Saturday, 16 March 2013

The Fully Monty

Earlier in the week Jan cleaned the shower door and I gave a commitment to clean the cubical walls and fittings.  I decided to do this today whilst she went to the boatyard to check if any post had arrived.  After assessing the task I realised my jeans and shirt were going to get very wet in such a confined space.  The obvious solution was to wear my birthday suit.  However not wanting to scare the neighbourhood I first placed the bungs in all the portholes.  Everything was going very well and the liquid cleaner was doing a wonderful job of removing the accumulated, soap, salts and other unmentionable by-products of showering.  What I had not anticipated was the caustic properties of the cleaner on the more delicate parts of my anatomy.  However thinking laterally I took a shower as part of the cubical rinsing down process.  Now I’m just a little red in some areas.  Perhaps I should leave the shower to Jan’s tender ministrations!

When developing the original specifications for Waiouru three years ago we considered including hinged footplates welded to the hull below the gunwale in the cratch.   The idea was they would add entry and exit at the cratch.  It’s an idea we discarded until we were living on nb Molly and discovered Gary & Ruth had a removable step for the same purpose.  Bill subsequently told us where we could get one made.  In the afternoon I measured the gunwale and drew a sketch using Google SketchUp.

Not to scale

The step would hook under the internal gunwale lip and then fold across the top of the gunwale before lying against the side of the hull.  Bill has suggested the step be held in place by a length of adjustable chain on either side and that there should be a hole in the chequer plate on the step so the end of the gangplank can be secured to it.  I’ve designed it so it all folds into as compact a size as I can make for ease of storage. 

Jan has finished knitting her cardigan and now faces the task she dislikes the most…… sewing all the seams!  Photo tomorrow I hope………

Now on to the boring battery stuff.

Part 3 – Time Required to Re-charge our Batteries

We have a 900A/h capacity battery bank and usually don’t discharge it lower than 60%. This means we need to put 360 amps back into the batteries. However nothing is 100% efficient and usually an additional 50% above the required recharge amount is required to fully re-charge a battery.

As discussed in Part 2 the Absorption stage takes much longer than the initial Bulk stage as the batteries can no longer accept a high volume of charge.

Looking at the technical information on our battery manufacturers website I find that the Absorption stage commences when our batteries are 80% charged. Moreover they provide some information on the time it will take to complete the Absorption stage

Absorption Time

  • Where : T = 0.42 x C /I
  • T = Absorption Charge Time
  • C = 20 hr Rated Capacity (of the Bank)
  • I = Charging Current ( 10% of C20 max)

Our battery manual states our battery Capacity at the 20 Hour Rate is 450 and amps of 22.5.  The time to complete the Absorption stage is therefore….. Time = 0.42x(450÷(450x10%)

Time = 4.2 hours

Assuming the batteries are 60% charged before recharging commences then 20% of the recharging will be Bulk (to reach 80% charged) and the remaining 20% will be at the Absorption rate and be completed in 4.2 hours irrespective of how much charge is being generated by the alternators. We already know the combined output from the engine alternators is 225A.

The battery bank has a capacity of 900A/h but us at 60% which means the bank requires 360 amps to fully charge or 180A to reach Absorption at 80%. However and additional 50% of charge is required when recharging. Therefore to reach the Absorption stage 270A (180+50%) at the bulk rate are required.

Bulk Time

Time for Bulk = Required amps ÷ alternator output (270÷225) = 1.2 hours

Total Time

Total time to charge the batteries (Bulk + Absorption) = 5.4 hours.

Interestingly the Smartgauge reports the batteries have been fully charged in approximately 4 hours. So either my calculations are incorrect or the Smartgauge isn’t accurately reporting the correct state of charge. If the latter is correct it’s of concern because failure to fully recharge the batteries will lead to sulphation and reduced total capacity.

I have calculated the Absorption charge rate by dividing the required amps by the required time (270÷4.2) to give 64 amps per hour maximum charge during the Absorption stage.

Next….. Charging source options

1 comment :

nicknorman said...

Hi Tom

Just to mention that batteries changing to absorption at 80% is a very much a rough rule of thumb - it depends a lot on the relationship between the battery bank capacity and the alternator (or other charging device) max output. For us with a big alternator and modest bank (440AH) absorption is reached within a few 10s of minutes, long before 80%. So whilst you calculation are useful for planning purposes, the best way to work out when the batteries are fully charged is to look at the charging current.

The SG is not that good on charge - basically it just times it - since it can only detect voltage and the voltage is determined more by the charging system than by the batteries.