Battery Capacity/Total draw?

[Mings74]

Hi all,
Firstly I know nothing of Batterys or electrics. I have two linked batterys on my UFO 31. They are charged by the alternator on my Perkins 30hp.
The batterys say on them they are type 643 675 amps. They also say "cold crank 480 amps" and "reserve 180 mins at 25 amps".
What is the total power I have available to me?, do you just add the 675 amps? (unlikely to be that straight forward!).
I have looked at the draw on my electrics on board and they are as follows:
GPS 2w
Autopilot 0.6a
Wind log 50ma
Depth/distance/speed log 15ma
VHF 0.3a
If I add these together is that my total draw on the batterys?.
How long would they last without charging (roughly)
How long would I have to run the engine to replace the used amount?.
I appreciate this last question depends on my alternator, I have not checked it but was told the engine had been altered to have a Ford Escort one fitted for cheapness and availability should it expire!.
One last thing how many milliamps in a watt and how many watts in an amp?. (probably not that easy again!). Any answers/suggestions greatly appreciated. Cheers DB

 

[captainslarty]

suggest read up on 12v electrics, mucho available on the net... or get a copy of calders boatowners electrical and mechanical manual.

 

[KenMcCulloch]

Your batteries sound - from the labeling - as if they are heavy duty starter batteries rather than proper deep cycle batteries. I can recommend 'The 12 volt bible' available from any good nautical bookseller, as a source to help you sort out watt an amp is and how to keep your volts safe.
Cheers

 

[GeorgeTina]

Hi (basic info only as this topic requires a book!!)
The capacity of your batteries is 75 amp/hrs (180mins * 25amps) so if you have connected the 2 batteries together you have 150 amp/hrs. It is bad practice to discharge a lead acid battery below 50% of its total charged capacity, so in reality you have a usable 75 amp/hrs. (This assumes batteries are fully charged, which takes a long time with an alternator as the rate of charge decreases as the batteries approach full charge). Connecting the batteries together is not good practice as if you discharge the batteries too far you may be unable to start your engine. You need a way to connect the batteries for charging and then isolate them for use. One for starting and one for "domestics" i.e. instruments etc. This is usually achieved with a 1/2/Both switch or a diode splitter (if you don't understand you need some local help to sort it out).
Your current draw is (approx) 0.15+0.6+0.05+0.015+0.3= just over 1 amp per hour. In theory (if you isolate the batteries) you would have 50% of 75 amp/hrs =37.5 hours at 1 amp draw.
Hope this gives you a starting point but this is a complex subject and (as suggested) a good book would help.

 

[VicS]

Battery capacity is quoted in ampere hours (amp hours for short) abbreviation Ah.

Basically it is the current in amps multiplied by the time in hours. So a 100Ah battery can supply 1 amp for 100 hours, 10 amps for 10 hours, 50 amps for two hours etc. Except that it is not quite that simple because the useful capacity depends on the current being drawn. The quoted figure is usually at the "20 hour" rate which means you'd get 5 amps for 20 hours but the effective capacity reduces as the current rises. Furthermore you should not really run a battery totally flat or anything like it in the case of a starter battery.

It is tempting to interpret your battery markings as 675 Ah capacity but they would be huge. Think, a smallish car typically has a battery around 60 Ah. Are yours 10 times as big and 10 times as heavy or thereabouts. Typically a 60 or 70 Ah battery will measure about 10"x7"x7" and have a cold cranking rating of 400 to 500 amps

Your batteries may be dual purpose leisure batteries. The fact that cold cranking amps is given (which is the current they can supply to a starter motor) implies that they are not pure deep discharge traction batteries. Some leisure batteries have A CCA figure on them some do not.

My guess is that you have a couple of batteries around 60 or 70 Ah (That would be about right for an Escort) which are engine starting batteries.

To compare your requirements (which are pretty small by some standards) muliply the the current draw of all uses (Don't forget cabin lights and Nav lights) in amps by the time in hours each will be in use. Add the results together to get a total and compare that with your available battery capacity in Ah BUT take that as only about half of the quoted capacity if you have leisure or semi-traction batteries and somewhat less if they are pure starter batteries.

(If necessary calculate the current from watts divided by volts ie a 6 watt bulb draws 6/12 = 0.5amps if 12 volt. A milliamp (mA) is 1/1000 or 0.001 amps, so 50mA = 0.05A

It's a good suggestion to get a good book and "The 12volt Bible for Boats" is often recommended Its about £15 from mail order chandlers, maybe less from Amazon.

While waiting for your book you will find the notes from TB-Training at http://www.tb-training.co.uk/cover.html very useful at explaining the basics.

You do add the capacity, in Ah, of the two batteries together to get the total.

The length of time you have to run the engine to recharge them will depend on many factors chiefly the output of the alternator. It is temping to say that a 50 amp alternator will take about 1 hour to replace 50Ah worth of power but in practice it will take appreciably longer unless you have a fancy charging regulator rather than the standard automotive type.

Hope that helps rather than confuses.

Edit:
George63 is right about them being 75Ah I'd guess.
Regarding your questions about units they should all be answered by Tb training.

But relationship between volts amps and ohms is volts = amps x ohms
(or amps = volts/ohms, or ohms = volts/amps)

Watts = volts x amps,
but watts = amps² x ohms and watts = volts²/ ohms are usefull to remeber at times.

 

[pvb]

A type 643 battery is typically 90-95Ah, often specified for truck use.

 

[Mings74]

Excellent info guys thank you all, helps to start getting my head around some of the issues. George I forgot to mention that I have a 1/2/both switch and use both to start the engine, then switch to 1 or 2 so that I always have one good for starting. PVB I think you are right I recall they may have been advertised as truck starting batterys. Thanks all Ming.

 

[VicS]

[ QUOTE ]
often specified for truck use.

[/ QUOTE ] You could be right but I thought the quoted CCA would be a bit low for a truck. A little truck perhaps!

Mings74 can hunt around the internet and discover tha actual rating as well as we can!


Mings74,

I am surprised that you need both for starting, but if you do you do. I dont know what battery is recommended for your engine. I'd like to be able to start on one, increasing battery size to do it if necessary, It might pay you to check and clean all the connections from batteries via switch to starter and the earth from engine back to batteries.

Be absolutely sure that you always switch batteries by going via the "both" position if the engine is running. If you accidentally switch them both off there is a real danger that a voltage surge will blow the diodes in the alternator.

 

[pvb]

[ QUOTE ]
You could be right but I thought the quoted CCA would be a bit low for a truck. A little truck perhaps!

[/ QUOTE ]Remember that most trucks - even smallish ones - operate on 24v, so the cranking current is reduced compared with 12v systems.