Batteries and Battery Monitors Part 1 – Why we needed one

In this multi part post we are going to look at why you need a battery monitor in the first place, the alternative types of batteries available and then how they are rated for capacity.  It is then useful to know how your battery charger works before rounding up by looking at what a battery monitor can calculate for you.

Our Battery Monitor
Our Battery Monitor

Having been electric car users for a number of years we understand how careful use of our batteries can extend our range. It still came as a surprise to us when we hired a boat and the lights would go out quite early if we stayed for a couple of days without running the engine. Hire boats are designed to be used, they have just enough battery capacity to do the job, they normally don’t include solar panels, or even a larger inverter.

Part of our upgrades and modification of our ex-hire boat,, was to add an additional leisure battery and upgrade the inverter to an 3500W inverter charger with a shoreline charger.

The batteries are one of the most expensive consumables on your canal boat, good management of them can increase their life by many years, and the best way of keeping them in good condition is to know how much power you have available.

Why voltage is not enough

When I started looking at battery monitors, it was suggested “Why bother all you need is a voltmeter to see your state of charge” and while mostly true, you can tell a lot from the battery voltage. The voltage will drop if you are using the battery, and most of us cannot be bothered to isolate the batteries before taking a reading.

A good battery monitor not only displays the current battery voltage, but it also displays the current in amps and the power in watts being used. This can be extremely useful when trying to find out how much power you are using and on what device. On the monitor we installed we also get a time remaining display which shows us in hour and minutes how long the battery will last at the current usage.

When battery size is not the FULL answer?

The biggest confusion with battery monitors and what causes the most misunderstanding is the State of Charge/Voltage displays. In use, a battery’s voltage will drop depending on the load it’s providing, and batteries do not really have a fixed capacity; it all depends on how you use them. Even when new, it is unlikely your 400Ah battery bank will give you a true 400Ah, and then will only degrade as they get older.

Your battery’s capacity will vary widely depending on how fast you discharge it. The slower you discharge the battery the higher its potential capacity and the faster you discharge it the lower its capacity.

We tend to leave the battery monitor on its “Time Remaining” display as this very quickly shows me if what I am doing will cause me a problem, and if I will need to run the engine before we move on.

Inverters eat your power

Inverters use quite a lot of energy converting your batteries 12V or 24V up to the 240v, most seem to be between 80-90% efficient, so that would mean that for 200w you are really consuming up to 240W. Inverters become more efficient when they reach over approximately 30% of their capacity, so best not to use them to just power your mains phone charger.

Monitoring what’s using your battery

The battery monitor we fitted has a Bluetooth connection to a smartphone (currently both Apple and Android apps are available). Using this, I captured some screenshots showing the battery usage (Click on the image to enlarge).

Fridge and Laptop usage Inverter Switched On Vacuum Cleaner
Just the Fridge and Laptop Inverter switched on Vacuuming

When using our mains vacuum cleaner via our inverter from the battery bank, the monitor dropped to just under 2 hours left at that usage, but this soon returned to days after I had stopped cleaning. Even I would find it hard to hoover for 2 hours on our narrow boat.

Different battery technologies have different profiles when it comes to their capacities. Lead Acid batteries should only be discharged to 50%, else permanent damage can occur, so even when you have a battery that states 100Ah capacity, you may only have a usable 50Ah capacity, and this will drop as the battery ages.

Your Battery C-Rating

Batteries are often marked with a C rating which gives the number of hours that the battery is designed to deliver its fully capacity over. Our Leoch Adventurer SFL-110 batteries have two C rating marked for their capacity 100Ah C100 and 85Ah C20.

Battery C Rating
Battery C Rating

This means that the battery will provide 100Ah if discharged over 100 hours or just 85Ah if discharged over 20 hours.


C20 85Ah The battery can discharge at a rate of 4.25 amp for 20 hours 4.25 X 20 = 85Ah
C100 100Ah The battery can discharge at a rate of 1 amp for 100 hours 1 X 100 = 100Ah

So far, we have been talking about Lead Acid Batteries, which are still the most common batteries found onboard and the most limited in terms of usable capacity.  In the next part of this series, we will look at the available battery options and their costs.

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