Tech: Beating the battery blues

Reading about cruisers who sail around the world in engineless sailboats leaves me in awe. For sailing mortals though, having an engine makes sense and is well worth the extra weight and cost. The only caveat is that an engine requires an electric starter motor to crank start, and the motor in turn requires a battery for power. Having a reliable battery on board is therefore as critical to the safety of your vessel as having a reliable anchor. On land, a dead car battery is merely an inconvenience, while you sip your cappuccino and wait for the automobile service to rescue you. At sea, things are more serious. You’re in trouble if your starter battery is dead and you need to urgently move your vessel - and cappuccinos are harder to come by too! (Of course sailboats have sails for propulsion too, but the wind is not always agreeable).

I experienced this during my previous sailing trip around Kangaroo Island; Arriba’s house batteries and starter battery had fully discharged overnight. This is not supposed to happen in a well-designed marine electrical system, since the house batteries and the starter battery are isolated via a paralleling switch.
Arriba's electrical distribution panel.
In Arriba’s case, the +ve terminal of the starter battery is connected via an isolating switch to the (left) starter busbar. The starter motor(s) and alternator(s) also connect to the starter busbar, enabling engine starting and charging independent of house batteries. Similarly, the +ve terminals of the house batteries are connected via another isolating switch to the (right) house busbar. The battery charger and solar panel controller also connect to the house busbar. Finally, a Voltage Sensitive Relay (VSR) connects the two busbars, subject to both isolating switches being ON.

NB 1: In general, nothing should be wired directly to the +ve terminal of any of your batteries except for a mission-critical devices that should never be isolated, such as bilge pumps.
NB 2: Don’t switch off the starter isolating switch when the alternator is on, as running an alternator open circuited will fry it.

The VSR has three modes: ON means the starter and house batteries are in parallel; OFF means they are isolated. I seldom use the ON setting as it means all batteries are discharged at once, which increases the chances of inadvertently draining your starter battery. It should only be used in an emergency. In the third AUTO mode a sensor measures the voltage of the starter battery and connects all batteries in parallel only when the former has reached a so-called “cut in” voltage, typically ~13.3V. This way the starter battery gets first dibs at the energy coming from the alternator. When in parallel the house batteries will also charge, however if there is insufficient incoming energy, the starter battery will also be discharged. This is because battery banks in parallel always equalize in voltage. Simply put, electrical current, like water, flows to the lowest potential, which in the case of batteries is the one with the lowest voltage. The VSR defends against the starter battery from losing too much charge by isolating it when it detects a so-call “cut out” voltage, typically ~12.6 V. It remains isolated until the cut-in voltage is again detected.

Actually, the picture is a bit more complicated, because Arriba was built to charter boat survey standards and has a dedicated battery for just the VHF Radio. There is a second VSR between the radio battery and the house batteries, designed to charge the former from the latter.

Fortunately Arriba also carries an extra starter battery, for my Hookah, which we used to get going.

Note: Arriba uses identical dual-purpose batteries for house and starter batteries for both interchangeability and unified charging, since mixing battery types can result in undercharging or overcharging. Dual-purpose batteries also have the advantage of tolerating deep discharges that would wreck most pure starter batteries, and typically last longer as a result.

So what went wrong? When I got back to port I checked the health of all my batteries using a battery analyzer like the own shown below.
Battery analyzer
To use the battery analyzer you need to know the Cold Cranking Amps (CCA) of the battery, not just the Ampere Hours (AH). Note that you can’t rely on just a voltmeter to assess battery health. Firstly, battery voltage does not fall in linear proportion to the amount of energy discharged. In fact, the voltage will have dropped only a fraction of a volt when a battery is 50% discharged. Even so, one can correlate voltage vs. percentage state of charge, for example, something like the following (YMMV):

12.66V = 100%
12.45V = 75%
12.24V = 50%
12.06V = 25%
11.89V = 0%

Secondly, when batteries are unhealthy they can fool you by supplying a reasonable voltage at very low current draws, only for the voltage to drop precipitously the moment a normal load is applied. The reason is complicated, so feel free to skip over the explanation.

Batteries are made up of number of electrochemical or voltaic “cells” which consist of pairs of electrodes a small distance apart, filled with an electrolyte, i.e., a liquid or gel, that conducts electricity. For example, a 12V lead-acid battery comprises 6 cells in series, each of which produces ~2.1V when fully charged, for a total of ~12.6V. Discharging involves a chemical reaction at the positive electrode, the anode, releasing electrons and another reaction at the negative electrode, the cathode, absorbing electrons. The cell cannot function however until an external electrical circuit, or electrical “load”, connected across its terminals completes an electrical path. The electrical circuit enables electrons produced at the anode to flow as electrical current to the cathode where they are consumed.

In a lead-acid battery electrode plates are made of lead (Pb) and the electrolyte is a mixture of water (H2O) and sulphuric acid (H2SO4). Anodes are covered in a paste of lead dioxide (PbO2). When a lead-acid battery is discharged, lead sulfate (PbSO4) forms on both plates, in a process known as sulfation. When the battery is recharged, the lead sulfate is transformed back to lead and sulfuric acid at the cathode or lead dioxide and sulfuric acid at the anode.

Over time residual lead sulfate accumulates on battery electrodes, which weakens the chemical reaction and reduces the battery's ability to store energy. All it takes is one cell to weaken to pull the whole battery down, since electrical current will keep flowing to the cell with the lowest voltage. Further, with a battery bank, if one battery’s voltage is unable to equalize with others in parallel, it will keep drawing electrical current until all the batteries are fully discharged, even with no external load.

In summary, a bank of house batteries will drain even if there is just one bad cell, and if the starter battery is also in parallel, it too will suffer.

So how did my batteries drain, despite the fact the paralleling VSR was in AUTO?

All of Arriba's (4) house batteries and (2) starter batteries, are all less than 2 years old and were 100% healthy, but I’d completely forgotten about the radio battery, which is located in a different closet. Mea culpa! The radio battery VSR was ever only intended to stop the radio battery from being drained by house batteries, not vice versa. The radio battery, while producing a respectable 13V (under no load) was down to 22% health. That normal voltage ensured that the radio battery VSR always cut in, but the poor health meant the house batteries were continuously being drained. This explains how the house batteries drained, how did the starter battery drain?

I think the explanation is simple enough, and it’s the reason that charter boat operators always remind you to run the engines for an hour or two every day. The day before our incident we’d been sailing the whole day. The engine was hardly used, so the alternator did not run long enough to substantially recharge the starter battery. Further, it was cloudy most of the day so the PV panels did not contribute much energy either. When we moved anchor the previous night there was just enough charge to start both engines a second time each. Unfortunately I did not run the alternator long enough to recharge, as it was the middle of the night. I believe those last 2 engine starts discharged the starter battery below 50%, which killed its ability to deliver usable cranking amps the next morning.

Note: Rated “Cold Cranking Amps” applies only to a fully charged battery, and decreases as a battery is discharged. A half discharged battery generally lacks the CCA to start an engine. In a shore test, just 3 engine starts reduced the voltage of the fully-charged starter battery from 13.14V to 12.50V.

I'll leave you with a few recommendations:
  • If you’re away from port and you start to experience power problems, set your paralleling switch (VSR) to OFF as a precaution. Your starter battery will then get all the power from the alternator.
  • To test a battery that is part of bank, disconnect one of the terminals so it is not in parallel with other batteries.
  • Don’t simply replace a bad battery with a fresh one unless you understand the root cause, otherwise you may end up trashing the replacement as well.
  • On long trips, carry a spare battery or a portable generator (genset) that can be hand started by recoil.
  • Don’t let the fact that a battery is out of sight, mean it is out of mind!
Finally, here's a short video I made which describes how the paralleling switch works.


PS You may also be interested in my blog post on emergency power at sea.