Friday, January 30, 2015

Battery Voltage Monitor

Our Tartan 3500, Intuition, hull number 115, has the standard battery voltage and amp meters installed in the nav station.  Since purchasing Intuition, one of the mysteries is determining when to charge the house battery.  The general guideline is don't deplete battery more than 50% charge.  However, there's gotcha's with this, in that the battery voltage needs to be resting voltage.  For our first few years of ownership, I monitored the voltage according to this chart,

My general guideline was 50% charge is reached when open-circuit voltage is 12.32 volts and the load voltage is 11.8 volts.  In practice, this is difficult to monitor when sailing or at anchor, with refrigerator and/or electronics running.

When sailing on an extended delivery on a friend's boat, he monitored the battery bank usage with a Link Pro Battery Monitor,

These gauges provide

- Main and aux battery voltages
- Amp draw
- Accumulated amps used
- Percent capacity remaining
- Alarms

I was hooked - no more guessing when to charge!! 

In researching battery monitors, there are a few suppliers: Xantrex, Victron, and others.  I decided on installing the Victron BMV 702.  This item came with all necessary cables and was less expensive than the Xantrex. I also read that folks have had problems with the Xantrex units, while feedback was generally excellent for the Victron.  

The way these gauges work is that a shunt is installed between the house battery and boat's common ground, along with +12 volt probes on each battery.  As electrical items draw amps from the battery, the current runs through the shunt and the battery monitor measures the voltage drop.  This voltage drop is converted to amps and accumulated by the gauge.

Installation was pretty easy.  There were several items to address to install.

I needed to find a suitable location for the shunt. I decided to put this next to the DC ground bar.

My next challenge was that my house battery had several ground wires connected directly to the house battery.  Since current through these wires or cables don't pass through the shunt, their amps would not be measured by the monitor.  One cable that caused me to think through Intuition's wiring was the DC ground from the Link 1000 battery charger and inverter. After thinking about this for a while, I decided to move the black cable (ground) from the Link 1000 to the DC common bus.  This required ordering two custom 00 gauge cables with the proper M8 and M10 ends. I find it odd that the shunt used M10 sized bolts, while the negative battery terminal and ground use the smaller M8. I also moved the ground for marine ssb/ham radio.  Now all the grounds are on the common DC bus and pass through the shunt.

Next, I needed to decide where to install the gauge. Installing the gauge was very easy - all it required was a 2" mounting hole.  It was pretty painful to drill this 2" hole in our nav-station.  I didn't have much choice as to where to install the gauge.  I went for it and drilled a 2" hole, mounted the device.

One of the advantages of the Victron BVM is that an RJ11 cable is used to connect the shunt and the gauge.  Connecting the wires was very easy - snake the RJ11 cable through the conduit and plug into the back of the gauge, just like a telephone.

Setup only requires setting the house battery capacity and what feature to monitor on the 2nd battery - I chose volts.  The gauge defaults to 200 amp-hours.  I changed this setting to 300 amp-hours.  I also disabled the 'watts' display.

At this point, I thought I was done and began celebrating!!!  Like everything on a boat, there's always something that goes wrong.  I turned on the boat's electronics and the gauge measured positive amps (great) BUT NOT any accumulated amp-hours (not great).  I let the gauge accumulate amps for about a 1/2 hour hoping it would begin registering. It didn't.  I thought for sure I had a bad gauge.  I was not happy.

After reading the manual for the 10'th time, I found one-sentence that clued me in to the problem,

Charge amps are positive, e.g., putting amps back in the battery, and consumed amps are negative, e.g., using amps.  I had the shunt wired backwards...  A quick fix, swapped the cables and noticed amp-hours increasing with usage.

How did this happen - when installing the shunt, I decided to flip it over so the RJ11 and battery wires are plugged in from the top.  That effectively flipped the battery and ground connections.  Because of the delay in ordering the custom cables, I forgot about this last-minute decision.

This is the single most useful gauge on the boat, even more so than fuel (since we rarely use diesel, but always are using battery).  In practice, knowing that we have 300 amp-hours, it is very easy to check the accumulated amps.  When approaching 150 amps consumed, it is time to charge the house bank!

I find it a bit difficult to view the LCD display.  Like most current electronics, the display is very small and is backlit with blue LED.  This is a minor concern, since the accumulated amps is so important to monitoring  your battery capacity.

Don McLennan
Intuition - 3500 - #115

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