Batteries and Battery Monitors Part 4 – Battery Monitor & The Shunt

Our Battery Monitor

I have installed a Victron Energy BMV-712 Smart battery monitor, but there are many other options from other manufacturers, see the listing at the end of this article.

I selected this one as it gave me the opportunity to connect to my onboard Raspberry Pi which I am using to give some smart boat features and provide live location tracking for my website in the future.

Our Battery Monitor Display
Our Battery Monitor Display

The battery monitor comes in two parts, a display which provides a readout of the battery status and a means of programming/setting up the system for your configuration.

The Shunt

Victron Shunt
Victron Shunt (c)Victron

The second part is called a shunt, this needs to be placed, in our case, on the negative side of the battery bank, so that everything must pass through it to reach the battery, there should be no other connections to the negative side of the batteries else your measurements will never be accurate.

A shunt is a resistor of very low but known value that is placed in parallel with a voltmeter so that all the current being measured flows through it. The voltage drop across the shunt’s resistor is measured; this voltage drop across the shunt is proportional to the current flowing and can then be calculated using Ohms law (Current = Volts / Resistance).

Shunts are rated for the maximum current they can measure, in our case 500A which at 12V is 6000W, more than enough for our boat.

Basic Battery Monitor Wiring
Basic Battery Monitor Wiring

The battery monitor also has an extra cable that connects to the positive side of the battery bank to power the electronics and the display, but also to measure the current battery voltage, we have an extra wire connected to the starter battery, so we can monitor its voltage as well.

Peukert’s Law

To then calculate the remaining capacity of the battery, the monitor uses an adaptation of Peukert’s Law** which can be used to calculate the capacity of lead acid batteries at different rates of discharge.  As we discussed earlier, the discharge rates affect the battery capacity.

** Developed by Wilhelm Peukert (1855-1932) Peukert’s law is used to calculate the batteries deliverable capacity at the current given rate of discharge, His law describes the batteries capacity at a constant discharge until it reaches its cut off voltage, below which you can damage your battery, this constant is called ‘K’, for example K=1.25 is used for our flooded lead acid batteries. There are however some limitations to this law as it does not consider the batteries temperature or age. I expect each monitor manufacturer modifies this to consider these extra factors when displaying the results, our system records each battery charge/discharge cycle.

The capacity of a battery falls at higher rates of discharge because the chemical reaction within the battery reaches its maximum speed for the given plate size and therefore the voltage drops. If left to recover, that missing capacity will return.

Using these calculations, a battery monitor can calculate the available power remaining while the battery is in use (under load) and as that load changes or even as the battery is charging, it can display the current State of Charge (SOC).

Now you know what is happening?

Armed with this information, you can then decide how you want to operate your boat and if you will need to start the engine to charge the batteries. One of our future projects is to work out the size we need for some solar panels, and we will use the data from the battery monitor to help calculate the size system we need, but that is a topic for another day

Battery Monitors Suppliers

Victron –
NASA Marine Instruments –
Advanced Yacht Systems –
Simarine –
Votronic –

Boat work and around and about in Droitwich

We have come up to the boat for a few days, I have some boat work to complete, we are fitting a battery monitor, but more on that in a later post as I also wanted to touch up the paint from the few scrapes we have had over the summer.

Battery Monitor Display
Battery Monitor Display

While I have worked on the boat Sonia and Mary have been on a trip by train to Birmingham New Street Station and the Bull Ring Shopping centre, for both of them on the train was about £12.50 off peak return, much simpler than driving and trying to find parking.

Droitwich Lido
Droitwich Lido

They also tried the Droitwich Lido an open air pool, which is a rarity these days, when I was young we had one in Camberley, but it was very run down and soon closed, but it seems the Droitwich Lido is a bit of a find, very clean and tidy, I am sure we will be back and bringing the grand kids.

My thanks also goes to the very helpful Facebook group “Droitwich Spa Marina Friendly Forum“. A quick post as I needed a hole cutter, and within the hour I had two offers of one on loan. Thanks to the owners of NB Cedrik, it was a tool my wife was very pleased I did not have to buy.

Returning back to Black Prince

If you have been reading our blog you will know we had a little problem with the paint on the roof, we purchased the boat from Black Prince only a few weeks ago, they have looked at the peeling paint and are going to repair it.

Paint Problems on roof
Paint Problems on roof

It is an odd problem, looks to me, a non-expert, like some surface contamination under the primer as caused it to lose adhesion, I have seen similar problems this caused by the oil used to lubricate the air tools used when removing the old paint.


Worcester to Stoke Prior

We have been on a trip to Worcester and are going to drop the boat off at their base in Stoke Prior before we set off home.

We broke the trip up this time, turning around at Diglis Basin just in front of the Diglis Lock, we had decided to try and get past all the locks and then rest up  for the night just after the M5 bridge at Tibberton, we would then have a simple trip to our mooring at Droitwich Spa Marina the next day, before heading off on the Saturday to the Black Prince Base at Stoke Prior.

Sonia and Obi the dog collected the car from the marina and drove to Stoke Prior while Mary and I moved the boat. Once Sonia had arrived at Stoke Prior, she let them know we were coming and check where they would like us to moor before walking down the canal to meet us.

The trip took us just under 3 hours at a leisurely pace,  with just the 3 locks at the top of the Droitwich Canal and then 7 on the Worcester and Birmingham Canal. We finally arrived and moored up next to another Black Prince customer boat, “Pub Trawler II” which was known as Ava when she was a hire boat.

We have since heard that they will completely strip the roof and repaint it, should be ready in a couple of weeks. Furthermore, we are hoping to get a battery monitor fitted while she is with them. I have selected the Victron BMV-712 as I can talk to this via the onboard monitoring system I am developing (More on this later).