2 batteries in a prius

Ok, so im taking my old set up(well sub, amp and second battery) from my scion and putting it in the prius. I have a 07 prius with the jbl system but no nav. Now i had components and amps in my scion but i have done tons of research and decided not to mess with the factory amp. so im still running the factory HU,amp, and speakers. now i have installed a jl cleansweep and hooked it up to my JL 1000/1v2 amp which powers my JL13w7Prowedge. I tested everything and it all works and sounds great. then i went to install my second battery and realized i dont have an alternator so i cant use the relay(atleast i dont see how i can) also wont be able to use and isolator so it leaves me with no option but to wire it parallel. now i have a kinetik 1400 as my second battery and the stock battery as my main. so here is my question: Can i use the kinetik battery with my stock battery ( my understanding is it has to be the same 2 batteries but i have also read that they dont have to be identical they just have to be the same amperage)? also is there a way to hook up the second battery with a relay or isolator without having an alternator? what are my options what would be best? i dont listen to the stereo with the car off that often and when i do its for about 10-20 mins max. my last question is: the guy at the car audio shop told me that i have to ground each battery and amp to their own ground(but i thought when running parallel you just do + to + and - to - then ground the amp, although i have seen some diagrams of the amp being grounded to the neg terminal on the second battery and obviously the main battery is already grounded). so what is the proper way to run the battery parallel? thanks for the help!!!!! cant wait to get this all figured out and hooked up.

Don't parallel them with the vehicles batteries. For maximum life, they should only be paralleled when being used; they should be identical (same age, batch, history, temperature); have equal discharge (& charge) rates - ie, +12V from one, GND from the other, with 2 equal cables connecting + to + and - to -; and hence a dual pole isolator (or 2 relays) between them.   

the thing is that the prius dont have an alternator, so i dont know how i would install a relay or isolator. i can get a another kinetik 1400 (thats what my second battery is right now) if needed. but they have to be same age, batch, etc. like u said, so then i guess i would have to buy 2 kinetik 1400 and sell me current one? the main 12v is in the trunk on the right side in the cutout i was going to install the second battery on the left side in the cutout or in the storage tray under the trunk floor. i would be running 0 gauge wires. you said dont parallel with car batteries, why what will happen? also what will happen if i hook up the kinetik and my stock battery? what would happen if i just bought one more kinetik 1400 and hooked it to my 1 year old kinetik 1400? Thanks

An isolator is merely a relay (switch), but how it is controlled varies. An alternator is not required. IN your case I would suggest the "on" button for your sound system actuates it. The relay should be rated for the max current draw of the (audio) system plus some overhead (eg, normally assume worst case one flat and one full battery, or just one battery - plus charging current). And fuses to match the relay and cable. Fuses near each battery unless they are adjacent. [PED: If batteries are housed adjacent, arguably only one or no fuses can be used. And to maintain inter-connection resistance/impedance, the same fuses in the -ve interlink, else longer cable to simulate the +ve leg's fuse resistance. If no fuses between the batteries is acceptable, then a single fuse from the +12V out terminal to the load (to protect the cable; or batteries) is all that is required - there is no balancing or matching required. ] Surely the Prius is not a 12V system - it's much higher...? (Or is there a specific 12V battery that mimics normal car batteries?) If not, what are the Prius batteries worth? Do you want to risk wrecking them (with NO warranty)? When you replace a Prius battery, is it one, or several/all (banks or by individual bank)? As to not paralleling batteries, I have explained what to do for maximum battery life. But you'll have to search for reasons why - I'd simply ask you "what will it NOT do?". But that is merely an opinion (backed by experience, research, investigations, documented cases etc).... The convincing arguments IMHO for NOT paralleling batteries (for long periods unless being charged or loaded) includes that - for 2 batteries - it DOUBLES the failure rate - noting that that failure rate refers to self-failures including manufacturing defects - ie, totally independent of HOW they are used). AND, if one battery fails, they ALL do (assuming copious hours or days). So if my common hearings and readings of a certain popular "O" AGM battery failing within 3 months is correct, if such a 3-monther were paralleled with a non-defective identical battery, that'd be TWO batteries failed after 3 months - ie, a failure period of 1.5 months, or 2 per 3 months - double that of the faulty 1 per 3 months failure battery. [ The latter gets complicated - it is Probabilistic Theory and maths etc. The above is a simplistic treatment to explain the concept. ] Some disagree with the above. Hence paralleling 2 batteries must INCREASE their reliability. I'll let others argue that one. (It cannot have zero effect unless NOT connected or they both/all have "infinite" reliability (ie, R = 1; f = 0; etc).) And there are some that reckon you can parallel ANY battery regardless of age, condition, size etc (provided eg lead-acid & 12V etc) - eg, see batteries, how many, what kind?....

anon wrote: Not only is it done all of the time, with mismatched batteries alot of the time....

Even haemphyst thought isolators were a waste - but that was over 5 years ago...

i had a relay in my scion and one of the slots is for the alternator..... i believe an isolator is the same way? so your saying i could just run a line from the "on" button to the alternator spot on the relay? the prius has a 12v battery and then it also has the battery pack which is for the electric motor and a few other things. so i have a normal car 12v battery in the trunk. no warranty would be voided by what im doing and if it is im not worried about it. i will check that link in a few minutes but that is why i started the thread for help because i have read a lot of both sides and both are so convincing so i dont know which is correct and which is safest and best for my setup. seems like everyone has a different opinion on what shoudl be done with installing batteries in cars and they defend what they believe so well. if your sure i can run that line from the "on" button to the relay then i will do that. but after doing that i will still need to get 2 new 12v batteries or can i use what i have? thanks for your help and explaining things.

You should be able to use any +12V source. The UIBI is merely a 12V relay. Instead of a manual switch, ACC or IGN +12V, or a voltage or current sensor, it uses the charge light circuit. Usually the alternator charge lamp terminal is GND, hence grounding the IGN +12V "hot" charge lamp and lighting it. When the alternator charges, the charge lamp terminal swings to +12V hence extinguishing the charge lamp, but also turning on a grounded relay - eg, UIBI, electric fuel pump relay, fuel-stops, running lights etc. One concern is the relay coil current and if the source can handle it - ie, can the charge-lamp circuit supply 250mA for a "normal" auto relay, or 1A for 20A relay etc. But that can easily be fixed. I prefer a $2-$3 MOSFET with 80A - 120A capability that only uses uA (micro-Amps) to turn on. That can be used without a relay, or power a relay or relays with 80A of coil current (eg, 400 typical 30A Bosch relays).    You can use as many batteries as you like. The selection depends on capacity, load, desired reserve time and recharge interval & duration. Keep in mind that if including whatever Pius battery, you are also discharging that battery. (What if that flattens? Can the engine bank recharge it? Would a low-voltage cutout work?) And keep in mind what I have said if paralleling "different" batteries. AFAIAConcerned, that is also what reputable experienced people say - including some honest or scrupulous battery sales people. Those that say different may have different requirements - eg, total loss systems where replacement cost is not a concern, or for wet-cells where (eg AGM) thermal runaway is not a concern. Others are generally those that profit from increased battery failures. Others seem merely opiniated, or ignorant. I gave some supportive references in a link above (and that was FAR from exhaustive - and I did not include older hardcopy or restricted reports), and I have yet to see any meaningful references provided by others - they usually just regurge the same crap - like the "100AH per 1,000W" rule, or more batteries rather than a bigger alternator (for street use), or higher pressure fuel pumps for boosted performance.   (Until I understand otherwise, I still find those extremely amusing (else I cry for all those ripped-off victims).) But I cannot comment on "the other side" if I don't know their arguments or bullsh. I too can say how we'll all migrate to Venus as the Sun ages and Venus cools. Many totally agree - it is simple logic. The fact that as the Sun cools, it EXPANDS and is expected to engulf Earth before it dies or novas will do nothing to kill that belief - except for those that know otherwise. (And many of those enjoy watching the others....)

honestly i read your post three times and im lost, i will try again tomoro as i have alot on my mind today. I talked to a old friend of mine that i worked with at this car audio shop a few years back. he told me to get another kinetik battery so i ordered a kinetik 1400 (to replace original 12v) and a kinetik 1800 (for the audio equip) im still waiting to for them to arive. I sold my old kinetik for $75 to help make up the costs of the new batteries. he told me to run parallel without relays and i should be fine. now i told him i had a prius and what not but he hasnt installed anything in a prius either. he had 5 batteries in a Subaru Tribeca powering idk how many amps which powered 7 jl 13.5w7's and his door speakers and w.e else(the suv was sick!!) and he said i dont need a relay with low esr batteries unless i was running something crazy like what he had. he also said he had no idea how to hook up a relay without the alternator as well. i will ask him to translate what u have told me as im completely not comprehending. is what he told me right: that i can just hook up these two new kinetik batteries parallel without a relay? my goal is to keep my voltage, not have my lights dim, get good life out of the batteries and not mess up the electrical system.to do it right and not have to worry that the car is guna go into flames. everyone i talk to tells me not to mess with the prius but its my car so i dont have a choice unless i didnt want my system but i do and i will get it in there one way or another. the battery pack that runs the motor etc is 240v. from what i understand there is a computer box that acts kinda as the alternator but the guy at the dealership said not to touch that box, that i would void the warranty and prolly kill myself because the box has 240v going in and out. while im nto to worried about the warranty i dont want to lose my life messing with the electrical in this car if i dont need to install a relay. let me know what you think.        thank you for all your help! :)

i found somemore stuff on the battery and charging system : the factory battery is 36ah There is a DC/DC inverter that charges the 12v via the HV battery Further clarification: The Prius HSD has two motor-generators, call MG1 and MG2. MG1 is smallish and used to spin the ICE and to transfer power from the ICE to MG2. MG2 is larger and is used as a prime mover and for regenerative braking. Both MG1 and MG2 are sometimes used as electric motors, and sometimes as generators. The electricity generated from MG1 and MG2 is either used to move the car or charge the HV (traction) battery. The small 12V battery, which is like the battery on a "normal" car, is charged from the HV battery by means of a DC to DC converter, which steps down the high voltage to that of the 12V battery. The DC to DC converter also supplies the 12V accessory needs when the Prius is in run mode (lights, radio, MFD, etc.). http://www.eahart.com/prius/psd/ It has no timing belt, alternator, or starter to wear out and need replacement.

Unless you know what the rating of the dc-dc converter is and that that is big enough for your load, then I don't understand why you want to connect to your existing system (battery) at all. If the dc-dc conv handles it, then fine, connect whatever batteries and off you go. They will stay charged as long as the main (240V) bank can charge them. (YES - it reduces your range.) Otherwise - eg:

ogjlbh21 wrote: my goal is to keep my voltage, not have my lights dim, get good life out of the batteries and not mess up the electrical system.to do it right and not have to worry that the car is guna go into flames.

...you connect whatever batteries you want as a "total loss" system for whatever running time you want - until you recharge them. That is a case of suitable battery sizing - IMO preferably with low-voltage protection.

ogjlbh21 wrote: is what he told me right: that i can just hook up these two new kinetik batteries parallel without a relay?

oldspark wrote: For maximum life, {batteries} should only be paralleled when being used; they should be identical (same age, batch, history, temperature); have equal discharge (& charge) rates - ie, +12V from one, GND from the other, with 2 equal cables connecting + to + and - to -; and hence a dual pole isolator (or 2 relays) between them. ....... Those that say different may have different requirements - eg, total loss systems where replacement cost is not a concern, or for wet-cells where (eg AGM) thermal runaway is not a concern. Others are generally those that profit from increased battery failures. Others seem merely opiniated, or ignorant.

ogjlbh21 wrote: a old friend of mine..... told me to run parallel without relays and i should be fine. he said i dont need a relay with low esr batteries. ..... he had no idea how to hook up a relay without the alternator.

Whilst your friend translates for you, ask him how ESR effects whether you need a relay or not (to isolate parallel batteries). Then ask at what stage of discharge does a low ESR battery need the relay? EG - as an AGM becomes discharged, its ESR increases and is eventually bigger than that of a fully charged wet cell (of similar capacity) which - apparently - required a relay. And for what sizes? EG - a wet cell 150AH has a lower ESR than a 35AH-75AH AGM - so do they require relays? My final point of batteryconfusion is, if you don't need relays for paralleled low-ESR batteries, why is it recommended to isolated them to prevent thermal runaway which is something exhibited by AGM batteries (low ESR) and not wet cells (which typically have twice the ESR or same capacity AGMs)?    My final confusion is how can someone that knows how to connect an isolating relay to an alternator NOT KNOW how to connect it to an on-off switch - eg, an amp or HU? I can understand the reverse situation! [ Dare I question an ICE without a timing belt (maybe it uses a chain or gears? Or it's a 2-stroke!). The Prius does have alternators - you mentioned two. And what starts the engine - a push start - you can't start it is the 240V bank is flat? You can use the same machine as an alternator and a starter-motor - in fact that is the current - er, present aim in vehicles (especially the 42V systems). Call it a generator, alternator, combo - it's all the same....] Asides from the [....], the above were merely some things I thought pertinent before risking damage. I still recommend keeping any battery you introduce completely separate from ANY Prius battery. That is simply wrt to its operation rather than any technical complications.    (If your Prius can adequately charge the load, then maybe add caps or battery(s) to preserve the Prius 12V battery.)