Dual Batteries - shoot holes in my idea please!

[ericbendler]

As someone who definitly sees his headlights dim when the bass hits, I was wanting to get some opinions on a dual battery setup in the 525i.

We are all lucky enough to have two mounting points. Either under the hood or under the back seat right? Whether you are using one or the other.

My factory spot is under the hood, but my rear seat mount is not being used.

Instead of the 3 farad cap that I have under there currently, I was thinking about using the lucky setup that I have and installing a second battery under the rear seat as well.

I have read (exhaustively) other articles on dual battery setups, and wanted to get the final word from this board.

I have two power leads (one black and one red) that I will attach to the new secondary battery at the rear seat, keeping their 12volt power supply in tact.

I will vent it as it should be with the factory tubing and the tubing supplied by the battery maker.

I have identical (age and type) batteries as I am not going to be isolating them. I am aware of the possibility of having two batteries and stil needing a jump without the isolator.

My question is this........

Do you forsee any problem specific to BMWs? I have had the two rear power wires connected to my cap for years, so why would connecting them to my rear battery be any different.

I searched the board for "dual battery" and so on with little success. If anyone knows of a good thread, I would appreciate it.

Other than that, just wanting to hear people shoot holes in my idea to increase my amps while keeping it at 12 volts.

(I run two soundstream amps that really make my lights dim).

Hopefully there are no threads about dual batteries in the e34, so this will be the first.

[RallyD]

before you go into dual batteries:
Being that you have soundstream amps and a capacitor, it appears you may have a clue about quality and such. I always run 2ga welding cable off the alternator and make sure my earth ground system can handle the power. 4ga wires to the amp and 4ga grounds to chassis from the amps.
If you have an upgraded alternator cable already you might want to go with 2 batteries after all.
http://www.pirate4x4.com/tech/billav.../schematic.jpg


i used 2 batteries in one car that I had. I ran the Odyssey PC 680. It is compact and quite powerful.
http://www.odysseybatteries.com/applications/auto.htm

[Tiger]

What kind of cap do you have? The cap is supposed to prevent this from happening. How did you hook up the cap?

Battery will never give you fast enough of power to handle the bass. What amperage is your alternator? What is alternator power output? Have it been tested?

[pingu]

What kind of cap do you have? The cap is supposed to prevent this from happening. How did you hook up the cap?

Battery will never give you fast enough of power to handle the bass. What amperage is your alternator? What is alternator power output? Have it been tested?

I'd be very surprised if a battery couldn't give enough current - we're only talking a few tens of Hertz for bass, not tens of MHz. The internal resistance of a car battery is of the order of a few milliOhms and while it will rise with frequency (due to inductance, amongst other effects), I think this will be trivial.

On the other hand, I'd be wary of connecting two batteries in parallel. I wonder if the one with the lowest open-load voltage will hogg all the charging current - in other words one of the batteries might end up fully (over?) charged while the other is almost devoid of charge.

[ericbendler]

What kind of cap do you have? The cap is supposed to prevent this from happening. How did you hook up the cap?

Battery will never give you fast enough of power to handle the bass. What amperage is your alternator? What is alternator power output? Have it been tested?

I don't know about the internals, but the cap is 3.0 Farads. I have what seems to be an OEM 8 guage wire running from under the hood to the rear seat. That is what I was going to use for the positive supply for the rear seat battery.

Then I could just run the power leads to the amps from there (with the correct fusing).

My alternator is a french 130 amp.

I had the cap hooked up under the seat as though it were a battery.

[ericbendler]

Don't diode based isolators drop the voltage on your lines? I don't want that to happen. If I can get past the charging issues - (one battery getting all the charge, while the other one discharges) without putting my impedance on my 12v lines, I can move forward.

[Phatty5BMW]

8ga from under the hood is not nearly big enough. Run atleast 4 guage to your back seat and they sell adapters to hook that to 6-8 ga. which is fine to hook up to your amp. You loose power the farther you get from your supply. I would run the 2/4 guage to your capacitor then run the other guages to your amp.

Pat

[Tiger]

8 ga is too small for big amp... I don't know what your total wattage is but I would like to know. 8ga is good for up to like 300 true watt I think... I am not sure. If you have more draw than this, then you are starving the amp.

As for the cap, the only way I hook them up is positive cable only. From battery positive to negative on capacitor and from positive capacitor to amp. With this way, capacitor is always in work.

[Tiger]

Bass draw massive amount of power in ms timeframe. Battery just cannot give that out that massive amount in that timeframe... that is the job of capacitor.

Second battery is used mainly for storage in the engine off condition. Majority of people with serious system usually upgrade the alternator for higher output capacity.

130amp or 145amp as in US is plenty of power.

[AngryPopTart]

On the other hand, I'd be wary of connecting two batteries in parallel. I wonder if the one with the lowest open-load voltage will hogg all the charging current - in other words one of the batteries might end up fully (over?) charged while the other is almost devoid of charge.

I don't think that happens. The batteries may very well output different voltages, but I don't think overcharging one is a concern here.

If you're going to run two batteries, you may as well take this opportunity to just throw in a deep cycle battery with a cut-off switch (to disconnect it from the charging system) and use this to power the stereo as there will be a significant difference in quality when it is seperated from the factory system. Batteries are a good thing to run audio equipment off of, but the charging system as well as the ignition system introduce alot of garbage into your audio. You may want to play with different routes, like running only the sub amps off the second battery.

In the case that you decide to run any of the audio equipment off of the secondary battery alone, make sure you completely isolate the grounds on those pieces of equipment. Don't ground them to the body, and don't use screws which will effectively ground the chassis of said equipment to the body, use nylon washers or whatever you have to do to keep them away from the ground of the main battery and run seperate wires to the negative post of the secondary battery. Otherwise you will have ground loop problems, oscillations, etc.. Don't forget about the audio cables that run from the main deck to your amps either. The ground will find it's way in this way as well so you'll have to alter your RCA cables and lift (cut or otherwise disconnect) ground from both channels.