Advice on Parallel Battery Charging Options

[Redbone]

Looking for advice on wiring to keep the primary/starter and secondary/accessory battery charged. I'm running them in parallel (positive to positive/negative to negative), but found conflicting options on keep both charged or nothing at all about alternator connections. (Yes, I have the Big 3 wiring). Which option is best on keeping both charged while running them in parallel?
Option #1
Run batteries in parallel. Connect only the primary battery to the alternator. (I heard this will work but the secondary battery will lose charge faster.)
Option #2
Run batteries in parallel,. Connect a separate positive wire from the alternator to the primary battery. Connect a separate ground wire to the accessory battery.
Option #3

Don't run batteries in parallel. Connect each battery separately to the alternator.​

I've also seen where I need to add an inline fuse to the positive wire close to the battery. Someone said to add an inline fuse to both ends. Thought on this?

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[CarAudio.com - Hal AI Bots]

Looking for advice on wiring to keep the primary/starter and secondary/accessory battery charged. I'm running them in parallel (positive to positive/negative to negative), but found conflicting options on keep both charged or nothing at all about alternator connections. (Yes, I have the Big 3 wiring). Which option is best on keeping both charged while running them in parallel?
Option #1
Run batteries in parallel. Connect only the primary battery to the alternator. (I heard this will work but the secondary battery will lose charge faster.)
Option #2
Run batteries in parallel,. Connect a separate positive wire from the alternator to the primary battery. Connect a separate ground wire to the accessory battery.
Option #3

Don't run batteries in parallel. Connect each battery separately to the alternator.​

I've also seen where I need to add an inline fuse to the positive wire close to the battery. Someone said to add an inline fuse to both ends. Thought on this?

When wiring two batteries for a car audio system, especially in parallel, it’s crucial to ensure both batteries are charged effectively and safely. Let's break down your options and the concerns regarding inline fuses.

### Option Analysis

1. **Option #1: Run batteries in parallel, connect only the primary battery to the alternator.**
- **Pros:** This is the simplest method and will charge the primary battery effectively.
- **Cons:** The secondary battery may not receive sufficient charge, leading to a faster depletion, especially if the audio system draws significant power.

2. **Option #2: Run batteries in parallel, connect a separate positive wire from the alternator to the primary battery, and a separate ground wire to the accessory battery.**
- **Pros:** This method allows the primary battery to charge while providing a dedicated ground to the secondary battery, which can help in reducing voltage drop in the system. However, the secondary battery may still not be charged effectively unless specifically wired to the alternator.
- **Cons:** The secondary battery still relies on the primary battery for charging, which may not be sufficient.

3. **Option #3: Don’t run batteries in parallel, connect each battery separately to the alternator.**
- **Pros:** This is the most effective way to ensure both batteries are charged fully and independently. Each battery can be monitored and maintained properly.
- **Cons:** More complex installation and may require additional wiring and space in the vehicle.

### Inline Fuse Considerations
- **Adding an inline fuse:** It's a good practice to add an inline fuse to protect your wiring from overcurrent. This should be placed as close to the positive terminal of each battery as possible.
- **Fusing both ends:** Generally, using one fuse close to the battery is sufficient, but if you have long runs of wire or if the wire is particularly thick, using an additional fuse on the other end can provide extra protection against shorts or overloads.

### Recommendation
For the best performance and safety:
- **Consider Option #3**: This will ensure that both batteries receive adequate charging from the alternator without one battery depleting the other.
- **Use fuses on both batteries**: This will help safeguard your installation.

### Additional Notes
- Ensure both batteries are of the same type and capacity to avoid issues related to charging and discharging rates.
- Consider using a battery isolator or a smart battery management system if you want to maintain parallel wiring while ensuring both batteries are charged properly.

Feel free to ask more questions or for clarification, and I encourage other members to chime in with their experiences or additional advice!

 

[dragon.breath 10+ year member]

You definitely need a fuse on both ends of the power wire between the two batteries. Think about it. The power wire gets chafed and shorts out on something ( or, you have an accident) The fuse at the front battery blows. But the second battery is still shorted out. No rear fuse, I hope you like fighting fires.

 

[Redbone]

When wiring two batteries for a car audio system, especially in parallel, it’s crucial to ensure both batteries are charged effectively and safely. Let's break down your options and the concerns regarding inline fuses.

### Option Analysis

1. **Option #1: Run batteries in parallel, connect only the primary battery to the alternator.**
- **Pros:** This is the simplest method and will charge the primary battery effectively.
- **Cons:** The secondary battery may not receive sufficient charge, leading to a faster depletion, especially if the audio system draws significant power.

2. **Option #2: Run batteries in parallel, connect a separate positive wire from the alternator to the primary battery, and a separate ground wire to the accessory battery.**
- **Pros:** This method allows the primary battery to charge while providing a dedicated ground to the secondary battery, which can help in reducing voltage drop in the system. However, the secondary battery may still not be charged effectively unless specifically wired to the alternator.
- **Cons:** The secondary battery still relies on the primary battery for charging, which may not be sufficient.

3. **Option #3: Don’t run batteries in parallel, connect each battery separately to the alternator.**
- **Pros:** This is the most effective way to ensure both batteries are charged fully and independently. Each battery can be monitored and maintained properly.
- **Cons:** More complex installation and may require additional wiring and space in the vehicle.

### Inline Fuse Considerations
- **Adding an inline fuse:** It's a good practice to add an inline fuse to protect your wiring from overcurrent. This should be placed as close to the positive terminal of each battery as possible.
- **Fusing both ends:** Generally, using one fuse close to the battery is sufficient, but if you have long runs of wire or if the wire is particularly thick, using an additional fuse on the other end can provide extra protection against shorts or overloads.

### Recommendation
For the best performance and safety:
- **Consider Option #3**: This will ensure that both batteries receive adequate charging from the alternator without one battery depleting the other.
- **Use fuses on both batteries**: This will help safeguard your installation.

### Additional Notes
- Ensure both batteries are of the same type and capacity to avoid issues related to charging and discharging rates.
- Consider using a battery isolator or a smart battery management system if you want to maintain parallel wiring while ensuring both batteries are charged properly.

Feel free to ask more questions or for clarification, and I encourage other members to chime in with their experiences or additional advice!

 

[Redbone]

3. **Option #3: Don’t run batteries in parallel, connect each battery separately to the alternator.**

- **Pros:** This is the most effective way to ensure both batteries are charged fully and independently. Each battery can be monitored and maintained properly.

- **Cons:** More complex installation and may require additional wiring and space in the vehicle.

I understand more wires would be involved, but what would make it more complicated? Wouldn't be just wiring like the main battery?

Are you saying an isolator isn't needed if wiring them independently?

 

[Redbone]

You definitely need a fuse on both ends of the power wire between the two batteries. Think about it. The power wire gets chafed and shorts out on something ( or, you have an accident) The fuse at the front battery blows. But the second battery is still shorted out. No rear fuse, I hope you like fighting fires.

I need a fuse at both ends only if I wire both batteries independently (Option #3)?

 

[supernov4]

Running positive from main battery to second battery
Will give you same results as Running separate power wire from alternator to second battery
It's just a few extra feet of wire that you will pigtail off of, if going off main battery
Also running from main battery to rear battery will be easier to fuse

As far as ground wire
Ideally I would run a zero awg ground from alternator mount bolt to main battery
And zero awg from batt to chassis

And secondary battery find lowest possible resistance ground point anywhere in the car
If there is nothing good to ground to in the rear just run it up front to main battery ground

 

[1aespinoza Premium Member]

Option 1

Run batteries in parallel. Connect only the primary battery to the alternator. (I heard this will work but the secondary battery will lose charge faster.)

Option1 works 100%. Only reason to use an isolator is if you use the system with the car off for extended periods.

 

[Doxquzme Premium Member]

^this^

Option 1, every time (with a parallel ground).

 

[Redbone]

Running positive from main battery to second battery
Will give you same results as Running separate power wire from alternator to second battery
It's just a few extra feet of wire that you will pigtail off of, if going off main battery
Also running from main battery to rear battery will be easier to fuse

As far as ground wire
Ideally I would run a zero awg ground from alternator mount bolt to main battery
And zero awg from batt to chassis

And secondary battery find lowest possible resistance ground point anywhere in the car
If there is nothing good to ground to in the rear just run it up front to main battery ground

Should I add an inline fuse between the alternator and main battery?

 

[Redbone]

Option 1

Option1 works 100%. Only reason to use an isolator is if you use the system with the car off for extended periods.

Should I add an inline fuse between the alternator and main battery?

 

[Redbone]

You definitely need a fuse on both ends of the power wire between the two batteries. Think about it. The power wire gets chafed and shorts out on something ( or, you have an accident) The fuse at the front battery blows. But the second battery is still shorted out. No rear fuse, I hope you like fighting fires.

Should I add an inline fuse between the alternator and main battery?

 

[Doxquzme Premium Member]

If there is one, yes; if not, IT'S optional.

 

[supernov4]

Should I add an inline fuse between the alternator and main battery?

I personally don't
But it would be a safer bet
Logic says if that power wire gets cut or unbolts itself from alternator it can short out on the frame

If you choose to add it, it should be close to the battery

 

[1aespinoza Premium Member]

Should I add an inline fuse between the alternator and main battery?

I have never had one or seen any OEM vehicle that has one. Do it if it gives you peace of mind.