charge 2 batteries with isolator?

I have a question about a dual battery set-up that I could use that would isolate a second ( stereo ) battery, from the

start battery while engine is off, such as an isolator would do. Although tThe question I have, is how could I charge both batteries from

a home charger, if the isolator seperates the batteries when engine is off? Thanks for your ideas

Why? Otherwise, the same as usual. Isolate BOTH batteries from their loads and connect them together. Are you using a $5 UIBI (Ultimate Intelligence Battery Isolator), or a more expensive "smart isolator" etc? The latter should connect the 2 batteries automatically when charging. (The UIBI requires an spdt/changeover switch unless already configured with optional controls.) Let me know if this is another where a "bigger battery" was suggested over a bigger alternator. If you are externally charging for other reasons, that's cool. (EG, total loss; faulty alternator; desulfation; etc.)

oh this is for a marine ( boat setup ).. It is usually a good idea to get on the water will fully charged batteries.  I would like to use an isolator that would charge the stereo battery when engine is cranked, and also be able to charge the primary, and stereo ( second ) battery via a home charger. thanks

But don't put the (salt) water on the batteries! (Ka-Bang! Or in marine-speak - Boat-Bang!) (Ha ha - as opposed to your "It is usually a good idea to get on the water will fully charged batteries".) Being marine I assume a stator system or permanent magnet rotor etc - not the normal automotive current controlled rotor which has a charge lamp. Hence you probably have a "Smart" battery isolator - aka voltage sensing - ie a relay controlled by a voltage sensing circuit. (A UIBI is a relay controlled by the charge light (circuit).) Hence if you connect your charger to the main or first battery (that the marine alternator charges), the isolator should automatically connect the aux/2nd battery. [ IE - when the marine engine charges, the main battery voltage goes above (say) 12.8V to 13.6-14.4V etc, hence closing the isolator.    The same thing when "home" charging the main battery - its voltage should exceed the isolator's turn-on or connect voltage, hence also charging the other battery. Unless you have a bi-directional sensor, charging the 2nd or "dwwnstream" battery will NOT close the isolator. ] The main reason for disconnecting loads when mains (AC, home) charging is to protect loads in case of high-voltage spikes etc. (I sometimes don't bother, but I have older vehicles (no EFI/EMS etc), know my charger, and rarely get lighting strikes etc.) FYI: If your main battery is fairly discharged or you have a small home charger, it may be that the voltage takes a while to rise above the isolator's connect voltage.   That's not a problem.... (though max total charging would probably occur with both batteries connected)... But when the 2nd battery connects, the voltage could drop thereby turning the isolator off. Then back on, then off, & on & off &.... But that's why "smart" isolators should have delays and hysteresis (eg: on at 13.5V, off at 12.8V). (Ahh - but what switching voltages, and how long the delays? LOL - the problems with "smart isolators". The UIBI has no such issues.) tresvatos - I originally asked "Why?" thinking "another dumb application and implementation". Thanks for proving yet again that most posters and questions hereon are not like those I find elsewhere. It is so nice merely confirming or explaining that yous already have the right idea or implementation etc. (No arguments etc - the only time-wasting is my "let's check every detail/assumption" rambling.) Thanks for pulling my forum-day back & well into the positive!

Thank you.. I believe that post above answered my question.  My next order of business is to find a smart isolator that will connect the parallel circuit when the primary start battery is charged sufficiently.  I am using Optima Brand Marine batteries as well in this set-up.

tresvatos wrote: .... a smart isolator that will connect the parallel circuit when the primary start battery is charged sufficiently.

No such thing! Well, not without current sensing at the main battery or VERY sophisticated AC diagnostics! They simply connect when the voltage exceeds their "connect" threshold, and that should happen within seconds of charging irrespective of how flat the main battery is. (Unless the alternator cannot supply the total load.) That is generally the best behaviour anyway because that is the MAXIMUM total charge replacement in the quickest time (and with least battery wear). In fact I don't know why they bother with advertising their alleged "prioritisation" when that is worse for the system - unless perhaps you consistently crank and only run for a few minutes... Many claim such battery prioritisation - eg - see the video for Projecta's 12 Volt Electronic Dual Battery System for their DBC100K etc. If that has any truth to it, please explain how the "two wire" voltage sensor determines the main battery's state of charge! Alas the extra extra cow dung can have a huge inflationary effect on isolator prices! I'd consider using the MW728 (~$20) though I might increase its dropout voltage from 11.2V (with series diode(s) and a small relay if the "on" voltage is not to increase by the same amount). The MW728 drives whatever "plain" relay you want to use (it can supply 10A). (aka a UIBI using an MW728 instead of a charge light!) The main snag is whether the timing/delays of the MW728 are suitable. But otherwise get a reputable isolator - forget their hype, read what others say. And probably just buy on lowest price with adequate current capacity. (Noting that the current can be increase by simply adding a bigger relay - is, the isolator or MW728 energised the relay instead of connecting the batteries.) Hopefully dispelling "priority charging" - and the need for it - will save you a $hundred or two! And I just realised - that Projecta bullsh is an Australian site. Another $10,000 fine coming up? Does anyone want to sue? LOL! I like suckers that think we are suckers!

I understand exactly what you mean with the voltage sensor cutting on -off-on-off..  I am wondering now if the circcuit may be easier and simple if i were to just put a switch to trigger a relay to connect two 12 volt batterie in parallel.. I could just flip a switch when I want to use one battery to power the stereo and accessories while the engine is not running..

That is the manual way... But should you forget.... But it sounds like you have tasted one of the drawbacks with the voltage sensing units. Not only are different voltage levels desirable (eg, disconnect at 12.8V if NOT charging but maybe 12.0V if charging), but also different delays depending on the reason (cranking, indicator or other "sags"). My MW728 was oscillating on-off at about 15-30 second intervals with a ~5A load despite the #2 ~60AH battery being fully pre-charged on an 8A charger. (Engine was NOT running/charging.) But that ceased after a short run - the alternator's 14.4V at whatever Amps (up to ~75A less ~20A max for the vehicle) presumably blowing away the cobwebs (ie, soft-sulfite) of the 2nd battery. Hence the advantage of the UIBI (charge-light controlled). But voltage-sensor delays can be extended to many tens of seconds. Before the UIBI, I had a "smart" isolator to connect the 2nd battery, but that was a $22 kit with an 80A "latching" relay. (Hence the circuit only drew 4uA except when toggling the relay. Or higher if using the flashing status LED.) Its voltage is easily changed as per kit instructions - eg for 12 or 24V operation, or as a "high voltage connect" or "low voltage disconnect". [The former being a "smart battery isolator", the latter a "battery protector" to disconnect the load before excessive battery flattening. (Never use 555 timers for that unles they disconnect themselves, and their 10mA draw is ok!)] I still use that kit as my low-voltage cutout (ie, isolate the load if battery #2 gets too flat), but the front "high-voltage cut-in" is now replaced with a $10 relay (~140A capacity) connected to the alternator charge light terminal. The point being...? Oh yes - the kit explains the circuit operation and the various delays. (I shied to it the same way ImAnIdiot shies to my l o n g explanations.) But it should be possible to vary the delays... (The kit is the Oatley Electronics AUD$22 K227 - 12-24V DUAL BATTERY ADAPTER FOR A VEHICLE. Consider a spare 80A latching relay for another AUD$6.50 RL8. NOTE that it is the PCB, components and relay only - not the case nor connectors.) Alas I've been meaning to re-examine that explanation... There is also the "cig socket" 10A MW-728 which is available for under $20 here (Australia). (If 10A isn't enough, use it to control a bigger relay. Snip off the cig plug & socket as desired.) It is based on an LM3900 or similar "quad comparator" or op-amp (distant memories...) and equivalent circuits are not difficult to build (provided you have the circuit). Not that I condone reverse engineering brilliant and successful commercial products (that's why I suggested the Oatley kit), but DIYers might be keen to modify an MW-728 to suit their voltage & delay requirements.... (FYI - for the price of an MW728, it is NOT worth "ripping off" to do it cheaper!)    Anyhow, a manual switch saves you about $250 for a Projecta 100A "smart priority charging" male-Bovine-dung isolator if bought here in Aus. Cheaper isolators here are usually $80 upwards. (That's why I used the $22 Oatley kits!!) [Dare I mention a "Redarc" isolator that claims better "battery charging" from its 80A isolator than a straight relay of 140A or higher rating? Yes - that confuses me too - it does NOT include a dc-dc converter - they cost $300 for 20A and $500 for 40A (from other suppliers - and all Aussies know you must have a dc-dc converter to properly charge your second battery. TRUE! - I've read it on Aussie web sites!! {Yes, seemingly unscrupulous commercial sites.}).] Selecting a "smart isolator" depends on what you want. BlueSea include a starter input (to parallel batteries during cranking). Several offer variable voltages for both connect & disconnect. (There must be some hysteresis - the MW729 f.ex is typically 11.2V & 12.5V.) BTW - increase "fixed" voltages with diodes - eg, add a 0.2V Germanium or 0.3V Shottky or 0.6V normal diode to boost the 11.2V MW728 "cutout" to 11.4, 11.5, 11.8, 11.2+0.6+0.6=) 12.4V etc. Use an output-controlled SPDT relay to short out some or all of those diodes for a 12.5 or 12.7, 12.8, 13.2V etc "cut in" voltage. The diodes are inserted in the ground from the sensor (MW728) and the supply (cig-plug to socket). So many possibilities - all cheap "building blocks" - albeit with some "simple" electronics (diodes & relays).

I have decided to use a battery combiner..this will disconnect the parallel circuit between the two batteries when voltage at the starting battery drops below 13.3 volts and lets the other battey that stereo amp cables are connected to reamain supplying power.  My question now is can a RED top Optima starting battery, and a BLUE top Deep cycle be wired in  parallel..?? I have heard not to parallel two unlike type batteries, and then I have heard it will not affect the batteries..so that is my question..thanks

Battery Combiner... Smart Isolator - same thing. Paralleling different batteries when charging does not matter - ie, blue, red, pink, wet, AGM, new, old, 35AH, 100AH etc. (As long as same voltage (12V) and lead-acid.) It would only be if that clashes with manufacturer requirements. EG - if the reckon their battery needs 14.6V charging as opposed to the normal max of 14.4V etc. If paralleling when not charging (and to a lesser extent - when being loaded) then matching batteries becomes important - but you are not in that situation. Just be wary of using non-marine batteries in marine applications - ie, salt environments.