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


Ampere hour

Engine Running Time Minutes

Giga Box Standby



Giga Box on



TV on



TV Standby



Coffee Machine



Laptop 1 On



Laptop 1 Standby



Laptop 2 On



Laptop 2 Standby













12 Volt Appliance


Ampere hour

Engine Running Time Minutes

Lights 4 off









Water Pump
















  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


External Generator


Wind Generator


Solar Panels








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.       


5 Responses to Battery Management

  1. Will Chapman says:

    These are really interesting statistics and fit in with a point I have raised with HMRC and am awaiting their clarification.

    The point I made was that a typical boat needs to run its engine between 2-4 hours (x hours) a day just to provide essentially domestic needs. Doesn’t it therefore follow that said boat could cruise for x hours a day on diesel purchased at the domestic rate?


    Will Chapman

  2. nblazydays says:

    Will, Pleased to meet you. I have never thought of it like that but you know I think that you might have a point. I suppose the usage forms a dual basis under these circumstances and only adds to the complication and highlights the ridiculousness of the legislation.

  3. John Allebone says:

    Hi there, does your 95amp altenator really put back 175amps into the batteries in 110 mins? Surely it only starts off at 95amps/hr but drops off as the batteries come up to full, therefore taking longer than the calculated time.(Or is my system/battery bank, faulty) I thought that it was only AGM & Gell batteries that would charge at the higher rate for longer? I look forward to your reply, thanks, John.

  4. nblazydays says:


    You are correct in that the alternator current will fall as you reach full charge. But in our case we try and maintain the batteries at between 50 and 80% and we are using a smart charger and therefore my estimation is a close approximation, ok it would be closer if we were going to 70% but I am not an expert on this, in fact it is a new field for me and as you know boating is about learning. I have meant to do a revised version of this in the light of what I have learned over the last year by actually living on the boat.

    A smart charger gives three states of charge Bulk, absorption and float. The bulk charge is at maximum current until the battery reaches 70% of charge, this can be adjusted and I do not know what ours is set to but will research it.

    Also I would be very suprised that the alternator gave out a max of 95amps when we are moored up. The 95 Amps (or less) is also approximately a tenth of the battery capacity which is a good figure to charge the batteries at. As I said I need a second edition to this document.

    BTW thank you for your comment.

  5. Neil Corbett says:

    I would be very surpised if your alternator put out its rated amps for more than a minute or two. My 70 amp alternator only puts out 30 amps after a couple of minutes then holds fairly steady until the batteries come up. I think perhaps that might because if the batteries are 50% charged when you start, they already offer some resistance.

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