Friday, 15 March 2013

Heart in mouth moment

Jan had her head in the big white box this alpha male hunts in when his stomach roars.  She withdrew one hand and said “Is this important?”

A little out of focus but it’s a crimped female spade terminal.  I had an instantaneous “heart in mouth” moment thinking Jan had accidentally ripped the wiring out of our new fridge.  Almost immediately I realised that wasn’t possible……… but we might have a broken wire.  Once it was in my hand I could see the cable had been cleanly cut rather than snapped, and the crimped end was a different type to those we used during the construction of Waiouru.  As it’s not a type of crimp we’ve used and the fridge still works I’ve concluded it’s a discarded terminal left in the fridge during the manufacturing process. <phew> Sun

The other piece of good news is the replacement 12v relay is performing correctly which has enabled Jan to control the operation of the Hurricane heater using the Empirbus system.

Now for more boring stuff……..

Part 2 - Lead Acid Traction Batteries

A common rule is a lead acid battery should not be discharged below 50% of its capacity. Our domestic battery bank has a capacity of 900A/h so we have a maximum of 450A/h available to use before recharging the battery bank. We’ve never yet discharged the batteries to 50% although on some days they have gone as low as 60% (according to the Smartgauge).

There are three main stages of recharging a lead acid battery; Bulk, Absorption, and Float. Returning to the water analogy. Imagine the battery to be a bucket that is filled with sponge. The water added to the bucket is the available power. When you first start charging the battery (Bulk stage) you are pouring as much water as you have available from your alternators into the bucket. Initially the sponge readily absorbs the water. However after a while the sponge simply cannot absorb all the water being poured into the bucket. When this occurs the Bulk stage has been completed and the Absorption stage commences. At this time the bucket has probably reached 80% of its total water storage capacity. The last 20% of capacity is made up of tiny air pockets throughout the sponge. Given time these air pockets will gradually fill with water. However most of the water you are pouring into the bucket just overflows. The bucket and sponge only accept what it can absorb.

There is another part to the structure of the bucket. It has a small hole which allows water to slowly drip out. To ensure the bucket stays full after the Absorption stage is complete the battery enters the “Float” stage. During the Float stage the water that’s leaking from the small hole is replaced.

One potential problem with a lead acid battery is if the battery isn’t fully recharged and maintained on a regular basis sulphate crystals can start to form on the surface of the lead plates.  The crystals act as an insulator preventing the affected portions of the lead plates from accepting a charge.  Our available 450 A/h  would gradually reduce.  The batteries would start to get “tired” 

The other thing that will happen to varying degrees is some of the lead on the plates in the battery during the discharging and recharging process will gradually flake off and fall to the bottom of the battery case.  This will also reduce battery capacity.  Over time this lead can build up on the base of the battery case until it reaches the height of the bottom of the plates.  It then “shorts out”  the plates in the cell and the battery is effectively “dead”.

As we have opted for an expensive domestic battery bank we want it to have a long life.  Therefore identifying and implementing a process for reducing the effects of crystallization on the battery bank is something I’ve been considering.  But more on that later.  

Tomorrow I’ll explain the theoretical charging time for our battery bank.


Halfie said...

Tom, how does the Smartgauge know when a battery has discharged to 50% (or any other percentage)? Is it merely a voltmeter, in which case, what voltage corresponds to 50% (dis)charged? Or is it more sophisticated? Perhaps it senses the current drawn and relates that to voltage to come up with a reading.

I hope you know the answer!

Tom and Jan said...

I don't know the answer to your question. It has to be something based on voltage readings. However I recall reading somewhere it can be up to 10% inaccurate.
I like it as an easy way of getting an approximate idea of the battery state of charge but I'm getting to be a battery nerd and feel I should also have an amp/hr meter. Maybe a Victron BMV600!