Date:         Sun, 5 Aug 2007 16:19:12 -0600
Reply-To:     Martin Jagersand <jag@CS.UALBERTA.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Martin Jagersand <jag@CS.UALBERTA.CA>
Subject:      T3 Westy's; batteries; how to have your cake and eat it
Content-Type: text/plain; charset=ISO-8859-1

Hi,

Following my dual 6V deep cycle install in a T4 Westy and the
recent flurry of battery emails to the list, I started thinking of
what would be the best way to maximize deep cycle Ah, while
minimizing space use in a T3 Westy.

Removing the constraint of not converting storage space into
battery space, one can of course put the tried and true golf cart
size deep cycles in the locker under the bench seat.

However, for many of us the under seat storage is an important
locker to squeeze in bulky outdoor equipments, that won't fit elsewhere.
For me so important so I even removed the heater in there on my '85.

So what remains to utilize are the two regular battery lockers behind
drivers and passenger seats. One is already taken by the starting
battery, and in the other one cannot fit a larger than 55-70Ah
deep cycle. So what can one do to get 100+ Ah deep cycle capacity?

My proposed solution is to replace the typical charge relay with a
battery voltage cut out relay (or "battery protector"). That is one
of those devices that are advertised to protect your starting battery
from discharging to a voltage below what's required to start a car.

Now instead of having one dedicated starting battery and one deep
cycle battery, you need two deep cycle batteries with enough CCA
to start your vehicle. They will be connected together in parallel
for long periods, so they should be of same type and age. (Otherwise
small differences in chemistry might make one battery leak current into
the other, and thus parasitically discharge)

Armed with a couple of 12V gel cell batteries (from our robots at school),
a "battery protector" relay from Canadian tire, my 1000W inverter, a charger
and some test loads, I set out to test this idea. (On the bedroom floor, to
wife's dismay when she came home... I was quickly booted out)

I wired it together as follows: Battery 1 was wired to charger and would
act as the starting battery in a vehicle setup. Battery 2 was wired to
the inverter (and any other camping loads). The "battery protector" was set
up between the two batteries according to the enclosed manual.

In operation, it works like this: When charging, the battery protector
puts both batteries in parallel, just like a dual battery charge relay.
The difference is that when it stops charging, and switches to
discharge during camping, the two batteries remain connected,
but only until a certain voltage/discharge %-age is reached. After
this, the relay cuts, and battery 1 becomes isolated from battery 2.

The experiment started well. Batteries both charged up. I disconnected
the charger, and I could use the combined CA of both to run loads.
On reaching 12V (corresponding to about 50% battery capacity) the
relay would cut the connection as advertised on the battery protector
box. Experiment successfully concluded I thought, and prepared to
wrap up. Whoops, moments later, the battery protector cut back in
again, then it would oscillate in and out, upping and lowering the
voltage around the set point 12V, causing an alarm buzzer to go off in the
inverter. (Alerting wife...)

So what happened? When the battery protector cut the connection
at 12 V, battery 1 that had been loaded, but now was unloaded, would
recover after some time, voltage go up above 12V again, and the battery
protector did exactly what it is supposed to do: enable the connection
again.
Now being loaded, but not recharged, the voltage would quickly drop below
12V, causing the relay to cut, and this would go on over and over again at a

rapid rate. (presumably until the battery was discharged enough to remain
under 12V even unloaded)

Not a pleasant effect! So how to solve? Turns out the solution was
rather easy. Instead of having the negative "sense" wire on battery 1, the
starting battery, (this is the normal setup as it then measures the voltage
of that "protected" battery.), i put the sense wire on battery 2, the
camping battery. Now it worked great: When the voltage dropped below 12V,
the relay would cut and separate the batteries. Battery 2 remains loaded,
hence the sense voltage does not pop above 12V, and the starting battery
remains protected, without any on-off oscillation.

So end effect was that from my two 10Ah robot gel cells got 10Ah while
both were connected, and another 5Ah from battery 2, after disconnect,
while "protecting" 5Ah in battery 1. Scaling this to two 70Ah deep
cycles (about the biggest size that can be squeezed into the T3
battery boxes.), one would get 70Ah while in parallel (at a very high
max current capacity, no problem to run your cappuchino maker!),
then another 35Ah in single battery mode, making a total of
105Ah deep cycle capacity, and still fitting in the standard spaces!
50% remains in battery 1 for starting. (Other mixes of starting/camping
Ah percentages can be had by varying  the cutoff voltage.)
Like both having your cake and eating it...
An added bonus, is that in winter use (when we drive to ski places,
but don't camp in the Westy), there is double the CCA of one battery to
start the Diesel in my '82.

Any cons? Well now both batteries will be cycled in semi-deep cycle mode,
so the one used for starting will likely wear out a bit faster than if it
had not been used for caming supply. On the other hand, if the typical
use is using 70Ah or less, then the combined life of both batteries
will be much better than if a single 70Ah deep cycle was used to 70Ah,
since each battery is only cycled to 50% of capacity instead of one to 100%.

I've got some bodywork to do on the '82 Westy, so probably won't
splurge on the 2  deep cycles until next season, but will keep looking
for the best battery (most Ah, least cost) that fits the T3 in the
meanwhile.

Martin

PS "Battery protector" was bought for $27 at Canadian tire on sale.
It is rated at 275A continuous, 1100A intermittent (starting) current.
Everything at CT goes on sale cyclically so it's just a matter of looking
and waiting.
--
Martin Jagersand
University of Alberta
http://www.cs.ualberta.ca/~jag/