The following sections have been developed to help you with the battery portion of your renewable energy project. Click on one of the following links to jump to that section of the battery toolbox.
Battery Safety – How to safely work with batteries and battery banks.
Battery Bank Sizing – How to properly size a battery bank.
Battery Selection and Wiring – How to determine which is the right battery for you.
Battery Charging – How to properly charge your battery bank.
Code Compliance – View the code “check list” for safe battery bank installations.
Venting – Calculate the volume of hydrogen gas your wet lead acid battery bank releases under charge, the method of venting, and the required size of your vents.
Battery Short Circuit Current – The Danger, and Safety first!
Battery Safety
Safety Precautions When Working With Batteries
- Follow all instructions published by the battery manufacturer and the manufacturer of the equipment in which the battery is installed.
- Make sure the area around the battery is well ventilated. Battery enclosures should be designed to prevent the accumulation and concentration of hydrogen gas in “pockets” at the top of the compartment. Vent the battery compartment from the highest point. A sloped lid
can also be used to direct the flow to the vent opening location. - Never smoke or allow a spark or flame near the engine or batteries.
- Use caution to reduce the risk or dropping a metal tool on the battery. It could spark or short circuit the battery or other electrical parts and could cause an explosion.
- Remove all metal items, like rings, bracelets, and watches when working with lead acid batteries. Batteries produce a short circuit current high enough to weld metal to skin, causing a severe burn.
- Have someone within range of your voice or close enough to come to your aid when you work near batteries.
- Have plenty of fresh water and soap nearby in case battery acid contacts skin, clothing, or eyes.
- Wear complete eye protection and clothing protection. Avoid touching your eyes while working near batteries.
- If battery acid contacts skin or clothing, wash immediately with soap and water. If acid enters your eye, immediately flood it with running cold water for at least twenty minutes and get medical attention immediately.
- If you need to remove a battery, always remove the ground terminal from the battery first. Make sure all accessories are off so you don’t cause a spark.
- Never charge a frozen battery.
- If a remote or automatic generator control system is used, disable the automatic starting circuit and/or disconnect the generator from it’s starting battery while performing maintenance to prevent accidental starting.
Important: Baking Soda neutralizes lead lead acid battery electrolyte. Keeping a supply on hand when working with batteries is a good idea.
Battery Bank Sizing
Battery banks can be sized based on the greater of either load demand or generation input. To view each method, please see below:
Sizing Battery Banks Based on Loads:
To estimate the battery bank requirements on load, you must first calculate the amount of power you will draw from the batteries. This power draw is then translated into amp hours (Ah) — the unit of measure to express deep-cycle battery capacity. Amp hours are calculated multiplying the current drawn by the load by the length of time it will operate.
Sometimes, the energy consumption of your loads are expressed in watts. To calculate amps when the power consumption is expressed in watts, use the following equation:
A = W/V (where W = watts, and V = volts)
For example:
A 100 watt (110VAC) light bulb can be expressed at 0.90 Amps (100/110=0.9)
If the light runs for three hours it will consume approximately 2.7 a/h of power (0.9 x 3).
The above example shows that the amount of energy used is directly related to the voltage. The same 100 watt light bulb running on a 12V battery bank will consume 8.33 a/h per hour, or 25 a/h for three hours of use!
If we are powering that same size light bulb with the same 12V battery bank but with an inverter then we also have to include the inverter efficiency in our calculation. If we assume that the inverter is 85% efficient, then we can say that the amount of energy consumption for that same light bulb will be 9.76 a/h for three hours of use.
As you can see, the battery bank voltage you select is important. It will impact the amount of current (A) that your inverter draws to supply your loads, and it will also impact the amount of current your solar array puts into your battery bank. Generally speaking, larger inverters and solar arrays will always use higher voltage (24 or 48V) battery banks.
There are many load calculation worksheets online (see our calculator section). All of them will help you record your connected loads, calculate your consumption (in amp/hours) and apply an inverter correction factor. Once you have that number, then you can determine your overall battery bank size in the next section.
Sizing Battery Banks Based on Rate of Discharge
Another method for determining battery bank size, particularly when there may be only one large load is to discover the battery rate of discharge in amps.
for example:
A typical wet lead acid golf cart (GC) battery has a rated capacity of roughly 220 a/h at the 20 hour rate. The manufacturer is telling you that the battery can sustain an 11 A load (220 / 20) for 20 hours for full discharge. Note that this value is theoretical since we can’t fully discharge a wet lead acid battery without damaging the battery. It would be better to say that the battery can sustain the 11 A load for 10 hours, or 50 % DOD.
Note 1: 50 % DOD is the best value to calculate usable battery capacity for wet lead acid batteries in order to prevent premature aging and early failure of your battery bank. The number of available charge/discharge cycles in the battery bank will be significantly reduced by regularly discharging below 50%.
Sizing Battery Banks Based on Generation:
If renewable energy sources (solar, wind, hydro etc.) are going to be used for battery charging, then the amp-hours of the battery bank needs to be 5 times (bare minimum) the size of the charging source. For example, if you have a wind turbine that can produce 100-amps DC, then size the battery bank to a minimum 500 amp-hours. This allows the charging source to safely recharge the batteries without overcharging.
Also, particularly with wind generation, some charge controllers cannot respond quick enough to prevent over-voltage conditions in rapidly changing input levels (i.e., wind gusts or grid power interruptions). Small battery banks cannot absorb large spikes in input power that can occur under those conditions. Larger sized battery banks can provide a buffer to prevent equipment damage until the charge controllers take over.
Battery Selection and Wiring
So, you’ve read the above information on battery bank sizing and maybe filled out a few load calculation worksheets and you have a daily energy (load) consumption number. Now you are ready to select your batteries and build your battery bank.
Batteries are available in different sizes, amp-hour ratings, voltage, liquid or gel, vented or non-vented, chemistries, etc. They are also available for starting applications (such as an automobile starting battery) and deep discharge applications.
Recommendations for renewable energy applications:
- Use only the deep cycle batteries for inverter applications.
- Use the same battery size and type for all batteries in the bank.
- Recommended battery types for renewable energy applications are: Flooded Lead Acid (FLA), Sealed Gel Cells (GEL), and Sealed Absorbed Glass Mat (AGM).
Some bad news:
Batteries can not be fully discharged! You must leave some energy in the battery bank to keep it healthy. Different battery chemistries have different maximum “Depth of Discharge” or “DOD” allowable numbers. In general, wet lead acid batteries have a maximum DOD of 50%. That means that we can safely (without damage) use 50% of the batteries rated capacity. Also, batteries will last longer if they are not discharged deeply.
What does that mean to you in real terms? It means that you must take the maximum DOD into account when you size your battery bank.
For example:
Let’s assume that you have an energy consumption of 220 a/h per day. You will need a 440 a/h “rated” capacity battery bank to ensure you have enough energy in the bank for one days’ worth of storage.
Let’s continue with our example number of 440 a/h and select and example battery bank. Batteries are manufactured in (generally) standard sizes such as “group 24” or Golf Cart (GC), or L-16. These sizes refer to the BCI Group number of the battery and determine the physical size of the battery case. These sizes don’t expressly refer to the battery capacity, but since battery capacity is relative to amount of acid storage which is relative to case size, then there is a correlation between case size and AH rating.
Common Battery Sizes:
| BCI Group | Size (l x W x H) | Rated Capacity | Voltage |
|---|---|---|---|
| Group 24 | 11.13″ x 6.60″ x 9.25″ | 80 a/h | 12 VDC |
| Group 27 | 12.00″ x 6.63″ x 9.06″ | 90 a/h | 12 VDC |
| Group 31 | 12.9″ x 6.75″ x 9.27″ | 125 a/h | 12 VDC |
| GC2 | 10.38″ x 7.13″ x 10.88″ | 225 a/h | 6 VDC |
| L16 | 11.64″ x 6.95″ x 15.73″ | 400 a/h | 6 VDC |
Looking at the above chart numbers, we see that our example load (440 a/h) is too big for one battery. The closest size would be two GC2 batteries. That would give us 450 a/h of storage for our example 440 a/h load.
Just like solar panels, batteries can be wired in both series and parallel configurations to increase voltage (series) or parallel (capacity). So, our above example will use two GC2 batteries wired in parallel. However, notice that our GC2 batteries are only 6 V nominal. Therefore, we will need to wire two GC2 batteries in series to increase our voltage to 12V, then parallel these two batteries with two more (also wired in series) to create our needed battery bank size. Please see the examples below (courtesy of Trojan Battery Company):
Battery Bank Charging
Charge Rate: The maximum safe charge rate is related to the size and type of the batteries. Flooded lead
acid batteries (with removable caps) can be charged at a high rate. Small batteries may require a lower charge rate. Check with your battery vendor for the proper battery charging rate for the batteries used in the system.
Bulk Voltage This is the maximum voltage the batteries will be charged to during a normal charge cycle.
Gel cell batteries are set to a lower value and non-sealed batteries are set to a higher voltage setting.
Float Voltage: The Float voltage is set lower than the Bulk voltage and provides a maintenance charge on the batteries to keep them in a ready state.
Temperature Compensation: For optimal battery charging, the Bulk and Float charge rates can be adjusted according to the temperature of the battery. This can be accomplished automatically by some inverter/chargers by using a battery temperature sensor (BTS). The sensor attaches directly to one of the batteries in the bank and relays temperature information to the inverter/charger through the BTS communications cable. Not all inverter/chargers have this capability. When battery charging voltages are compensated based on temperature, the charge voltage will vary depending on the temperature around the batteries.
Equalization: If you have liquid lead acid batteries (non-sealed), you may need to periodically equalize your batteries. Equalization is an overcharge performed on flooded lead-acid batteries after they have been fully charged. This maintenance step helps eliminate stratification and sulfation. Equalization causes water in the cells to evaporate. Check the water level after each equalization cycle and fill to the appropriate level. Also, hydrocaps should always be removed before equalization, as the high heat and vapor discharge can melt the hydrocaps and release their chemicals into the cells, thus destroying the battery.
Using the right battery charger is the first step in protecting and maintaining your expensive deep cycle battery bank.
The Cardinal Rule of Battery Charging:
Do not mix battery types (gel, flooded or AGM). Mixing battery types will cause incorrect charging, reduced battery life and possible damage to the battery bank if charged by an incorrect charge setting.
Sizing your Battery Charger:
Know the size in amp hours of your battery bank. If you under estimate the required charging capacity of your battery bank, the charger will take longer to charge your batteries. If you over estimate the required charging capacity, the charger may deliver too much current. Excessive charging current can cause battery overheating, accelerated water loss in flooded type batteries, and damaged batteries. Many battery manufacturers recommend a maximum charging rate of 20% of the amp hour capacity of the battery. For example, a 220 a/h battery bank (a small golf cart battery bank) should be charged at 44 amps per hour.
If you are connecting two different battery banks to a multi-bank battery charger, then calculate the charge rate based on the smaller of the two battery banks to avoid damage to the smaller bank.
Charging batteries from a “dumb” charging source:
Sometimes you need to charge your batteries from a source that does not taper charge current based on battery voltage. When this happens, use the following formula:
- Discover your subject battery 20 hour rate from available documentation
- Determine battery bank size
- Estimate current depth of discharge
- Calculate required Ah needed for complete recharge
- Compensate for charger inefficiency (typically add 15% to value in step 3)
- Divide the value discovered in step 4 by the charging rate of the charger (gas generator, inverter/charger, or other means).
Code Compliant Residential Battery Bank Installation
Checklist:
- Batteries need to be mounted on an insulating material (Rule 26-550)(1)
- Free-standing batteries need to be spaced apart by 10 mm (Rule 26-550)(2) to allow for air circulation and battery wall flexing.
- Conduit or No Conduit? – If wiring between battery box and other equipment is installed in rigid conduit or EMT, the end of the raceway must be sealed (with electrical sealing putty), and the raceway exit must be located at least 300mm above the highest cell terminal (Rule 26-552).
- Flexible, jacketed battery interconnection cables are preferred over bare rigid connectors because they are easier to install, and safer.
- Batteries should be installed in a room, building or battery box built specifically for that purpose.
- Adequate ventilation must be provided to disperse the hydrogen that is generated during charging of wet lead acid batteries(see below).
Ventilation
A battery will produce both hydrogen and oxygen gases when being charged. Hydrogen is lighter and will tend to become concentrated at the top of the battery enclosure. A concentration of greater than 4 % constitutes an explosion hazard. Care must be taken to ensure adequate ventilation to disperse the hydrogen gas that accumulates. The maximum recommended concentration is between 1 and 2 %. Ventilation can be achieved either by an active air fan or by natural methods.
The following information can be used to determine how to properly ventilate a battery enclosure:
First, regardless of which method you adopt, you need to know how much hydrogen your batteries are capable of producing in one hour. For a PV system, the maximum amount of hydrogen that will be produced in one hour is given by the following formula:
Vh (cu/ft of hydrogen per hour) = (a/h rating) x (no of cells) x (the maximum charging current*) x (0.016)
*the maximum charging current is expressed as a percentage. Since we use C/5 as our max charge rate, this value should be 20% of the a/h rating. If you know the maximum charge rate to be less than the maximum charge rate, enter that value as a percentage of the a/h rating.
(the above formula determines the volume of hydrogen gas expressed in cubic feet)
Now that you know how much hydrogen is produced, you need to determine the concentration of hydrogen in % that will accumulate in your battery box per hour. The formula for figuring this out would be:
Vh x (100)/Free air space in the enclosure in cubic feet.
(of course, in order to figure this one out you will need to estimate the free air space in the box)
If this calculated concentration were 1 %, then the air would have to be replaced once each hour to maintain the 1 % deemed safe by code. If the calculated concentration were 2 %, then the air would have to be replaced twice each hour etc… This leads to the following equation for the required number of air changes per hour:
Ac (per hour) = Calculated % Concentration in 1 hour/Required concentration in %
Now that you have discovered the amount of hydrogen production per hour, the concentration of hydrogen build up in your battery box and the required number of air changes per hour needed in your battery box, you can make an informed choice between active or natural ventilation.
Active Ventilation:
There are many pre-engineered in-line battery fans on the market today. They are designed to be installed in the outlet vent pipe of the battery box. They have the advantage of brushless motors (to prevent igniting the hydrogen gas as it passes through the motor) and air seals to prevent unwanted outside air infiltration when the fan is not running. In order to reduce energy usage, most of these fans are installed with a relay that is designed to turn the fan on based on battery voltage. This way the battery fan only runs during charging cycles. Many of the higher end model inverter/chargers such as Outback and Xantrex sine wave units have programmable relays built into them that will drive these fans. Otherwise, Morningstar and Solar Converters manufacture relays and relay drivers for this purpose.
Make sure your selected active vent is capable of moving the required amount of air volume determined above, and that you install at least one intake air vent on the opposite side of the exhaust vent in the battery box. The intake air vent(s) should be at least the same size as the diameter of the fan to allow proper air flow.
The Zephyr Power Vent has a rating of 360 cu/ft/h at 12/24V or 480 cu/ft/h at 48V. The following chart shows….
| BCI Group | Rated Capacity | Batery Bank Voltage | Charge Rate* | VH** |
|---|---|---|---|---|
| Group 24 | 80 a/h | 12 VDC | 20% (16 A) | 153.6 cu/ft/h |
| Group 27 | 90 a/h | 12 VDC | 20% (18 A) | 172.8 cu/ft/h |
| Group 31 | 125 a/h | 12 VDC | 20% (25 A) | 240.0 cu/ft/h |
| GC2 | 225 a/h | 12 VDC | 20% (45 A) | 432.0 cu/ft/h |
| L16 | 400 a/h | 12 VDC | 20% (80 A) | 768.0 cu/ft/h |
* Maximum Recommended Charge Rate is the rated capacity of the battery bank in a/h (20 hour rate) divided by 5, or 20%.
** (Rated Capacity) x (# of cells) x (20) x (0.016)
Natural Ventilation:
Rooms used for battery storage typically have natural ventilation due to window and door leakage. Typically, a room would have approximately 1 air change every 4 hours. Battery boxes typically have no natural ventilation, so you must provide an inlet at the bottom of the box and an outlet at the top. Since hydrogen is lighter than air, natural circulation will take place providing the required ventilation. The following equations are used to calculate the vent sizes:
V = 836A(h)
V = the required volume of air in m3 per hour (determined in the equations above)
H = the vertical height difference between inlet and outlet in M
A = the combined area of the vents in m2
Some sort of protection will be required to keep out bugs and lint. A screen could reduce the effective area by as much as 25%, so the hole size needs to be increased to compensate for the screen. Also, vents should be placed on opposite ends and sides of the battery box to provide uniform air flow.
Some adjustment can be made to the vent size by extending the height difference between the inlet and outlet vent holes. This can be done by installing a stack pipe on the top vent. A greater inlet/outlet height differential will provide greater venting force. The size of pipe should be determined to have at least the same area as the vent hole area itself.
Battery Short Circuit Currents
The Danger: In DC systems, a shorted battery has the potential to deliver an extremely high current in a very short amount of time, typically within 5 to 15 milliseconds. Without some form of protection such as a fuse or breaker, a short circuit condition can cause serious damage and is very dangerous. In effect, the battery itself can become a fuse. If the weakest link in the circuit is external to the battery, then the circuit will melt at that point and (hopefully) open the circuit. If however the weakest link is within the battery itself, then the melting and opening of an internal connection has the potential to ignite the hydrogen/oxygen gas mixture in the battery which is devastating to the battery, your skin and the surrounding environment.
Safety First!: Knowing the danger is not enough. Everyone that has a battery bank, particularly a wet lead acid battery bank should have a battery acid spill kit containing acid absorber and neutralizer, gloves, goggles and even an apron, gloves and boots. Also, a portable eyewash station or emergency eye and skin wash bottle close to your battery bank. If possible, never work on a battery bank alone. If you have to be alone, make sure someone knows where you are and can contact you to make sure you are safe. Make sure to always wear gloves and goggles when working directly with batteries and use insulated tools where possible. Remember also that sulfuric acid accumulates to the top of the battery around the vent holes. Wash your hands!
Known Battery Short Circuit Currents:
Trojan T105 Battery – 220 ah rated capacity: 4000 A Short Circuit Current
Surrette 2V 1050 ah rated capacity: 9250 A Short Circuit Current