Plugging in the Numbers

With our long-awaited replacement solar panels, and publicly available charging points on the side of the canal in Banbury, I have been thinking about the relative costs and efficiencies of the different means of propulsion. I’m going to share some of those thoughts below.  If that isn’t your thing, you might want to look away now.

Bartimaeus has a large bank of batteries.  We need to look after them – replacing them early due to neglect would be wasteful and costly.  If looked after, we’re told they should last ten years at least (they are nearly four years old already).  The main 48V battery bank consists of 24 lead acid cells.  The capacity of the bank is equivalent to about thirty times the capacity of our 12V 100Ah starter battery (we also have two identical “starter” batteries in the bow to power the bow thruster).

Stern deck of narrowboat with a panel lifted showing dozens of battery terminals across the width of the boat.  A small hose runs in to the compartment from a large plastic bottle.
Battery Bank and Watering System

There are a number of different ways that the batteries get charged.  Each is controlled by an electronic system which ensures the batteries don’t get over-charged.  The nominal 48V system is fully charged when pushing a current through it takes the voltage up to 57.8V.  This is where all the controllers stop charging. One important consequence of this is that as the batteries approach full, they have to be charged more slowly.

The main way we charge the batteries is with the HybridMarine system.  This comprises a 50 horsepower (38kW) diesel engine and a 10kW electric motor.  We only rarely need more power than the electric motor can deliver, getting in or out of a heavily silted lock and manoeuvring on fast flowing rivers have been the only cases so far.  Most of the time when we are cruising on electric we are using about 3kW – the equivalent of boiling a kettle.

When we are running on diesel, the HybridMarine system uses the electric motor as a dynamo.  The diesel engine turning the propellor at ordinary canal cruising speeds is very lightly loaded.  The electric motor is connected to the propellor shaft by a drive belt.  The control system starts to generate electricity when the propellor starts spinning.  The amount of power being extracted is gradually turned up until it reaches maximum or starts to noticeably affect the propellor speed.

The maximum power that can be extracted is limited by the characteristics of the batteries.  At all times the maximum charging current is 100A, of which the HybridMarine system will only produce about 77A.  This means the maximum amount of electricity being generated is 77A x 57.8V = 4.4kW. A display in front of the steering position shows the current state of play.

Helm's view of the HybridMarine control panel.  A small display screen with a row of buttons below.  Nearby is the throttle lever and the joystick for the bow thruster.
Control Panel for HybridMarine System

My normal preferred cruising speed is the slowest speed we can manage whilst also generating this much electricity, thus minimising the amount of fuel needed to charge the batteries.  This turns out to be a similar speed (2 to 3 mph depending on the canal conditions) to driving on electric at 3kW.  We are able to go twice as far using the same amount of diesel.

An alternative charging mechanism is a mains plug-in at a marina, or sometimes on the canal side.  We make use of these opportunities when we can, but they are not as frequent or reliable as we’d like.  Last night, for example, we returned to Banbury having deliberately driven on electric throughout the day.  When we got here, we found there was a boat tied up next to all the charging posts – even though only one was plugged in (we managed to find a spot the next morning).  Charging points are usually limited to 16A (about 3.8kW, or using the toaster and the kettle at the same time).  This charges the batteries at a similar rate to running the engine.

A charging point for narrowboats on a brick and concrete wharf. A cable leads to a moored narrowboat in the canal.
Public Charging Point in Banbury

The solar panels on the roof are rated at 1.3kW.  So far we have seen a maximum charging rate of about 800W in early April. (I’m amused to note this is about one horsepower.)  We can hope to get a little more than this in the summer months, but it is still only a quarter of what we use (or generate) while driving along.  So for every hour we spend driving, we need to bask in full sun for a further three to replenish the batteries.  In lock flights this can be a reasonable ratio, at other times it suggests we should do more sight-seeing.

The cheapest charging method is obviously solar.  When the boat is unoccupied it is enough to keep the batteries topped up.  Once we start using the fridge and the kettle it might keep up in the summer months.  If we start driving (electric) too, it gets a lot less likely.

There is about 10kWh of energy in a litre of diesel.  Our HybridMarine system can make use of about 3kWh of that (we use about 1.5 litres/hour).  At a current pump price of £1.50 that works out at about 50p for a usable kWh.  Plug in electricity at marinas usually costs a little less than that, but there is usually a mooring fee to pay on top.  We are paying 95p per kWh on the canal side in Banbury, but no fixed fee.  So on the face of it, diesel is the cheaper option, but driving in electric is much more pleasant when we have the option.

The charging characteristics of the batteries are also a factor.  To fully charge the batteries using diesel would involve running the engine for an extended period but using only a fraction of the available power.  Ideally we should hand over to solar at that stage instead.

So on sunny days, we should start early(-ish), drive on diesel for a bit, then moor up in a sunny spot before noon and go sight-seeing.  Apart from the “early” bit, that sounds like a good plan!