Electric motors are great, they have few moving parts to maintain or fail, and they can run effortlessly forever…or until the power runs out!

The heart of an electric setup is not the motor, but actually the batteries. These boxes of energy house the electricity that the electric motor needs to propel your yacht. The more power you can carry in your batteries, the more power you can use with your motor.

When we did our conversion to electric propulsion back in 2014, we chose AGM batteries, simply because they were the most cost effective at the time. They provided us with a lot of power in a large space and required little maintenance.

Fast forward to the present, and our AGMs are old and outdated. Better battery technology has come down in price and the “better batteries” of 2014 are no longer prohibitively expensive.

At the moment, there are two companies that seem to be reigning as king over the “direct replacement” lithium batteries. These companies sell lithium batteries that are the same size as the more popular lead acid and AGM batteries, but at a fraction of the weight.

The first company is Battle Born

Battle Born Batteries

These batteries are the same size as a Group 27, but contain the power of a Group 31. Moreover, they only weigh 31 pounds! Our current AGMs are Group 31 and weigh 67 pounds each, meaning these batteries weigh less than half of what our batteries weigh.

The next battery company that we will look at is Renogy.

Renogy Batteries

Renogy Batteries are also direct replacements for lead acid and AGM batteries, but these are even lighter in weight! Their 100ah battery weighs in at only 28 pounds!

Now that we know the major players in “direct replacement” batteries, lets look at why they are more sought after than a regular lithium battery. The first thing is familiarity. For our entire lives, batteries have been this rectangular box that sits in our boat or car and has two terminals on it. Lithium batteries can come in a huge variety of shapes and sizes, which sadly leads to people wondering “which side is the top?” Direct replacement batteries do away with this metal block to lithium by packing it into a familiar shape and size.

The second advantage comes with the voltage. Lithium batteries are made up of cells, and the entire battery pack can have any voltage value you want! This once again becomes a problem since people are used to a battery being 12 volt. In our case where we run a 48v motor, we simply have four 12 volt batteries hooked up in series to create the 48 volts we need to operate. Instead of Battle Born and Renogy offering their batteries in “custom voltages” they packaged them as 12 volt batteries.

You might be wondering, why switch to lithium at all if they take up practically the same space and give you the same amount of power? There are two answers to this question. The first is weight, and the second is usable power.

Lithium batteries weigh less than half of a regular battery. The best way to visualize this is to look at the Amps per Pound at 12 volts.

AGM batteries hold 105 amp hours and weigh 67 pounds
This is to say 1.567 amps per pound

If you want to have a 800 amp hour battery bank, it will weigh (800/1.567) 510.5 pounds.

Battle Born batteries hold 100 amps and weigh 31 pounds.
This is to say 3.225 amps per pound

That same 800 amp hour battery bank would now only weigh (800/3.225) 248 pounds

Renogy batteries, being the lightest, hold 100 amps and weigh 28 pounds
This is to say 3.571 amps per pound

The same 800 amp hour battery bank would weigh just a smidge less at (800/3.571) 224.0 pounds.

So, you might be wondering now, aside from weighing half as much, why bother with expensive batteries? They take up about the same space, and if the boat isn’t overloaded, why bother? Well the truth is, all batteries hold power, but they don’t all let go of the power the same.

Think of batteries as a jar of jelly. They all hold the same amount of jelly, but they differ in how the jar opens.
Lead Acid and AGM batteries have a small opening at the top of the jar, so you can get your spoon straight in, but you can’t get to the sides of the jar. When you have gotten all the jelly you can reach, you notice that there is still a lot of jelly left on the sides, but you just can’t seem to get to it!
Lithium batteries have a much larger opening at the top, so you can get to the sides of the jar much more easily. This lets you get to more jelly and leave the jar more empty when you are finished.

Lead Acid and AGMs can safely discharge down to 40%, while Lithium batteries can safely discharge down to 10%. This means that in a 100 amp hour battery, an AGM can give up 60 amps while a Lithium can give up 90 amps.

So, back to our hypothetical 800 amp hour battery bank, made up of eight 12 volt batteries. The batteries sitting in their box hold 800 amps hours. If these batteries are AGM or Lead acid, they could only give out about 480 amp hours. This identical battery bank made of Lithium batteries could now give out 720 amp hours. That’s an additional 50%, from half the weight!

So, bringing the battery bank and electric motor out of the hypothetical realm and into reality:

Our electric motor draws from our battery bank composed of eight AGM Group 31 batteries, each with 105 amp hours. We can motor at a slow speed of 2 knots for about 20 hours, giving us a range of 40 miles. With a Lithium bank, we would be able to motor 60 miles! That might not sound like much, but that is a lot of miles from the same size of battery bank. Yes, more batteries would give more miles, but space is an issue on a sailboat.

The switch to Lithium will be coming, and we will then be able to compare lithium batteries to AGM in terms of usability and performance.