Monday, 9 January 2012

Assault and Battery (3)

Continued from Assault and Battery (2)

Lead acid battery life can be extended if an equalising charge is applied every 10 to 40 days. This is a charge that is about 10% higher than normal full charge voltage, and is applied to the battery for about 2 to 16 hours. This makes sure that all the cells are equally charged, and the gas bubbles mix the electrolyte.

During each charge and discharge cycle, the lead content in the batteries plates gets gradually eaten away (grid corrosion). The material then forms a sediment that builds up on the battery floor. There is little or nothing you can do about this gradual change and the manufacturers leave a small space at the bottom of the battery for the sediment to fill.

Charging a battery to full capacity is a time consuming process. Lead acid batteries should be charged in three stages. The first stage is the bulk charge. The second stage is the absorption charge and this is followed by the third stage which is the float charge. The constant-current charge applies the bulk of the charge and takes up roughly half of the required charge time, the topping charge continues at a lower charge current and provides saturation, and the float charge compensates for the loss caused by self-discharge.

Over time a lead acid battery will become less efficient. Its capacity will begin to reduce through a gradual ageing process. Batteries can still fulfil a good percentage of their function well into old age. However, there comes a point when the law of diminishing returns dictates that the battery is past its useful life.

Ring 3 Stage Charger.
A high charging voltage above 14.4 volts will shorten the service life of your battery due to increased grid corrosion on the positive battery plate. A periodic fully saturated charge (all three of the above stages) is essential to prevent sulphuring of the battery plates. Leaving a battery in a discharged condition causes sulphation. The more discharged a battery is, the faster the sulphation will build up. In fact, keeping lead acid below 12.6 volts will cause the buildup of sulphation. The battery must always be stored in a charged state with periodic top-up charges.

Voltage Check
A normal automotive battery charger will charge a lead acid battery. Some of the chargers are quite crude and will have a charging voltage that exceeds the 14.4 volts. If we charge a 100 ah battery with a standard battery charger at a constant 10a for 10 hours we might assume that the battery would be fully charged. However, batteries are quite inefficient when charging even when new. A new battery would require about 10 hours charging time to get to about 70% of its capacity. The next 30% of capacity would take almost as long again. As the battery gets charged up, the voltage goes up, so the amps out of the charger goes down. They charge OK, but a charger rated at 10 amps may only be supplying 5 amps when the batteries are 80% charged.

However, a specialist charger with the three stages listed above, is a much better option.

But what happens if we are not connected to a mains power supply with access to a state of the art charger. What if we have to rely on the engine alternator to provide the battery charge. How can we monitor how well our batteries are functioning?

To get a good idea of the state of charge, you can monitor the voltage at the battery bank terminals with a volt metre. However, the battery bank will require an hour without use to settle out at the true quiescent voltage.

Lead-Acid batteries do not have a memory, and the rumour that they should be fully discharged to avoid this "memory" is totally false and will lead to early battery failure. Inactivity can be extremely harmful to a battery. It is a poor idea to buy new batteries and save them for later use. Either buy them when you need them, or keep them on a continual trickle charge.

On Rosie we have a Beta Marine marinised Kubota 38 engine complete with 2 alternators rated at 45 amps for the starter battery and 90 amps for the leisure battery bank. We have a Victron Inverter with a three stage land line mains charger. Plus the Beta battery monitoring system. The Beta engine was provided with an alternator controller which is also a three stage charging system. The controler is user configurable for different types of battery.

Our new battery bank is rated at 500ah as we have a bank of four 125ah leisure batteries fitted. With few exceptions we have converted where possible everything on board to 12 volt operation. We have gone for a number of power conservation options such as LED lighting.

Last but not least. The sealed, or maintenance-free, battery are now a popular alternative to the old wet top-up batteries. The construction and materials used in the maintenance free batteries vary quite a bit. The most popular is the Absorbed Glass Mat type of sealed lead-acid battery that uses AGM between the plates. It is sealed, maintenance-free and the plates are rigidly mounted to withstand extensive shock and vibration. Nearly all AGM batteries can recombine 99% of the oxygen and hydrogen. There is almost no water is loss.

On a personal note, I still prefer the wet cell lead acid battery that requires topping up from time to time. Whilst the zero maintenance types are more popular, I feel that I can extend the battery life cycle by being proactive. Doing a monthly check ensures that I will become aware of problems much sooner. But that's what I did in the past and it falls within my comfort zone. None of this new fangled zero maintenance electrickery stuff for a crusty old curmudgeon with electrolyte in his veins like me, and wet cell batteries are also much cheaper.


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