Tuesday 10 September 2013

Battery Chart (2)

Continued from 'Battery Chart (1)'

The first thing is that we would never want to discharge the batteries to the fully flat stage. If only to protect the number of life cycles which will vary from charged to depth of discharge. The smaller the rate of discharge from the battery, the longer the discharge power will be available.

I have a rule of thumb for protecting the battery bank life cycle and a power budget. This is 20% of total capacity of the battery bank. 100ah will give me a low discharge rate when spread over 24 hours.  500/5 = 100ah /24 = 4ah continuous rate of discharge. Over a 24 hour period that takes care of the power consumption. Fridge/lights/television/pumps/radio and other items.

So we have calculated a power budget for the boat and them made improvements to reduce the power consumption. The first thing we did was to change all the lights to LED. The TV is rated at 12 volts. We have done away with as many mains appliances as possible. Leaving just a hair dryer and a washing machine. The batteries and inverter can cope with the occasional use of the mains hair dryer. 

Another upgrade was to improve the capacity of the engine alternator pair to 100ah and 70ah. When we need to use the washing machine we run the engine at the same time. This reduced the discharge load on the battery bank.

Another upgrade is adding solar panels. The output depends on the weather the time of day and the time of year. But we have obtained over 12,500ah into the battery bank so far. Which is about 25 times the total battery capacity.

If you choose wet cell batteries 'the kind that you top up with deionised water' you can check the specific gravity in each cell. This will let you know how well each cell is performing. Problems with a battery would be highlighted much sooner.  You would let the batteries rest for 24 hours before testing with a hydrometer. 

The next item is the battery charger. Some battery chargers are described as being 'smart 'or 'intelligent'. They can be configured for the type of battery in use. Usually this is by changing a pair of Dip Switches. The battery charger will vary the rate of charge applied to the battery bank depending on the switch settings. 

Our mains battery charger is a three stage type.

Bulk: This is when the charge pours amps at the battery. The current and voltage are unregulated. Its not unusual to see 50/60/70 amps flowing into the battery bank. As the battery bank starts to return to its 60/70% charged state. The rate of amperes will have started to tail off. The charger switches to the second stage.

Absorption: The charging voltage stays at a predetermined level (14.1v) depending on the type of battery the charger has been configured for. This stops the batteries from overheating and also reduces 'gassing' and water loss. 

Float: As the battery bank reaches about 90% charge the predetermined voltage level falls to (13.8v) This slowly charges the battery to full capacity without overcharging. A side effect is to equalise the individual battery cell voltages. The cells which are at a lower specific gravity will absorb more of the charge than a cell which is closer to its correct specific gravity.

Some boats have a split charger system. That takes the output from the alternator charging the starter battery and then connects it to the leisure battery bank after  the started battery has charged. Because Rosie has two alternators, the final upgrade was to add a Stirling A to B (Alternator to Battery) charge controller. This device controls the output from each alternator. After the starter battery alternator has charged the starter battery and has reached full charge. The controller takes the output from each alternator and combines them together to recharge the battery bank. The A2B charges the battery bank much quicker than using the alternator voltage controller to control the charging rate. 

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