Battery FAQ: The Most Requested Battery Questions Answered

Batteries are essential for our electric devices to operate properly. They give us autonomy and power everywhere – from our cars to our cell phones.

Over the last ten years, new features were introduced to improve their performances, but in the meantime, it added a lot of complexity.

Therefore, for you not to be lost into technical considerations, our battery expert is going to answer in a clear way the most frequently asked questions about batteries.

If you are still left with unanswered questions, feel free to comment in the section below the article. We will be glad to clarify any points for you.

1. What Does Ah and mAh Mean on a Battery?

Black Board with Maths Formulas

One of the key specifications of your battery is its electricity storage capacity. Currently, two terms are used to quantify the storage capacity: Ah and kWh.

Ah stands for Ampere-hour. It describes how many Amperes (Amps, measure of electric current) your battery is able to provide continuously during 1 hour at its rated voltage.

Let’s take an example, a battery rated at 60 Ah and 12V means that this battery is capable of providing 60 Amps during 1 hour (at 12V) before being completely depleted.

It is important to note that the total capacity of a battery is affected by its discharge rate – C. The higher the discharge rate (C), the lower the total capacity available.

The term mAh stands for milliAmpere-hour. It is used for smaller capacities – 1000 mAh is equal to 1 Ah. A phone battery or a portable battery bank needs much smaller current than car batteries, for example. Therefore, their capacity is rated in mAh.

2. What Is kWh in a Battery?

kWh stands for kilowatt-hour. This is an absolute measure of the energy storage capacity of your battery.

One kWh is equivalent to a power of 1 kilowatt provided during 1 hour.

Remembering your college lessons: Power (W) = Current (A) * Voltage (V)


Kilowatt-hour (kWh) = Ampere-hour(Ah) * Voltage(V)

The term kWh is gaining popularity with the rise of lithium batteries. You can now find batteries rated at 12V, 24V, 48V, and more. Therefore, the capacity in Ah doesn’t make sense to compare the energy capacity of batteries with different working voltages.

For example, a 60Ah, 12V battery has a total capacity of 12*60= 720Wh, whereas a 60Ah, 24V battery has a total capacity of 24*60=1440 Wh. They both have the same Ah but different energy storage capacity.

kWh is an accurate and absolute measure of your battery energy capacity.

3. What Is the C Rating of a Battery?

The C rating is the rate at which a battery is discharged, one of the most important specifications.

As mentioned before, the total capacity of a battery in Ah is affected by its discharge rate. The higher the discharge rate (high C rates), the lower the total capacity of the battery.

As an example, a battery with a capacity of 60Ah rated at 1C means that this battery is capable of providing a current of 60A continuously in 1 hour. Therefore, if you are aiming to fully discharge this battery at lower C rates, for example, in 20 hours (1/20=0.05C), the capacity of the battery will be greater than 60AH.

Below, I have summarized the discharge time at different C rates.

Discharge rateDuration
2C30 minutes
1C1 hour
0.5C2 hours
0.1C10 hours
0.05C20 hours

The capacity of GEL and AGM batteries is greatly affected by the discharge rate. Therefore, it is usually measured at 0.1C.

For example, a 60AH, 0.1C AGM battery will provide 60Ah over the course of 10 hours. Divide the battery capacity by the time of discharge to get the output current – 60/10=6 Amps.The continuous discharge current will be 6 Amps during 10 hours.

It is possible to discharge the battery quicker, but the overall capacity will be lowered. On the contrary, if you discharge the battery at 0.05C (during 20 hours), the overall capacity will be higher. Manufacturers usually provide charts with the capacity at different C rates.

Lithium batteries are almost not affected by the discharge rate. Therefore, their capacity is always rated at 1C.

4. How Much Does a Car Battery Weigh?

Green Car Battery

Your car batteries are lead-acid battery types. As they are named, they contain lead (chemical symbol Pb) as their main active material which is one of the heaviest metals.

A typical car battery contains up to 60% of lead in weight. Therefore, they are quite heavy – on average between 32 and 45 pounds.

Lighter car batteries will soon be available thanks to the development of lithium batteries. Lithium is the main active material and one of the lightest metals. Therefore, it is expected that car lithium batteries will only weigh around 10 pounds.

At the moment, we cannot use lithium batteries in cars as the car alternator is not designed to properly charge lithium batteries.

5. Why Is My Battery Discharging by Itself?

Batteries are electrochemical devices, which means that the electricity is stored and released through chemical reactions happening inside the battery.

When your battery is not in use, there are still some chemical reactions going on but at a much lower scale. Therefore, your battery capacity will slowly decrease with time. This is called self-discharge.

The self-discharge rate of your battery depends on its battery technology, age, storage temperature.

Typically, deep-cycle AGM and GEL batteries have self-discharge rates between 2% and 15% per month at 68°F. However, every 15°F increase doubles the self-discharge rate.

Lithium batteries, like LFP (LiFePO4), have the lowest self-discharge rate between 0.35% and 2.5% per month.

6. How to Charge a Deep-Cycle Battery Properly

Deep-cycle batteries are a type of lead-acid batteries. Unlike lithium batteries, they don’t support fast charging and need to be charged in a three-step process to guarantee their highest capacity and the integrity of their components.

First of all, you have to use a charger dedicated to lead acid batteries. Connect the red cable of your battery charger to the positive (+) terminal of your battery, then connect the black cable of the charger to the negative (-) terminal of your battery.

You can now start charging in a three-step process.

Bulk Charge

In this first step, the charging voltage will increase steadily to its maximum value, while the charging current will keep the same high value. Typically, during this phase, your battery will reach 80% state of charge.

Absorption Charge

The charging voltage will keep the same value, while the charging current will decrease until the battery reaches 90–95% of its full capacity.

Float Charge

Finally, the charging current will slowly decrease to zero until full charge is accomplished. Applied voltage will also decrease until the battery reaches full-charge equilibrium.

The whole charging process could take between 8 and 14 hours.

7. Can You Fully Discharge a Battery?

You might have noticed that manufacturers recommend not to fully discharge your battery to extend its life duration.

For example, GEL and AGM batteries are advised to be discharged to 50% only and lithium batteries to 80% of their full capacity.

The main reason is that a deep charging and discharging cycle deteriorates the components of your battery, therefore reducing its performances over time.

GEL and AGM batteries are the most vulnerable to full discharge. For example, if fully discharged, a GEL battery will only last 12 months. At 50% depth of discharge, the life duration could be doubled.

The same applies to lithium batteries, but their depth of discharge is larger (80% recommended for a life duration up to 10 years). Full discharge is possible, but life duration might only be around 4 years.

8. How to Build a Battery Pack

Lithium DIY Battery Packs

A few years ago, building a battery was only possible for companies with heavy manufacturing equipment, high safety standards, and specialized engineers. This was true for GEL and AGM technologies, but with the advent of lithium batteries, everything is more simple, and it is possible to DIY a lithium battery with a high storage capacity.

You will need the following:

  • Lithium battery cells, type 18650
  • A battery management system (BMS)
  • Electric wires
  • Casing

Lithium battery packs are made of an assembly of single elements – the battery cell. The standard size is the 18650 cell with a voltage of 3.7V, available in various capacities. Without getting into too much detail, you will simply connect your cells in series or in parallel to adjust your battery pack’s final voltage and capacity.

The second most important component is the battery management system (BMS). The BMS is essential for the safety of your system as it will balance the state of charge of your cells, prevent overcharging and discharging, monitor the temperature, and shut down the system if necessary.

Finally, your battery pack and BMS should be installed in a protective case.

All the building blocks of your DIY battery are available online. We recommend this book to guide you step by step in your realization.

9. How to Tell If a Deep-Cycle Battery Is Bad

Follow the three steps below to find out the condition of your battery.

A Visual Inspection

If you notice that the battery is bloated, has corrosion marks, or, even worse, is leaking, then do not attempt to charge it or to use it. You will have to dispose it properly

Check the Battery Voltage

If your battery is intact, then you will measure its voltage with a digital voltmeter. On your voltmeter, select the 10–20V gauge, then connect the black probe to the “-” of your battery and the red probe to the “+.” You can now read the output voltage.

If the voltage of your deep-cycle battery is around 12.6V to 12.8V, it means that your battery is fully charged. If the voltage is above 12.9V, your battery is overcharged. On the contrary, if you read a voltage below 12V, then your battery is empty.

Note that discharging your battery too deep will damage and reduce its overall lifetime.

The Load Test

If your battery has a voltage of at least 12.5V, you can perform a load test. You will need a battery load tester. As it is named, it will impose a load to your battery while measuring its output voltage. If your battery voltage falls below 9.6V, then your battery is not in good condition.

DIY tip: If you don’t have a voltmeter, for a quick check, connect the “+” of your battery to an electric cable, then gently touch the “-” of your battery with the other side of the electric cable. If it sparkles, your battery has power. If nothing happens, then your battery is empty or damaged.

10. What Causes Battery Corrosion?

You might have noticed that after using your battery for years, visible corrosion (white or blue powder) appears at the battery terminals. This is caused by chemical reactions inside your battery.

Indeed, batteries are electrochemical systems in which electricity (electrons) is stored in the form of a charged chemical. Every time you charge and discharge your battery, it creates a chemical reaction inside the system.

Your battery electrolyte is a strong acid (generally sulfuric acid). Small gas leaks will react when they come in contact with air and will accelerate the corrosion of the copper terminal. Therefore, the bluish material is hydrated copper sulfate, and the white powder is anhydrous copper sulfate.

If you notice corrosion at the positive terminal, it could be due to overcharging. If the corrosion appears at the negative electrode, that’s because of undercharging.

11. How Long Will a 100 Amp-Hour Battery Last?

The capacity of a deep-cycle battery is affected by its discharge rate (C). The higher the discharge rate, the lower the capacity.

Manufacturers usually indicate their deep-cycle battery capacity for a discharge rate of 0.05C (20hours). It means that the battery capacity is rated for a continuous full discharge in 20 hours.

Therefore, for a 100AH battery, it will take 20 hours to completely empty the battery at a continuous output current of 5 Amps.

It is possible to get higher output current, but the battery will be depleted in less than 20 hours.

Useful tips:

  • Never discharge your deep-cycle battery more than 70% of its total capacity.
  • Higher discharge rates are possible, but your battery capacity will be reduced.

Check the specification of your deep-cycle battery. Manufacturers provide the overall capacity (Ah) in function of the discharge rate.

12. How Many Watts of Solar Panels Does It Take to Charge a Deep-Cycle Battery?

A 12Volts, 100Ah deep-cycle battery has an energy capacity of 12V*100Ah= 1200 Wh.

A 400W solar panel will, on average, produce 1400 Wh per day. Therefore, we recommend using one 400W solar panel to charge a 100Ah deep-cycle battery in a day.

Useful tips:

  • You can check your solar energy production potential using the Global Solar Atlas website (it varies depending on your location).
  • As mentioned before, to properly charge a deep-cycle battery, the battery charger has to undergo three charging steps. The same rule applies when charging a battery with a solar panel. Consequently, aside from solar panels, you will need a solar charge controller.
  • It can take up to 14 hours to complete the three charging steps. Therefore, full charge might not be achieved in one day.

13. How to Tell Positive and Negative on a Car Battery

Car batteries have two terminals because the current (electrons) only flows from the negative (-) terminal to the positive (+) terminal.

The positive (+) is red, and the negative (-) is black. The positive terminal should be connected first, then the negative one which is also grounded.

14. How to Read a Battery Charger Amp Meter?

Read Battery Amp Meter

Your battery charger is now properly connected to your battery. You can follow the four steps below to start charging your battery and read the charger Amp meter:

  • Select the appropriate charging voltage on your battery charger Amp meter. Usually, this is 12V.
  • Turn on your battery charger.
  • The needle of the Amp meter will go up to the maximum charging current, usually in the range of 2 Amps to 20 Amps depending on the battery you are charging. This is the bulk charge.
  • After some time, the needle of the Amp meter will go down (absorption and float charge) until it reaches zero. This means your battery is fully charged.

Useful tip: Charging a deep-cycle battery involves a three-step process managed by the battery charger. It can take up to 14 hours to complete those steps.

15. How Long Will a Deep-Cycle Battery Power an Inverter?

Deep-cycle batteries can be used in an off-grid power system. They are connected to an inverter, which converts DC (direct current, 12V) from the batteries to AC (alternative current, 110–220V, 50Hz) to be used by your common appliances.

Your deep-cycle battery duration will depend on

  • Your battery capacity
  • The output current delivered to the inverter

Let’s take an example with a 100Ah, 12V deep-cycle battery connected to an inverter.

As mentioned before, this battery is able to continuously provide 5 Amps for 20 hours before being completely drained. However, manufacturers recommend a 50% depth of discharge to maximize your battery life duration. Therefore, in real work conditions, your battery will be able to provide 5 Amps for 10 hours.

In terms of energy, this is equivalent to 50Ah*12V= 600Wh. 600Wh will be your available energy, but bear in mind that it has to go through the inverter (DC to AC), which has an efficiency of 90%.

In the end, your total useful energy will be 540 Wh (600Wh*0.9) for 10 hours, equivalent to 54W continuously for 10 hours.

To give you an idea, this is roughly the total energy needed to power a medium-sized refrigerator for 10 hours.

Don’t forget: With deep-cycle batteries, the battery capacity is affected by the discharge rate. It is possible to get more than 54W out of your battery, but that will decrease its overall capacity.

16. How to Charge a Car Battery with Home Electricity

Car batteries are 12V deep-cycle batteries. They can only be charged with DC (direct current) via a three-step charging process. Your home electricity is AC (alternative current, 110–220V, 50Hz). Therefore, you cannot directly use home electricity to charge a car battery.

There are two options:

  1. Employ a car battery charger: A dedicated device that will use your home electricity to properly charge your deep-cycle battery.
  2. Use a home off-grid inverter: If you have a solar system installed at home, you certainly have an inverter that will convert the DC power from your solar panels into AC power for your home appliances. The inverter is also capable of properly charging car batteries. Just plug in your battery, and leave it until it’s fully charged.

17. How to Tell If My Battery Is AGM?

The two main battery categories are lithium-ion and lead-acid. Lead-acid battery is the oldest technology. AGM battery is a type of lead-acid battery. It falls under the category of deep-cycle VRLA (valve-regulated lead acid) battery.

There are two types of VRLA deep-cycle batteries – GEL batteries and AGM batteries.

AGM means absorbent glass mat. In this type of battery, the electrolyte is absorbed on a fiberglass mat. Therefore, AGM batteries are sealed and maintenance-free.

In the end, if your deep-cycle battery is sealed, it will be either a GEL or an AGM battery; from the outside, they look the same, but manufacturers always indicate the type of battery on one of its sides.

19. How to Charge an SLA Battery

SLA means sealed lead-acid battery. They are also called VRLA (valve-regulated lead-acid) batteries. There are two type of SLA batteries:

  • AGM battery
  • GEL battery

Both of them are deep-cycle batteries. To charge them, you will need a dedicated battery charger. The charger will follow a three-step process (bulk, absorption, and float charge) to ensure efficient charging without damaging the battery.

For a more detailed explanation of the charging process of your SLA battery, you might want to refer to our mini guide.

20. Is Lithium Battery the Best?

Collection of Different Batteries

There are currently two types of battery technologies commercially available: the oldest one, lead-acid battery, and the most recent, lithium-ion. To cut it short, lithium batteries are far superior from lead-acid batteries like AGM and GEL types. I have summarized below the six reasons why lithium-ion batteries are better than deep-cycle AGM and GEL batteries.

Higher Energy Density

Thanks to its technology, a lithium battery can store around five times more energy than deep-cycle batteries for the same weight and volume.

High Discharge/Charge Rates

Lithium batteries support fast charging/discharging without affecting the total output energy. On the contrary, the capacity of deep-cycle batteries is strongly related to the discharge rate (C) and needs to undergo a specific charging process that can take up to 14 hours.

Deep Discharge Capacity

You can fully discharge a lithium battery, but it is recommended not to excess 80% depth of discharge for an extended life duration. On the other hand, deep-cycle batteries should not be discharged more than 50%. In the end, half of the capacity of your deep-cycle battery will not be used.


Lithium batteries last between 2000 and 4000 cycles (5 and 10 years), whereas lead acid will only last 2 years if properly used.

Tolerant to Intermittent Charging/Discharging

Lithium batteries have no problem with intermittent charging (sudden power variation), for example, when paired with a solar panel or when supplying power to an inverter, whereas deep-cycle batteries will need a continuous charge over time.

DIY Is Possible

Last but not least, you can build your own customized lithium battery at home! This is clearly not feasible for deep-cycle batteries.

21. How to Charge a 12-Volt Battery Using Solar Power

Solar power can charge a 12 Volt battery, to ensure proper charging you will need two things.

Solar Panels

Solar panels below 25W won’t be able to charge a 12V battery. For efficient charging, we recommend at least 60W, with an ideal being 220W.

Solar Charge Controller

The solar charge controller will ensure smooth and safe battery charging. Be sure to select a MPPT solar charger. This technology will get the highest output from your solar panel at all times and will ensure that your batteries are properly charged. They are fit to charge 12V deep-cycle batteries (via a three-step process) and 12V lithium batteries (high-speed charging possible).

First of all, be sure that your solar panel is exposed to full sun to maximize its output, then connect the solar panel to the solar charge controller.

Finally, connect the charge controller to your battery with crocodile clips, for example. Be sure that the “+” of your battery is connected with the red cable of the charge controller and the “-” to the black cable.

22. How Long Does It Take to Charge a Deep-Cycle Battery?

As previously mentioned, deep-cycle batteries are a type of lead-acid batteries. They are found under two names – AGM and GEL.

Deep-cycle batteries unlike lithium batteries don’t support quick charging and need to undergo a specific charging process involving three steps:

  • Bulk charge (to recover up to 80%)
  • Absorption charge (up to 95%)
  • Float charge (up to 100%)

The charging speed will depend on two factors:

  1. The battery capacity to be charged (usually max. 50% of the total rated capacity)
  2. The output of the battery charger (from 2 Amps to 20 Amps)

In the end, even with the most powerful battery charger, charging your deep-cycle battery from 50% to 100% might take between 8 hours and 14 hours. This is mainly due to the absorption and float charge that can take quite a long time.

23. How Do You Test a Marine Battery?

Marine batteries are deep-cycle batteries found in boats. They are strongly built to overcome rough conditions.

Most of the time, they don’t undergo full charge and are subject to corrosion because of the marine environment. They can also be left unused for a long time. Therefore, they need to be tested to be sure that they will provide enough power when sailing.

First of all, perform a visual inspection. If you notice corrosion on the battery connectors, gently remove them.

Then, you will need a battery load tester. This is the most accurate way of testing your battery.

A load tester, as it is named, will apply a load on your battery. The output voltage will be monitored.

Follow the two steps below for a proper use of the battery load tester:

Measure the Voltage of Your 12V Marine Battery

If it is below 12V then your battery is empty. Try to charge it, the voltage should now be above 12.6V and below 12.9V.

Plug in Your Battery Load Tester

Connect your load tester to your marine battery’s positive (+) and negative (-) outputs. Then, apply a load equal to 50% of your battery CCA (Cold Cranking Amps) for 15 seconds. CCA is the maximum peak current that your battery can provide. Typical CCA range from 350 to 600 Amps. Therefore, you will apply a load of 175 to 300 Amps. Look for the CCA value of your battery in the manufacturer’s specifications.

In the meantime, monitor your battery voltage. It should not fall below 9.6–9.5V.

If that’s the case, then your battery is damaged.

24. Why Are Lithium Batteries So Expensive?

Lithium-ion is the newest battery technology, and lead-acid battery (GEL and AGM types) is the mature one. As previously mentioned, Lithium batteries have a lot of advantages compared to lead-acid.

However, their buying price is significantly higher, but is it really?

Let’s take an example. We will compare two 12V batteries, rated at 100Ah capacity.

It is recommended to discharge lithium batteries up to 80%, whereas lead-acid only 50%. Therefore, the actual available capacity is 80Ah for the lithium and 50Ah for the lead-acid.

The life duration of a lithium battery is, on average, 2500 cycles, whereas for deep-cycle, it is a maximum of 600 cycles.

In the end over its lifetime, the lithium battery will have a total capacity of 2500*80= 200’000 Ah, and the lead acid deep-cycle battery is 600*50= 30’000 Ah.

You will get around seven times more energy out of the lithium battery. Then, let’s have a look at the buying price – on average, $200 for the lead-acid and $500 for the lithium battery.

In the end, the upfront cost of the lithium battery is 2.5 more than the lead acid, but the total restituted energy is seven times more than the lead acid.

The cost per total Ah for lithium batteries is $0.0025, and for the lead acid, it is $0.0066.

Conclusion: the lead acid battery is 2.6 times more expensive than the lithium battery.

Final Thoughts

Batteries benefit from constant technical improvement and dropping prices (for lithium-ion) driven by the success of the electric car industry. In the coming years, thanks to the energy transition, they will be even more present in our lives, as part of a smart energy grid combining information and energy.

We tried to answer your most common questions about batteries in a simple and clear fashion. If you are still left with more questions or need a personalized answer, don’t hesitate to reach us in the comment section below.

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