what is the difference, relay, isolator

I was looking on ebay for isolators to add a 2nd batt to my car and they had relay/isolators.what is the difference?and how would you wire it?

http://www.bcae1.com/battiso.htm

// POST EDIT about 28.5 hours after the above timestamp - added an all-important missing NOT to paragraph 8 below. Hopefully also improved some other expression; trivially. end POST EDIT //    I was going to say $50-$100 and no IQ points. Not that I consider that BCAE reference good on that issue - it really does miss a lot.... All systems are the same in that they require fuses at EACH battery end unless the batteries are collocated/adjacent etc. Some say just parallel the batteries, maybe even any batteries. In the interests of safety and cost, I am not so bold. If you think you can hardwire them, then do it. However, I suggest not unless being loaded or charged most of the time. But no matter how you look at it, you at-least double the best battery's self-fail rate, and when one fails, they both fail. But, if you do hardwire them, then IMO match everything (battery batch and history, temperature etc) and ensure symmetrical currents (ie, diagonal and equal length connections for 2 batteries) and include regular monitoring (voltage, temperature etc). Forget diode isolators - ridiculous except for low power systems (like Amps, not tens of Amps!). Plus they require voltage correction. If manual control is ok, then a manually switched relay, or from IGN - else ACC if the interconnection does not gracefully tolerate contributing to the cranking current. If you don't have a charge light, then get a voltage sensing isolator - ie a voltage sensor with relay. They are often called "smart (battery) isolators" but that is an oft misunderstood name. The "Smart" refers to their marketing for non-marine and non-stator charging applications - it does not refer to their operation being "smart", though they are the smartest readily available solution when the charging system is NOT the typical rotor-controlled alternator with charge-light as used in most typical vehicles. Another option is current sensing, but that seems rarely used. (Why? It is cheap and does not involve the complex voltage thresholds or timings required by voltage sensing systems.) If you do have a charge light, then you are a contender for the simplest and most reliable system which has been recently nicknamed "The Ultimate Intelligence (battery) Isolator" or UIBI. Alas nothing bling here - just a relay big enough for the required current that is energised by the vehicle's charge-lamp. It is simply a larger version of the electric fuel-pump control used in some carburetored cars!     A very big relay might need an intermediate smaller relay or transistor or FET etc to buffer the charge-lamp circuit, though other than recent EMS-interactive alternators, most alternators - or rather, their voltage regulators - have little problem with the typical 250mA of smaller relays, and can usually supply 500mA to 1A. (Older mechanical regulators could supply several Amps!) A later development is to add or use a MOSFET. For a few dollars, MOSFETs capable of switching 100A or more can be used without risking to any alternator or dash/EMS. Only a few nA or uA are needed to connect the 100A current, though often such $3 MOSFETs are simply used to drive ANY relay from any system. To summarise the UIBI operation, if the ignition is on and the charge lamp is off, the batteries will be interconnected. [ Actually the lamp itself is irrelevant - it is the alternator's voltage regulator's charge-lamp circuit +12V that turns on the relay/isolator. If not charging, that circuit will not be +12V unless you disconnect the alternator's D+ or S&L etc "small wires" to allow the charge-lamp (and sometimes other lamps) to "pull up" to +12V and hence energise the UIBI relay. ] Searching may find limited info on the UIBI or "charge lamp" battery isolation system. Should a search yield nothing, IMO there is little point me repeating it here. If successful, you are likely to find my descriptions, or perhaps where I describe the problems with other systems, or even expose them. (EG Pro**cta and others' "priority charging isolators" LOL - what crap! Deliberate misleading advertising perhaps... who knows?)