Friday, 6 January 2012

Assault and Battery (1)

It's been some time coming, but its time for a direct assault on Rosie's batteries over the next few weeks. Batteries on a boat are a consumable item. They have a life, the harder you work them the shorter their life expectancy. But as with most things a bit of "Tender Loving Care" will make their working life and performance last much longer.

Hydrometer check
Many years ago, part of my job was maintaining batteries used for emergency power. I remember sitting in a class room for a period of six weeks being bored to death with a load of mumbo-jumbo technobabble.  At the end of the period, what I learnt was that general battery maintenance and battery life span, are two very different things. Especially in an environment where batteries were changed on a regular maintenance schedule. Care of the batteries went little further than weekly topping up of the electrolyte levels with de-ionised water and the odd check of the specific gravity of the battery acid. 

What I have learnt since is that if you want a cost effective life span for your batteries. Then the maintenance regime needs to be a regular task completed on a monthly basis. There are also a few other things that you can do to improve the cost effective life cycle of the battery bank.

Typical Battery Construction
The technobabble relating to the care and welfare of your typical battery bank is a black art. Unless you take a great deal of pleasure from shoving needles into  your eyes. You don't need to understand all the technobabble associated with them. Lead-acid is the oldest rechargeable battery in existence, (150 years and counting) so the technology is not new. However, over the years improvements have taken place in construction, materials and our understanding. There are however some simple and easy things that you can do to help you to become a caring owner, of that battery bank hidden away under your feet.

In an ideal world, what you should do on your boat is try to provide the optimum operational conditions for their longevity. Ever tried to start your car on a cold and frosty morning. The starter motor labours to turn over the engine and it can take much longer than normal to start. This will have as much to do with the temperature of the battery as its state of charge.

Batteries have a range of temperatures in which they like to work. Not too cold and not too hot. They even have a preferred temperature at which they will perform at their best. The optimum operating temperature for the lead-acid battery is around 25 Centigrade or 77 Fahrenheit in old money. If you use your battery bank outside the normal temperature range the battery bank will continue to work, but its length of life may be reduced.

Batteries self-discharge faster at higher temperatures. Battery lifespan can also be seriously reduced at higher temperatures. Most manufacturers state a 50% loss in life span for every 15 degrees F over a 77 degree cell temperature.

Vibration can and will damage a battery. Due to the improvements in internal and external construction batteries are much improved, but long term vibration can still be a significant problem. However, batteries do not bounce and are not very good at absorbing such harsh treatment.

I use a thin sheet of foam material under the battery bank to insulate the batteries from the cold hull and help reduce the vibration caused when the engine is running. However, as heat reduces the life span of batteries at a faster rate than the cold. I do not insulate the batteries in any other way.

When batteries are being charged they create internal heat. That heat needs to be able to escape. I have small 10mm spacers between the batteries that allows some air to circulate in between the batteries which are mounted side by side in the bank. This helps to cool them down. Some batteries come with inbuilt spacers in the shell of the battery. How effective is this? I have no idea, but I know that when a battery gets hot it uses up some of its internal fluids and this shortens the batteries life span.

Batteries are rated at a nominal 12 volts. However, depending on the construction and the materials used this voltage can vary slightly. There is lots of technobabble about various voltages. All you need to know are the rough rule of thumb values.

The nominal voltage of a standard lead acid battery is generally stated as being 12.00 volts. However when measuring the (open circuit) voltage of a fully charged battery, it  should give a reading of 12.65 volts. When the battery is being used (under load) as the battery discharges and its capacity falls so will the voltage across the terminals. The end-of-discharge voltage for lead acid is 10.5 volts.

Voltage on a fully charged battery will read 12.65 volts for a 12 volt battery. At 50% the reading will be 12.18 volts, and at 0% will be 10.5 volts or less. Specific gravity measured with a hydrometer will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell.

Technobabble Alarm: Because of something called the Peukert Effect which is directly related to the internal resistance of the battery. The higher the internal resistance, the higher the losses while charging and discharging, especially at higher currents. This means that the faster a battery is discharged, the lower the AH capacity. Conversely, if it is drained slower, the AH capacity is higher.

The state of charge, or the depth of discharge can be determined by measuring the voltage or the specific gravity of the acid with a hydrometer. This will not however tell you how good the battery condition is. Only a sustained load test can do that. It is a good idea to have your batteries load tested before replacement. If there is a fault in the charging system the battery may not be charged fully and the symptoms would look like an old battery.

A battery "cycle" is one complete discharge and recharge cycle. It is usually considered to be discharging from 100% capacity to 20% capacity and then charged back to 100%. However, there are often ratings for other depth of discharge cycles, the most common one is 50%.

Battery life is directly related to how deep the battery is discharged at each cycle. If a battery is never discharged more than 50% of its capacity it will last twice as long as if it is cycled to 80% of its capacity. When calculating your battery bank capacity and you have some idea of the loads. You should look for a battery bank with a discharge capacity of around 50% for the best storage vs life cycle.

Continued in Assault and Battery (2)


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