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Why a Battery Monitor?
A Voltmeter provides a measurement of only the battery’s voltage. A Battery Monitor with a shunt gives you a clear indication of not only voltage (V), but also how much current is moving into and out of the battery (Amps), and how much stored energy capacity remains in your battery (Amp Hours and percentage.) No more guessing how much current is being consumed by appliances, or how much current is being recharged to the batteries, or wondering about the batteries’ State of Charge (SoC). A Battery Monitor makes all that info available at the touch of a button. |
Most Battery Boxes with power outlets include a Voltmeter. So why add a Battery Monitor?A Lithium battery changes everything
Over the years we have used an AGM battery in a Battery Box as a portable power supply when not using our Campervan for weekend trips.
The Voltmeter on the battery box has helped to provide an estimate of the State of Charge (SoC) of the AGM battery – the charge of an AGM battery declines at a linear rate that is reflected in the decline in voltage.
We recently replaced the AGM battery with a Lithium battery. This makes the Battery Box much more ‘portable’, reducing the weight from over 30kg to under 15kg – much easier (and safer) to lift into and out of vehicles. The Lithium battery has also considerably extended the run time of our portable fridge. (About a week in mild weather, if the battery is only powering the fridge. Longer if being recharged with a solar panel.)
However, given the relatively constant voltage of Lithium batteries as they discharge, the Voltmeter is not able to be used to provide an estimate of the SoC of the battery. However, a Battery Monitor with a shunt can measure the incoming and outgoing current from a Lithium battery, and use this information to provide a reasonably accurate estimate of the battery’s SoC, along with other useful electrical information.
So, after our experience of installing and using a Battery Monitor and shunt in our Campervan we have fitted a similar device to our Battery Box.
Please note: The information below isn’t intended as a comprehensive step-by-step installation guide, but more as an overview that may serve as an idea starter for those with similar needs. Only consider this DIY project if you have suitable tools, skills and knowledge. Incorrectly fitted electrical components can cause overheating which may result in a fire.
Here is what you will need…
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- Lithium Battery (eg – Kings 120Ah Lithium Battery)
- Powered Battery Box (eg – Kings Battery Box)
- Battery Monitor and shunt (eg – Kings Battery Monitor & Shunt)
- Heavy gauge Black and Red cables with terminal rings (6 or 8 AWG) (See notes below)
- Light gauge Red cable with inline fuse holder and 1 amp fuse. (See notes below)
- 6mm marine plywood
- Screws, cable ties, etc.
Plus these tools….
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- Electric drill
- Jig saw
- Drill bits
- 54 mm hole saw
- Clamps
- Spanners, screwdrivers, pliers, wire cutters, files, sand paper, paint, etc
- Plywood/timber off-cut (see notes below)
How does it all work
All the equipment used in this project is available from Adventure Kings. Watch these Kings videos for a comprehensive overview of the battery monitor and installation procedures.
It is also highly recommended to fully read and understand the manual supplied with the Battery Monitor.
What is a shunt?
A shunt is an accurate, very low resistance resistor – in the order of 0.002 ohm – which is placed in-line in the negative cable from the battery, so that all the current going out of the battery (discharging) or into the battery (charging) must pass through it. The tiny voltage drop through the shunt due to its resistance is measurable, but so small that it does not have any impact on connected appliances.. Using Ohm’s Law (V = IR) a calculation can be applied, based on the measured voltage drop (V) and the shunt resistance (R) to display the amount of current (I) going through the shunt. Make sure your shunt is rated to pass enough current (amps) to support all connected appliances, particularly if you are using an inverter. For a single 120Ah battery the Kings 500 Amp shunt is more than enough. (A 120Ah Lithium battery can only supply around 100 Amps of continuous current.) |
A shunt is the brains of a battery monitor.Putting it all together….
There are three steps in completing this DIY project…
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- fitting the shunt in the lid
- fitting the monitor display in the front wall of the box
- joining it all together
It should only take a couple of hours to put together, plus time to collect all the parts, and time for the paint to dry on the spacer ring.
Here is a quick before and after circuit overview (click for a larger view)….
Step 1 – Fitting the shunt in the lid
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- Open up your battery box.
- Remove the battery and separate the lid from the main box.
- Remove the inner cover from the lid of the box to expose the internal cabling.
- Fit the shunt under the lid using button head stitching screws with a wide head. You will need to use a drill to increase the size of the existing mounting holes in the shunt body so as not to crack/split/damage the perspex block with larger screws.
- Connect the the data cable and the temperature monitor cable to the shunt as described in the video above. The temperature monitor cable can be connected to the negative terminal on the lid. (Not sure if the temperature sensor is of any use in a battery box environment – it is probably optional.)
- Connect a light duty red power cable with an inline fuse to the appropriate terminal on the shunt. (See video above). We couldn’t find a ready-made fused cable to purchase, so we purchased a cable adaptor set from Jaycar, which included a fused cable with a ring termination, and cut off the red fused cable to use on the shunt. (The power cable connector terminal on the shunt is a bit fiddly/tricky – it is definitely easier to connect this cable to the shunt before fixing the shunt to the lid.)
- Connect the ring terminal end of the fused cable to the switched side of the battery box’s on/off switch, so that power will be disconnected from the shunt when the battery box power switch is turned off. (The manual indicates that user settings are retained in the monitor’s memory.)
- Now the main bit – connecting the heavy duty negative power cables to the main shunt terminals. The existing cables in the Kings Battery Box are 8AWG. When fitting new cables use at least 8AWG gauge cables. Important – watch the video above for details of connecting the negative ‘Product’ and ‘Battery’ cables to the P- and B- terminals on the shunt respectively …..
- Connect the ‘battery end’ of the existing negative battery cable to the P- terminal on the shunt.
- Connect one end of your new heavy duty black cable to the B- terminal on the shunt. The other end of this cable can be fitted with the rubber terminal cover to be connected to the battery when the assembly is complete.
- Hunting through the spares box we found suitable red and black cables left over from a previous inverter project. These cables are 6AWG, a little heavier than the cables already installed in the Battery Box, but also a little longer. We connected the black cable to the B- terminal of the shunt (as discussed above) and took the opportunity to replace the existing short red positive battery cable in the battery box with the longer red cable from the spares box, so that the lid can be more easily removed from the box when required. (Cables like these cables at Amazon may be suitable for this project. Check eBay, Supercheap, Repco, etc too)
Step 2 – Fitting the Monitor Display to the Box
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- You will need a 54mm hole saw to drill a hole to mount the monitor display. We have mounted the monitor on the front of the box, but it may also be possible to fit it to the top or the end of the box. Check the mounting space under the lid. (It would be more difficult to drill a hole in the lid considering all the wires fitted there, See a couple of alternate options at the bottom of this page.)
- Use the hole saw to drill a hole in the front of the box. We centred the hole on the ‘O’ in the word ‘BOX’ on the label. Clamp a timber or plywood offcut behind the plastic when drilling the hole, so you have something solid to drill into…
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- Now – we could fit the monitor display directly in this hole – but the rear space is very cramped, and will compromise the cables at the rear of the monitor. So, I suggest making a spacer ring to move the display out from the box….
- You will need some marine plywood…
- Make a suitable spacer ring using the 54mm hole saw and a jig saw.
- Sand smooth and paint black.
- Fit the collar to the rear of the monitor assembly. The locking collar protruding beyond the back of the monitor will now provide some protection from the battery pressing on the monitor cables. Though we need to provide a relief space in the collar for the cables to leave the monitor collar….
- After fitting the collar and checking where the opening should be located, remove the collar and file a small opening in the side of the collar…
- After fitting the collar and checking where the opening should be located, remove the collar and file a small opening in the side of the collar…
- Refit the collar, then click the data and temperature cable terminators into place.
- To provide some additional protection, slip the foam ring from the original Kings packaging over the monitor housing when fitting the battery…
- Check the alignment of the monitor on the front of the box.
- Now – we could fit the monitor display directly in this hole – but the rear space is very cramped, and will compromise the cables at the rear of the monitor. So, I suggest making a spacer ring to move the display out from the box….
C. The Hook Up
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- With the battery fitted in the box, we can now connect the main battery cables to the battery terminals. As noted above, we also fitted the foam monitor packaging and a thin piece of packaging foam over the top of that, along with some thicker foam at the end of the box, since we didn’t have any divider panels to locate the battery lengthwise in the box..
- The longer battery cables make it much easier to assemble the lid on the box with the cables connected. Just check that the cables curl up neatly on top of the battery when fitting the lid.
- Tidy up and cable-tie (or twist-tie) the data cable and the (optional) temperature cable connected to the monitor – tuck the tied bundles into spare battery box space.
- Double check that all connections are firm, and nuts are tightened. Also check that any crimping of cable ring terminals is robust. Spring washers should be fitted to prevent nuts rattling loose in transit.
- Strap the lid down tight, then fully charge the battery.
- Once charged, set the Ah and % battery capacity on the monitor, as detailed in the Kings video above.
- It is now easy to see the SoC of your Lithium battery in the battery box (remaining Ah and %) as well as how much current is flowing into and out of your battery at any given time. The Kings videos above, and the monitor manual explain how to use the arrow buttons to cycle through the display options.
- With the battery fitted in the box, we can now connect the main battery cables to the battery terminals. As noted above, we also fitted the foam monitor packaging and a thin piece of packaging foam over the top of that, along with some thicker foam at the end of the box, since we didn’t have any divider panels to locate the battery lengthwise in the box..
But wait – there’s more…… Alternate monitor display locations?
Here are a couple of ideas, if locating the monitor display on the front of the box isn’t suitable for your situation…
