Date: Sun, 22 Aug 2004 23:54:46 -0400
Reply-To: Dennis Haynes <dhaynes@OPTONLINE.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: [Syncro] dual battery isolator
In-Reply-To: <JBEOJOBKGBDBEAIBOCBEMECFDJAA.syncrowestytd@mindspring.com>
Content-type: text/plain; charset=us-ascii
Karl,
If you buy a cheap RV, it may use a battery isolator of some sort. If
you get any RV of significant size or cost, it will use a relay or
multiple relays. The major issue next is how it is controlled and the
charging system.
In most automotive or RV applications, the alternator is a relatively
stupid device that tries to maintain a fixed voltage. It does not know,
sense, or care what the charge state of the battery is and most do not
even compensate for temperature. It is really a power source for vehicle
and accessory operation. Battery charging is a secondary benefit.
Therefore, once the batteries are connected in parallel, (relay closed),
and the system is above the batteries base voltage, (~12.6 volts) some
charging or at least maintenance will occur to all. The Bosch alternator
is set to maintain 13.8 volts (2.3 volts/cell). This charge level is
used as it is an acceptable "float" charge level that will sufficiently
charge a battery and can be maintained indefinitely, at most
temperatures without damaging a flooded cell battery. Unfortunately, it
will not bring a deep cycle battery to 100% capacity, quickly. Many
American cars will float the battery at 14.25 to 14.5 volts, (2.375-2.42
v/c). This does provide better charging, but extended operation will
shorten battery life, particularly during hot conditions. A smart
battery charger will actually set a charge rate based on current, (The
chargers capacity), Then as the battery approaches 14.7 volts, (2.45
v/c), it will sense the current dropping as the battery begins to peak
and start gassing, then taper the charge current. Allowing the battery
to gas for a short while is needed to stir the electrolyte and re-coat
the plates, and break the sulfation. This step must be carefully
controlled. Obviously this will not happen with alternator charging.
Now for the relay control strategy! One method is to control the relay
off the ignition switch. If done this way, the relay and wiring must be
sized to handle the load of the starter as the house battery will try to
supply some current during the starting cycle. If the main battery is
low enough, it may try to do the whole job. The advantage of this is
simplicity and both battery banks are available during times the
alternator can't keep up or if the alternator should fail. The
disadvantages are the size of the wiring required and the surge load on
the alternator immediately after engine starting. Also, if the house
bank is very low, it may draw off some starting capacity from the main
bank.
Another choice is to use the accessory contact of the ignition
switch to control the relay. This is the method used by most RV
manufacturers. This provides all the advantages of the above method, but
keeps the house bank isolated during the starting cycle. Thus, wire and
relay size can be reduced if desired. An 8 gauge wire and 40 amp relay
is sufficient unless you are going with a really large house battery or
bank.
The next choice is to use the westy method for the refrigerator
and control the relay off the alternator trigger circuit. Works as above
except will not connect the house bank until the alternator begins
charging. If a large relay coil is used, you may need to rev the engine
a bit higher to get the alternator to "kick in". You should also have a
capacitor (VW uses the same cap as used for noise suppression), present
to keep the spike from the relay de-energizing feeding back to the
alternator or ignition system. Circuitry can be added to turn on the
relay during an alternator failure so you can use both batteries to
extend driving distance or give your self a jump if the relay and wore
sizing is adequate, (see choice 1).
The ultimate choice is to use a battery "combiner". This is now very
popular on high end RVs and boats. These devices are basically self
controlling and are readily available at marine supplies. It is
basically a relay controlled by battery voltage. Most are set to operate
when either battery goes above ~13 volts, and then open when the
combined banks drop below 11.5-12 volts. The better ones will have a
remote trigger circuit so you can force the two banks together if the
charging circuit fails or if you need to give your self a jump. The
major advantage here is that after starting, the circuit does not close
until the main battery has at least started charging. This reduces some
of that surge load on the alternator. The next benefit is that after
shutdown all the batteries are still available to support electrical
demands. As the battery bank voltage drops, the combiner will open
separating the house and main battery leaving plenty of power in the
main battery for starting. If a battery charger is supplied to charge
the house battery, (you should do this), as the house battery charges,
the combiner will close to also charge the main battery. No additional
controls required.
Another important step is to provide upgraded wiring from the alternator
to the batteries. At least a #8 charge line is needed. If you are going
for golf cart batteries or something really big, go for #6. Get the
alternator voltage upped to at least 14.25 but do not exceed 14.5.
As for charging times, the alternator will typically supply 20-25 amps
(voltage and wire size effect this), to a battery. The battery you
selected is usually rated ~110 amp/hours (20 hour rate). After
discharging it 80%, you will need to drive 5-6 hours to recharge it. A
bulk charge may happen in 2-3, but you will not get the 100% capacity.
You should really get a "Smart" battery charger-power supply for when
you have the opportunity to plug into shore power. The Progressive
Dynamics 40 or 60 amps would be my choice, available at Camping World.
http://www.campingworld.com/browse/skus/index.cfm?deptID=3&subOf=215,55&
skunum=21995 You may also want a combo inverter charger but bring
money. If you get this unit, go for the charge wizard controller. It
adds even more intelligence including temperature compensation for
longer battery life and less watering.
One thing to consider, these batteries will off gas, especially after
deep discharging and proper charging. Be sure to enclose and ventilate
it to the outside. The Optima and Lifeline batteries eliminate this
concern but have cost and performance disadvantages.
Some how all this seems like a big expense for 1.7 cu. feet of food
storage. How about a nice cooler and some ice? Or better yet, you can
enjoy some restaurants.
Dennis
--Original Message-----
From: Westyman [mailto:syncrowestytd@mindspring.com]
Sent: Monday, August 23, 2004 1:15 AM
To: Vanagon list; VWVANFULLTIMERS; Syncro list
Subject: [Syncro] dual battery isolator
Hope this doesn't start a whole new argument on the subject, but I seek
input from those who may have done this:
I'm installing a group 29 deep cycle battery as my main camping battery
in
my Westy [no, it won't fit under the front seats!] to power my new
12-volt
Norcold DC-0040 fridge. Yes, I finally had enough of dealing with the
unreliability of the old Dometic! What I'm concerned about is the
different
charge rate required between the deep cycle and the main battery. The
main
will likely never be drained below 75-percent, but the deep cycle may
become
very drained, and often. I currently have a 70-amp relay connected as in
the
stock Westfalia method for dual batteries. Will that work 'OK', or do I
need
to use some other method or isolator? It seems to me that the alternator
would not charge two batteries of different types and levels of drainage
efficiently. Comments? Thanks.
Karl Mullendore
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