can you run two car batteries in parallel

[SophieLynn]

Yes, you can run two car batteries in parallel, which involves connecting the positive terminals together and the negative terminals together. This setup increases the total ampere-hour (Ah) capacity while keeping the voltage the same as a single battery (12 volts for standard car batteries). It's a common solution for vehicles with high electrical demands, like those running powerful stereos, winches, or auxiliary lighting, without the need for a larger, single battery.

The primary benefit is extended runtime for accessories when the engine is off. However, it's critical that the batteries are of the same type, age, and capacity. Mismatched batteries can cause one to drain the other, leading to premature failure of both. A key component for a safe and effective parallel system is an isolator or a dual-battery controller. This device ensures the starting battery is prioritized for engine cranking and prevents the auxiliary battery from draining it.

Here's a comparison of key considerations:

Consideration	Single Battery Setup	Dual Battery Parallel Setup
System Voltage	12V	12V
Total Capacity	e.g., 70 Ah	e.g., 140 Ah (with two 70Ah batteries)
Primary Use Case	Standard vehicle operation	High-demand accessories, camping, off-grid power
Installation Complexity	Simple	Moderate; requires proper cabling and safety gear
Risk of Failure	Standard	Higher if batteries are mismatched or improperly isolated

To do it correctly, use heavy-gauge battery cables designed for the task, install appropriate fuses close to each battery's positive terminal, and secure the batteries firmly to prevent movement. For most DIY enthusiasts, consulting a professional or a detailed guide specific to your vehicle is highly recommended to avoid costly mistakes and ensure safety.

[VanHadley]

Yeah, it's totally doable and pretty common in trucks and RVs. You just hook positive to positive and negative to negative. The big thing is to make sure both batteries are identical—same brand, same model, same age. If one is old and weak, it'll just kill the new one. It’s a great way to power a fridge or lights all night without worrying about your truck not starting in the morning. Just get the right thick cables and maybe a battery isolator; it’s a solid weekend project.

[MacRiley]

From an electrical standpoint, connecting batteries in parallel increases the current-supplying capability and capacity while maintaining a constant voltage. The critical factor is internal resistance. If the batteries are not perfectly matched, one will bear a disproportionate load, leading to inefficient charging and reduced lifespan. This isn't a simple plug-and-play modification; it requires calculating the total expected load, selecting cables with sufficient cross-sectional area to minimize voltage drop, and integrating overcurrent protection. Proper installation is paramount to prevent short circuits.