​The Basics. Battery Types and Battery Efficiency. 

​Basically, batteries are electrical storage devices. They do not make any electricity, they only store it for future use. A key point to keep in mind; they are not 100% efficient. The energy stored in a battery is lost during charging and discharging as well as through heat and chemical reactions. If you use 1000 watts from a battery, it may take more than 1200 watts to recharge it and you still only get 1000 watts from it. There is also a limit to the usable energy available based on the battery storage type. 

​Efficiency is related to the materials making up the battery storage elements. ​​Alkaline and NiCad batteries like we use in flashlights have a low efficiency, typically around 60-65%.

Modern lead-acid batteries can vary from 85%-98% depending on quality of materials used in manufacturing and type. These include Flooded batteries, Gelled, and Sealed Absorbed Glass Mat (AGM) batteries. 

​Lithium Ion (Li-ON) batteries can achieve a 99% efficiency. In addition, you can use almost 90% of the stored energy in a Li-ON battery, compared to only ​30%-50% of what is stored in lead-acid batteries. The trade off is much higher cost and some increased safety concerns. 

​The most common battery type used for marine deep cycle purposes are lead-acid. The offer the best price-to-power ratio. AGM's are the best type for use on cruising boats due to their ability to handle a large number of charging cycles, little chance of any leakage if cracked, low off-gassing emissions, and improved energy efficiency. 

​Flooded type are the very least expensive, but require regular watering, higher maintenance requirements, give off large amounts of hydrogen gas when charging (which will set off your carbon monoxide detector), and typically do not hold up as well with multiple discharge/recharge cycles seen in a marine environment. These are typically advertised as "heavy duty" or "commercial" batteries for large trucks. OK for those looking to go cheap but be aware they are poor choice for long term use on a boat. You get what you pay for, always. 

Compared to other battery types, AGM's have a very low self-discharge rate, typically 1%-3% per month. This means they can sit in storage for much longer periods without a need for trickle charging or re-charging as other lead-acid battery types. ​

​Battery Lifespan

​The lifespan of a deep cycle battery will vary considerably with how it is used, how it is maintained and charged, temperature, and other factors. It can vary if the battery is in extreme heat or extreme cold conditions, if it is overcharged or undercharged regularly, or if wet cells are allowed to dry.  How often a battery is charged and discharge and if it's done correctly each time also has an impact. Each of these factors will ruin the lifespan of a battery quickly.  Because of this, buying a used battery is never a good idea. Knowing this, you can help to prolong the lifespan of your batteries by treating them well.  

As a general rule, you can expect an AGM battery that is well maintained to last approximately 5-8 years.

A lead-acid battery is considered DEAD and not rechargeable once it reached 10.5 volts. This is related to the specific gravity of the acid in the battery and is a chemical limitation. If you find a battery at or near 10.5 volts, it's unusable, unsalvegable, and not worth any time or money to try and revive it. Consider yourself informed.

​A new battery should be given a full charge before use and needs to be cycled  ​multiple times before reaching full capacity. Never let a new battery get below​ 70% ​state of charge before it's been cycled at least 20 cycles or you risk significantly shortening it's lifespan. 

To maintain a healthy battery, deep cycle batteries will last longest if they are routinely kept from reaching less than 80% ​state of charge. As a rule you never want a deep cycle battery to reach less than 50% depth of discharge.

One of the biggest killers of batteries is sitting stored in a partly discharged state state for a few months, especially at higher temperatures. 77 degrees F is the ​reference-line for battery life. For every degree ​above 77, the lifespan diminishes by ​approx 3.5%. Likewise, the lifespan improves at the same rate for temps below 77 degrees F.

Battery Voltages

​All lead-acid batteries supply about 2.14 volts per cell. (12.6 -12.8 volts for a "12 volt" battery) when fully charged. Batteries stored for long periods will eventually lose all their charge. This "leakage" or self-discharge varies considerably with battery type, age, and temperature. It can range from 1% to 15% per month.

State of Charge (or Depth of Discharge) Explained

State of charge (SOC), or conversely, the depth of discharge (DOD) can be determined by ​measuring the voltage of the battery. Keep in mind the voltage of a battery will not tell you the condition of a battery, except that of a dead one. Only a sustained load test can determine if a battery is still usable when in question. Voltage on a fully charged lead-acid battery as stated previously is about 2.14 volts per cell (VPC) or 12.84 volts on a 6 cell/12v battery. A battery at a 50% SOC will read approx 2.03 VPC (12.18 volts). A battery that is dead will read 1.75 VPC (10.5 volts)

"False" Capacity

​It is possible for a battery to show a full charge and yet have a significantly diminished capacity. This happens when a batteries cells are damaged from age, plates become ​covered with lead sulfate, or gel-cells develop bubbles. When voltage tested they appear fully charged, but once but under a load they rapidly discharge all their energy. This is why a load test is the only way to fully examine battery health when it's in question. Don't rely on voltage readings.

When To Charge A Battery

​To maximize the life of a battery, always recharge it immediately after using it, and you shouldn't let it sit in a discharged state for more than 24 hrs. The longer a battery sits in a discharged state after use has a direct impact on it's lifespan. Recharge should start anytime the battery reaches 80% SOC routinely and it should never be allowed to get to the dreaded 50% SOC.

A battery is considered fully discharged when the voltage reads 11.9 volts. Ideally, you never want your battery to get to this level, as it will shorten the lifespan and reliability significantly each time you let it fully discharge. ​​​

Why Marine Batteries Fail

When a lead-acid battery is discharged, a soft lead sulfate material forms on the battery plates. During the battery's recharge, this material is lifted off the plates and is recombined into the battery's electrolyte solution. If, however, the battery is left in a partial state of discharge for as short as 3 days, the lead sulfate materials will begin to harden and crystallize, forming a permanent insulating barrier. As the barrier gets thicker and thicker from repeated periods of sitting dormant in a state of discharge, the ability for the battery to accept a charge or deliver energy is diminished. This accumulation is called sulfation, and is the most destructive process in the life of a lead-acid battery.  Once this sulfation reaches a certain point, the battery is no longer viable and will not hold a charge. Dead batteries cannot be magically "reclaimed" with any expectation of longevity and should be replaced and recycled. Don't waste money on "magic" fluids that claim to revive dead lead-acid batteries.

Replacing Batteries In A House Bank

​Yes, it's expensive, but our goal should be to replace all connected batteries at the same time. Adding a new battery to a bank of old batteries is the best way to waste money. When batteries are connected in parallel or in series, connecting a new battery will not energize the stack to the level of the new battery. Unfortunately, the new battery will quickly be degraded down to the level of the older batteries. This is also the case if you bring in an old battery to a new battery bank. Because of the way electricity works, old batteries always ​pull energy from, and ruin the life of a new battery when they are tied together.  If you have to replace a single battery in a bank for some reason, it's best to find one that is the same age and usage as the one it replaces or you'll be wasting money. Unfortunately, that's just how the chips fall.

​How Batteries Are Rated And Grouped

In order to have a constant standard for rating how much power a battery can store, manufacturers indicate how many amps of current draw it takes to drain their battery to ​10.5 volts. As previously mentioned, a battery that falls below 10.5 volts is considered dead, so this voltage specification is nominal for the purposes of determining the amp-hour rating of a battery.

Understanding Amp-Hour Ratings (AH)

Typically for a deep cycle lead-acid battery, the time period is 20 hours ​so the standard is a 20 amp hour rating, but it's not uncommon to see a battery label with 5 and 10 amp hour ratings indicated.

IF you see a "100 amp-hours (AH)@20 HRS"  you need to divide the 100 AH by 2o HRs to determine this battery promises it will deliver 5 amps every hour for 20 hours until it reaches approx 10.5 volts and needs a full recharge.

Reserve Capacity (RC)

Some deep cycle batteries provide "Reserve Capacity (RC)". This specifies how many minutes the battery will provide a 25 amp current at 80 degress F until the battery reaches the 10.5 voltage. 

Amp-hours vs Reserve Capacity

While these both provide good information, it's not the same information.  

Reserve Capacity tells us how long our battery will last with a 25 amp load attached to it. 

Amp-hours tells us what the max current we can draw and still get 20 hours of service from our battery.

​BUT; there is are caveats to this information many people do not consider. ​Two specific issues will affect the amount of amp-hours you will actually have from your battery system:

  • Peukert’s Law applies here. As the law indicates, battery capacity decreases as the current draw increases. Similarly, battery capacity increases as current draw decreases.
  • Battery loads tend to be dynamic. In other words, they may be changing all the time depending on what devices are on and how they are being used. Coupled with Peukert’s Law, figuring out how long a battery will last can represent a pretty monumental task.
  • ​Because of these, you may get a better idea of how much battery you need using the RC for marine applications due to the higher number of amp load typically found on boats.


    That's a lot of information to digest, so when you're ready, just click below to start learning about how to build a House Battery Bank and how to attach multiple batteries to create your perfect system.


    {"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}

    Subscribe to our newsletter now!

    © Copyright 2022. All rights reserved.