Hey there I was looking for some help here
I have a stock battery in my car which I want to upgrade to a newer larger battery. I wish to keep the stock battery in the car and connect it to the newer battery.
I want to have both batteries connected so that If i were to drain one somehow the car would always crank.
Also, I wish to use the stock battery as a capacitor for my stereo system, and also for a tank circuit so that when I crank the car the carputer stays on.
Have done a quick search, can anyone help me out here? Need to know: can I have the battery do all these things and also how should I wire it?
2007 Honda Fit Sport 1.5L SOHC-VTEC
Going back to my days in a car audio shop here...Originally Posted by powerslide
First thing to remember, is that if you connect another battery to your vehicle's electrical system, when the engine is running, it will be another load on your alternator; When the car is running, the alternator is putting out approximately 14.4 Volts, and attempting to charge the battery(ies) since they have a lower potential (approximately 12.6 Volts typically). I have seen more than one alternator smoke under the extra load of charging a second battery. Also, this will take some of the power you were using for your car's stereo and power accessories to charge the additional battery. Be prepared for this contingency. An aftermarket high-output alternator may be in your future.
To have the second battery be able to take assist in a crank situation, they would have to be in parallel somehow. Not difficult, but reference the above paragraph for related perils. That being said, you would have to tie them together with large guage wire (size is dependant upon the load the battery will see when it "assists" the other for cranking, as well as the distance of the cables). Also, in parallel, any device (Amplifier, headlights, etc) would pull from both batteries, not just the one. You could add a high current isolator (Very large relay/solenoid) that only connects the 2 when the engine is running (or during an emergency "boost"), and that would allow them to function separately when the engine is off.
On the the function of capacitor. A car battery is generally not useful, as most car batteries do not have a low enough ESR to assist a powerful car stereo in its more intense passages (the maxed out blast of volume at a stoplight). This is the whole reason .5, .75, 1, 1.5, and even 2 Farad capacitors have made their way into high end aftermarket stereo installs. Capacitors of this type generally have an ESR 10-100 times lower than a car battery (brand and model dependent). The ultra low ESR of a capacitor allows it to deliver current at massive amounts (200+ amps) for short bursts of time without a significant voltage drop. That is why your car's voltage dips when you crank it; The battery has a much higher ESR and cannot deliver starting current without a significant voltage drop (anywhere from 1-4 volts avg). Also, batteries are generally located a large distance (from the hood to the trunk) from the amplifiers and associated high current audio devices. AT that point, the wire between the battery and the device starts to play a large roll in the voltage drop. That is why capacitors are located close to the device they are working to power; to keep the power cable runs as short as possible and lower the total resistance in the run, reducing voltage drop. Once you start putting distance between the source of power and the device, the wire used can make or break the system.
In case you were wondering, ESR means "Equivalent Series Resistance". If we throw Ohm's law into the mix (E=IR), we can mathmatically prove car batteries make bad capacitors, and long cable runs cause significant voltage drop under high current loads.
One site you could visit for parts that can accomplish this is http://www.stinger-aamp.com/. They make high-output alternators, capacitors, isolators, etc. taylored for audio systems.
Weeee... Hope that wasn't too much in one post.
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I've got batteries in this same configuration. The main battery is a normal automotive battery. THe secondary battery is a Optima Yellowtop. I'm using an isolator from these guys, http://www.hellroaring.com . I think I may have figured out the problems I was having. Assuming I have, this configuration works great! If it keeps failing, them I'll keep relying on the generous Optima warranty
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Dual Battery with Optima Yellowtop | Hellroaring Battery Isolater
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the quick answer is yes you can do all this. a 2nd batt beats all the "tank" ideas. but be warned, like what philstuf were saying, i mean just 2 days ago i got stranded in the parking lot 'cause my main batt. died. i tried to use my tank batt(40ah deep cycle) to crank the engine but the 2.5 meter length, 8 gauge wire didn't do the trick. i ended up taking the tank batt to the front replacing the main batt and trying to fit the terminals being both of different size, a greasy 1 1/2 hour job specially when all i got is a swiss army knife to do the job.Originally Posted by powerslide
maybe i should replace the wires that connects the two batt with something like a thick 4 or 2 gauge wire, but the length is still debatable.
how you connect your two battery would make a big difference. my setup goes like this: the two batt is connected by 8g wire with a relay connected on the positive (i got this relay idea from ricky23?) now unless there is a 12v present on the relay, the two batt were both isolated from each other. when the car is running, the two batt is being charged, well experience later showed me that this is not right 'cause during redlight i noticed the voltage would drop and the engine hesitates. one time on a 10 deg highway the engine actually stop. i later installed a switch so when the voltage drops to less than 13v, i can cut the connection to the tank batt thus concentrating power only to the main batt. messy.. i am now thinking of installing a voltage monitor kit to do all this automatically.
i also noticed that when both batt are connected, they "fill" each other to have both the same voltage, so it does equilibrium. this is the reason why when the first time my main batt died, i could'nt even crank the engine.
a week ago i purchased the cheapest car batt charger, i installed a batt monitor kit and i can now leave it connected without worrying that the batt would overload. i only use it for my tank batt. this is cheaper that upgrading my alternator plus there's no sweeter feeling that knowing that your tank is fully charged.
all the tech stuff i mentioned is readily available in the market, but if you know basic soldering, you could save hundreds of dollars, exactly what i did.
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