Printer Friendly Forum Posts - adding another battery, isolator needed?

IMO - DEFINITELY an isolator. Especially since you are adding new to old, though I recommend an isolator anyhow.

In your case, maybe a simple manual "battery isolator" switch will do. They are typically rated for 200A-400A and are common in competition vehicles. They usually cost ~$20. Put that between the battery +ve interconnection (though the ground could be used instead).
But you will need to remember to manually disconnect when not in use. The longer batteries are paralleled when not being used, the greater the chance of both batteries failing.

Similarly, a relay that is manually switched - probably from the ignition. (That may be your best solution?)

If you have a charge light, I recommend the UIBI (search the12volt else google "UIBI isolator").
(But...) The UIBI is a DIY isolator that is merely a relay sized to your requirement which is turned on when the charge-light goes off - ie, the vehicle is charging. It can be easily modified to connect when cranking if that is the reason for your extra battery (merely add 2 diodes (20c IN4004 etc) and connect to the starter solenoid or cranking switch.
And it assumes the alternator's (regulator's) charge-light circuit is able to energise the relay or a smaller intermediate/buffer relay (typically 60ma - 250mA @ 12V).

If you have an electric fuel pump, chances are it is controlled the same as the "whilst cranking and whilst charging" modified UIBI. The you could energise your isolating relay's coil from the fuel-pump relay's output. (Fuel-pump relays are the same as UIBIs - except for those that also work on oil pressure - not that that is an issue for battery isolation, but it is farken stupid for fuel-pump control!!!!)

There are also voltage-controlled isolators, but get one with a cranking or manual trigger if you want paralleled batteries for cranking, and if you don't mind paying the extra money (and IMO, putting up with unnecessary voltage-controlled isolator issues). But that wold be the common option if you did not have some charging indicator (ie, a charge or not-charging light).

If you decide to hard-wire the 2 batteries, then they should be matched - ie, the same type, size, age, history, temperature etc with "symmetrical" or balanced connections (ie, same +ve to +ve gauge and length as the -ve to -ve, with the vehicle's +12V coming from one battery and ground (-ve) from the other).