Battery & Amp Hour Meter FAQ

We hope the following questions and answers will help you use our products and get the most out of your "Watt's Up" DC Amp & Amp Hour power meters.

See the RC market focused FAQ here: RC / Radio Controlled Cars & Planes FAQ

You may also be interested in our Battery Electronics 101 content that focuses more on general electric and electronic notions regarding electrical measurements.


Why do I need one of your meters?

Here are some applications where you will benefit from having one of our meters.

DC Amp & Watt Hour Meter Applications

e.g. "Watt's Up" replaces a DC ammeter, voltmeter, wattmeter, Amp hour and Watt hour meter and does more while costing less. It is a precision instrument and the tool to show you what's really going on with DC power systems and it's easy to use.

For a fraction of what you've already invested in your equipment, our affordable meters give you answers to electrical questions that protect your investment and help you get the most out of it.

If you're having a problem, the measurement abilities our meters provide help you diagnose the problem. One can pay for itself in a single incident saving the cost of a battery, charger, inverter or related component. The possiblilities are endless.

Will a "backwards" or reverse connection damage the "Watt's Up"? E.g. connecting a charger to the LOAD side and a battery to the SOURCE? What will the display read?

No. Connecting in this manner causes current to flow in the reverse direction, but will cause no damage up to the rated current of the meter. However, while connected this way, the meter will not indicate the actual current nor any value derived from the current i.e. Watts, Amp hours or Watt hours. E.g. the Amps and Watts will read 0.

Any accumulated Ah or Wh displayed will not increase further until power again flows from SOURCE to LOAD, i.e. in the forward direction (meaning current is flowing in the black wires from LOAD to SOURCE).

About measuring current flow in both directions (e.g. bi-directional measurement)?

The "Watt's Up" and Doc Wattson meters only measure current flowing in the black (negative) wire from their LOAD side to their SOURCE side as labeled on the meter's case. Current flowing in the opposite direction is not measured and does not affect accumulated or peak readings. Current flowing in the red wires is not measured at all.

To do a net current flow "bi-directional" current measurements (e.g. measure both directions of current flow) at one point in a circuit, we recommend using two meters connected together and oriented LOAD to LOAD (or SOURCE to SOURCE) as that is a more flexible and easily interpretted arrangement, than a bidirectional meter usually offers.

Alternatively, the meters can be mounted in separate parts of the system to measure two currents simulaneously. For example, the charge current into a battery and the discharge current out. Those currents may differ and this arrangement will show that. In this case meters are connected as usual ensuring that the current to be monitored is flowing in the correct direction in the black leads.

These dual meter arrangements allow independent measurement of the two current flows and their peak values. It also allows their respective accumulated values (e.g. Ah, Wh) to be reset separately. A bonus is that the voltages of the two separate systems can be independently monitored as well as the currents.

Note that with the dual systems, the red wires can also be connected using the "3-wire" arrangement. The red wires only need to carry enough current to power the meters and measure the voltage at the desired points.

How can I reset the meter's accumulated readings?

Meters in current production (e.g. versions 2.1 or higher) have a reset signal on their Auxiliary connector's pin 3 (The orange lead on our CONN100 cable). Briefly connect pin 3 to 0 volts (minus lead) to reset the meters accumulated measurements to zero. This is also described in the meter's user manual.

What happens if I exceed the "Watt's Up" maximum current specification?

Briefly exceeding the maximum specification by a small amount isn't a problem because the meter connections are, essentially, straight through wires that are "tapped" to make measurements. One lead has the current shunt in it and so all current must flow through it. The other wire is unbroken all the way through. There are no fuses.

Currents greater than what the meter can measure just show the highest value that was measured. This value will differ on individual meters.

Exceeding the maximum specification for an extended duration, heats the wires and the current shunt, eventually to failure. To avoid this, minimize duration of any overloads and keep the wires cool. The latter can be done by actively cooling the wires (moving air, heatsinking, etc.) or greatly reducing their gage number so they heat less for a given current. e.g. a 10 gage wire will heat 6.8 times less than 14 gage as its resistance is 0.0010 Ohms /ft. versus 0.0026 Ohms /ft. for the 14 gage and heating is proportional to the square of resistance!

What happens when the value I am measuring exceeds the "Watt's Up" maximum value? Will the meter be damaged?

Exceeding the voltage or current maximum values can permanently damage the meter.

Exceeding the maximum voltage, even briefly, will probably cause permanent damage.

Infrequent and brief (a second or less) current spikes up to 150% of the maximum value should not cause permanent damage. The maximum current (Amps) an individual meter will display varies from meter to meter. Some may show up to 110% of maximum (e.g. 110 A on a 100 A maximum specification meter) while others may have a maximum of 101 %.

You can exceed the Watts, Ah, and Wh maximums without any damage as these are just calculated values. All that happens is that the values "wrap around" above 6553.6. This means they reset to 0 and begin counting again from 0. So if your system has really accumulated, say, 7500 Wh (7.50 KWh), the display will actually show only (7500.0 - 6553.6=) 946.4 Wh (0.95 KWh).

How do "inductive loads" like motors cause circuit damage?

Loose connections and long wires on circuits switching high currents can damage electrical components. The problems can be largely eliminated by ensuring that connections are secure/not intermittent (e.g. not just twisted together) and to a lesser degree by keeping wire lengths short and by twisting wire pairs together to reduce their inductance. Always disconnect first/connect last the meter from/to the load (or highest inductance connections) first. Another way to think about this is to keep a battery or power source connected to the meter when ever potentially inductive loads are connected or disconnected. The battery or power source helps to clamp any voltage spikes created.

Here's a little more about why this all happens.

Wires exhibit a property called inductance. Inductance is an indication of the amount of energy stored in a wire's magnetic field due to the flow of current. If you interrupt the current flow, say with a switch or pulling apart a connector, the magnetic field collapses and induces a voltage in the wire to try and oppose the drop in current. Depending on the specifics of the wire, current and materials near the wire (like iron) the induced voltage can be quite high. In fact, it can give you a shock and damage electrical components still connected to the wire by generating voltages that exceed their ratings. This is one of the reasons capacitors and diodes are needed on brushed motors in addition to reducing RF noise.

Cars use this ability to generate voltage spikes in their ignition coils to generate the thousands of volts their spark plugs need from the car's 12 volt battery.

When using auxiliary power, why do I see small measurement values when the SOURCE and LOAD leads have nothing connected or are shorted? Will this affect performance?

It is neither unusual nor a problem for there to be small values displayed in this situation. Our meters have been optimized to provide highest performance for real measurements of connected power systems and loads, but with all leads unconnected may produce erroneous readings.

With real connections, accurate readings will be produced.

Note that shorting the SOURCE leads together and/or the LOAD leads together (DO NOT SHORT WITH A POWER SOURCE CONNECTED TO EITHER SET OF LEADS!) may show a very small voltage (e.g. 0.01 Volts). These small "shorted lead" voltages are well within specification and don't affect the accuracy of real measurements.

Wire gage seems too small. Is that a problem?

Probably not. Because the wire and insulation we use can handle the heat and the voltage drop across the wire's length is small because it's short. Read more about these issues below.

The correct wire gage in an application can depend on many factors. Typical tables (e.g. NEMA) for wire gage and current assume a particular kind of insulation and that the wires are confined in conduits, walls, etc. Our meters are seldom confined that way and, so, different wiring standards apply.

The key issue is that wires heat up from their resistance to current flow. Lower gage wires have less resistance and therefore, generate less heat at a given current. Resistance doubles every 3 gage increments. e.g. 13 ga. has twice the resistance of 10 ga.

When heat can't escape fast enough the temperature rises until it causes a problem like the:

  • insulation melting or failing,
  • wire heat causing something else to fail or burn,
  • wire conductor melting like a fuse.

Our "Watt's Up" WU100 and Doc Wattson R102 meters use AWG 14 or heavier gage wire and high temperature silicone insulation.

The wires used are chosen for their:

  • handling flexibility,
  • resistance to breakage from bending,
  • ability to handle high temperatures.

With normal ventilation, this wire can easily handle the heat generated from continuous operation at the meter's maximum continuous current rating as well as the peaks specified.

With the recommened 3-wire meter connection meter heating is reduced because only the black wires carry current since the single red wire only carries the meter's operating current.

You can, and people often do, splice in lower gage wire to further reduce heat generation.

I'm a technical person and want to modify my meter. Can you help?

Take a look at our special meter modifications page. It discusses some topics that may be of interest.

Can I use an external xxx Amp current shunt resistor?

Take a look at our special meter modifications page. It discusses a modification for use with an external shunt.

How do I determine my meter's version number?

The power up screen shows the Watt meter's version number. E.g., for versions 2.1, "Watt's Up" shows: RCE v2.1 and "Doc Wattson" will show: RCE HE2.1.

I have a question not answered here. What should I do?

If you weren't able to find what you need in our support specifications or on-line manuals, please send us an email for more information. Visit our contacts page for contact info.

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Learn more in the FREE — "Watt's Up" user's manual.

It contains tutorials and other useful information on electrical parameters, measurements and system testing.

Meter FAQ