Some questions come up regularly about batteries for off-grid solar systems (not connected to Hydro-Québec). So we took the initiative to group them on this page.

Which category of battery should be used in an off-grid solar system?

Deep-cycle batteries should be used because they are designed, unlike automotive batteries, to power electrical devices on long periods of time. Some models of “marine” batteries can also be used.

The 2 most popular types of deep cycle batteries are:

  • Lead Acid
  • AGM (VRLA)

What are the advantages / disadvantages of these 2 types of batteries?



  • Economical


  • Need to check the electrolyte level and add distilled water as needed on a monthly basis.
  • Be careful when handling it because it is not sealed
  • Risk of sulfuric acid spillage (can not be installed in a supine position)
  • Relieves hydrogen when recharged. Require ventilation in enclosed areas
  • Frost freeze if discharged (will freeze at -69C when 100% charged and at -7C when fully discharged) – Source Trojan Battery Storage

Note: low temperature will decrease temporary the capacity of energy storage but extend their life



  • No maintenance required
  • Sealed. No risk of spill. Can be installed in a recumbent position
  • No emanation of hydrogen when recharging
  • Less prone to sulfation (accumulation of lead sulfate on electrodes)


  • More expensive

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The size and capacity of the batteries

The batteries exist in several dimensions. To classify them, group codes have been assigned to them. These codes are not based on capacity but on the physical dimensions and location of terminals. Typical BCI codes are U1, 24, 27 and 31. Industrial batteries are generally designated by a model number such as “FS” for “Floor Sweeper” or “GC” for golf cart ( “Golf Cart”). Many batteries have no particular code and are only manufacturers’ references. Other standard size codes are 4D and 8D, large industrial batteries, commonly used in solar electric systems.

U1 34 to 40 Ah 12 volts
Groupe 24 70 to 85 Ah 12 volts
Groupe 27 85 to 105 Ah 12 volts
Groupe 31 95 to 125 Ah 12 volts
4-D 180 to 215 Ah 12 volts
8-D 225 to 255 Ah 12 volts
Golf Cart & T-105 180 to 225 Ampères-heure 6 volts
L-16, L16HC etc. 340 to 415 Ampères-heure 6 volts


Although they are deep cycle, it is not recommended to over-discharge these 2 types of batteries on a regular basis since it will shorten their life prematurely. Here is a graph produced by Rolls illustrating the lifetime in number of cycles (discharge / recharge) of its batteries according to the depth of discharge:

Durée de vie des batteries acide-plomb selon le pourcentage de décharge - dod

We therefore recommend to our customers to do not discharge their lead acid and AGM (VRLA) batteries at more than 50% of  on a regular basis in order to maximize their investment. Thus, depending on the level of discharge, maintenance, temperature, the number of charge / discharge cycles, this type of batteries can last up to 8 years or even longer.

Lithium batteries, meanwhile, can be almost completely discharged without much impact on their life. For example, Blue Planet Energy’s Blue Ion 2.0 batteries are backed by a 15-year warranty or 8,000 cycles at a 100% depth of discharge

How to know the state of charge and health of batteries?

To accurately determine the charge level of a lead-acid battery, a hydrometer (also known as a hydrometer or acid tester) must be used when the battery is at rest (wait approximately 6 hours after charging). The density of the acid will determine the state of charge of the battery. Although it is not the most accurate method, one can also know the approximate state of charge by measuring its open circuit voltage using a voltmeter
Source: Trojan battery maintenance guide

Here is the chart for lead-acid batteries:

12V solar deep-cycle lead-acid battery state of charge according to voltage

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How to store batteries during the winter if they are not used?

Periods of inactivity can be extremely harmful to lead acid batteries. When placing a battery into storage, follow the recommendations below to ensure that the battery remains healthy and ready for use.

NOTE: Storing, charging or operating batteries on concrete is perfectly OK.

The Most Important Things to Avoid

  1. Freezing. Avoid locations where freezing temperature are expected. Keeping a battery at a high state of charge will also prevent freezing. Freezing results in irreparable damage to a battery’s plates and container.
  2. Heat. Avoid direct exposure to heat sources, such as radiators or space heaters. Temperatures above 80° F (26.6º C) accelerate the battery’s self-discharge characteristics.

Step-By-Step Storage Procedure

  1. Completely charge the battery before storing.
  2. Store the battery in a cool, dry location, protected from the elements.
  3. During storage, monitor the specific gravity (flooded) or voltage. Batteries in storage should be given a boost charge when they show a 70% charge or less. See the table above.
  4. Completely charge the battery before re-activating.
  5. For optimum performance, equalize the batteries (flooded) before putting them back into service. Refer to the Equalizing section for this procedure.

Serial / parallel battery connection

Applications often demand more voltage or more ampere capacity than the capacity of one battery. By connecting multiple batteries in series, parallel or series parallel configurations, you are able to increase the output voltage or battery bank amperage as needed.

To increase voltage, batteries are connected in series. Capacity of the battery bank remains the same as voltage increases. To increase the available amount of current and capacity, batteries are connected in parallel. In this situation it is best to use lower voltage, higher capacity cells to  minimize the amount of parallel strings

Serial connection to increase the voltage:

Ex: 2 x 6 Volts connected in series

Battery voltage: 6V each

Battery capacity : 400Ah

System voltage: 12V

System capacity: 400Ah



Series parallel connection to increase capacity and voltage:

Ex: 4 x 6 Volts batteries connected in series parallel


Battery voltage: 6V each

Battery capacity: 400Ah each

System voltage: 12V

System capacity: 800Ah



It is not recommended to connect more than three (3) series strings together. Multiple parallel connections will often create uneven resistance between strings, causing an imbalance of charging and discharge currents, resulting in possible cell damage or premature failure.

How much energy can my battery bank store?

The capacity of a battery is measured in Ah (Ampere-hour) or in kWh (Kilowatt-hour). The ampere-hour number is normally indicated on the battery (ex: 400Ah). The simplest way to know the total capacity of its battery bank is to calculate the capacity of a single battery in kWh and to multiply by the total number of batteries constituting the bank.

Ex: The capacity of a 6V 400Ah battery is 2.4 kWh. For a bank with 4 identical batteries, the total will be 9.6kWh. It should be remembered, however, that it is not recommended to use more than half of this capacity with lead-acid or AGM batteries.

How long will it takes to charge my battery bank with my solar panels?

Panneaux solairesJust take the total capacity of your battery bank in kWh, divide it by two (to account for the fact that it will never be discharged more than 50%) and divide it by the total power of its system solar in kW. This will give the number of hours needed to recharge the bank of batteries.

Ex: 4.8kWh (4 batteries 6V 400Ah / 2) / 0.81kW (3 panels of 270W) = about 6 hours of full sun. Naturally, this is a number of hours in optimal conditions (sun perfectly aligned with the panels). In our example, this would correspond to about 1 perfectly sunny summer day. On an annual basis, the number of hours of sunshine at full power is rather about 4 hours per day.

ÉcoSolaris Solar Energy Panels Electric Charging Stations Air-Heater

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