BEWARE – This is an ongoing piece of work.
One of the many problems with the battery system on a narrowboat is recharging the battery bank. Any time the engine is running the bank will be charging. But, there are rules!
rule 1
The most important rule is that you should never leave a lead-acid battery not fully charged. After a short while chemical changes take place that permanently reduce the battery capacity. If you are going to charge it, you should fully charge it. So, for instance, when over-wintering in a Marina it is standard practice to leave the charger on, keeping a trickle-charge in the batteries, so they all remain fully charged. Without access to a reliable mains supply, solar cells are a must.
rule 2
There is a rule of thumb for an ordinary lead-acid battery, that the maximum charge current in Amps is about 20% of the rated Amp-hours of the battery. Anything higher reduces battery life.
So in my case, with four 115Ah batteries, 460Ah in the whole Domestic bank, I should be charging at 92Amps maximum. ( 0.2 x 460) Which is close enough to the 110A of the Domestic Alternator.
rule 3
Never stop the engine until the battery is fully charged. See Rule 1. The problem is: it’s difficult to know when that is.
A parallel universe
Chestnut has two alternators. A 90A unit that charges the Starter Battery, and a 110A unit that charges the Domestic bank of batteries.
I will ignore the Starter Battery in this article. It is only used when starting the engine and will generally be recharged again within 10 to 15 minutes. Unless you have an old engine that takes a LOT of starting…. But this does mean that within 15 minutes of the engine starting the Engine Alternator is doing nothing more than feeding a small trickle charge to the Starter battery.
The Domestic battery is the issue. Assuming I have discharged it to 50% overnight, so that’s 230Ah or 2.7KWHrs, I need to put that energy back. A 110A alternator will, theoretically, take 2 hours to do that, in practice, about 5 hours, as lead acid batteries have to be charged carefully. You can bulk charge them with a maximum current to start with (see Rule 2), but once they get to around 85% you need to finish the charge with a slowly reducing current to avoid damage. And it’s this last bit that takes the time.
There are devices available that will parallel up BOTH the alternators to recharge the Domestic bank at 200A, but I would need to double the size of the Bank for this to be a safe charging current.
People who do over-charge their bank get the advantage that the Domestic bank recharges very quickly. So if, like the rest of us, they have no idea when the bank is fully charged, charging it quickly reduces the chances of leaving the bank only partially charged.
They are effectively making a trade-off of reduced battery life by over-charging (rule 2), versus reduced life by not fully charging (rule 3).
So a goal of this new monitor is to let me know, with a simple visual indication, when the bank is fully charged so I can turn the engine off.
The new monitor
There are various battery monitors on the market that make all sorts of claims, but the only ones that can work reliably are expensive. This is because the only way to know, reliably, when the bank is fully charged is to measure both the voltage and current. And that requires re-jigging the boat’s main cabling to install current shunts. Also most monitors are either full of technical data or over-simplistic. So I decided to design and build one myself. Anyone who can handle a soldering iron should be able to follow suit.
Design goals
I’m going to put three shunts on the boat.
- One monitoring the current in and out of the Domestic battery
- One monitoring the current out of the Domestic alternator
- One monitoring the current from the Solar cells (when I get around to fitting them)
I’m going to put a control unit, based on an arduino device, in the electrical cupboard.
I’m going to put a simple “traffic light” display on the control panel, to give me an “at a glance” view of the state of the bank.