumes a fully charged battery
(unlikely without external charging or enough fancy hardware that you'll
already know the answers to this question) under ideal conditions
(unlikely), to a point where damage to the battery may be likely. It's a
useful *relative* measurement in terms of how long you could nurse a car
along the road drawing 25 amps after it quits charging. In fact 25 amps is
probably more current than it's wise to draw from a battery this size at
all (again, for best battery longevity). The typical rule of thumb is C/4
for conventional batteries, and maybe up to C/3 for gel and AGM (like the
Optima) batteries.
david
--
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
