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Why would Deep Cycle Battery not Fully Charge


Some reasons for the battery not being able to fully charge include bad battery connections, improper battery care, and of course, the battery itself. Using a proper deep cycle battery charging system (that will charge in stages) is vital to keep it in proper condition.

Now, let’s troubleshoot your deep cycle battery:

Visual inspection of deep cycle battery

  • The top of your battery should be clean and dry. If it’s not, that could be the first sign of possible uncontrolled discharge, which could lead to a shortened lifespan of your battery.
  • The battery case should not be damaged. Cracks on the battery, commonly occur if the battery was overcharged and overheated quite often. This is another sign of possible battery failure.
  • Visual observation of sulfation. This condition is typically caused by overcharging or undercharging and exposure to air battery plates. If you visually see a lot of sulfation, your battery is either dead or will be really soon.

Battery testing to get voltage reading

Using Digital Voltmeter, Multimeter or Wattmeter is the easiest way to find out the charge level of your deep cycle battery:

Q&A: How to check your battery charge level and troubleshoot issues?

Following is 12 volt AGM battery example:

Charge Level Voltage
100% 13.00 v
90% 12.75 v
80% 12.50 v
70% 12.30 v
60% 12.15 v
50% 12.05 v
40% 11.95 v
30% 11.81 v
20% 11.66 v
10% 11.51 v
0% 10.50 v

This table could be used as a general guide as well (voltage may be a bit different for other types of batteries). If your test shows a voltage of 11.66 volts or lower, you just found your problem and you need a new battery.

Battery discharge test system

This type of test will give you a lot of information about your battery:

Battery Amp-hour Discharge Test Using an 8500 Series DC Electronic Load

Reading the results of the voltage test

So, what do we have here?

  • Voltage is below 10.50 (0%). Your battery is dead and you need to get a new one!
  • Voltage is below 11.95 (40%). You can sometimes improve a battery in this condition by using a desulfation device (see video below), otherwise, you’ll have to live with the low capacity or replace the battery.
  • Voltage is below 13.00 (100%). Your battery is not fully charged. Make sure you always get to that level or the level suggested by the manufacturer** in order to avoid problems in the future.
  • If your battery quickly loses charge down to 11.00 volts. You have a faulty cell and need another battery (this is difficult to repair and it is usually caused by excessive vibration).

** If your manufacturer’s instructions say something different, follow the instructions!

Here is a video on everything about desulfators from a pro!

The Truth About Battery Desulfators

Here is an infographic to help you out (if you are in a hurry) .

Common reasons for battery failure

Batteries are sensitive things! Just like living organisms, if you don’t “feed” them properly (or take more energy than they can safely give you), they will just die on you!

If you are not taking proper care of your battery and it starts giving you less or no power, there is a big chance that you “killed it”! We may not think about it, but it is so easy to ruin a battery just by not charging it fully enough or letting it discharge more than it should (maybe both).

Using a good battery management system (also known as BMS) is good help here. If you are in a situation where your battery does not take or hold the charge, there is a big chance that you have done any of the following:

1. You let your battery sit too long without charging after the discharge

Even if it’s a little as 24 hours, your battery will not appreciate it and start losing its capability due to sulfation build-up. Sulfation begins at about 12.5 v and the longer your battery sits at this and lower level, the faster it will become useless.

2. You discharged your battery deeper than allowed by that specific battery type

The deeper the discharge, the faster the lead plates of a battery will be coated with sulfate crystals until the point where it will become unusable.

3. You are constantly undercharging your battery

It is very common for people to see a battery charge of about 80% and start using it right away. Big mistake!

Even though the finishing 20% will take about the same amount of time as an initial 80%, if your battery charge is incomplete the sulfation process will begin with future consequences.

4. You did not notice that your electrolyte plates are exposed to air

In this situation, your plates will start sulfating immediately and before you know it, it will become useless. Adding distilled water to your wet cell (or flooded) deep cycle battery just enough to cover the exposed plates before each charge, will help you avoid this situation in the future.

5. You did not notice that your battery is slowly discharging

There are several things that can contribute to that:

  • Parasitic loads. This is anything that you leave on your RV or boat that is taking power from your battery. It is usually something small (like a gas leak detector) that could actually drain your battery if left ON long enough.
  • Self-discharging. Regular self-discharging (about 10% per month) is something that happens over time to all kinds of batteries while in storage.
  • Hot weather. Another reason for the battery losing its charge could be because it is sitting in high heat! Temperature over 100+°F will increase internal discharge.

** Important note! Sulfation is the number one reason for the battery going bad! In reality, the lead sulfate is always formed on the battery’s plates during discharge, but it dissolves quickly into electrolytes when you charge your battery again.

Sulfate becomes a problem only when the battery is sitting around without a charge for too long. Prolonged periods without recharging or constant incomplete recharge levels will make sulfate stick on your battery permanently and therefore it will stop working.

Charging your battery fully every day may not be always possible for you, but you can try to do it at least 2 times a week from the shore power supply in order to keep sulfation at bay.

** Voltage maintenance! It might be a good investment (especially if you get an expensive battery) to get a good float charger that will keep it 100% charged all the time when not in use. If you are using a good deep cycle battery charger, it should be able to do this as well.

Other things to check

If your battery doesn’t fit in any of the above descriptions, there might be another reason for not being able to charge the battery to its full capacity:


You need a charger that will charge your deep cycle battery slowly and in stages. If you having trouble even starting charging your battery, then this could be a problem.

Only a specialized charger will actually work for your deep cycle battery. Smart Chargers – are computer-operated chargers that are designed to charge your battery in stages, for maximum performance and longevity.

They are also designed to automatically stop charging upon reaching about 14.4 volts. Charging a deep cycle battery in any other way may overcharge and damage a battery.

Did you charge it fully?

One of the reasons for battery failure could be that you may not have given it enough time to charge! Did you charge your battery to its full capacity or stopped short?

Most people don’t have enough patience (or time, really) to get it to the manufacturer’s voltage suggestion (usually 14.5 volts) and start using their deep cycle battery of around 12 volts. If you want your battery to last, do not do that!

Find some time to charge it correctly and it will last (if maintained properly as well) no less than 4-5 years, vs 1-2 years (like for most people that don’t take care of it). Investing in solar power may be a good solution if you are on the road a lot (or cruising waters) and don’t have constant access to the main power.

If you are just using an alternator to charge your battery, you will need to be driving for at least 3-4 hours before the charge is restored (depending on your discharge level). It is mentioned that alternators have a tendency to overcharge your battery, especially if it is deeply discharged (that’s why regular car batteries have no more than 10 deep cycles available). 

If you are using a voltage-regulated charger (DC Alternator onboard charger), you will not have a problem with charging from the alternator. It will not allow voltage to go above 15 volts, which is damaging for most batteries.

You can get a fully charged AGM from shore power in about 12 hours using a smart charger and the about the same with the solar smart charger. If the alternator is your main power source, but it does not produce enough volts to cover your manufacturer’s suggestions, just top it off with shore power (when you get there) or some other means (like solar or generator), since it is really important to get to that level for your deep cycle battery to last.

While you are waiting for that “fully charged” light to go on, please note that it is not easy to fully charge a deep cycle battery, especially with a low amp charger (which is actually good for a battery). In order to replenish a 150 amp-hour battery at 2 amps (for example), you will need 75 hours.

Charging a deep cycle battery does take time and it happens in stages. If your battery charge is below 12 volts, there is a big chance that it is not fully charged yet!

Do not start using the battery that is charged below 12 volts! Constant undercharging is the main reason for a deep cycle battery to lose its capacity very quickly.

The first charging stage is usually the quickest and it will bring your battery up 80-85% to its full capacity. If you remove a battery now, there is a big chance that you will cause your battery to lose its ability to charge well in the future.

You have a powerful battery on your hands that deserves proper maintenance, and YES, a lot of patience!

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