Date: Thu, 22 Feb 2007 07:50:42 -0800
Reply-To: Michael Elliott <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Michael Elliott <camping.elliott@GMAIL.COM>
Subject: Re: was fav 12 V acc, now 2nd battery relay system
In-Reply-To: <d0a.96ef9fe.330ed6d6@aol.com>
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(Is this a top post or a bottom post or a don't care group?)
Hi Frank, Yeah, I see what you mean, but I don't see it as a real risk.
If a 1315 is being used, and if the aux battery is deeply discharged,
and if the wire between the two batteries is fat, the aux battery will
suck current from the starter battery when the two are connected. If the
alternator is unable to provide enough current to keep the voltage
sensed by the 1315 above 12.8 volts, the 1315 will disconnect the aux
battery, wait a bit, then try reconnecting it. A smart system, an
advantage.
In a relay system, like mine, the batteries are not bridged until the
alt light goes dark (I am still not clear, exactly, in electrical terms,
what conditions cause the lamp to go out, but presumably it indicates
that the alternator is powering everything sufficiently well to be
charging the engine battery). If the aux battery is deeply discharged,
and if the wire to it is fat, then there will be a lot of current
through that wire when the relay closes. Now, if the alternator has
enough current capacity to provide the needed current, then the aux
battery will not be dragging down the engine battery because the
alternator will be providing what is needed to keep voltages where they
need to be, at the same voltage. If they are at the same voltage,
current will not flow from one to the other. The engine battery might
not be getting much charging at this time, since the majority of the
alternator's current will be going into the aux battery.
[If you put 13 or 14 volts onto two batteries that are connected in
parallel, the battery that has been more deeply discharged will draw
more current -- from the source, not the other battery unless the source
can't keep the charging voltage at 13 or 14 volts. If the charging
source can't and the voltage drops then the more fully-charged battery
will be providing the charging current into the less-charged battery.
But if all things are equal, like battery types, and wires having
sufficiently low resistances so they don't limit currents and all that,
the strongest source provides the charging current.]
If, however, the alternator can't provide enough current to be charging
my engine battery and aux battery at the same time, or if the wire from
the alternator to my engine battery is skinny while the wire from my
engine battery to the aux battery is fat, the engine battery might be
called upon to provide some or all of the current that the aux battery
wants. If a 1315 was monitoring the voltage at the terminals of my
engine battery, it would disconnect the aux. If it was monitoring the
voltage some other place it might not, depending on whether the whole
system sags or not. The relay system is dependent upon the alt lamp to
tell whether it can connect the aux battery. I presume (again, don't
know) that the alt lamp will light if the alternator can't drive the
system voltage up then it would disconnect the aux battery. Again,
voltage drops caused by wire losses affect the operation of this: a fat
wire between the two batteries and a skinny wire from the alternator to
the engine battery would cause the alternator to think thing are better
than they are and maybe not light the alt lamp. That's one situation
where the 1315 is better, since it monitors the engine battery while the
alt lamp seems to know more about the conditions back at the alternator.
But if the alt lamp ain't happy, the relay disconnects the aux battery.
A second advantage to the 1315 is that it does wait a bit to try to
reconnect the two, while the alt lamp might go right out as soon as the
relay disconnected the aux battery, causing the relay to reconnect,
causing the alt lamp to light again. Wash, rinse, repeat.
I do see those as two distinct advantages that the 1315 possesses.
However -- and here is where folks who know Vanagon wiring real well can
correct me -- I don't think that the engine battery is at much risk if
it is connected to a deeply-discharged aux battery because I am assuming
that the wire from the alternator to the engine battery is fat enough to
deliver the current the aux battery wants so that the engine battery
isn't much bothered. If the aux battery is totally hosed, then all bets
with the relay system are off, as I described below in my post of a
couple days ago.
My aux battery is a deep cycle Group 27 under the bench seat. While it's
not identical to the starting battery it's not so different that I'm
worried about overcharging or undercharging one or the other. Besides,
the long wire run to it from the relay provides a bit of a voltage drop
which limits charging current if the battery is deeply discharged. I
don't normally let it get below 50% depth of discharge anyway.
I'm not arguing that the relay system has all the advantages of the
1315. I think the 1315 is cool, and see that it has two clear features
that make it desirable, but I don't see the risk of connecting a
deeply-discharged aux battery to the engine battery as long as the
alternator can provide the current and that current can reach at least
as far as the engine battery.
--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR
Frank Condelli typed:
> In a message dated 21/02/2007 11:56:49 A.M. Eastern Standard Time,
> LISTSERV@GERRY.VANAGON.COM writes:
>
> Hi Frank, I'm not seeing any normal conditions where my relay system is
> at a disadvantage when compared to the 1315. My starter battery is
> always connected to the charging source, just like in a system using the
> 1315. My aux batter is connected to the starter battery/charging source
> under only two conditions:
>
> 1. The alternator is putting out enough voltage to extinguish the dash
> ALT lamp. What's that -- 13 volts or more? Bridging the aux battery to
> the starter battery under these conditions is appropriate unless the aux
> battery has suffered a major failure, in which case all bets are off
> with my system.
>
> 2. I have operated a switch to manually bridge the aux battery to the
> starter battery/charging source by applying voltage from the aux battery
> to the relay coil. This would be done in the case where the starter
> battery is low and I need to steal some power from the aux battery to
> pump up the starter battery. There also should be no problem unless the
> starter battery has suffered a major failure.
>
> I did use the phrase "normal conditions" in my opening paragraph, and I
> noted above two unusual conditions which involve the near or total
> meltdown of one or both batteries. In the situation where the aux
> battery was hosed, the 1315 would allow a fellow to keep driving because
> it would refuse to connect the bad battery, whereas my system has no
> such safeguard. If the starter battery is low, both systems allow a
> fellow to bridge the aux to it for starting, but if the starter battery
> was massively failed neither system would help.
>
> The 1315 does not require metering: it appears to be smart enough to not
> connect ugly-dead batteries to the system. So that's an advantage -- but
> to me, not much of one because I like meters -- even with a 1315 I'd
> monitor battery voltage.
>
> So, unless I'm missing something, the 1315 only provides two advantages
> over the relay system: no-brainer operation (always useful when you have
> a squirrel's brain) and it refuses to connect a hosed aux battery. Those
> two may be advantages enough for some folk to recommend the 1315 to
> them, but to say that "relay systems work but not well," is a bit much.
>
>
>
>
>
> Mike, this is all well and good and correct, however your missing one
> important point. Using a relay system has both batteries connected together as
> soon as the engine starts and the alternator red light goes out we all agree on
> that ? OK, so if that is true and the aux battery is dead and your staring
> battery is full what happens ? Well, what happens is the main battery will
> try to dump it's charge into the aux battery until they both become equal. The
> rate at which this happens depends on the size of the wire used to connect
> the two batteries and the speed at which the alternator can keep up replacing
> the missing voltage in the batteries. So, this is not a problem if you don't
> mind changing two batteries more often than usual as ordinary car batteries
> do not like to be discharged and charged constantly. That was the reasoning
> behind using a deep cycle battery as the aux battery. So then we're back
> where this thread all started. The only deep cycle battery that sort of fits
> under the seat is the OPTIMA Yellow Top which we see by recent experience is a
> piss poor battery ! So.......some of us have gone back to using a regular
> car battery with a good warranty as the aux battery. Those of us who have
> done this most likely already have the 1315 as it is required to charge the
> OPTIMA correctly in a two dissimilar battery situation. It also make more sense
> to charge the batteries independently and this is what the 1315 does best.
>
> Cheers,
>
> Frank Condelli
> Almonte, Ontario, Canada
> '87 Westy, '90 Carat, '87 Wolfsburg (Forsale) & Lionel Trains (_Collection
> for sale_ (http://members.aol.com/Fkc43/trainsal.htm) )
> Vanagon/Vanagon Westfalia Service in the Ottawa Valley
> _Frank Condelli & Associates_ (http://members.aol.com/Fkc43/busindex.html)
> _Vanagon Stainless Steel Exhaust Systems_
> (http://members.aol.com/Fkc43/stebro.htm)
> _BusFusion_ (http://members.aol.com/BusFusion/bfhome.htm) a VW Camper
> camping event, Almonte, ON, June 07 ~ 10, 2007
>
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