Thursday, 12 January 2012

Assault and Battery (4)

Continued from Assault and Battery (3)

Standard lead acid battery type is normally the cheapest type, these are cost effective but need more 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 thousands 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 many 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.

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. Sealed batteries are very sensitive to overcharging which results in gassing. This causes water loss that can never be replenished, loss of capacity and premature aging.

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.

One reason that I like wet cell batteries is due to the effect on the life cycle by over charging a battery. Overcharging will cause a rise in temperature, but more seriously, overcharging can also cause the release of gases. As the gasses are vented from the battery, the electrolyte fluid level will diminish. Requiring that water is added to replace that lost to gassing. It is almost impossible due to the design of the battery to add lost fluid to a low maintenance sealed battery. Manufacturers have no control over how the user treats the battery. For safety reasons, pressure release vents are built into the cells to provide a controlled release of internal pressure to prevent reaching dangerous levels. It is recommended to keep the charging current below 20 % of the rated capacity.

Other maintenance issues are sometimes encountered with batteries. As batteries begin to come to the end of their life it is possible for them to suffer a number of ageing issues. All rechargeable batteries have a finite lifespan and will slowly lose storage capacity as they age due to secondary chemical reactions within the battery whether it is used or not. Some cells in a battery may deteriorate sooner than others, but the overall effect is to reduce the capacity of the battery.

The boater can get a much longer life out of the battery bank by specifying a battery bank with a capacity that is larger than that required to meet the in between charge from the calculated power budget. However, the bigger the capacity the longer it will take to charge the battery bank. The bigger the capacity the bigger the purchase cost.

I based my capacity (Ampere hours)  on the estimated power budget.  The budget figure should not reduce battery bank capacity when new, by more that 35% in a 24 hour period. This also took into account the battery ageing would also reduce capacity over time. I wanted to use other sources such as solar generation to provide a daily top up charge. Thus reducing the need to run the engine when off mains. I calculated a solar panel size of around 100 watts. This would provide at best around 8 Ah into the bank. However due to the vagaries of our weather and the lack of charge during the hours of darkness, the real input into the bank would be reduced.

My worst case power budget figure was to consume 120 Ah at an average of 5 amperes per hour, over a 24 hour period. With the new capacity of the battery bank being 500 ah. This would be around the 25% discharge point when the bank was new, ignoring the solar top-up. This would give me a predicted life cycle of around 800 cycles. I estimated that we would be off mains for a maximum of 200 days a year. So I hope to achieve around 4 years for the battery bank life.

Battery performance diminishes due to deterioration of the active chemicals as the battery ages. There are published processes which claim to rejuvenate a battery. The process will never bring a battery back to an as new condition. I have played around with a number of the chemical rejuvenation processes, some have a limited effect but most are as much good as the snake oil of the old travelling salesmen. Some processes will extend the working life of a battery for a short period in time. But like all the anti-ageing creams, lotions and potions we see advertised on television. Ageing will always run its course, but it might be delayed a little.

Depending on how you use you battery bank, end of life will occur at different points in the life cycle. The actual ageing process results in a gradual reduction in capacity over time. When a battery reaches its specified lifetime it does not stop working suddenly. In a low capacity environment it may be as low as the 50% of the "as new" capacity point. In a  high capacity environment end of life might occur at the 80% point.

The cycle life depends on a number of factors and assumes that the battery is always fully recharged. However, how long a battery bank will last is also down to the internal construction of the batteries. It is down to the level of discharge in each cycle. If the battery is only partially discharged each cycle then the cycle life will be much greater. It is down to the extremes of temperature that the battery is exposed to. It is down to the level of under charging as well as the level of over charging. It is down to how the battery is stored. More than anything else it is down to how it is used and abused as well as the care and maintenance given throughout its life.

To be continued in Assault and Battery (5)
Previously Assault and Battery (1)
Previously Assault and Battery (2)
Previously Assault and Battery (3)

Other posts on the subject of batteries and battery banks.
Boat Battery Maintenance Pt 1
Boat Battery Maintenance Pt 2
Boat Battery Maintenance Pt 3

Post on Solar and Wind Power
Piss power for your boat.


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