The Ultimate Deep Cycle Marine Battery Guide

Choosing the right deep cycle marine battery for your boat can be tricky. Consider this your ultimate guide to the deep cycle marine batteries on the market.

Batteries have come a long way since 250B.C.E., Sumeria. Deep cycle batteries, in general, are everywhere we look around us, but we don’t notice.

The ones you’re more familiar with are on your sailboat or yacht and are quite large and heavy. Did you know a kind of deep cycle battery is in your pocket? That’s right—the Li-Ion battery that powers your cellphone is a deep cycle battery!

Want to know more about how a deep cycle marine battery works? We thought you might—so don’t stop here and keep reading!

Deep Cycle Marine Battery Types

As a boat owner, the days of messing with the chemistry of your battery are over. There are many types of batteries available these days that are plug-and-play.

That’s not to say you don’t have to think long and hard about what kind of battery you will use. Suppose you go with Lithium-based batteries. You also need to think about the Battery Management System (BMS). But let’s not get ahead of ourselves.

Flooded Lead-Acid (FLA)

FLAs or Flooded Lead-Acid batteries are a reliable standby, even if they aren’t the latest and greatest technology that ever was. The idea of an FLA is the first battery design, as found in Sumeria.

Many people swear that FLAs are the best for them. While that might be true, it is likely because they are familiar with the technology and how to maintain it. And yes, you will have to maintain it. It will off-gas, and you’ll need to measure the fluid inside with a hydrometer.

Remember that you are dealing with liquid, fluid acid, but there is also vapour to consider. Keep your eyes behind goggles, wear gloves, and it’s a good idea to wear respiratory gear. Boats are not known for their spaciousness, and vapours can be very harmful.

These also tend to be the heaviest battery type and most challenging to maintain.

Sealed Lead-Acid (SLA)

A Sealed Lead-Acid battery, or SLA, is a no-spill “no maintenance” battery type, which is very popular. It emits gas only when under a certain pressure. One of the drawbacks to this kind of battery is, of course, that it contains lead.

Another drawback is that it won’t charge as intensely as an FLA battery type. However, the lack of maintenance on an SLA is a considerable benefit, as you won’t need to keep a hydrometer with your other boating tools.

VRLA (or Valve Regulated Lead-Acid) and SLA batteries are often regarded as the same but not so exactly. While an SLA is a VRLA, Gel and AGM are also considered under the VRLA umbrella at times.

Absorbed Glass Mat (AGM)

Battery technology is getting quite impressive. A great testament to that is AGM or Absorbed Glass Mat technology. It is a micro-fibreglass porous mat between each of the lead cells, so there is no spillage.

These batteries are lighter than most other Lead-Acid batteries, and that alone is a great advantage. Another benefit is the cycles and lifetime are higher than most other LA batteries too. Because of their construction, they’re touted as being mountable in any orientation.

It’s still a good idea to get professional advice about this kind of thing.


Gel batteries are very similar again to SLA batteries and are a no-spill technology. The difference with this kind of battery is that they add silica gel to the acid in the battery to stabilise it.

You can consider Gel batteries safer than normal fluid batteries in general. If the casing is somehow damaged, nothing leaks out.

The downside is that it is completely sealed, so there is no maintenance that you can do, should the need arise. The solution, if there’s a problem, is to recycle it and replace it.

Lithium-Based (LiFePO4)

Lithium-based batteries (especially LiFePO4) are an incredible technology. It’s routine for compounds in this family to have more than 2000 cycles!

The way it works is by putting a paste of Lithium-Iron-Phosphate on a positive aluminum electrode.

There’s a polymer membrane in the middle that allows only the Lithium ions to pass through. They move into a carbon lattice with a negative copper electrode while it charges. It discharges by going back to the FePO4 side.

It is usually rolled together on skinny layers into a coil and stuffed into a steel tube with a positive and negative end. It looks pretty much like a AA or LR6 size battery. These cells are then connected in series to increase the voltage to the equivalent of a 12V battery—viola!

Some words of warning:

  • Keep the battery under 45 C
  • Keep battery temperature above 0 C when discharging
  • Keep currents under 0.5C (0.2C preferred)
  • Don’t cycle below 10% – 15% SOC (State of Charge)
  • Don’t store the battery at 100% SOC
  • Don’t charge to 100% unless you have to
  • Never use without a good BMS

This battery pack is very similar to what you would see in a Tesla, for example, and needs a highly sophisticated Battery Management System. Each of these cells needs to be balanced and managed, which is what the BMS is for. Otherwise, it won’t get near the cycles or charge as quickly.

Connecting In Parallel VS Series

A quick word on connecting your deep cycle batteries: they usually only have about 100AH (Amp Hours) or C20 equivalent. To increase capacity (or voltage from 12v to 24v system), you need to figure out how to connect your batteries.

Connecting in parallel looks about the way it sounds, negative to negative terminal, and positive to positive. It is like taking several jugs of water and lining them up, so the one at the top is pouring into the one below it. The water will flow from the top jug into the next one and so on.

In this way, it discharges at only 12V, but you get 100AH multiplied by the number of batteries.

Connecting in series links negative to positive and positive to negative of the next battery in the series. It is like pouring all the jugs at once, side by side. They’re all draining at the same time, even if you only need to fill a pint.

A lot gets wasted this way unless you need to discharge quickly, since it is 12V multiplied by the number of batteries.

Fully Charged: Anchors Up

The world of the deep cycle marine battery is a very diverse landscape, and it’s difficult to know where to turn for your needs. It’s difficult to know which battery to install, how to install it, and why this one is the best.

We know this guide will be helpful the next time you need to think about a deep cycle battery. Make sure to replace all your batteries at once for the best effect. Otherwise, it’s like a chain with a weak link.

Speaking of chains, this ship is sailing! Contact RB Battery to get help with your specific project today!

Reliable Power Delivered Worldwide