Batteries

[MedDreamer]

If I have a two battery set up what rating should each battery be?

I have picked up from other threads that one battery should be solely for starting the engine (petrol) and the other should be a "deep cycle" type to power the domestics.

Can anybody help by being more precise about suitable batteries (the one I currently have is rated 110 amp hours, which means very little to me - is this a deep cycle battery?

Thanks for any help offered.



<hr width=100% size=1>Martyn

 

[andyball]

<A target="_blank" HREF=http://www.uuhome.de/william.darden/>here</A> is a good place to start with battery faq's.

110 Amp Hours is the notional capacity of the battery in amp-hours usually based on a 20 hour period (notionally your battery can produce 5.5 Amps for 20 Hours then it's empty. Reduce the hours/increase the current & the capacity drops ; quite markedly with higher currents like heaters,bow-thrusters, inverters etc.....so if you draw say 15 Amps, you won't get 110AH over a shorter time, but rather less, say 80AH (guessing here).

Good practice is to never fully dscharge a lead-acid battery, so many systems work on using no more than 1/2 the rated capacity of each battery....the deeper the discharge, the fewer discharge/re-charge cycles before the battery loses significant capacity.

"starting" batteries are constructed to provide very high currents w/o damage for short periods & are not meant for deep discharge, they're often much smaller in size & capacity for the same starting ability ....."domestics" are meant to cope with lower current for longer periods & shrug off being run to 1/2 capacity or less. (use of inverters drawing very high currents falls somewhere inbetween).

In practice, a "domestic" battery of sufficient size can easily start many engines (esp petrol) 'cos lower compression: a large car starting battery would be say 60AH, so a domestic style 110AH would probably cope ok, but it depends on the size/compression of your engine & how cold it is.

A 90AH "leisure" battery very easily starts my 2.0l four-stroke o/b.

As to what makes are suitable...all depends on money really. you can pay from £35 to £200 or more for a 90AH leisure type, likewise for starting ones......if space is short, a smaller "starter" one + larger domestics is good, imo. I've also always been quite impressed with cheapy leisure batteries from caravan shops etc, finding they last ok even with the occasional (or sometimes regular) complete discharge & they're v.v.cheap to buy.....but often need a battery box since they can leak a little if the boat leans over/shakes around a bit (as boats do).



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