DISCHARGING A DEEP CYCLE BATTERY

Harold Brochmann

In the last article I described the installation of a digital voltmeter that allowed me to determine the exact voltage of each of GYPSY’s two batteries both under load and no-load conditions. The reason for doing this was that an accurate voltage measurement can tell you the state of charge and general condition of the battery. The analogue voltmeter already installed on the electrical panel did not have sufficient resolution for usable readings.

In the next article I intend to talk about some experiments I’m planning to discover the optimum procedure for charging the “house” battery.

In this article I describe my investigations into battery discharging. The reason is to provide some background so as to better understand what happens during the charging process.

If I run my engine until the batteries are completely charged and read the voltage as soon as the engine is stopped, I get around 14 Volts. If no load is applied, then this will slowly drop to 12.6 or so over a matter of hours. The higher reading is called a surface charge. If a load such as a light bulb is applied then this drop occurs very rapidly. Very little energy is liberated during this initial phase.

Here is a table I came across in my research:

The table purports to show the remaining battery capacity versus the open circuit voltage readings. The actual readings would depend on the temperature of the battery.

We assume that the voltage reading of a fully charged battery is 12.7 V. As energy is drawn from the battery, the voltage drops. In practical terms this means that when I leave the lights on in my boat at night they will gradually get dimmer as the battery becomes more and more depleted. At some point I have to turn the lights off so as not to damage the battery by over-depletion.

If no current is drawn when I read the voltage reading of my battery, I get the open circuit reading. The closed circuit reading varies with how much current is drawn.

I decided to experimentally determine the actual practical capacity of my deep cycle battery.

At Radio-Shack I got 3 - 10 Ohm, 25 Watt resistors and soldered them in parallel. Using a friend’s highly accurate meter, I determined that their combined resistance was just about the 3.3 Ohms it ought to be. These were then connected in series with a switch and an ammeter across the terminals of the freshly charged battery.

The voltage of the battery was read at irregular intervals throughout the day with the switch closed and again after leaving the switch open for around a minute.

Then I fully charged the battery again and repeated the experiment with a resistor which I determined was pretty close to 2 Ohm.

The results are plotted in this graph:

The vertical axis shows voltage, the horizontal axis, elapsed time in hours.

The two upper traces show the open and closed circuit voltage readings from the first experiment, with the 3.3 Ohm resistor. The current here is in the range 3.5 – 3.8 Ampere. The two lower traces show readings using the smaller resistor yielding currents of 5.6 to 6.3 Ampere.

Note that in both experiments, applying a load causes a sudden voltage drop to around 12.2 volts, after which the voltage curves are roughly linear.

In the case of the slower discharge rate, it got to be past my bedtime before the experiment was finished, so I extrapolated the results shown by the dotted lines.

All the articles I read on battery usage recommended that lead-acid batteries not be discharged to a point where less than 20% of capacity remains. Doing so unduly decreases the useful service life of the battery. So I decided that the practical limit of my battery has been reached when the open circuit voltage dropped to 11.6. At this point the battery had yielded 37.4 and 33.6 Ah… about one third of the advertised capacity of this battery!

Incidentally, this is equivalent to approximately 1 kWh of electric energy – or about six cents worth at BC Hydro’s present rates.

The rate of energy conversion at the slower discharge rate was in the range of 45 Watts; about what it takes to illuminate GYPSY’s cabin at night. At the higher rate, around 70 Watts was produced.

So; what have I learned? Well, I have learned that the practical limit of energy that can be drawn from my deep cycle battery is about 1 kWh which is quite a bit less than what the specs suggest. I also learned that the slower rate of discharge yields more energy; and I have enough capacity to illuminate GYPSY’s cabin with two 20 W halogen bulbs for a total of 12 hours when at anchor.