Date: Tue, 14 Mar 2006 08:17:29 -0500
Reply-To: dhaynes@OPTONLINE.NET
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: Isolator vs Seperator (mechanical relay) was Re: Sure Power
1315 battery separator
In-Reply-To: <1b2e01c646fd$285ab020$c901a8c0@w2ktopcat>
Content-type: text/plain; charset=us-ascii
In the real RV world, extremely cheap RV's and trailer set ups will sometimes get the diode based isolators. Anything respectable these days gets a relay and some control method for charging of Auxillary or "house" batteries.
Except for some specialty units such as those by Hell-Roaring, diode isolators have a forward voltage drop of ~1.1 volt cold. As they heat up, the voltage drop increases. So lets say you need the 90 amp output of your alternator, the diode will consume over 90 watts of power just making waste. Now, at full load, the limit of the alternator is really the internal resistance. As such, at full load, the alternator is only ~50% effecient. It needs to produce another 1.1 volt or more to overcome the the voltage loss of the diodes. So the alternator will also disspate another 90 watts of heat so the net loss is 180 watts. You pay for that in some increased fuel consumption and of course hotter alternator operation leading to longer life.
As for the diode providing a better battery charge, not so. Unless there is aseparate voltage or current control for each bank, the charging system is basically a constant voltage source limited only by capacity of the alternoator, wiring, and control devices. Basically, during the charge process, the alternator is supplying a fixed voltage and whether a diode or relay is used, the battery bank with the least charge resistance will get the most current.
As for reliability, yes, relays can fail. Properly sized, the contacts will last for thousands of cycles. Sometimes large relays will get the contacts fouled since battery cahrging does not always draw enough current to burn the contacts clean on larger relays. Quality relays will silver plated contacts can take care of this problem. Proper battery control relays will often look like older Ford starter solenoids. Big enclsed contacts.
Another advantage of the relay is the ability to load share the batteries. During periods of low alternator output or the surge load of an inverter, the relay will allow boht banks of batteries to carry the load. The diodes will not.
No the only real question is what control strategy to use.
Dennis
----- Original Message -----
From: TC <trclark@SHAW.CA>
Date: Monday, March 13, 2006 7:14 pm
Subject: Re: Isolator vs Seperator (mechanical relay) was Re: Sure Power 1315 battery separator
> > Installing the isolator in a Vanagon has too many drawbacks that the
> > relay or separator don't have. Please explain exactly what you
> now think
> > an isolator can do better than a 1315.
> I dunno how much more clear i can get with my laymans knowledge:)
> but here goes a couple of assumptions
> A-the battery must always be recharged at the proper voltage as
> spec'd by
> mfg
> it must be maintained at FULL charge at all times.
> B-under/over charge will damage a battery
> C-a batteries current demand(amps) is directly related to is
> discharge level
>
> Longer device life & Longer battery life if the aux is discharged
> regularlysince
> 1) Isolator is solid state vs seperartor mechanical relay so it
> should have
> a longer theoretical life
> -more efficent it has no contacts that build resistance over
> time, so
> your going to get correct volts/amps
> to both batts over the long haul (see point A)
> 2) Each batt gets it volts/amps directly from the alternator
> -the aux battery current demand (amps) is supplied from the
> alternatornot from the alternator AND the 1st battery
> in heavy discharge state the aux may not get the required
> amps/voltsit needs for recharge
> (see point A/B above) since its in competion with the 1st
> battery for
> amps/volts
> and the sudden surge from battery to battery when the
> mechanical relay
> is activated can
> damage the more discharged battery
> __________________________________________________________
> But in the vw having to modify the alternator is a real drag 1
> find myself
> in today:)
> And with my config I believe i will never be using the aux to a
> low levels
> since i have prostar 30m in now
> ps. i just ordered a new 1315 & alternator
> (the busted spade on the diode board that costs $80 and i got a re
> mfg alt
> for us110
>
> > >>ARE THERE THERE ANY ADVANTAGES TO AN ISOLATOR at all ?
> > >>OK I just found this any comments ?
> > >>http://www.smithae.com/surepower.html
> > >>
> > >>
> > >luv this talking to myself thx for the sounding board
> > >So I think the rule of thumb is
> > >if your useage pattern is more towards
> > >staying & using your aux batt & discharging more fully so that
> when the
> > >charge
> > >cycle starts your aux batt has a substantial recharge requirement
> > >then you should consider an isolator solution.
> > >if your discharge cycle
> > >1) does not tend to drain the aux right down
> > >OR
> > >a low voltage disconnet is installed
> > >OR
> > >a solar charge system is charging at the same time as discharge
> so that
> the
> > >aux never realy has a full drain
> > >the battery seperartor is a better solution
> > >
> > >hopefully this is the bottom line on isolator vs mechanical relay
> > >
> > >
> > >
> > >
>
