# Adding second battery?



## maxxx (Feb 4, 2009)

I am planning to add a second battery to the rear of my car, in parallel with the starting battery, to help with the stereo. I was going to add the Odyssey ER30 because it is the perfect size for my space, but they said I cant because the charging characteristics are too different from my starting battery.

My starting battery is a group size 94R, Motomaster Eliminator (Canadian Tire) and it does not have the six screw caps for water on it but has a small circular window within which you can see green. I'd change it but the starting batt is new with 9 yr warranty, and it wasn't cheap. I will try and find out the manufacturer, but in the meantime can anyone recommend a small battery for the rear for my application? Looking for 20aH+, not higher than 6 1/4" including terminals. I have some length and width restrictions too but not sure, biggest restriction is height.

Thanks in advance!


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## Timelessr1 (Feb 12, 2010)

The charging isnt where the problem will occur....its when the car sint running that the problem happens. One battery will draw from teh other batery if the batteries are diffrent, and in your case they will be. within no time at all both batteries will go bad. what you need is a battery isolator which will disconnect the batteries when the car is off, and when the car is on (charging) both will be charged. 


Jsut a quick question also...how do YOU think the seocnd battery is going to help yuor system? im just curious? Are you looking to play your stereo with the car off?


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## maxxx (Feb 4, 2009)

I was told by odyssey that the two batteries could not be connected to the alternator at the same time, based on the quicker charging capabilities of the second battery.

Yes, there may be odd occasions where I will have the stereo on for brief times with the engine off, so some extra reserve would be good. It would not be a habit, I realize that frequently drawing down the batter(ies) would shorten the life of both.

I have four amps and a high-farad cap in the back of the car. The alternator has been upgraded to a 120 amp version from 90, and I figured a second battery would only help the situation. What can you tell me?


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## quietfly (Mar 23, 2011)

Check out these isolators they will do what you need in allowing your batteries to charge separately.


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## Timelessr1 (Feb 12, 2010)

maxxx said:


> I was told by odyssey that the two batteries could not be connected to the alternator at the same time, based on the quicker charging capabilities of the second battery.
> 
> Yes, there may be odd occasions where I will have the stereo on for brief times with the engine off, so some extra reserve would be good. It would not be a habit, I realize that frequently drawing down the batter(ies) would shorten the life of both.
> 
> I have four amps and a high-farad cap in the back of the car. The alternator has been upgraded to a 120 amp version from 90, and I figured a second battery would only help the situation. What can you tell me?


I can tell you that the second battery will put an ADDITIONAL strain on your alternator. When a car is running, and your electrical draw is less then your alt output, then the battery is a load on the alt since its getting charged. Batteries are used in two man circumstances....1. to start the car 2. if your current demands exceed your alt output.

If you are using the battery on a constant basis for number 2, you might need a bigger alt...or lower current drawing amps.


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## Timelessr1 (Feb 12, 2010)

quietfly said:


> Check out these isolators they will do what you need in allowing your batteries to charge separately.


One negative downfall to using a dio9de isolator is therey is usually a voltage drop due to its use


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## SQooter (Apr 23, 2011)

I'd like to do this on my boat. Then I could relax. I'm always afraid to run the stereo too long while parked.


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## quietfly (Mar 23, 2011)

My boat already has 3 huge batteries.... your boat should have atleast 2...


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## maxxx (Feb 4, 2009)

Sounds like keeping my one battery may be the right thing to do. But a couple of points for further consideration, as I am trying to understand this and make the right decision.

I understand that an alternator is a steady provider whereas a battery is a "deeper well" of at least momentary current. I expect that my ~900W stereo would have peak moments that would, on a frequent but only momentary basis, require more current than the alternator puts out. 

Also, assuming, as I suggested above, that my stereo has musical moments of hunger for current (i.e. not flat, wide-open demand) that exceeds the alternator's output capability, the voltage in the system is going to drop to the point where the alternator is (sort of) out of the picture and the battery is dumping current into the potential created by the amp. Would the voltage not drop less if there were two batteries instead of one? If this is true, would the entire system - alternator and batteries - not last longer and perform better with a more modest voltage fluctuation that comes from the support of multiple batteries?

In my particular case, my alternator is at least 30 amps larger than my car requires in stock configuration, so I figured a small second battery in the back could be supported by the alternator without overloading it.

Would this not be the right reasons to add a secondary battery? I would love to hear from people who run secondary batteries in the rear of their car, and how they are configured. From what I have heard strapping a second battery in the back is common (kinetic, etc.).


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## quietfly (Mar 23, 2011)

maxxx said:


> Sounds like keeping my one battery may be the right thing to do. But a couple of points for further consideration, as I am trying to understand this and make the right decision.
> 
> I understand that an alternator is a steady provider whereas a battery is a "deeper well" of at least momentary current. I expect that my ~900W stereo would have peak moments that would, on a frequent but only momentary basis, require more current than the alternator puts out.
> 
> ...


For the most part you are right with your system shouldn't strain your alternator much, if at all. However you need to keep your batteries isolated from each other because chemically your smaller battery with less resistance will cause a steady current run without isolation. Eventually this will ruin both batteries. The isolator will prevent that and allow both systems to charge, but at the small penalty of some voltage drop. Really nothing to worry about, and in my opinion totally worth it. 
JMO YMMV 
Sent from my PC36100 using Tapatalk


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## maxxx (Feb 4, 2009)

Thanks quietfly. 

If the smaller battery in the back is the only battery for the stereo, I doubt the ER30 would be worthwhile. In other words, if I am going to isolate a battery dedicated for the stereo, it should probably be a large one? My amplifier RMS adds up to about 900 watts, I am wondering what minimum size battery is worthwhile for an "isolated" setup.


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## Chaos (Oct 27, 2005)

Timeless1 & quietfly are both absolutely right.

That said, I frequently see people drop in an auxiliary battery (like a small Kinetik) in order to help compensate for excessive current draw of amplifiers which exceed the capability of stock alternators.

In the short term, this appears to help because it keeps the voltage up and minimizes dimming. On the other hand, charging two batteries does put more constant strain on the alternator, and if you play the system without the engine running, it will drain any & all batteries on the circuit. 
Although you don't strictly need the isolator for any of this to work, the fact is that it all comes down to power one way or another.


If you have amps that are rated for more than the charging system can supply, you'll never get maximum performance out of them for very long at all. Period. Likewise, anything short of upgrading the alternator is more of a temporary band-aid than a long term solution.


The good news is that 900 RMS should be well within the ability of the alternator you already have. If anything, one good battery like an Odyssey or larger Kinetik would be my recommendation for long-term performance.


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## maxxx (Feb 4, 2009)

OK, I am working on a plan to add a second isolated battery to my system, and I am confident that my upgraded alternator can handle it. 

I understand that diode-based isolators create a voltage drop. Some devices boast not having a voltage drop. What hardware can I use to run the second battery for the stereo that creates as little voltage drop as possible? 

In planning my system this way, I would like to consider occasions where the second battery is substantially drained while the engine is off. When I start the car again, the alternator would be faced with two batteries of considerably different voltage / potential / demand / whatever, even if the batteries are isolated from each other. Should there be some sort of "buffer" (i.e. I imagine a sophisticated isolator that moderates load??) between the alternator and the second battery? Does anyone have recommendations for "smart" hardware for use in this application?


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## quietfly (Mar 23, 2011)

maxxx said:


> OK, I am working on a plan to add a second isolated battery to my system, and I am confident that my upgraded alternator can handle it.
> 
> I understand that diode-based isolators create a voltage drop. Some devices boast not having a voltage drop. What hardware can I use to run the second battery for the stereo that creates as little voltage drop as possible?
> 
> In planning my system this way, I would like to consider occasions where the second battery is substantially drained while the engine is off. When I start the car again, the alternator would be faced with two batteries of considerably different voltage / potential / demand / whatever, even if the batteries are isolated from each other. Should there be some sort of "buffer" (i.e. I imagine a sophisticated isolator that moderates load??) between the alternator and the second battery? Does anyone have recommendations for "smart" hardware for use in this application?


first off, even if your second battery is substantially drained, it will still only charge at the same rate. Assuming you truthfully have 30 amps of overhead, then this is not even a consideration. your battery would charge at around 15 amps at most at 14-16 volts ( this is dependent on your regulator) no real "smart" hardware needed. 
Second off for your second battery to be that drained you'd have to be playing your system with out your alternator running. even with INSANE musical peaks you moderate 900watt rms system is not going to be able to significantly drain your battery while the alternator is running. and during the musical ebbs the battery would quickly recharge. 

hope this helps..


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## mrflamboynt (Apr 23, 2011)

i disagree, i have used a lot of different kinds of batteries in conjuction with standard starter batts. if that were the case, then caps wouldnt work very well.... the chemical difference between a cap (or even a 'batcap') is substantial. but the use of these types of 'power cells' has never had adverse effects on the rest of the system. i have used a stock 60A alternator (w/big 3 upgrade) to juice 3 batteries for over 1 or more years, and never had any issues with the alt... just my experiences

and for most setups i am against the use of isolators.... an isolator should only be used if absolutely necessary....


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## quietfly (Mar 23, 2011)

mrflamboynt said:


> i disagree, i have used a lot of different kinds of batteries in conjuction with standard starter batts. if that were the case, then caps wouldnt work very well.... the chemical difference between a cap (or even a 'batcap') is substantial. but the use of these types of 'power cells' has never had adverse effects on the rest of the system. i have used a stock 60A alternator (w/big 3 upgrade) to juice 3 batteries for over 1 or more years, and never had any issues with the alt... just my experiences
> 
> and for most setups i am against the use of isolators.... an isolator should only be used if absolutely necessary....


There is a huge difference between caps and batteries. Caps store electrical energy, batteries use electricity to reverse chemical reactions (or conversely create electricity as a result of a chemical reaction) the alternator is not where you'll have the problem; rather its the batteries. Your batteries will always "leak" current down towards the cell with the lowest potential. This is why they say never mix new and old batteries. Now just because your cells have never experienced this in a way that you could determine, doesn't mean its a good idea to discount sound advice. 
Personally, I hate condoms, but I always use them just in case. I feel the same way about isolators..... :b

Sent from my PC36100 using Tapatalk


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## ttocs388 (Jun 25, 2010)

mrflamboynt said:


> i disagree, i have used a lot of different kinds of batteries in conjuction with standard starter batts. if that were the case, then caps wouldnt work very well.... the chemical difference between a cap (or even a 'batcap') is substantial. but the use of these types of 'power cells' has never had adverse effects on the rest of the system. i have used a stock 60A alternator (w/big 3 upgrade) to juice 3 batteries for over 1 or more years, and never had any issues with the alt... just my experiences
> 
> and for most setups i am against the use of isolators.... an isolator should only be used if absolutely necessary....


how long did you use them for? Most people I know that say that did not keep the car long enough to see if it was good or bad.


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