# Primary and secondary battery do's and dont's?



## Lyons238 (May 19, 2016)

Hello,

I searched but didn't find what I was looking for so I figured it may be helpful to others.

When selecting a primary and secondary battery should they be the same? If they are not what are the affects?

I heard that you should use the same size battery. For instance I have a XS Power D3400 under the hood, and an XS Power XP950 in the trunk. I was told this may be causing some unwanted stress or potentially why I'm still experiencing some dimming occasionally. Aside from more capacity, would there be any benefit to my primary battery and electrical system overall if I had a matching D3400 in the trunk?


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## Onyx1136 (Mar 15, 2009)

Lyons238 said:


> Hello,
> 
> I searched but didn't find what I was looking for so I figured it may be helpful to others.
> 
> ...


Ideally they should always be the exact same battery. Also they should be right next to each other, not one in the trunk and one under the hood. 

The point here is to minimize any differences between either of the batteries. If there is any difference in voltage, current capacity, discharge rate, resistance, temperature, etc. one battery will always present a parasitic load on the other battery. Essentially one battery will constantly be trying to "jump start" or recharge the other battery.


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## cobb2819 (Jan 13, 2011)

Onyx1136 said:


> ...Also they should be right next to each other, not one in the trunk and one under the hood...
> 
> Essentially one battery will constantly be trying to "jump start" or recharge the other battery.


In there case where statement number 1 is not possible, using a Marine style charge controller and isolating loads for specific purposes would be ideal. Think about boating, you have a starting battery and a "house" battery. Both of these systems need to be recharged by the alternator, but are truly independent. A charge controller would be fed by the alternator and then send current where current is needed. This will eliminate the second statement. It's also about the most expensive way to do this, but...


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## Lyons238 (May 19, 2016)

So would I benefit from replacing my XP 950 with a D3400? The batteries won't be next to eachother but would it help? I was under the impression with these style batteries you don't need an isolator?

Also, using a smaller battery near my subs and amps, does that mean that battery is constantly pulling from my front battery, which may be why my lights are dimming even with a pretty good electrical system? 250a alt, Big 3, D3400, XP950, running 2600w MAX more like 2200.


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## Locomotive Tech (May 23, 2016)

Dimming may be unavoidable if you car is newer. Many new cars come with CPU controlled charging systems. Based on the Battery Voltage, Battery Temperature and operation the CPU will essentially "turn off" the alternator output and place the alternator in "idle". Once the CPU senses a predetermined voltage, in my civic its about 12.4 -12.8, the Alternator is turned backed on and will stay on until the upper voltage setting is reached. I noticed the dimming the day I bought my car brand new. 

There are ways around this on some models but then you will also shorten the life of the alternator and the battery. 

Looks like you have a pretty stout electrical system already. When you say that you are "running" approximately 2200 watts, are you sure or is that a guess. That's a lot of power and most do not usually ride around cranked up. 

Easy way to tell if dimming is because of your system or the design of the vehicle, wait til night and park your car facing a wall or something, turn of your system and watch the headlight intensity as you gently apply some throttle, and see if it changes. Then try some more aggressive applications. believe it or not some cars can draw 200+ amps on hard accelerations.

I ended up getting rid of my class AB amp and went with class D which are vastly more efficient. The JL's I can speak for, they still produce rated power down in the low 12 volt range. This why I asked if you are truly running at 2200 watts, if your basing this on 14.4 or 14.8 numbers, it is likely that 2200 watts may be stretch.


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