idea, isolating batteries when car not running

Hi folks. Jed here from Lancashire England.
For many years my parents have weekended in a sited caravan in the countryside. It's near a farm for water etc but does not have mains electric. They manage vey well with leisure batteries for the 12v led tv and lighting.   Dad is now 82 and taking the batteries home to charge is getting too much for him. A generator spoils the tranquility and a wind generator is subject to theft. Solar isn't really an option.   Here's my idea.   He has a volvo s60 which has the battery in the boot. I thought of mounting a leisure/marine dual cycle battery securely in the boot in a box. I want to hook it up so it's charged in tandem with the main car battery whilst the engine is running. I want it to become isolated from the main battery when not running. I intend to rig up a tow socket both on the car and the caravan and make up a lead so dad can park near to the caravan and hook up. The alternator will charge the battery as he drives and ther will be no need to lift it off and charge it up on the bench.   I know frequent boosting isn't good for batteries but marine dual cycle ones seem fine as I use them on my boat and it'd has an alternator.     My issue is that I don't want the battery connected to the car battery when the engine is off as it the draw from the caravan may flatten the starter battery. I want to isolate it some how with an automatic relay or something. Any thoughts folks. Any other ideas.   Many thanks. Jed

I presume you mean deep cycle? (I'm not aware of dual cycle.)

Google "uibi oldspark" to find threads here & elsewhere about a commonly used charge-light controlled isolation relay. That's very popular with alternators until ~1990 or 2000. [ Care is needed with the alternator (regulator's) charge-Light circuit's ability to supply the relay coil current in newer alternators. ]


If your vehicle is very new, it may have a DP or ECU controlled alternator.
Tho by adding some circuitry (eg, resistors, capacitor, MOSFET) they too can use a mere relay, most use voltage sensing isolators - aka "smart isolators" - tho these days they have to be a lot smarter than merely sensing voltage levels (thanks to ECU control).

In your case, failing the UIBI type of isolator, I'd consider an IGN +12V relay - especially if you have an IGN-2 type circuit that is OFF during cranking.
If not isolated during cranking, the battery interlink (isolator) needs to be capable of carrying the shared cranking current.

I would think that a 60A relay with 50A self-resetting circuit breakers would be fine.
If your deep cycle battery has a lower maximum charge current limit than the 50A breaker, one could be reduced in size.
[ Note that battery interlinks have fusing at EACH end except when they are adjacent to each other or have physical security. ]

You should find more info and explanation in my UIBI & isolator writings elsewhere.

ACC will do if no IGN 2 after checking that ACC dumps during start cycle.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I was thinking that, but I'm so used to those with dual batteries for audio & they keep ACC on for audio...
Or they forget to turn ACC off.

Good point but there's loads of stuff plus I think fuseboxes up the Volvo arse.

-------------
Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

You mean those Swedes still haven't gotten their electrics together? Ha - they can shoot down the SR71 with a 45 year old plane, but...
Of course I think they are now Ford, or DAB, or whatever. (I know SAAB is/was GM because it shares the modern variant of my Isuzu floorpan.) And I guess whether Ford is UK or US, electrical issues are still alive & well (or a-dead & still sick).

Fraud Mondeo/Fusion until 2 years ago! now Chinese.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

thanks for your help. will give it a go and see if we can get it too work. cheers jed

Sorry Jed, we got carried away.


I think an ACC energised relay will be fine since it's not a permanent set up (it's only for occasional battery charging) and HOPEFULLY it's unlikely that ACC or IGN wil be left on on without the engine running.

And probably a 30A relay with (say) 30A breaker (~$4 + $8 here) tho I'd suggest 60A relay & 50A breaker ($10 +$8). (That's assuming the 2nd battery is placed close to the main battery. If it's a long run (more than 0.5m etc), then add a 2nd breaker - one at each battery with the relay in between.)     


Then hope that if they are still using a Bosch alternator that they are stronger than the crap they produced until the 1990s. (Bosch alternators would often not handle jump starts nor bad batteries.)