2 batteries

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STOP PRESS - the amp is connected to the nearest battery - ie, boot/trunk battery. (As opposed to both batteries next to each other and remote from the amp which is a crisscross connection.)

See below if more info needed, or for clarification of 2 "truly parallel" batteries.

A second battery for audio systems is usually placed next to the amp (and is AGM etc). Placing a battery remote from the amp does little other than provide longer reserve time - it won't do much to remove cable dips, and a low ESR battery is wasted. (I mention this in case that is your purpose.)

If you are running them in parallel, they should be matched batteries and connected as per:

(From Rainbow Power Company - Deep Cycle Batteries FAQ - a great company, great information etc.)

Normally I recommend against parallel batteries for audio etc (except when paralleled during charging which can be done with most alternators by adding a relay - see my posts elsewhere), but assuming your batteries are matched (same type, age, batch, condition) and located together (same temperature), and with the connection above, you have the most favorable parallel situation. [2 batteries in parallel have at least TWICE the failure rate of a single battery (where failure rate is for internal cell collapse and failures that will cause discharge of the 2nd good battery).]

Hence running loads from parallel batteries, you run them from the +ve of one, ad the -ve of the other.     

And now re-reading your OP, I see that you probably have the 2nd battery in the rear (good - that's better!)....

Then they should only be paralleled when charging.
And your amp is connected to the nearest battery. (That should be obvious.)

The advantage with the near-amp 2nd battery is that only the wiring between the 2nd battery (ground & +12V) to the amp need be fat (low resistance) - the alt or 1st battery to the 2nd battery is less critical - unless you want full voltage from the alternator (ie, the alternator is large enough to supply the amp and keep it above the full-battery voltage of ~12.7V).

This was probably all covered in the post above...

oldspark wrote:

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And your amp is connected to the nearest battery. (That should be obvious.)

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...when you have a second battery dedicated for the sound system, it is already connected to the primary battery.  You would connect your amplifier to this second battery as it is nearer to the amp.  This keeps wires at their shortest.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

PS - I liked some of the threads found under Anon's search string.

I loved the one about running 2 alternators - some people just don't get it. (your don't "just" run 2 alternators to the same system. 2 alternators for 2 different supplies or batteries - yes, but not a commoned battery or supply!)

Then there are others that mention older methods - eg smart isolators etc, when a charge lamp does the same usually better, more reliably, and cheaper. Else a voltage sensing circuit with timer (ie, smart solenoid/relay/isolator).
But as to alleged current sharing or charging priority; and diode isolators.... Oh well, I came along well after them (I think!).

thanks everyone, so basically i should parallel the two batteries and get the exact same batteries, correct?

No.

Use a relay. And have the appropriate protection (ie - fusing at each end of the interconnecting hot wire).



The "Charge Lamp Terminal" is labeled "D+" on most single-wire alternators, and "L" on others.

If your alternator does not have a similar chargeLamp circuit, a voltage sensing circuit (with appropriate time delays) can trigger the relay - that's what commercial Battery Isolators tend to use.   


I thought I had relevant writings here on 12Volt, but most seem to be on other forums....
(One day I'll organise a better diagram.)

To recap what I said earlier, batteries should not be paralleled (except when being charged, or being used for a specific purpose - ie, not just left connected together.

alright cool, thanks man

would i be able to run one deep cycle battery in the stock location without any other batteries?

I am currently.... However, I should not be because my Frontier is not generating enough power to supply the upgraded system and the vehicle's normal operating draw. Deep Cycle is a method in which energy is utilized. How do you plan to use your energy? Multimedia systems take advantage of Deep Cycle energy, but any system type can find a use for it. Your question is a easy YES. The real question should be- "should you be"?

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This is what I do for FUN!

I take that back.... I may be fine running the system after the Alt upgrade. I am changing the batt because I will want a higher level of current available to me than what I am capable of now...

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This is what I do for FUN!

Deep cycle batteries are not suited for cranking (2 different construction methods).
But if you can change starters - ie, I have a reduction starter that draws ~140A instead of the standard >250A.

And obviously if the deep cycle is much larger than the original cranker, it may be ok. EG - my battery shop recommended a 780CCA 100AH twin-terminal for crank & deepcycle - though that was a wet/flooded battery; apparently very popular with the 4WDers (~AUD$180 versus a typical 55AH 480CCA for $120).

But the same old problem - even though a deep cycle as the main battery may last longer because it handles greater discharge depth - what happens if it is too discharged to crank & start the engine?

so, really i could run a deep cycle, but its just not too great of an idea, but running two batteries i know the two should be matched, so then the stock one would be a deep cycle, so is two deep cycle better than one?

It's more like if you run two batteries, they should NOT be paralleled.
If you do, as soon as one fails, so does the other.
And despite my simplistic "double failure rate" model, the reality is a yet greater failure rate due to the constant albeit tiny interactions between the paralleled batteries.

Hence why (permanently) paralleled batteries should be matched. Taking that to its ultimate means same battery batch, housed adjacently at the same temperature with matched diagonal connections (ie, matched + & - interconnection, and +ve output from one battery and -ve/GND from the other).

By doing that, you merely double the unreliability (for a 2 battery system) - ie, you have removed external sources of imbalance that cause mismatching of the batteries behaviour.
(By placing one in the boot, you introduce temperature hence charge & discharge mismatches, plus whatever cable & load inequalities exist.)


Now, if you want what most people seem to want - namely two independent batteries that are charged together, none of the above apply.
Two independent batteries are used when the user wants assurance that they always have a cranking battery etc.
The second battery may simply be a spare for emergencies, or to power loads like audio systems, refrigerators, light shows, computers etc.
Either battery can die and it won't effect the other (within reason!).

When the vehicle begins charging, the otherwise independent batteries are connected to the charger - ie, paralleled and connected to the charger.
Other than voltage and hence usually type - eg, 12V lead-acid - they do not have to be matched as none of the "discharging each other" conditions apply. Have a charging 1.2AH paralleled with a 1000AH - whether AGM, gel or wet cell - it doesn't matter.


All other issues are the normal ones of sizing and practical considerations. EG - current limiting of charge if required, how stressed a battery type/size is (eg, deep-cycle cranking), etc - but these are normal considerations independent of how batteries are connected.

Many multi-battery vehicles hence retain their main cranking battery and use deep-cycles for their others.
Although I intend to do the same, when my main cranker expires, I will probably use deep cycle for all batteries assuming I install my "parallel whilst cranking" bypass (which involves an extra 2 cranking relays, or one with extra starter cable). Hence I share the cranking burden across at least 2 deep cyclers with hopefully no worse than a 1:2 current split.   

Excluding sulphation and (hence?) exercise considerations, the less the current (magnitude) demand placed on a battery, the longer the battery survives.
Hence car batteries are usually "rated" to last 3 years or thereabouts. I tend to get more than 6 years from them (despite my abuse and 40-45 year old vehicles!!)

wait, i reread that a few times and am i kind of confused about what your saying

I'm too fast on the paradiddles am I?

It's yet another paraphrasing of my usual blurb.

Re-read. Ignore what you don't understand. Ensure you are at ease with what you understand and go with that.
Ignore the additional background information - it is only there to help explain else justify the big picture.

ok man, so would it work just to series the batteries?

No.
2 series 12V batteries is 24V.

If you don't understand what I am saying, just parallel them.

As jmelton wrote in batteries, how many, what kind?....

jmelton86 wrote:

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

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On mp3car.com, Optima's eCare Manager (OptimaJim) said that Optima batteries can be paralleled. Based on that, why can't others? (Though matching etc was involved.)

There are heaps of other web writing that say the same.
I know many that hard-parallel batteries too.


Don't worry about trivial details like mismatching, temperature, and connection methods - that stuff comes from knowledgeable people that try to increase the life of the batteries.

The reality seems to be that people that parallel batteries using any technique do not complain.
Just go with that latter reality.

I think I am in love with Mad Scientists... forget whatever i said about not following scientific methods. (Though that was a generalisation that referred to mainstream scientists.)

Mad Scientists has/have stated so succinctly what I have been trying to say.
And for newer ECU controlled alternators - sure, get an isolator.   
For those with charge lights, use its superiority - use a relay.

so say if i had an optima yellow top for example under the hood and had it in series a kinetik 2400 and that in series to another kinetik 2400, would that be fine?

Explain what you mean by series.

A 12V battery in series with another 12V battery means 24V.
24V tends to be detrimental to 12V loads.

3 x 12V batteries in series = 36V. That tends to be even more detrimental - not that it has much extra impact. (How your 12V loads blow on 24V compared to 36V is somewhat academic, even though the "power of destruction" is over twice that at 24V.)

For 12V loads, you can only parallel 12V batteries.

so with my example would i parallel them or use possibly a combination of series and parallel?

Do you mean connect two 12V batteries in series?

And what loads do you have that are 24V?

good point. i guess none!

Cool.

Then DON'T series connect any 12V batteries.