Why Solar Battery Drains Fast and How to Avoid It

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Solar batteries provide more capacity than ever, yet it seems like they drain even faster than before. If your battery bank is draining rapidly, there might be an underlying problem in your solar panel system. This guide will show the most common reasons for rapid battery power loss and what to do about it.

A solar battery will drain quickly if it isn’t recharged for a long period or if the charge controller is faulty. Leaving a battery fully discharged without charge for extended periods will lead to rapid draining too.

The most common reasons for solar battery drain are the following.

  • Insufficient charge
  • Too long between charges
  • No charge controller
  • Storing batteries without a charge
  • Overheating
  • Poor maintenance
  • Increased load
  • Faulty charge controller or inverter

Let us take a closer look at each one and what preventive steps you can take.

Not Enough Charge

Conventional wisdom says to never fully charge or discharge a battery, and that is true. Recharge at 50% for lead acid and 35%-40% for lithium. While most recommend not topping off at 100%, do not top it too low either.

For instance, if you recharge an AGM battery at 50% and top it off at 75%, that is only 25% usable power. If you use a microwave, solar welder or other power hungry electronics, that battery will drain fast. So while charging to 100% is not recommended, you should charge up to 85% to 95%. That will allow you to use the battery for longer periods without needing to recharge.

With lithium ion batteries such as the PowerTex 100ah 12V, you should expect the battery to last longer with its better discharge rate. Just remember to recharge at the recommended level.

This is where the need for balance is needed. You should not fully charge or discharge solar batteries, but neither should you avoid filling it with power. As long as you keep it at 85% full, the battery should be able to give you the power you need.

Too Long between Battery Recharges

Batteries should be recharged within 24 to 48 hours in warm weather, and 2 to 3 days for cool weather. Recharge solar batteries as soon as possible, especially if it is fully discharged.

Fully discharged batteries that are not recharged after a long period results in sulfation. The sulfur molecules inside the battery get discharged and begin to cover the lead plates. Sulfation makes it impossible for the battery to charge and discharge properly.

Charging and discharging small amounts can also lead to battery drain. For instance, recharging and discharging by 10% can damage the damage the battery so it cannot hold a charge like before.

Even modern batteries are vulnerable to this condition. With lithium it is less likely, but storing a fully discharged battery should still be avoided. All batteries will discharge at some point, and if there is little to no power left, it will damage the internal circuitry.

Not Using a Charge Controller

As many solar panel users will point out, using a charge controller is one of the best ways to prevent unexpected battery drain. A charge controller regulates the flow of power in the battery and prevents overheating, one of the main causes of power drain.

There are two types of charge controllers, PWM and MPPT. MPPT controllers are more expensive but allow you to use high voltage solar panels with low voltage batteries. They are also more efficient compared to PWM. PWM controllers are more affordable and work well for small solar panel systems.

if you have a small solar system, a PWM controller is going to be enough. But if you have a large battery bank and a solar array, an MPPT controller is required to reduce energy loss. The bigger the system, the more energy you need to conserve as the cost will add up. In this case we recommend the BogueRV MPPT 40ACharge Controller as it is compatible with gel, lithium and AGM batteries.

Make sure the charge controller is large enough for your battery bank. The controller amp capacity and voltage have to be greater than the battery bank for it to work. if you have a 12V 35ah battery, your charge controller should be 20A. Do check the charge controller specs to determine what battery sizes it is compatible with.

Putting a Fully Discharged Battery in Storage

Batteries self discharge over a long period. Without sufficient charge, the battery will continue to drain its power until it is empty. This can cause permanent damage and make recharging impossible.

If you are not going to use the battery for a while, charge it up to 85% or so. Store the battery, but check on it every once and while. Apply a charge whenever the level drops close to the discharge rate level, 50% for lead acid and 35% to 40% lithium. This is a general guideline for lithium since the discharge rate varies by manufacturer.

The point is you should never leave store an empty battery. Even leaving a battery at the recommended discharge rate should be avoided.

Suppose you have a lead acid battery and you store it with 50% capacity. If you leave the battery in storage for several weeks or months, will it still be at 50% when you decide to use it again? No, the charge would have dropped below 50%, by how much it depends on the temperature and storage conditions.

But it will drop and below the recommended discharge rate. This will make it harder to recharge the battery and when it does charge, it won’t hold up as well as before. Now imagine leaving the battery with only 10% charge or less. By the time you remove it from storage, the charge could b at 0%. So charge your battery before storage.


Do not store batteries in temperatures higher than 95 F / 35 C. Doing so could lead to internal discharge and drain the power.

Heat is not good for batteries especially when carrying heavy loads. A lot of batteries, especially lithium, have been designed to work in extreme heat and cold. However, lithium batteries are expensive so most solar power owners use lead acid.

Lead acid batteries work fine with solar panels as long as you properly maintain it. However they are more vulnerable to heat than lithium, so do not place them near hot or combustible objects. Doing so could rapidly drain its power even if it is not running.

There may be unavoidable circumstances where you have to run the battery at high temperature. As long as you don’t do it often, it should not cause problems.

Poor Battery Maintenance

Lead acid batteries require maintenance, and without it the battery will cease to run efficiently. A poorly maintained battery will eventually lose its capacity to hold a charge. So remember these:

  • Lead acid batteries have to be refilled with water every two weeks.
  • Clean the wires and terminal connections regularly.
  • Flooded lead acid batteries have to be equalized every 90 days. Do not equalize sealed lead acid and lithium batteries.

Those are general maintenance guidelines for batteries. Your battery manufacturer may have additional maintenance specified in the manual so check it out.

Increased Load

Another possible reason your battery drains quickly is it has a heavy load. If you have been using the same battery bank for a while but increased the load, the system will lose power quicker.

This is why you must always plan ahead for solar power. Determine how many solar panels you will need and what batteries to go along with it. Here are some pointers to keep in mind.

  • Calculate how many total watts you will need. Add all the appliance wattage plus 20% reserve power.
  • Will you be using your solar panels to run your appliances during the day? Or will you run them on the batteries and use the panel to charge the batteries?
  • If your power needs grow, increase the battery bank. If you want to, say, set up a solar shed you must add more batteries to meet the new power demand. Repeat the steps you did before but add the new power requirements.

Defective Charge Controller or Inverter

Sometimes the problem could be in the charge controller or inverter, not the battery. If any of those two are defective, it might affect the battery bank’s capacity to hold a charge.

The charge controller is connected to the battery and solar panel. It serves to regulate current flowing into the battery. It also adjusts the voltage so the solar panel and battery matches up. An inverter is used to convert DC power (which solar panels produce) into AC. Once converted, the power is transmitted to the battery and your appliances and devices.

Because of how closely integrated the components are, failure in one could affect the other. A faulty charge controller could lead to sudden voltage spikes or drops, affecting the battery internal charging system. The inverter is probably the most sensitive part of a solar system and problems with it could disrupt the battery charging capacity.


Regardless what battery type you use, proper maintenance and use are essential. Knowing when to discharge and recharge batteries is the key to extending their life cycle and saving dollars in the long term.