Battery
Backup for the Z3801A GPS Receiver
This
article by Richard Bailey describes a low cost and practical
method of getting a battery backup power supply for your GPS
receiver. Many of the components are available from a Wal-Mart
store!
While
marine deep discharge batteries are more suitable for very long
power-off times, the less expensive automotive types shown here
are usable for shorter periods where the battery is not deeply
discharged. The relatively light load of a GPS Z3801A receiver
helps in this regard.
I
decided to build a battery back-up for the Z3801A. I wanted
to have a system with at least 24-hour capacity that was relatively
inexpensive with readily available parts. What I came up with
cost about $70 and all the parts were bought at Walmart. First
of all I should say I'm not a fan of gel-cell batteries most
often used for back-up and I have had very poor luck with them.
A 12 volt 20AH gel-cell cost about $30 plus shipping and you'd
need four of them to power the Z3801A. Walmart has 12 volt Garden
tractor sealed lead acid batteries for $15 each (may have $5/battery
refundable old battery fee-great if you have old batteries to
dispose of) and four of those will fit in to a Sterlite plastic
container with a drawer that will hold and protect the batteries.
I bought an inline fuse holder and 3 amp 2 pin auto fuses in
the automotive section and ΒΌ"-20 1" long screws, flat washers,
and nuts in hardware, and supplied my own wire. I had the wire
going to the Z3801A come out the back and put the fuse holder
in the recessed pull area on the front for accessibility. I
used 1" solid foam for spacing but cardboard or just nothing
will work. This sits underneath my bench out of the way and
the wire to the receiver feeds up along the wall to the shelf
the receiver lives on.
All this is powered by a supply like the simple one shown
on this web site with just a transformer, bridge, large
filter capacitor, and two fuses. I found that I had a 120 to
39 volt 1.5 amp transformer that would "float" the batteries
at 54VDC or about 13.5VDC per battery which is just about perfect.
With the extra load from charging I used a 2A fuse for the load
and a 1.5A slow blow line fuse. I also used a higher rated bridge
rectifier.
If you try to use the 36V 1.5 amp transformer specified, check
what the voltage to the receiver is without batteries. If it
is about 50 VDC after the receiver has been on long enough to
warm up you will want to adjust it by wiring half the secondary
of a 6.3 VCT 1.5A filament transformer in series adding to get
the correct value or just find another transformer. Note that
the output voltage will be less while the batteries are charging
or after a power outage.
50
VDC is the voltage I measured after 24 hours on batteries and
it may take 48 hours (about twice the outage time) to get back
to 54 VDC. If power outages in your area are rare this should
be adequate. A better way is to use a switching regulated supply
adjusted to 54 VDC. Be warned that some power supplies are intolerant
of voltage being fed into the output terminals when they are
not powered and having the batteries connected directly across
their output could cause severe damage to the supply. The simple
unregulated supply shown on
this site is not affected this way.
If you plan to use another supply, connect a 10A - 100 PRV diode
between the batteries and the power supply with the cathode
end toward the batteries. Sometimes called a "disconnect"
diode, it will be forward biased when the supply is charging
the batteries, and reversed biased when the supply goes off,
thus protecting the supply. The supply voltage will have to
be set about .6 VDC higher (54.6 VDC) to compensate for the
series diode drop.
Wal-Mart
Parts List:
Drawer 007314918068 $7.44 1 each
Battery U1 068113107874 14.93 4 each
Mach screw 005026725074 .83 2 each (nuts included)
Flat washer 005026725682 .83 1 each
Atm inline 005171281039 1.88 1 each
©
2002-2004 Richard Bailey
