Load testing with the Watt's Up and Doc Wattson Power Meters

We live in a world of batteries, especially living in an RV while boondocking. We have batteries in flash lights, radios, cell phones, GPS, laptops, HTs, and many more. New things like Blue Tooth add to the pile by changing a world of wires to a world of batteries. One very important battery in our RVs is the house battery. We care for the house battery to insure maximum life and capacity

We charge the house batteries by hookups to shore power, running the vehicle engine, generators, or solar panels. When boondocking where we have no connection to shore power battery capacity becomes an important issue. We may not want to run a noisy generator that annoys the neighbors. Overcast days limit solar output. Starting the big engine in the tow vehicle or motorhome just to charge a battery uses a lot of expensive fuel.

Battery capacity is the issue. Is there enough capacity to run what we want for as long as we want? It could be a single Group 24 marine battery or a pair of T-105 6-Volt batteries behind the propane tank. It could be a bank of gell cells under the couch or a pair of L-16s in the basement. No matter, they all have a limited capacity, some more than others but still limited.

One way to figure out if we have the desired capacity is to find out the individual current draw from each item we power in the RV, multiply that times the expected length of time it will be in use them sum them all up. There are sites on the web that give good approximations of the various loads but that is all they are, good approximations. It is much better if we had actual measurements. Power consumption is good information to have but consumption over time is even better.

There are control panels available that will tell you the real time current usage, battery capacity remaining, Amp Hours you put in or are using, etc. But the drawbacks I found with those are two fold. First is that they are expensive. The second is that they measure total current so individual loads are difficult do dig out. You could turn off everything then bring them up one at a time but some loads are intermittent. It can be difficult to measure total current consumption with intermittent loads while keeping other needed systems powered up.

Now I know we as RV'ers and Amateur radio operators love to find and play with new tools. Thinking about this problem I came across a tool, the Watt's-Up DC Power Meter. A higher capacity version is the Doc Wattson with the same form factor. They are available at Ham Radio and model airplane shops, and on the net from places like Powerwerx.com and rc-electronics-usa.com. Cost is under $60. There are other devices with similar functionality but they are more expensive.

RVs Also are self contained in power with 120 Volt AC agenerators and and 12 Volt DC subsystems. The generator charges batteries but many camp grounds have quiet times and other RVs use solar power. To use an AC device such as a computer battery charger requires a device to convert the DC to AC. Tis device is called an inverter. See my article on inverters for more information

Watts Up above, Doc Wattson below with Power Pole connectors.

Both measure and record peak Amps, peak Watts, minimum Voltage, real-time Voltage and Current and they can be ordered with Power Pole connectors. I use it to keep track of the current draw of various items such as inverters and radios. Are you fused correctly or do you have the correct wire gauge? Information from the Watt's up will let you make the correct decisions on those and many other things. The Doc Wattson is better at higher current long term loads such as we have in RVs. My only complaint is that there is no data link to preserve the data in an external computer. Once power is removed, all data is lost.

Doc Wattson powered up with external 9 Volt battery.

Both can be powered by the input voltage under test or you can attach an external battery with a range of 5 to 12 Volts. A 9 Volt battery works well. You will need a connector to hook the battery to the unit. Here it betrays its model airplane roots as the connector is the same as used for Futaba and other servos. The battery connector is available for under $3 from just about any hobby shop and where you buy the Watt's-Up. Pick up a snap connector for a 9 Volt battery at Radio Shack for under $1 for a package of several and you are in business.

I have made several adopters that allow me to connect to any DC load. A lighter plug and socket with Power Pole ends and a set with clip leads are a couple of examples. Using the lighter plug and socket I have tested several small Modified Sine Wave inverters for idle current. The clip leads worked for testing larger inverters and for testing total current draw on the main house battery. I am sure once you start playing with one you will find many more necessary adopters. The Watt's-Up and Doc Wattson are very handy little instruments and a must for any Ham Radio RVer.

Lighter socket adopters.

Alligator clip adapters.

If that were the end of its capabilities it would still be worth its cost. I wanted to know the current draw over time of the refrigerator. Does it vary as the refrigerator cycles? Is it the same under Propane or shore power? I looked at the capability of the Watt's Up and looked at the rc-electronics-usa.com web site for ideas and came up with the following.

All your DC loads should come to the load center or fuse block where each load should have a separate fuse. The current through the fuse is the information we want. Looking around I found a device at Autozone called Add-A-Circuit. It is a device that replaces a fuse and has a pig tail off one side for adding circuits. Looking at the fuse side of the Add-A-Circuit there are lower and upper fuse sockets with the pigtail off the upper. In operation you remove a fuse from the fuse block then place it in the lower fuse socket on the Add-A-Circuit. You connect a new circuit to the pigtail and insert the proper sized fuse in the upper fuse socket. The Add-A-Circuit then replaces the fuse removed from the block. There are Add-A-Circuits for the standard blade fuse and the mini fuses.

What I did was connect a black Power Pole connector to the pigtail. Then a blade connector was put on one end of another short wire and another black Power Pole connector on the other. The blade connector should be small enough to slip into one of the fuse sockets. You may have to file it a bit to make it fit. The blade fits into the lower fuse socket end nearest the pigtail.

Add-A-Circuit with Power Pole connectors.

Add-A-Circuit as modified connected to the Watts Up.

To use this device, pull out the fuse for the circuit in question. Insert the fuse in the upper socket of the Add-A-Circuit then put the Add-A-Circuit into the fuse back where you removed the fuse. Connect the Power Pole connectors to the black connectors of the Watt's-Up. Now all current going through that fuse will be registered on the Watt's-Up. You can leave it place for measurements over time.

The Watt's-Up is a very handy little device and the longer you have one the more uses you will find for it.