Wiring Batteries In Parallel Danger: What You Must Know

Wiring batteries in a parallel configuration is used in many applications. And with good reason, because it allows you to increase your battery capacity yet still operate with the same voltage.

For example, many people have a battery bank in parallel connection in an RV or a solar panel system.

But is it safe? Is wiring batteries in parallel dangerous? Overall, it is safe. However, there are some things that you need to be made aware of to avoid any danger. This post will review the common dangers of wiring batteries in parallel, including:

  • Connecting batteries
  • Overcurrent and heat
  • Uneven charging and discharging
  • Incorrect wiring

Knowing what to look out for and the proper way of connecting batteries will reduce your risk of danger.

Table of Contents


The first concern that we will review is the batteries themselves. Since wiring batteries in parallel involve at least two batteries, many dangers can occur when not selecting the correct batteries to connect.

Connecting Batteries

The first danger to be aware of is your battery setup and not having mismatched batteries. As you look into the secondary battery, try to avoid old batteries. Sometimes, when looking for more battery capacity, we look around our garage and find an old 12 volt battery sitting there. Connecting that old battery to a new single battery may be tempting, but I would avoid this.

Always try to connect batteries of a similar age and battery life. It is not a bad idea to test your old battery before trying to connect it to a second one. Many auto stores will test batteries for free (in hopes you will buy from them should you need a new battery).


The next concern with mismatched batteries is using the same battery type. Just because you have two 12 volt batteries, it doesn’t mean they are the same and can be wired together in a parallel configuration.

One issue to avoid is using a flooded lead acid battery connected to a lithium battery. Lithium ion batteries have become more popular in our everyday lives. You are seeing them in cars more, as well as for RV’s, solar batteries, or an inverter.

The key is not to mismatch the lithium battery with any other battery. The main reason is that lithium ion batteries have a different internal resistance than a lead acid battery or even an AGM battery. And when you charge or discharge batteries with different internal resistance, you will get a voltage imbalance that can cause overheating, reduce efficiency and give you hard or unpredictable voltage drop.

Car Battery vs Deep Cycle Battery

Another concern to look out for in the batteries connection is car battery vs deep cycle battery. We reviewed before that it is common to find an old battery and try to connect to a new one for more capacity. Besides the age of the battery, another concern is connecting a car battery to a deep cycle battery.

Do not connect a car battery to a deep-cycle battery.

It is advised never to connect these two batteries together. Even though they can both be a 12 volt battery, their internal resistance and power supply are totally different. A car battery provides a burst of power supply to the starter to rotate and start the engine. A deep cycle battery will give out slow consistent voltage until it almost drains completely.


As you create a battery bank, verify the battery capacity is similar to each battery. An easy way to do this is to check out the amp hour rating on each battery. Each battery should have a label giving you the amperage rating or amps.

Check amp hour rating

The reason for the same battery capacity is to have your batteries and charge and discharge at the same rate. This will give you better performance and even wear and tear on the batteries.

If you are unsure of the capacity, check the outside dimensions of the battery are similar. Usually, the bigger the battery, the more capacity. Ensure that both batteries are similar in size, which is a basic indication that they are similar in capacity.


This may sound obvious, but let’s cover it just in case. You want all the connected batteries to be the same voltage. In most cases, we have been reviewing 12 volt batteries and connecting them to each other. And when wiring in parallel, you get the same output voltage of 12 volts.

But what if you have different voltage batteries? What if one has a lower voltage? For example, there are 6 volt batteries out there that are used for golf carts or even, in some instances, a travel trailer (see my post on the 6v vs 12v batteries for more info).

Batteries need to be the same size and voltage.

The answer is to not connect low voltage battery with a higher voltage battery. You always want your parallel connection to be the same battery voltage for each battery.

Wiring Batteries

Previously, we reviewed the potential dangers that occur with the battery itself when connecting in parallel. Now let’s review the pitfalls that can occur with the wiring.


When setting up your parallel wiring, the main issue for danger can be the wrong wire size or jumper cables. It may be tempting to find any old wire to make a connection. But using an undersized wire to connect batteries can lead to overheating and fire risk.

Use a thick jumper cable to connect.

When you do parallel wiring, all the battery power is transferred from one battery to the next. If you have a smaller wire, that will create resistance and a large voltage drop. That will produce heat and cause a safety risk.

You want to use a thick battery cable for your parallel wiring. I suggest you use the same size or large of the main cables that were connecting your battery to your equipment. If using a 12 volt battery, many of these wires are 4 gauge or 2 gauge wires. Again, very thick wire. When in doubt, go thicker. It does not hurt anything to go with a thicker wire.

Incorrect Wiring

Another wiring danger can be incorrect wiring. A parallel connection is when you connect the two positive terminals of two batteries with one jumper wire. And then, connect the two negative terminals of two batteries with another jumper wire.

You need to be careful about how you wire those parallel batteries to your piece of equipment. The best way to do this is to take the positive cable from your equipment and connect it to a positive terminal on the battery (no trick here). Then take the negative cable from your equipment and connect it to the OTHER battery’s negative terminal (that is the trick). You want one cable going to each battery.

The advantage of connecting this way is that the two batteries will act more like one. Charging and discharging will be even as the batteries share the output voltage.

This is the same setup you want to use when connecting to a battery charger.  One cable should go to each of the connecting batteries. 

Connect one positive and one negative cable to the equipment

If you connect both cables from your equipment to just one battery, that first battery will drain quicker, giving you a chance for more inconsistency in voltage and charging.

Final Thoughts

Wiring batteries in a parallel configuration offers numerous advantages, such as increasing battery capacity while maintaining the same voltage level. This approach is widely used in applications like RVs and solar panels to create a battery pack. While generally safe, there are crucial considerations to prevent potential dangers. Mismatched batteries, overcurrent and heat issues, short circuits, and uneven charging and discharging can all pose risks in parallel battery setups.

Selecting identical batteries, avoiding mismatched types like lithium and lead-acid, using appropriate wire sizes, and correctly connecting positive and negative terminals are essential steps to ensure the safety and optimal performance of parallel battery systems. By adhering to these precautions, users can harness the benefits of increased capacity without compromising safety or functionality.