Construction.
Internally, a battery consists of a number of cells each with a nominal open circuit rating of 2.1 v fully charged, and connected in series to provide the required terminal supply voltage. Each cell consists of sets of positive and negative plates, each type connected in parallel, and separated by porous fibreglass insulators. The assembly is immersed in a sulphuric acid electrolyte.
The active material of the positive plates is lead peroxide, and of the negative plates, spongy lead.

Battery Cycles.

• Discharge Cycle. Sulphuric acid ionises in water to form positive hydrogen ions (H+) and negative sulfate ions (SO4 -). Lead atoms leave both pos. and neg. plates and combine with the sulphate ions to form lead sulphate. At the same time ionised oxygen atoms from the lead peroxide combine with the hydrogen ions to form water in the electrolyte. For each lead atom that leaves the negative plate an excess of two electrons is created on that plate, while for each oxygen atom that leaves the positive plate a deficiency of two electrons is created. From this it can be seen that, as the cell is discharged, the lead from both plates is used up and the electrolyte is diluted with water. Under normal conditions of operation the hydrogen and oxygen combine and no polarisation occurs. The ion transfer for cell discharge can be seen in Figure 4.6(a). A discharged lead-acid cell must not be left standing too long without recharging since the lead sulphate may fall away from the plates and if this occurs the cell can never be completely recharged back to its original state. Futhermore the sulphate may build up on the bottom of the cell and bridge the plates so destroying the cell. Excessive amounts of sulphate may be formed if the cell is discharged below about 1.7 V/ cell.
• Charge Cycle. The cell can be recharged by connecting it to a source of electrical energy as shown in Figure 4.6(b). During recharging the sequence described above is simply reversed and the chemical energy of the cell is restored. Once the cell is fully charged the hydrogen and oxygen liberated by the electrolysing of the water in the electrolyte cannot combine within the cell, and instead vent to the atmosphere. Overcharging leads to excessive gassing and the removal of acid with the wet gas. It is deposited on surfaces adjacent to the cell and gives rise to a corrosion problem. In addition, overcharging can lead to distortion of the plates and the generation of heat within the cell. If this condition is suspected, seek technical help, as there is a fault with voltage regulation. It is prudent to have a voltmeter fitted on your on board instrumentation to give some indication of the charge rate. Charging voltage will be a minimum of 13.5 v to overcome battery internal resistance.
  

• Amp -hours defines battery capacity, and for definition purposes, is measured over a ten hour period. A battery rated at 100 amp-hours is capable of delivering 10 amps/hour for ten hours.
• Cold Cranking Amps, CCA, for a fully charged 12 v battery may be defined as the amps it can deliver for 30 secs. at 0° C while maintaining a terminal voltage of 8.8 volts. i.e. 1.4 v/cell, although some manufacturers use other low temperature figures as a datum. It is common for manufacturers to apply a factor of x 6.8 to the amp hour rating to give CCA.
• Plate numbers defines the number of positive and negative plates to each cell. The more plates, the more current available during high current discharges, and the greater the cold cranking performance.

Battery Types.

• Starting Battery. - Large current drain for short-time periods, intermittent heavy loads, engines and generators, starter motors, anchor windlass or electric winches. These batteries have many thin plates to maximise plate surface area, which reduces acid diffusion time. This supports the substantial energy bursts needed for engine starting. They should be maintained at full charge, as the thin plates and low density material they contain, cannot handle repeated deep charges.

• Deep Cycle Battery. - Day to day service. house battery. - a battery that can withstand 'cycles' of long discharge then recharge, without significant voltage drop or plate damage. Used for lights, radios, navigational gear, 'in-house' and general electrical needs. These types have thicker plates with heavier plate grids and denser active material. The positive and negative plates are separated by stiffer and better engineered insulators. Heat and energy are created and as the battery plates react to chemical changes, plate particles fall off and float away into the electrolyte, falling eventually to the cell bottom. Once this accumulates to a certain height, short-circuiting may occur resulting in a 'dead cell'. Deep cycle batteries should incorporate protective barriers to prevent this.

Battery Maintenance Check List. On board, there are two ways to accurately test a battery. - with a good quality hydrometer, and with a digital multimeter. Allow the battery to stabilise i.e. at least 30 minutes after any charging or discharging. Note from the table below, that accurate s.g. reading is related to ambient temperature. Industry standard is 25° C. and a correction should be applied if the ambient temperature varies significantly. The S.G. reading should be adjusted by two ' points' for every three degrees above or below 25°. e.g. a hydrometer reading of 1250 at 13° C. would be adjusted to 1242, indicating the battery was approx. 92% charged.

• Battery Electrolyte checking - Using a quality hydrometer, check electrolyte specific gravity (SG) and levels of each cell weekly. A variation of more than 25 points will indicate a faulty cell, and the battery should be replaced.Testing is based on the principle that as a battery discharges, the electrolyte of sulphuric acid becomes less dense. This reading correlates accurately with the battery state of charge. S.G of pure water is 1.00 .
• On Load Terminal voltage. Measured with a digital multimeter. An analogue meter is not accurate enough for the range being measured. For a 12 v. battery, the on load terminal voltage should not fall below 11.4 v. during radio transmission, which is an important consideration.The difference between a fully and half charged battery is less than 0.5 volt. Open circuit voltage is an indication of battery charge level but not of battery capacity, and does not take into account the effect of battery internal resistance on load.
• Terminals - Clean thoroughly once a month with a wire brush. Coat with petroleum jelly (Vaseline). Discard oxidised bolts and renew.
• Keep batteries clean - Don't allow deposits to build on terminals or leads. If acid is spilled, clean thoroughly using goggles and rubber gloves.
• Temperature - Check battery temperature when charging. - If hot, cease operation. The battery may be excessively sulphated or overcharging. Either way, get the battery checked by a competent person. Preferably, a 'Heavy Discharge' test will analyse the battery condition accurately. This test indicates the battery's ability to supply an adequate current and to maintain an adequate voltage while delivering that current. The current is usually three times the amp hour rating of the battery for 15 - 20 seconds. On a 12 v battery, a reasonable condition is indicated if the voltage does not fall below 9.6 v. If the voltage falls quickly, suspect a faulty cell.

Note that open circuit voltage is only an indication of charge level, and does not indicate battery capacity or condition.
# Battery charge should never be allowed to fall below 50%.

Charging.
It is important that a battery not be left idle for extended periods not fully charged. Always top up your battery charge at least on a regular fortnightly basis.
Discharging the battery in excess of its discharge rate during use, (usually 10 to 20 amps per hour), can give up to 20% less amp hours available. On the other hand, too slow a discharge rate can give you much better figures. Batteries that discharge too slowly can be hard to charge if allowed to drop below 50% capacity. Ideally a battery will require 120% replacement by the charge source to bring it finally up to 100% charge value. The 'lost' 20% is due to losses incurred within the battery itself due to condition and internal resistances during the charge-up time.

It is of value to understand the observed conditions of battery charging, which are more apparent on a battery with a low level of charge, and if an amp meter and voltmeter are utilised. Refering to the above table, a battery can be assumed to have minimum charge when open circuit terminal voltage is 11.5 v, and electrolyte s.g. is below 1.100. Initially, the charge current will be highest, and the observed terminal voltage lowest. As the battery level of charge increases, amperage reduces, and the terminal voltage increases. If this sequence is not apparent, suspect a faulty cell.
The process of battery charging and discharge follows Ohm's Law. In charging, a voltage is automatically or selectively applied, the battery internal resistance is constant, and charging current I, is a function of V / R. e.g. A 'flat' battery may have a terminal voltage of 11.5 v. If connected to a charge source supplying a regulated 14 v, the effective voltage for charging will be + 2.5 v. A typical 100 a.h. battery may have an internal resistance of 0.13 ohm.
Then I = V/R, or 2.5 / 0.13 = 19.2 amps charging current. When the battery terminal voltage increases to 12.0 v, the current would be reduced to 15.4 amps. At full charge terminal voltage of 12.6 v, the charge current would be 10.7 amps. These figures are theoretical only, and the charge source (regulator) is designed to limit charge current (amperage), depending on the level of charge.
Conversely, during discharge, the load power P can be considered constant. P = VI . Therefore, as battery terminal voltage decreases, load current increases, and the amp hour capacity of the battery decreases at a faster rate.

When a battery is ' fast charged', charge voltage may be boosted to 16 v. This creates a charging current in the order of 35 amps. The maximum rate of charge that a flooded cell deep cycle battery will accept without internal damage is 25% of its rated capacity. i.e. a 100 a.h. battery can be charged at up to 25 amps. Starting battery charge should be limited to 20%. If sustained for an excessive period, the heat in the plates and the electrolyte gets to damaging point and lots of negative things begin to happen in a battery. In a lead acid battery the electrolyte sometimes boils and evaporates away (there's a fair bit of water in the acid solution) and extreme temperatures bend and buckle the thin plates. You can often see the bulge in the sides of an abused battery. Even if you don't boil the batteries you can still cook them and do permanent damage that results in shortened battery life and performance. In extreme cases, the acid boils off into a vapour, complete with lots of hydrogen and if a fire is caused or even a stray spark from the battery charger cutting in or out occurs, a big explosion and fire can follow. Therefore, it follows that intelligent and smart battery charging is an absolute must if the smart owner is to maximise his investment in batteries, and his life expectancy.

Many onboard charging problems come about because conventional 'automotive' alternator/regulators are used to do the job that is beyond their capabilities. On a vehicle, the alternator's job is to replace a relatively small amount of discharged power within a short space of time. When this job is done the altemator concentrates on supplying the vehicle's power requirements. On a boat this system invariably runs short as the altemator is required to supply and maintain virtually a 24 hour demand, especially in a 'live aboard' arrangement. The batteries are only charged for one or two hours daily, at best.
The alternator usually supplies a high initial charge rate that 'tapers off' during charging and the result is usually an 'under charged battery' about 30% short of its required charge. The answer to this is a heavy duty 60 amp alternator coupled with a "smart" electronic voltage regulation system that can balance and manage differing battery types and inputs sources such as solar panels, alternators,and wind speed generators. Such a unit would have two or three stages in the charging rates and cut in and out when it senses either a full capacity or the fact that the battery capacity has dropped so that it requires the battery charger to cut in again.
This has many advantages, the least of which is that the battery never overcharges and boils or that it is never left undercharged. A typical rate would be that a flat battery receives a first stage, brisk charge at about 15 - 20 amps up to say, 80% charged, then the second stage cuts in and charges the battery at a reduced charge rate, say 8 - 10 amps until it senses it is approx 95% charged and then 'trickles' it at a few amps up to fully charged. When the voltage drops after use, it repeats the performance and keeps the battery at a perfectly constant rate of full charge. Because the batteries are being charged efficiently, engine low load running hours are kept to a minimum. Typical equipment. | Outback Marine |

Batteries should be located in an accessible, well ventilated area, and be positively secured. Always use protective gear when servicing. Avoid smoking or open flames in the vicinity. Batteries emit hydrogen gas during charging, and a battery explosion can have serious consequences.

Gel Cell Batteries. In this type, the electrolyte is thickened by silica to form a thixotropic gel, and the oxygen produced by the positive plates, and the hydrogen gas created by the negative plates, is 'recombined' back into the electrolytic gel, allowing the case to be sealed, removing spillage dangers, and the need for general maintenance. Capacity is less than the lead acid type, and there can be problems created by fast charging.

AGM Batteries (Absorbed Glass Mat)
The day of the lead acid battery may be numbered. Originally developed in 1985 for military use, the AGM battery is a totally sealed maintenance free battery with the plates packed into, microfibre glass matt. As the electrolyte is captured within the microfibre matt there is a much lower resistance within the battery allowing a more efficient and faster reaction between the hydrogen from the negative and oxygen from the positive plates. There are also special voids designed in to the matt for efficient retention of gas without pressure buildup within the battery during a charge cycle, and more stable transformation from gas to water during use. They are maintenance free and there is no need to top up the electrolyte, ensuring no spills. Their greatest advantage is the fact they will accept a high rate of charge and are speedily charged from flat to full between three and five hours at 20 amps. Another great advantage is their high discharge rate, the low internal resistance allows discharge currents of up to five times the rated capacity of the battery. High discharge rate make them ideal starting batteries. Check them out at | APB Batteries |

Keep it charged. If laying the boat up for a long period, give yourself a head start. Disconnect the batteries and bring them home to the relative warmth of the garage. Clean them up and once a fortnight, give them a charge, ensuring they stay that way. Whatever you do, don't put your batteries on a damp or wet concrete floor. Recent tests have conclusively proved that batteries develop 'earth leakage' through the casing, much to the detriment of the battery's life. Surveyors often find batteries sitting on wet or damp surfaces. If a fully charged battery sits on a damp surface it will discharge right through the casing in a week or less. Often cheap battery boxes retain water, and these must be allowed to drain any excess water away.

Marine Batteries.
With any battery marked 'Marine Quality', be careful. You will pay a premium for any such rating, but in reality, many 'Marine Batteries' will not be much different to their automotive counterparts. You may get a couple of nice handles, maybe a rubber case, but look closely at ratings, capacities and plate numbers. More plates equals higher capacity and rapid recovery from starting discharges. Whatever you do, don't replace a 'Starting' battery with a 'Deep Cycle' battery. There are different batteries for different functions. Be sure yours are correct, for surely tears and gnashing of teeth will follow if you don't.

For battery service, contact a local VMRG sponsor | Battery World |

BATTERY FAULT FINDING. Remember that apparent low battery power may be due only to dirty battery terminals. Cleaning, or an application of WD40 or even Coke may be a simple temporary solution.
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Symptom	Fault
Won't accept charge.	Supphated plates.
Low specific gravity level.	Low level of charge.
Battery dies when load applied.	Low charge level/sulphated plates.
Battery hot when charging.	Sulphated plates.
High charging current.	One or more damaged cells.