Posted: Tue Jun 03, 2008 8:52pm
Post Subject:
Batteries
Dear Mark
I understand that you are contemplating spending around the £1000 mark on new batteries and do not appear to have given much thought to the systems you will be connecting them to. I will answer your questions as far as the info you give allows, but first I feel I must point out some things that immediately strike me.
First of all I can not square a 4 battery 12v domestic bank with a 4 battery inverter bank. The inverter bank suggests heavy mains loads which in turn normally indicates low 12v loads. When I say all my boat's domestics are running of a 3 battery domestic bank with no mains perhaps you can see that something may be out of balance here.
In general terms the larger the battery bank, the slower you discharge it and the slower you recharge it the more electricity it can store. This may be why you have two X 4 battery banks. Before you make any firm decision please got to http://www.smartgauge.co.uk/technical1.html and start reading through the list of technical topics. It will take time and probably make your head hurt, well it does mine, but then you will have van understanding about what you are doing. On the face of it I would say you would do far better to join the two large battery banks, use another split charge relay to join both alternators when the engine is running, charging the engine battery via the diode. In reality I would probably also change the diode for a split charge relay. Anyway it is all explained on that site.
I also have concerns about using a sealed battery on a narrow boat, especially one with an alternator controller unless I know a lot more about it. I feel you may well be spending money to get the sealed batteries destroyed. I know wonderful things are claimed by the makers of sealed batteries but unless you have a very good, well controlled almost permanent charging system you are likely to destroy them through never fully charging them, or if you do depleting the electrolyte. You will also only be able to test them with a voltmeter which is far from easy with such large banks.
When any lead acid battery is charged the surfaces inside it rapidly take a charge one molecule thick. This raises the voltage so the alternator system thinks the batteries are getting fully charged, so the charging current drops. The charging current is proportional to the difference between the battery voltage and the alternator's regulated voltage. A discharged battery may have a voltage of about 12.2 volts. A typical alternator will have a regulated voltage of about 14.3 volts. This gives a difference of 2.2 volts to push current into the battery. A fully charged battery could have a voltage of 13.2 volts so there is only 1.1 volts pushing current into the battery, thus the current falls. Given a reasonably well matched alternator I suspect you will get the batteries about 70% charged in between 2 and 4 hours, the other 30% will take very many hours and may never be fully accounted for. This is what the alternator regulator attempts to address.
Once the alternator output voltage reaches the regulated voltage the controller interferes with the regulation and starts to raise the charging voltage so the charging current is also increased. If it did this for long it would destroy the batteries - especially some sealed batteries, so the regulator goes through a number of timed cycles. Exactly how and what parameters varies from model to model so the testing data also varies.
The controller also seeks to overcome the lower charging rates associated with split charge diodes by converting the alternator from machine sensing (measures charging voltage at its output terminal to battery sensing (measure the voltage at the battery terminal).
To test it clip a voltmeter across the battery bank (minimum accuracy of 2%) and run the engine. watch the voltage. It will be low at first and gradually climb to about 14 to 14.4 volts according to the alternator make and model. Keep an eye on it and if the controller is working it should suddenly jump up to anything between 14.5 & 15 volts. After a period it should drop back to the regulated voltage. To get the exact method of testing you need to contact Sterling for the data for your particular controller.
Can you simply swap battery types? Without the regulator I would say probably yes, but with it the answer has to be "maybe" depending upon battery type and the possibility of altering the controller to suit the battery type.
If you are talking about fitting wet, maintenance free sealed batteries then you can just fit them, but if the controller voltage is too high it might deplete the electrolyte.
If you are talking AGM batteries the answer is "it all depends upon the exact design of the battery". To make matters worse many retailers do not have a clue about this when questioned.
If you are thinking about gel batteries you will probably ruin them with either the latest high voltage alternators or the controller.
I am going to duck out of answering the discount question, but I would think and internet search plus a few phone calls would give the answer.
I think you need to look into the whole system before spending any money - especially on expensive batteries. If you forced me to give an opinion based on your post I would say do not buy fancy batteries unless you also have a shoreline and a good marine four stage battery charger.
Tony Brooks
