Saturday, 20 August 2011

Electrical Audit

I’ve gone back to my original electrical audit calculations for Waiouru and updated them.  This time I’ve created an Excel spreadsheet where the variables can be adjusted.

The spreadsheet consists of four parts.

  • 12v DC consumers
  • 230v AC consumers
  • Actual available domestic battery bank capacity
  • Battery recharging times

I’ve listed all the 12v consumers by type (lights, appliances, pumps, etc); the quantity of each; the consumption in amps; and the average daily usage in summer and winter.

I then did the same with the 230v consumers.  As the electricity will be coming from the inverter I added an additional 10% consumption for the inefficiency of the inverter.

Next I calculated the energy available from the domestic battery bank.  I’ve been conservative and will only discharge them to 30% of their total capacity.  Any more might harm the batteries and shorten their life.

Finally, I calculated how long (in days) the battery bank would last before recharging as required.  Then I used the spreadsheet to calculate the daily hours the engine would need to be run to recharge the batteries in summer and winter.

By creating the spreadsheet with variables I am able to see the results of my “What if…?” ideas.  For example – what if I was to reduce the size of the domestic battery bank?  Or – I will cruise an average of 3 hours daily so how much charge can I generate in that time.  Fully recharging batteries isn’t a simple process so I may have to do more work on this part of the process.


Bruce in Sanity said...

Are you using traction type batteries with helical plates? The usual rule of thumb I was taught for flat plated batteries, and just repeated by Tony Brooks in this month's Canal Boat, is that damage will occur if discharged below 50% or 12.2 V.

Most of us CCers accept that we can't maintain optimum conditions for our batteries, because of the final charging problem, and so sulphation will nail them (typically after about two years' use) well before they reach their notional full time in terms of charge/discharge cycles.

For this reason, it's not worth spending too much on them, unless you are going to have an opportunity of overnight charging regularly.

All the best


Tom and Jan said...

Hi Bruce,

I had been planning on a very heavy duty domestic battery bank (1200VA) made up of Rolls 530VA traction batteries. The idea is there will never be discharged more than 30% and normally it will be a lot less than this. They should have a life of 10-15 years. Derek & Dot on Gypsy Rover did something similar.

Bruce in Sanity said...

Cool. In that case, have you checked out PB Batteries? They do a special range of deep discharge batts for narrowboats, and a neat topping up system.



Tom and Jan said...

Actually I was considering six of these click here although I’ve found a cheaper supplier

Davidss said...

I see no mention of solar charging as an input source. I have no direct experience, and perhaps for a continuous cruiser the charge input is considered to rarely exceed the battery discharge, so charging may as well be done from the engine or shore line.

I appreciate that maximum charge input varies with the size of the installation, as well as the sun, but I was wondering if there was a general principle that meant solar panels and a CC life style were not an effective mix.

Regards, David.

Tom and Jan said...

Hi David,

We haven't ruled out using solar energy. However it's on our list of desirable rather than essential! We will have to see how our finances hold up!

Bruce in Sanity said...

We're very glad of our solar panel, which is a BP Solar 160 watt beast. On a good day like today, it more than keeps up with daytime usage.

The other week, we spent three days on tixall wide without running the engine, and could probably have stayed longer.

Tom and Jan said...

Hello Bruce

What type of cells does your system have?
Polycrystalline’ or

Also, is it a flexible panel or rigid?

I can’t find any information on your model model but a panel with an output of about 160w appears to cost around £600-900. That’s a lot of diesel!

Bruce in Sanity said...


It's a rigid polycrystalline. The panel is rated at 135W, but the fancy controller claims to lift that to an effective 160W. Data sheet for the current version here.

Not cheap, as you say, (by the time you add in the controller and VAT, 4 figure territory) but long lived, and worth it for the much reduced need to run the engine on days we don't move.

Even on a dull day, we would typically run the engine for maybe half an hour, with the Hurricane on as well, so as to have a full tank of hot water.

Tom and Jan said...

The MPPT regulator isn't cheap!

I suspect solar energy is definitely going to be in our desirable basket rather than essential