charge 2 batteries with isolator?

15.7AH at what rate? (C1, C10, C100?) Or - how are you measuring AH? Do you mean Amps? (IE - 200W out should be about 20A, but it actually measured 15.7A???) 350A for 15 seconds is ~1.5AH, but assigning AH to a CCA test is meaningless - ESPECIALLY for a small battery. Instead, charge the battery and test it at its rated load - eg, 785mA for 20 hours - and confirm its end voltage. Or test it with the intended load until it hits its desired end voltage (20% or 50% etc) and decide if that is long enough.... But a bigger battery will last longer (lifecycle wise) if you use it for the same load for the same time - ie, the lesser depth you discharge a battery, the longer it will last. [ The less current you discharge and charge with, and faster you recharge it (after discharging), the longer the battery will last. And the less often you recharge it the longer it will last - ie, constant float is best - but you "must" recharge ASAP after discharge. These lifecycle considerations exclude specific reconditioning maintenance - eg, equalisation or >20A desulfation etc.

Well sorry for the delayed response, but I have decided rather than risk using an old battery, I just went ahead and purchased a nEW yellow top Optima to use in my boat for the stereo system.. The Blue ( Marine ) and Yellow, are essentially the same with only the battery post a little different for marine apps.  Now alot of people on stereo forums are telling me the regular Deep Cycle ACID batteries work better for long periods of listening to a stereo without any charge, or work better than the Optimas multi-use crank/deep cycle.. I already have the optima, but I have a fairly new Deep Cycle in my garage also.  So now my questions moves to should I use the Optima Crank/Deep Cycle Yellow, or a Plane ACID Deep Cycle for an hour-two hours of stereo use with no charge.. Remember this will be paralled via a smart voltage reg circuit (combiner-isolator )to an Optima Blue Crank/Deep Cycle.  Thanks for your help!

I thought wet batteries were not recommended for marine use (salt, chlorine, etc). When not isolated, it won't matter (ie, when charging) assuming the charge voltage is appropriate. Strange what those audio forums suggested - they seem to do the opposite of what I normally consider to be good advice - eg, suggest AGMs when there is a high current demand... If you are not going to recharge a fairly flat battery for a while, then a wet is probably the best choice. (It can be de-sulfated etc.) That is probably why they recommended the wet over an AGM. I rarely meet people that recommend Optima.

thanks for all the kind help! My next question is I have some steel cable used in aircraft( helicopter ) for the battery connections, being that I'm an aircraft mech. Would this work alright as a Ground cable in parallel circuit, or would copper be a much better option?? I always thought aircraft had the most optimal equip available, but I read that copper was alot better contuctor.

do not use steel cable. googling electrical conductivity shows copper being a rather close second to silver with steel far down on the list. mark

Steel? I would have thought aluminium for lightness, though that's even worse than copper for fatigue and creep.    Copper is the best conductor - beaten only by silver, but better than gold, ~30%(??) better than aly, and heaps better than steel. Steel is probably used in choppers - and helicopters - 'cos of vibration....

I bet you are right..I have never thought about that..I ws really curious why steel was being used in Multi Million dollar aircraft......I imagine that would be the reason, because of the vibration theat it is subjected to

my question now is would it hurt to have my reserve\house battery remain the one that i have now since all my stereo and lights are wired to it and on that side of my boat. If I do that I will place my starting battery on the other side of my boat with a long 4 ga. Wire running back to the main off\on switch which goes to the starter...basically backtracking with cable to where my original starter battery was located..I am using a smart combiner between the two batteries also ..thanks

Generally in vehicles, people (should) keep their normal (wet) battery for starting etc, and additional AGMs are used for the added loads (audio etc). This is for a combination of reasons including that AGMs do not like supplying high currents (cranking), and why have a low-resistance AGM far from the load (amplifiers)? Since an isolator is being used, battery symmetry does not matter - ie, matched batteries; symmetrical interconnections and discharge currents. (They both charge at whatever rate they can, and each discharges its own load.) So which battery where is one of life or performance...

yeah question is somewhat ambiguous..i guess to ask a simple version of the above question would be if it would matter if the starting battery were close ( 1\2 meter), or further ( 1 +1\2 meter) from main switch that supplies starter..this is using a 4 ga heavy cable..It woyld make sense to have closer but i would have to rewire my boat to place stereo and utility popwer wires to the otherside which would be a major project. Thank you for the wealth of knowledge