Tuesday, 27 May 2014

Charging our batteries

Following the blog post about the solar panels I though I might mention our domestic battery bank charging regime.  We have four large 450Ah 6V wet cell traction batteries connected in series/parallel to give a theoretical capacity of 900Ah.  Heavy duty traction batteries can be discharged to 30% but we’ve always used 50% as the lowest level we would discharge the batteries.  Having stated that, they are normally only discharged to 75% before being recharged.
To monitor the battery State of Charge (SOC) we have a Smartgauge which provides a percentage reading with an accuracy of +/- 10%.  Additionally we have a Victron BMV600 battery monitor which I use to measure the amps flowing in and out of the batteries.  Finally, the Tracer solar controller also provides the SOC as a percentage.
What interests me is each of these devices provides a different SOC when read simultaneously.  Normally the Victron and the Tracer show a higher reading (ie, a higher state of charge) than the Smartgauge.  I suspect the Smartgauge provides the most accurate reading as the Tracer and Victron will be showing 100% SOC yet the Victron will be also be showing amps flowing into the batteries.
There are four stages of charging the batteries (Bulk, Absorption, Float, Equalization).  I’ll ignore the latter because it’s a special stage. 
The engine has two alternators providing a combined theoretical output of 225A. 
When recharging the batteries they immediately go into the Bulk stage where they will accept a very high number of amps.  I’ve never seen our battery bank accept the maximum 225A produced by the engine alternators.  The most I have seen is around 168A.  There are two possible reasons for this.  First, the alternators are unable to achieve the theoretical maximum output. Second, the battery bank is already at 75% and is simply unable to accept a higher state of charge.  I’ve noticed it doesn’t take long for the amps flowing into batteries to drop down to 60-70 amps and my assumption is it’s the inability of the batteries to accept a higher state of charge which causes the amperes reading to fall.
The batteries change to the Absorption stage around 80% SOC.  At this point the amps reading drops to around 25-30A.  At 92% SOC on the Smartgauge the amps going to the battery are around 5-7A and we will be at the Float stage.  The Absorption and Float stages take considerably longer than the initial Bulk stage.
Unless we are cruising there seems little point in running the diesel engine for a long time just to recharge the batteries during the Absorption and Float stages. It’s not the cost of diesel that concerns me, but rather the wear on the engine.  We have a petrol generator which costs considerably less than the engine and what I tend to do is run the engine for the Bulk stage, then use the generator for the Absorption.  If it’s a sunny day the solar panels will complete the Float stage and then maintain the SOC until dusk. The process is then repeated the following day.
There are usually only two situations when we will continue to run the engine after the Bulk stage is complete.  It will either be because we are cruising or it’s when we want to use either the washing machine, vacuum cleaner or microwave. 
Our usual summer cruising routine is to move Monday to Friday and moor during the weekend.  Cruising for 3-4 hours daily is sufficient time to fully recharge the batteries.  Therefore we only tend to use the generator on the weekends or when not cruising.
The generator will provide enough electricity for our larger appliances but it “grunts” with the heavy load and as it is our alternative source of electricity I don’t want to use it in a way that might unnecessarily damage it.
Daniel heads off tomorrow and to his horror he discovered his laptop external USB storage drive had decided to fail.  After some frenetic activity he decided it would be better to purchase a replacement 2TB drive.  Whilst doing that he made the decision to purchase a 250GB SSD and a USB 3.0 enclosure.  I needed a walk and accompanied him to PCWorld and Maplin to watch him spend all that hard earned money.  I do love watching other people spend money.  Think of all that VAT going to the swindlers at Westminster!  It means they are less likely to look for ways of extracting it from me.  Unfortunately that theory was shot to hell when I also decided to buy an enclosure. 
At least my purchase was a mere (say it quickly) 20 quid on special at Maplin.  I’m recycling the 500GB hard drive out of the old laptop and it’s now an external storage drive.
Jan thinks I’ve previously written about charging the batteries but she doesn’t realize there have been some subtle changes! Smile


Paul and Elaine said...

I thought an enclosure was where you kept sheep!

Halfie said...

Tom, if you have four 450Ah batteries then I believe that makes a total of 1800Ah, irrespective of how you connect them.

Tom and Jan said...

You're confused Paul. Only Kiwi's think that!

Tom and Jan said...

You are going to have to explain your maths on that point. I always understood when you linked in series the voltage was combined and when you linked in parallel the amperes are combined. So linking two 6V batteries in series gives 12V and then linking two sets of 450A batteries in parallel at 12V gives 900Ah.

Halfie said...

Tom, as I understand it the ampere-hour rating of a battery is a measure of its capacity in terms of how much charge it can hold. Like a bucket of water. If you have four buckets you can choose to use them quickly by pouring them out two at a time, or more slowly, pouring them out one at a time. Either way, you still have four buckets-worth of water. It's the same with batteries.

Yes, if you connect two similar batteries in series you get twice the voltage, and connecting them in parallel gives twice the "lasting power". But, if you need, say, 3V, think how long two AAA cells in series will last compared with two AA cells.

Can you see what I'm saying?

Tom and Jan said...

My understanding is when you put two 6V batteries in series you double the voltage but the current stayes the same. When you connect them in series the voltage stays at 6V but the current doubles.
We have four 6v batteries that are connected in two pairs so both the voltage and the current doubles.