Tuesday, 17 August 2010

Boat Battery Maintenance Pt III

Boat Battery Maintenance Pt III

Most vessels use lead-acid storage batteries for starting engines and the operation of other onboard electrical equipment. Proper maintenance of the lead-acid storage batteries can and will lengthen their service life.

The number of times a battery can be discharged is known as its cycle life, and this is what determines its suitability for use with boats. Car batteries are the most common type of lead-acid battery, but will survive only 5 or 10 cycles so are unsuitable for our purposes. For boating applications a battery needs to be capable of being discharged hundreds of times. This type of battery is known as a deep-cycle battery.

Lead-acid batteries used on boats present several hazards to owners who are interested performing their own maintenance. Lead is a well known toxic heavy metal. It shouldn't be a danger unless the battery case is broken. The electrolyte used in the battery composed of sulphuric acid and water. Sulphuric acid will severely burn eyes and skin. Goggles and acid resistant gloves should be worn while servicing the fluid levels of conventional lead-acid batteries. Charging a battery produces explosive hydrogen gas. Avoid introducing sources of ignition near the battery and ensure good ventilation. Always wash your hands after working around batteries.

A lead acid starter battery is made up of a series of identical cells each cell containing a positive and negative plate. The resultant voltage of single lead acid cell is normally 2 volts. In order to achieve the voltage required for the equipment, each cell is then connected in series to form a battery. In a typical battery (such as that used in a boat for starting the engine) the voltage required is 12 volts. This is achieved by connecting six cells together in series and enclosing them all in one plastic box.

Leisure batteries as used on a boat where a sustained current requirement is needed, as well as the ability to be discharged to 90%, have a different make-up to that of a starter battery.

Each cell in a lead-acid battery is filled with an electrolyte solution made up of sulphuric acid and distilled water with a specific gravity of 1.270 at 60deg F (15.6deg C). There will be more on "specific gravity" later.

Boat batteries come in all shapes and sizes. They are often subjected to a wide range of adverse operating conditions. From the extremes of heat in the summer and the deep cold of winter. Add to this alternator undercharging or alternator overcharging and you will find that the typical day-to-day conditions will take a toll on your battery performance. Battery maintenance is often a chore that gets neglected and is only done when a dire situation dictates. Situations like, when you turn the engine starter key and the only sound is a dull click. Its still summer, the weather is conducive to doing a bit of boat maintenance, which will certainly become a pain in the rear if it is left until the winter. Now is the time to think about battery maintenance.

Battery maintenance always begins with a visual inspection:

Look at the exterior of the battery. Look for cracks in the case, dirt or corrosion on terminals, and leaking electrolyte. Batteries with any external damage must be replaced. Next check battery cables and connections, looking for any damaged components. Cables need to be clean, the insulation intact and the connectors not be frayed.

Using a wire brush clean the battery posts and terminals until they are nice and shiny. Bare metal-to-metal mating surfaces are required for good current conductivity. To prevent corrosion on terminals, thinly coat the terminals, terminal clamps and any exposed metal around the battery with high temperature wheel bearing grease or petroleum jelly.

The three most important aspects of care for all types of batteries are both charge and discharge cycle as well as keeping them topped up with distilled water, unless it is a sealed low maintenance battery. It is extremely difficult to accurately measure the state of charge of a lead acid battery and to predict the remaining capacity or overall condition.

External visual inspection doesn't tell the whole story of battery condition. Testing battery's open circuit voltage and specific gravity of its electrolyte will give a more accurate picture of the battery's health.

Open Circuit and Specific Gravity Testing.

The first test is to conduct an open circuit battery voltage check.

Note Accurate acid testing depends on the battery being left idle with no charging or discharging for at least 6 hours with 24 hours being preferable.

With a voltmeter check the actual voltage across the battery terminals. Make sure the engine start key is in the off position and any items using the boats 12 volts supply are also turned off as well. You could use the boats battery isolators to be sure that everything is disconnected. You should have 12.50 to 12.73 volts across the battery terminals. 12 volts is acceptable, but only if the battery is not fully charged but has sufficient power (50%) to start the engine. With the engine running and with a properly operating charging system and a fully charged battery, the voltage should be about 14.6 volts.

Charge / Voltage
100% 12.73 volts
80%   12.50 volts
60%   12.24 volts
40%   11.96 volts
20%   11.66 volts

The second test is check the battery acid specific gravity.

To complete this test you will need a lead acid battery hydrometer. They are inexpensive and you can get them on eBay for a few pounds. The battery hydrometer measures the proportion of sulfuric acid to water, which gives a precise measurement of the state of charge. You will first need to remove each cell cap and visually check the electrolyte level in each cell. The level should be just above the plates and just below the level of the filler hole. If you can see the plate tops, then you will need to top up the cell with a small amount of distilled water. NEVER USE TAP WATER. You can purchase a litre of distilled water from any good car spares shop - such as Halfords. You can even use the hydrometer as a pipette to aid topping up each cell.

To test the acid, place the hydrometer into the cell and draw up enough electrolyte into it to make the internal float - float. Do not add distilled water prior to testing if possible. (Otherwise fill and empty the hydrometer with electrolyte from the cell to be tested five or six times to aid mixing before pulling out a sample) On the hydrometer scale the specific gravity of a fully charged cell is 1.265. Each of the cells in the battery should have the same reading. If one cell is lower than 1.215 or higher than 1.310 you have a weak cell at that point in the battery.

Charge% / Specific Gravity

100%        1.255 – 1.275
75%          1.215 – 1.235
50%          1.200 – 1.180
25%          1.165 – 1.155
0%            1.130 – 1.110

The last test is a load or drop test.

Attaching a load to a battery and doing a drop test is something that is not easy to do without a specialist bit of kit called a load meter. However, you can get the battery drop test done at any good auto electrical outlet.

I know this sounds like a lot of work, but it really isn't. A visual check should be done once a month and a acid test and distilled water top-up every three or four months, especially after the second or third year of the batteries life. Get a drop test done on any battery before purchasing a replacement. Otherwise you might be replacing a good battery if the problem lies somewhere else..

Some Common Battery Problems.

It severely corrodes positive plate grids which weaken and finally disintegrate. Overcharging decomposes electrolyte into hydrogen and oxygen which causes excessive concentration of the electrolyte because of loss of water from the mixture of the battery fluid. This damages the plates and separators. High temperatures also show adverse effects on plates, separators and container. Excessive gassing creates the possibility of the active metal blowing away from the plate surface and fine acid spray escaping from the battery.

Perpetual undercharging results in sulphation of plates and running down of cells. Sulphate deposits are also seen on cell separators, which leak through and create short circuits between positive and negative plates. Undercharging also leads to buckling of plates.

Idle battery:
If left idle and self-discharge takes place, sulphation is observed on battery terminals. Sulphation of the battery kills battery life sooner than normal.

High Specific Gravity:
High SG destroys positive and negative plates and also reduces battery life.

Impure water top-up:
Impure water introduces impurities in the battery every time it is added. Iron and chlorine attack the plates causing to shorten their overall life. Chlorine bleaches separators which is harmful.

Negligence in water top-up:
The concentration of the acid increases due to reduced amount of water and so damages the plates and separators. The plate areas above the electrolyte level get hard and lose capacity.

When charged ions within a lead acid battery sink to the bottom of the cells. This leaves discharged electrolyte or diluted electrolyte at the top. The results is oxidization at the top of the plates and accelerated corrosion at the bottom of the cells due to higher acid concentration.

The depositing of lead sulfate crystals on the plates occurs as the battery is discharged. within a cell that permanently reduces the capacity of the battery. Deep discharging of the batteries can cause the sulphate to expand the negative lead plates separating the lead from the grid, or shorting it permanently damage the cell. Batteries, which remain partially, discharged for extended periods of time develop "memory" of the reduced state of charge due to sulfation. Sulfation accounts for approximately 85% of the lead-acid battery failures. Avoiding extended periods of deep discharge will reduce sulphation.

Replacement Batteries.

When you purchase a replacement battery, it is important to select the correct type. Remember, the more specialised the greater the cost.

NOTE Purchase batteries only from retailers with a high turnover on battery sales. Purchasing a battery that has been in stock for many months not on a maintenance charge. The battery can have already started down the sulphating route.

Standard lead acid battery type is normally the cheapest type, these are cost effective but need more maintenance and are more prone to damage from being discharged. This type of battery can be purchased from some retailers in a dry state. It will come with the electrolyte contained in plastic containers. When a battery is dry stored its shelf life is many years. I prefer this type of battery and to periodically do the routine maintenance.

Traction Batteries The term traction battery relates to batteries used to power electric vehicles. This can mean anything from a mobility scooter to a fork-lift truck, so encompasses capacities from 30 or 40 Ah to many hundreds of Ah. The smaller traction batteries are usually 6 or 12 Volt units, where the largest are single 2 Volt cells. Traction batteries are intended to be fully discharged and recharged daily and traction batteries can withstand thousands of discharge cycles. There are also batteries known as semi-traction batteries, which can be thought of as higher quality leisure batteries, exhibiting a greater cycle life. Marine batteries tend fall into this category.

Sealed lead acid battery type typically need no maintenance and will hold its charge for longer. These are also safer, as they cannot leak electrolyte as a standard battery may. Years ago, boat batteries lost water at a high rate and boaters were advised to check the acid level as one of their weekly checks. Improvements using calcium as a hardening agent in grids in place of antimony have caused less contamination of the acid and much reduced water loss. This makes the battery maintenance-free so no water needs to be added during its life under normal operating conditions.

Gel battery type are able to cope with more charge/discharge cycles during their lifetime and will hold their charge for longer having low internal resistance and so low self drain properties. Deep Cycle Batteries are designed to deliver constant power over prolonged periods of time.

Absorbed Glass Matt battery type are able to cope with more charge/discharge cycles and are able to hold their charge for longer having the same low self drain properties, they also benefit from being extremely resistant to vibration.

Recharge and discharge cycle.

All batteries have a charge - discharge cycle count. This is set by the depth of discharge.

If a battery is never discharged to more than 30% of capacity then the number of recharge - discharge cycles will be high. However, past this point the number of available cycles will be greatly reduced. See the typical leisure battery discharge-recharge graph from Lucas.

Replacing your boats batteries.

Calculating the power budget for your boat - should give some indication of the minimum Ah (Ampere hour) of the battery pack that is required. You will also need to take into consideration the space available as battery sizes do vary.

Rosie has 4 X 115 Ah leisure batteries plus a 100Ah starter battery. So that gives me 440 Ah or 132Ah before I reach the 30% discharged point. However, the Ah that a battery can give also depends on the rate of discharge. The greater the load placed on a battery the less Ah will be available. It is a complex formula (Peukert Calculator) which you will need to read up one anyway. So I take this into account by rounding down from 132 Ah to 110 Ah. (the same as a single leisure battery Ah rating)

Peukert's Law, was devised by the German scientist W. Peukert in 1897, it expresses the capacity of a lead-acid battery in terms of the rate at which it is discharged. As the rate increases, the battery's available capacity decreases. If you download the calculator, here is some info you will need to set the values. You will need to get the two hour/discharge rates off the battery panel.

C = "nominal" battery capacity. i.e. the number written on the battery or data sheet e.g. 100Ahrs
R = "hour rating" written on the battery or data sheet e.g. 20 hour
I = the "nominal" current at the given rate.
n = Peukert's exponent e.g. 1.3
Ip = the "Peukert" current. The equivalent current that the discharge will remove from the quoted battery capacity.
Cp = the "Peukert" capacity.

So what is Rosie's actual power budget.

Rosie has more lighting available than Blackpool. So I took the opportunity to reduce the wattage of the lighting system. The way I did this was to remove every other light bulb. I still have plenty of light available - but the power budget has been halved. It is not normal to have all your lights on all the time and so I have again used a suck-it-and-see method of rounding down to 20 %.  This gives me a rough 2Ah average figure for lights.

We have a toaster and microwave. This has a combined total of 1,800 watts, that's 12v at 150 Ah with a 100% conversion efficiency. However, I have only given this a 10% figure because we don't use it all that much, so that's 15 Ah average.

Pumps and other devices that are used infrequently I estimate at 2 Ah. The TV and satellite system which are 12v I estimate that at 3Ah as that is the current drain from the batteries when only these items are in use. So the total is about a 20 Ah budget. Or about 6 hours before I need to look at charging the batteries again. However, the Victron which measures the usage much more accurate than my rule of thumb calculations says... 22 hours of use before I need to re-charge. So rule of thumbing again I settle for a figure of 16 hours typical use per day.

Battery manufacturer web sites.


Part I

Part II


1 comment:

  1. The batteries need a care and proper maintenance with passage of time. It enhances the life of battery as well as the power of the vehicle.


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