Date: Sat, 16 Oct 2004 09:48:09 -0400
Reply-To: Dennis Haynes <dhaynes@OPTONLINE.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: Second Battery / Battery wiring / 110vt Inverter
In-Reply-To: <004101c4b30f$bd3428d0$e1d2ea42@ttower17>
Content-type: text/plain; charset=us-ascii
A 1,000 watt inverter for $69 probably can't really deliver 1,000 watts
and the output may be such poor quality that anything valuable plugged
into won't work properly but let me really answer the battery issue.
1,000 watts at 12 volts means you need to draw 83.3 amps, assuming 100%
efficiency. Considering the battery voltage may drop to about 10.5 volts
under extended load and an efficiency of ~ 90%, you really need 100 amps
available. Add a surge factor of at least 50%, (what is the inverter
surge rated at?) and now you got 150 amps. This is almost the same as
your starter motor cranking a warm engine. How long do you think your
present battery can crank the engine?
Ideally, batteries should be sized to be discharged at a 5 hour rate or
slower. You need to consider the load you normally plan to run on the
inverter and any other loads such as lighting radio, etc. If you only
plan to use the inverter at high load for short periods, like for a
microwave, then sizing the battery for a two hour discharge is adequate,
but battery life will shorter.
I'll avoid the math, but to truly support this inverter, the ideal set
up is two golf cart batteries in series. These will typically be rated
at 225 amp hours. The equivalent in Optimas requires four batteries or
you can also use two group 27 or 31 marine batteries. The golf cart
batteries are by far the best solution but they take a lot of room.
Now for battery charging! Since it is possible that you may want to
charge the batteries and use the inverter at the same time, the charging
relay and wiring have to rated to take that load, even if it exceeds the
alternator rating. This is because the main battery will also try to
support the inverter when the relay is closed. This is actually one
application where an isolator may make sense but I prefer the relay. I
suggest an RV style battery relay rated for 150 amps and 6 or 8 gauge
wire directly run from the alternator. A better option is to use a
battery combiner. These will connect the house battery when the main is
charged to ~13 volts. As the voltage goes below 12, it automatically
disconnects which will keep the inverter from draining all the batteries
even with the engine running. If the alternator can not keep up with the
load, the house batteries and inverter will be separated so the main
battery can get charged again. Another benefit of the combiner is that
if add a separated charger for the house battery, the combiner will also
use that charger to charge the main battery automatically.
Hope some of this helps.
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Doug in Calif
Sent: Friday, October 15, 2004 7:36 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Second Battery / Battery wiring / 110vt Inverter
Well Volks,
As I am moving my way around the 87 syncro GL to Westy conversion, I
have come to the front seat swivels and 2nd battery area of the
conversion.
I have decided I am going to install the swivel under the drivers side
as well even though most of you tell me you never turn the drivers seat
or set up the front table much.
I figure I need to "have it" so I can also "not use it". (grin)
I have come up with a way to bolt the swivels in place so I can still
pull the whole thing off and not have it welded in the way, which brings
me to my topic.
I am installing a nice 1000 watt inverter I picked up at Costco awhile
back for $69 It looks like it will fit nicely under the drivers seat and
I can make some large wire connections to the 2nd battery.
These inverters seem to really want good heavy connections to the
battery to produce decent amounts of AC current.
My main question is if you were wiring up the 2nd battery from scratch
how would you wire it in? I do like the simple relay design VW has it
seems to work well and do the job, no?
I also would like to use a smaller battery than stock size, and here's
my main question, are there any really good small deep cycle batteries
that will fit into the compartment?
I have seen some nice ones at West Marine for sailboats etc that look
good but good grief a hundred bucks?
I have heard there is a BMW battery that fits into the compartment and
is smaller.
I have found a fellow here in Sausalito (Arc Battery) who provides a
deep cycle battery that is the exact size of the vanagon battery. Its
darn heavy though.
What guage wire is used to connect the two batteries together?
Doug