Battery Management

To help with the decision about electrical power generation we need to calculate how much daily power we will take from the domestic batteries. I have measured the power taken by all of the mains equipment and converted this into a 12 volt power requirement based on an efficiency of 80% from the mains inverter. The 12 volt power equipment has been estimated from the equipment specification. All of these figures can be modified with actual values to establish an accurate model of the power consumption figures.

 

It is also important to establish the correct power profile in order to maximise and extend the useful life of the battery. The batteries are of a wet carbon fibre deep cycle construction with an ideal charge cycle of discharging to 50% and recharging to 80%. The battery capacity is 880 ampere hours so this corresponds to a discharge capacity level of 440 ampere hours and a recharge level of 704 ampere hours to give a discharge cycle of 264 ampere hours.

 

The tables below give the figures for the equipment that will run from the battery and using the 95 amp alternator attached to the engine the engine running time to recover the power used on a daily basis is also given.

 

Mains Appliance	Daily Ampere hour	Engine Running Time Minutes
Giga Box Standby	1.04	0.66
Giga Box on	9.17	5.79
TV on	3.28	9.08
TV Standby	14.38	2.07
Coffee Machine	16.67	10.53
Laptop 1 On	32.5	20.53
Laptop 1 Standby	1.67	1.05
Laptop 2 On	16.25	10.26
Laptop 2 Standby	2.08	1.32
Microwave
Toaster	6.77	4.28
Total	102.76	64.90

 

12 Volt Appliance	Daily Ampere hour	Engine Running Time Minutes
Lights 4 off	26.67	16.84
Fridge	42.14	26.61
Eberspacher	1.92	1.21
Water Pump
Impellar
Macerator	1.3	0.82
Total	72.02	45.49

 

  So the daily usage requires a total engine running time of about 110 minutes. In order to maintain the ideal charging cycle the engine would not need to be run until the discharge current reaches 264 ampere hours, which is about 90 ampere hours over the actual expected usage. So on days when we are moving there would be no problem with recharging the batteries, on days when we are stationary we would need to run the engine for just less than two hours per day.  So the battery system is definitely fit for purpose in that it is easy to achieve and maintain the desired charge cycle.

 

Looking at alternative power sources to recharge the batteries we have solar, wind power or an external generator. I have assumed 4 amps from the solar panel which is the maximum quoted for the Uni-solar PVL68 solar panel, this is probably being overly optimistic.  This would require sunlight days of 43 hours. 

 

Daily Usage Ampere hours	Engine alternator Minutes	External Generator Minutes	Wind Generator Minutes	Solar Panels Minutes
175	110	126	1049	2622

 

The wind generating power is 10 amps and is based on the Rutland 913 wind turbine and again I am being very generous as this current would require wind speeds of 30 mph for 17 hours per day. Neither of these options are worth considering for this particular setup, unless we were to get rid of all of the mains equipment and reduce the lighting and then a combination of wind and solar might be sufficient to meet the domestic power requirements.

The external generator is a viable option as far as power generation is concerned, there are petrol and diesel generators available with power outputs of around 1Kw. Petrol generators are quieter and cheaper than diesel but the problem then becomes a one of fuel storage safety and the availability of fuel. Diesel would be better from this perspective but diesel generators are generally more expensive and significantly noisier. I have found one that costs £260 but it has a noise level of 80dBA and is rather large.

 

In conclusion until demonstrated otherwise the alternator used with the boat engine will be used to generate the power needed to charge the batteries, this will also give the added advantage of supplying the daily domestic hot water. This will be revised if in the course of our travels if we discover that the alternator/engine does not meet our needs.