12 volt battery in parallel to the existing 12 volt battery

[Marki3]

Hi,

Any engineers/electronic experts willing to offer some advice?

I have been testing a sub woofer installation. While I was doing that I was running the sub from an external 12 volt battery while I evaluated the amperage draw. For my needs the average draw was only 2-3 amps which would mean I could theoretically run the sub from the cigarette lighter outlet which is good for 140 watts (or 11 amps).

But while I had the external battery wired to the sub I got to wondering if it would be a good plan to leave it in place. If I ran the cigarette lighter leads to the second battery and the sub power leads from the second battery terminals, then it would be effectively in parallel the the car's 12 volt battery. It would also be able to deliver the short high peak load that the sub might draw for transient high bass load. I assume that the car would see this as just a bigger battery overall and they would both be charged from the DC to DC converter.

Is this true or flawed reasoning?

Any reason not to leave a second battery in place? Assuming the second battery is fully charged and equal in voltage to the car battery then there should be no current flow between the two when you connect them. After that as the sub draws down the second battery it would be charged at a minimal rate from the car's 12 volt battery which in turn is maintained by the DC-DC charger.

The battery I had in mind is an AGM battery. Small footprint and only weighs couple pounds but can do high transient loads.

Thoughts and ideas would be much appreciated.

Thanks.

 

[jadnashuanh]

From a technical point of view, it will probably work. The only issue might be that there could be power places there shouldn't be when the car is off and the timer expires. I haven't tested it, but it appears that the 12vdc accessory outlet shuts off after a delay. If you were to put a diode in there to prevent power from being applied, that nominal 0.7vdc drop across it may never recharge your aux battery fully. It may not be necessary. I'd want to know the limits of the DC-DC power supply before I added a load, though, and if having voltage back fed into the circuit might power some things that normally would be off. You could use a relay to disengage, but the coil, while not drawing much, would be yet another load, and if on the EVSE, it may not be actually connected so it could recharge which would limit it to only being recharged while the car was in READY state. Lots of unknowns.

 

[Marki3]

Excellent points. Yes, the accessory outlets shut off after about 20 minutes, so you are correct that at that point the external battery would be applying load into the system with possible unintended consequences. A diode would prevent that and I think the small voltage drop associated with the diode would be unlikely to be a problem.

My thought was that if the sub draws 2-3 amp continuous while the car is in use, then that is a pretty small load and would be unlikely to ever put any real stress on the DC-DC converter since the outlets are rated for 11 amps. Having the battery in place wouldn't change that load. In the end the easiest might be just run directly off the outlet and not worry about having the battery in place.

The sub is rated at 200 watt but after I saw the low continuous amp draw of only 2-3 amps I replaced it's 20 amp fuse with a 10 amp fuse and with the levels I am using that fuse has been fine so far. That would be lower than the outlets are rated and might be the simplest solution.

Appreciate the input.

From a technical point of view, it will probably work. The only issue might be that there could be power places there shouldn't be when the car is off and the timer expires. I haven't tested it, but it appears that the 12vdc accessory outlet shuts off after a delay. If you were to put a diode in there to prevent power from being applied, that nominal 0.7vdc drop across it may never recharge your aux battery fully. It may not be necessary. I'd want to know the limits of the DC-DC power supply before I added a load, though, and if having voltage back fed into the circuit might power some things that normally would be off. You could use a relay to disengage, but the coil, while not drawing much, would be yet another load, and if on the EVSE, it may not be actually connected so it could recharge which would limit it to only being recharged while the car was in READY state. Lots of unknowns.

 

[ultraturtle]

Sorry for the repeated information from a different post, but I don't think you really need a second battery. I run an even more powerful amplifier from my Plug-In Prius 12 volt battery which is tiny, just like the i3. As an experiment, I even threw in a massive 2 farad "ultra" capacitor, and it had minimal impact on leveling the power draw, which was never much anyway:

• http://priuschat.com/threads/total-...essor-amp-and-subwoofer.112102/#axzz3KBDPF3cn

and

• http://priuschat.com/threads/total-...essor-amp-and-subwoofer.112102/#axzz3KBDPF3cn

The 12 volt system is more than adequate to power the 500 watt subwoofer amp and 4 X 100 watt sound channels. I power the system directly from the battery, with its own inline fuse. Power sensing comes from the car's accessory post power output to the JBL MS-8, which completely powers down the system when the car is turned off.

Folks tend to not make a distinction between power potential, and power actually used. While the system is capable of outputting as much as 900 watts, I've taken measurements at a sound pressure level louder than I would ever consider listening and found a peak of about 70 watts, and an average less than 20 watts - no strain whatsoever on the 12 volt system.

 

[ultraturtle]

Just took a look underneath the glove compartment and found that the sound system uses 3 fuses:

#68 5 amps
#61 20 amps
#76 40 amps

Suggest you keep it simple, wire the amp into either the 20 amp (12 ga wire) or 40 amp (10 ga for a maximum 10 ft, otherwise 8 ga) circuit, and call it a day.

By the way, while the car is powered on, the 12 volt system is typically getting its energy from the buck converter, and the 12 volt battery is charging, not discharging. The 12 volt battery is really not tapped to power the entertainment system under normal conditions.

 

[Marki3]

Thanks for the information. That is where I was leaning. The constant draw at even medium volume is less than 2 amps so only 24 watts. I am trying to make this plug and play and pulling power from the 12 volt outlet will make it easily removalable. I am parking the sub behind the passenger seat but wanted to be easily able to remove if I want to carry passengers from time to time.

Sorry for the repeated information from a different post, but I don't think you really need a second battery. I run an even more powerful amplifier from my Plug-In Prius 12 volt battery which is tiny, just like the i3. As an experiment, I even threw in a massive 2 farad "ultra" capacitor, and it had minimal impact on leveling the power draw, which was never much anyway:

• http://priuschat.com/threads/total-...essor-amp-and-subwoofer.112102/#axzz3KBDPF3cn

and

• http://priuschat.com/threads/total-...essor-amp-and-subwoofer.112102/#axzz3KBDPF3cn

The 12 volt system is more than adequate to power the 500 watt subwoofer amp and 4 X 100 watt sound channels. I power the system directly from the battery, with its own inline fuse. Power sensing comes from the car's accessory post power output to the JBL MS-8, which completely powers down the system when the car is turned off.

Folks tend to not make a distinction between power potential, and power actually used. While the system is capable of outputting as much as 900 watts, I've taken measurements at a sound pressure level louder than I would ever consider listening and found a peak of about 70 watts, and an average less than 20 watts - no strain whatsoever on the 12 volt system.

 

[Marki3]

Thanks. That is helpful info. I will likely pull a trigger for on/off from one of those fuses and then do power from the 12 volt outlet for easy plug and play. I have RCA on the leads from the head unit, a 12 volt cigarette lighter plug and then will put a quick connector for the trigger. Should be able to remove the sub in just a few seconds.

Just took a look underneath the glove compartment and found that the sound system uses 3 fuses:

#68 5 amps
#61 20 amps
#76 40 amps

Suggest you keep it simple, wire the amp into either the 20 amp (12 ga wire) or 40 amp (10 ga for a maximum 10 ft, otherwise 8 ga) circuit, and call it a day.

By the way, while the car is powered on, the 12 volt system is typically getting its energy from the buck converter, and the 12 volt battery is charging, not discharging. The 12 volt battery is really not tapped to power the entertainment system under normal conditions.

 

[ultraturtle]

... I will likely pull a trigger for on/off from one of those fuses and then do power from the 12 volt outlet for easy plug and play. I have RCA on the leads from the head unit, a 12 volt cigarette lighter plug and then will put a quick connector for the trigger. Should be able to remove the sub in just a few seconds.

Sounds like a smart plan. If you have not already purchased the sub amp, a cheap 60 watt (30 watt at 4 ohm) model like this one should work well for your needs. It's fused at only 10 amps:

http://www.crutchfield.com/p_500KTP190U/Alpine-KTP-190U-Power-Pack.html

 

[GodFollower]

Seems like you were already leaning away from adding a second battery, but I am going to stress how important it is that you do NOT add a second battery to this vehicle! The fact that your battery charging device(DC/DC converter in the EME) is controlled by how much has been removed from the vehicle battery(as measured/calculated by the IBS on your negative battery post) means that it knows what to expect for voltage and current draw during operation. While these converters are pretty stout, they are still operating in some pretty strict parameters. You do not want to mess with how the car would react to loads or charging currents during operation. If you are going to add a load, add it to a circuit that is controlled and monitored by the vehicle so it can properly compensate for that.

 

[Marki3]

Seems like you were already leaning away from adding a second battery, but I am going to stress how important it is that you do NOT add a second battery to this vehicle! The fact that your battery charging device(DC/DC converter in the EME) is controlled by how much has been removed from the vehicle battery(as measured/calculated by the IBS on your negative battery post) means that it knows what to expect for voltage and current draw during operation. While these converters are pretty stout, they are still operating in some pretty strict parameters. You do not want to mess with how the car would react to loads or charging currents during operation. If you are going to add a load, add it to a circuit that is controlled and monitored by the vehicle so it can properly compensate for that.

Thanks for the input. I ended up using the 12 volt outlet below the arm rest. That has worked well. Thanks also for posting the pin out on the amp. I wired mine with outputs tapped from the HU but now might go back and take the outputs from the front woofer outputs from the amp.

 

[Idleup]

Some things are better left alone, try just enjoying the the goal silence only this great car can afford.

Hi,

Any engineers/electronic experts willing to offer some advice?

I have been testing a sub woofer installation. While I was doing that I was running the sub from an external 12 volt battery while I evaluated the amperage draw. For my needs the average draw was only 2-3 amps which would mean I could theoretically run the sub from the cigarette lighter outlet which is good for 140 watts (or 11 amps).

But while I had the external battery wired to the sub I got to wondering if it would be a good plan to leave it in place. If I ran the cigarette lighter leads to the second battery and the sub power leads from the second battery terminals, then it would be effectively in parallel the the car's 12 volt battery. It would also be able to deliver the short high peak load that the sub might draw for transient high bass load. I assume that the car would see this as just a bigger battery overall and they would both be charged from the DC to DC converter.

Is this true or flawed reasoning?

Any reason not to leave a second battery in place? Assuming the second battery is fully charged and equal in voltage to the car battery then there should be no current flow between the two when you connect them. After that as the sub draws down the second battery it would be charged at a minimal rate from the car's 12 volt battery which in turn is maintained by the DC-DC charger.

The battery I had in mind is an AGM battery. Small footprint and only weighs couple pounds but can do high transient loads.

Thoughts and ideas would be much appreciated.

Thanks.