How to Tell If a Deep Cycle Battery is Bad

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RV batteries typically last for several years.

However, when they’re performing unreliably, it’s good to know how to tell if a deep cycle battery is bad. This can save you from using a battery that’s damaged, sulfated, or worse.

In this article, we briefly cover the types of RV batteries available and where they’re located. Then the remainder of it goes into depth about the testing procedures.

Types of RV Batteries

There are two types of RV batteries:

  • Starter batteries
  • Deep cycle batteries

Starter Batteries

Starter batteries are designed to provide a high surge of power to get the engine started.

They’re common in RVs, automobiles, and boats too.

There are hybrid batteries with both starter and deep-cycle functionality, the hybrid type is far more common in boating than in RVing.

The starter battery typically will perform a certain number of engine starts before the battery is low and needs to be recharged.

Deep Cycle Batteries

Deep cycle batteries are different from starter batteries. This is because they perform different functions.

Instead of a strong burst of cold cranking amps to get an engine started, 12-volt deep cycle batteries are designed to reliably provide a steady flow of electricity.

It’s exactly what’s required in an RV to power your lights, refrigerator, and heating without the battery presenting regular power problems.

There are different types of deep cycle batteries: AGM, lead-acid, gel, lithium, etc. However, a detailed explanation of each type is beyond the scope of this article.

However, we do touch on these types as they become relevant within this article.

An example of a popular lead-acid deep cycle RV battery is below:

Universal Power Group Sealed Lead Acid Battery (Model 45978)

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One commonality to all but the lithium type is that they should only be depleted down to between 40-60% to avoid damaging the battery’s cells.

Each manufacturer and product information differs on this, so check your battery’s manual or specifications to know the minimum safe charge level for certain.


Therefore, while a battery’s capacity might be 110Ah, it’s probably only safe to use roughly 50-55Ah of its full capacity from a fully charged state.

With the lithium battery type, it may tolerate being depleted down to 20% or less. It’s one of the advantages of lithium, however, they’re vastly more expensive, and need to be replaced as a pack.

The majority of RV owners won’t have lithium installed for their house batteries unless originally installed at the factory or a previous owner swapped over to them. In both cases, that’s rare.

The best lithium battery for an RV is the Renovy LiFoPO4 Deep Cycle 12V 100Ah battery. There is an optional monitoring screen, which is handy for RVers too:

Renogy 12V 100Ah LiFePO4 Deep Cycle Lithium Battery. 4,000+ Cycles. Built-in BMS. Maintenance-Free

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Read Also: How To Remove Sticker Residue from Camper

Locations for RV Batteries

The starter battery will be located up front as you might expect.

The house batteries (sometimes referred to as coach batteries) are 12-volt ones typically situated somewhere in the main coach area.

Under the dinette seating is common. But you may need to check in the RV’s manual or hunt around to locate them if you’re unsure.

How to Tell If a Deep Cycle Battery is Bad

There is a procedure to follow to test a deep cycle battery.

While deep cycle batteries are usually 12-volt ones, this isn’t a steady voltage, and the number is rounded down.

The voltage will begin a little higher (12.7 V) confirming a 100 percent charge. Then as the battery is drained of its stored power, the voltage will noticeably decline.

At a 50% level, the voltage reading should show around 12.2 volts, and with a 20% remaining charge, it will read approximately 11.98 volts.

To perform the procedure, two of 3 of these tools are useful to own:

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AstroAI Digital Multimeter TRMS 6000 Counts Volt Meter

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We will cover each of these through our explanation of how to test your batteries.

Ancel BST200 Car Battery Tester (12V 100-1100 CCA)

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Read Also: Charging Trailer Battery from Tow Vehicle

Protect Yourself When Working with Batteries

Wear thick gloves and eye goggles to protect yourself when working with batteries.

Depending on the battery type, they can contain acid or other substances that can spill out.

Therefore, wearing protective gear is important. Also, if you have kids, then have them play elsewhere for safety reasons.

Performing a Pre-check on Deep Cycle Batteries

To get more accurate test results, a full charge is best.

When the battery cannot be charged, rest it for 1-2 hours before returning to it.

The rest period will help to get better battery readings using the above-mentioned tools because deep cycle batteries need time to settle.

A visual pre-check is useful with batteries to look for irregular things that could indicate a problem.

Here are a few of the major things to visually look for in a pre-check:

Malformed battery case – When a battery case has a bulge or is otherwise not in the same regular shape it usually would be, that’s suggesting it is damaged. It would need replacing and not charging again. The battery is now a safety risk. It must be removed and replaced.

Leaking – Even minor leakage from the battery is a worry. Major leaking means the battery should be written off completely. While a battery can leak occasionally, it’s a bad sign and we’d want to replace that battery if we observed this during a check.

Terminal broke or damaged – The terminals that provide the connection to hook onto the need to be in good condition. When one or more is noticeably damaged or broken off, that battery is done. Replace it immediately and never use it again.

Plastic casing damaged – The battery’s casing could be cracked or broken in some way. This indicates either that the battery was previously bulging out and forced it to crack, or it was damaged during transit. Either way, the battery should be replaced because its safety for use is uncertain. Do not risk the use of a damaged battery.

Changing color – A change in the observable color of the battery or the case is a no-no. It suggests something is wrong inside, possible internal leakage, or worse. The battery again needs to be scrapped and not used.

If you see any of the above observables, remove, dispose of, and replace the affected battery. When doing so, it’s best to replace it with a matching battery of the same make, and model.

Damaged batteries pose a major safety risk. Do not risk continuing to use them for any duration.

Read Also: How Long Will A 12 Volt Battery Run a Refrigerator?

How to Test a Deep Cycle Battery

Testing the Battery’s Voltage

Following a detailed look over your battery bank, and you’ve satisfied yourself that they look okay, then check the voltage reading for each coach battery.

The DC voltage readings during a deep cycle battery voltage test will confirm whether there’s a problem with one or more battery cells.

It’s useful to wait several hours before testing the voltage once the RV batteries are fully charged. It allows the batteries to settle for more accurate readings.

Use either a multimeter or possibly a voltmeter, to test each deep cycle battery. Verify the voltage reading to confirm the charged state.  

Please refer to the table below for approximate voltage reading and charge percentages:

VOLTAGE:CHARGE (%):
11.98 V20
12.04 V30
12.12 V40
12.20 V50
12.28 V60
12.36 V70
12.46 V80
12.64 V90
12.70 V100

Typically, the voltage readings will be between 12.2 volts (roughly a 50% charge level) and 12.7 volts (approximately 100% charge level).

What Should a 12 volt Battery Read When Fully Charged?

Approximately 12.7 volts will convey a 100% charge level.

Low Voltage Readings

If the deep cycle battery won’t fully charge, then it’s useful to perform some tests on it.

If the problem is of a deep cycle marine battery not charging, check the connection to it. Otherwise, it is necessary to load test it.

The deep cycle battery may have become damaged due to excessive charge draining.

A multimeter or voltmeter can be used. They may confirm a reading at 11 volts or lower.

A valid volt reading below 11 volts suggests that the battery is now a dud and must be replaced.

A zero-volt reading would mean that something has short-circuited. That requires a more detailed review to discover why.

Black wide image with batteries showing different charge levels

Can a Deep Cycle Battery Be Load Tested?

Yes, it can.

Load testing is intended to confirm how well a deep cycle battery holds its charge.

When it’s struggling to do so or it discharges quickly, then it’s likely that either the battery is faulty, it’s sulfated (more on this in the next section), or the internal cells are damaged due to incorrect charging methods.

How to Load Test a Deep Cycle Battery

If you’re planning to do it yourself, battery load testing equipment is required to properly load test a battery. Here is how to perform the deep cycle load test:

  • Disconnect all the cables attached to the battery terminals
  • Connect the battery load tester to the battery terminals
  • Command it to provide a short load calibrated to half the Cold Cranking Amps (CCA) rating that the battery has.

Once finished, the load tester will confirm the readings.

You can expect a battery to retain over 9 volts and possibly below 11 volts for half a minute. This should remain until it’s used up.

However, when the charge goes into the battery during the test and then dissipated only seconds later, this confirms that the RV battery needs to be replaced.

Sulfated Battery Symptoms

Batteries suffer when they’re only charged up partially rather than always receiving a complete charging cycle.

Partially recharging a deep cycle battery can create sulfation. This is where sulfate crystals are created and remain inside the battery.

When not charged repeatedly for periods, the negative plates on the battery develop a sulfate build-up.

Later, you’ll notice a reduced performance as the charging ability suffers accordingly.

There are some ways to attempt to partially recover sulfated batteries. However, for the most part, it’s best to just replace them.

How Often Do Deep Cycle Batteries Need Replacing?

For RV owners who have purchased their motorhome or towable from a previous owner, it’s not uncommon that they’ve incorrectly charged the battery or repeatedly only partly charged it. Because of this, they may discover too late that the coach’s battery pack is done for.

It’s such a frequent problem, that adding the cost of replacing the entire battery pack with a new set of batteries is sensible for most purchasers of second-hand RVs.

For new battery purchases, their expected lifespan, or the number of recharging cycles (whichever comes first) varies with brand, model, and type.

With that said, new batteries should last for several years, at least. We’d suggest closely following the guidance from the battery manufacturer to get the most out of them.

A few hundred recharging cycles are common with deep cycle batteries. Lithium is the standout that usually runs into thousands of recharging cycles.

Nevertheless, batteries that are treated poorly from a recharging and discharging standpoint are prone to failure regardless of the potential for a greater number of cycles. Hence, a battery’s life cycle often depends on how its owner treats them.

The best RV battery for dry camping is lithium. This is because it can be heavily discharged when there’s little solar to boost its charge level.

When boondocking on an overcast day and not wanting to drive around wasting gasoline to recharge the batteries via the alternator, then lithium is invaluable.

Renogy 12V 100Ah LiFePO4 Deep Cycle Lithium Battery. 4,000+ Cycles. Built-in BMS. Maintenance-Free

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Closing Thoughts

When using the right equipment, it’s possible to tell if the RV battery is bad. Deep cycle RV batteries go bad more often, but lithium ones aren’t a special case.

They’ll tolerate more rough treatment with charging and discharging cycles, but they’re not invulnerable to being badly managed by the RV owner and damaged as a result of it.

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