“How do I connect my batteries in series? How do I connect them in parallel?” We get these questions more than we can count. In this Tech Talk, Lester Miller, Founder of Millertech, heads to our battery testing lab to walk through the difference, how to hook up your charger and trolling motor, which cables to use, and the setup we recommend.

An Easy Way to Remember It

An old friend gave Lester a trick he’s used ever since:

Parallel sounds like pair. You’re connecting a pair of batteries side by side. The voltage stays the same, and only the capacity goes up.
Series sounds like serious. The voltage goes up, and that’s serious voltage.

Parallel: Same Voltage, More Amp Hours

In a parallel connection, you connect positive to positive and negative to negative. The voltage doesn’t change, but the amp hours add up. In the video, Lester connects two 36V 50Ah batteries in parallel. The pack is still 36V, but now it holds 100Ah, twice the run time of one battery.

Series: Higher Voltage, Same Amp Hours

In a series connection, you connect negative to positive, battery to battery. The voltage adds up, and the amp hours stay the same. Lester’s example is three 12V 100Ah batteries. Connect two and you have 24V. Add the third and you have a 36V 100Ah pack. Your charger or trolling motor connects to the open positive terminal on one end of the string and the open negative terminal on the other end.

You can connect up to four MillerTech batteries in series or in parallel.

Wiring at a Glance

In series, the charger or trolling motor goes on the two open ends. In parallel, the charger and the motor each connect across both batteries.

Where to Connect Your Charger and Trolling Motor

Series pack. Everything you hook up to the 36V pack, charger or trolling motor, goes from the open positive terminal at one end to the open negative terminal at the other end. That completes the circuit through all three batteries.Parallel pair. You could connect everything to one battery’s terminals and it would work. But Lester recommends crossing the connections so the load is balanced between both batteries:

Connect the charger’s positive lead to one battery and its negative lead to the other battery.
Connect the trolling motor to the two terminals that are left, so its leads are also split across both batteries.

The rule of thumb: for any load, coming in or going out, connect it across different batteries.

Use Marine Cable, Crimped RightThe cables between your batteries matter as much as the batteries. Lester compares two:

600V welding cable isn’t a bad cable. We use it in our solar systems. But we don’t recommend it on a boat. Its copper strands are bare, with no tinning, so moisture will corrode them quickly.
MillerTech parallel cables, sold on millertechenergy.com, use tinned strands that resist corrosion. It’s a marine-style cable with a few thousand more strands than the welding cable, so it’s very flexible and easy to route.

How the cable is finished matters too. We hydraulically crimp our cable ends. “We’ve seen so many boats come in with battery issues, and a lot of times it’s bad connections,” Lester says. Don’t crimp cable ends without the proper equipment.

When you install, use ring terminals and torque them to 10 ft-lb.

Three 12V Batteries or a 36V Battery?

For a 36V trolling motor, you can wire three 12V batteries in series, or run one or two batteries that are built as 36V inside. After years of seeing both, MillerTech recommends 36V.Three 12V batteries do have two advantages:

If your cranking battery dies, you can jump your outboard off one of them.
If one of the three fails, most trolling motors will still run on 24V from the other two.

But the internals of a 12V battery aren’t made for a 36V system. It works, but over time the batteries age differently. Five, six, or seven years in, the pack is only as good as its weakest battery. If one is slowly losing capacity, the whole pack drops with it.Two 36V 50Ah batteries in parallel give you the same 100Ah. If one of them goes out, you still have a 36V battery to keep fishing on.

You’re never depending on just one.The proof is in our warranty calls. “Since we went to a 36V system, I can say we’ve had probably 99% fewer calls for warranty and problems than we did when we were using three 12Vs,” Lester says. “These batteries are made for 36V inside, and they just go and go and go.”

Questions About Your Setup?

Every boat is wired a little differently. If you’re not sure how to connect your batteries, call 855-MAX-LITH (855-629-5484) and a real person will walk you through it. You can also email [email protected] or visit millertechenergy.com