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Solar Battery Power

Off Grid Battery Systems

AGM - Gel Batteries - Flooded Batteries


Battery Thoughts

Application and Design

Maintenace Free, Non Freeze Batteries

Flooded Deep Cycle Batteries

Battery Charging

Your PV panels will produce electricity whenever the sun shines on them. If your system is off-grid, you’ll need a battery bank—a group of batteries wired together—to store energy so you can have electricity at night or on cloudy days. For off-grid systems, battery banks are typically sized to keep household electricity running for one to three cloudy days. Gridintertied systems also can include battery banks to provide emergency backup power during blackouts—perfect for keeping critical electric loads operating until grid power is restored.

Although similar to ordinary car batteries, the batteries used in solar-electric systems are specialized for the type of charging and discharging they’ll need to endure. Lead-acid batteries are the most common battery used in solar-electric systems. Flooded leadacid batteries are usually the least expensive, but require adding distilled water occasionally to replenish water lost during the normal charging process. Sealed absorbent glass mat (AGM) batteries are maintenance free and designed for grid-tied systems where the batteries are typically kept at a full state of charge. Gel-cell batteries can be a good choice to use in unheated spaces due to their freeze-resistant qualities.

Battery Thoughts
Batteries are the very heart of your solar electric system. They are where your power is stored - your reservoir.A battery storage bank is what allows your power system to deliver a constant level of power to your electrical loads. Without batteries you would have no power when the sun went down and maybe not even enough during daylight hours, depending on cloud cover, etc.
By running power from your solar panels through a charge controller and into your battery bank, power is available 24 hours a day, regardless of weather.
Even with several days in a row of bad weather, your battery bank can continue to store power through the means of a back-up generator. Most of your solar electric system needs little or no maintenance. The battery bank is the exception.
Maintenance is not difficult or very time consuming, but if the batteries are neglected, degradation can occur at a very rapid pace.
The right choice of the size, type and number of batteries is important to insure your power system performs properly and to maximize the life of the batteries. The following information is designed to tell you how batteries work, the difference in size and type of batteries, how to size, monitor and maintain your system and much more.

Deep cycle Battery and Shallow cycle Battery
A cycle in a battery occurs when it is discharged and then recharged back to its original level. How much a battery is discharged is called the depth of discharge.
A shallow cycle is when the top 20% or less of the battery is discharged and recharged. Batteries in your car or truck are designed for this type of cycling. They have relatively thin plates, with a large surface area that gives up lots of power over a short period of time,as when they start your vehicle.
These types of batteries should not be used in your solar electric system. Because of the kind of use they get they would not last very long. They are not made for constant deep discharging and recharging. A deep cycle is when up to 80% of the battery capacity is discharged and recharged. Deep cycle batteries are designed with thicker plates and with overall less surface area. These batteries give up just as much power for their rated size, but do it over a much longer period of time.Deep cycle batteries are what you want for your home solar electric system.
The depth of cycling has a lot to do with how long your batteries last. As the depth of cycle increases, the battery life is used up faster.
For maximum battery life it is best to shallow cycle your deep cycle batteries.

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Sizing your battery bank
The number of batteries in your battery bank depends
on many factors such as:
• The number of appliances you use and the amount of power they take.
• The number of days the batteries go without charging, due to bad weather or other factors.
• The level of discharge you wish to go to before re-cycling.
• The temperature of the area where the
batteries are stored.
• The size of your charging system.
• The size of your budget.
Generally speaking, the more batteries in your bank the better, because you will have more reserve power available and the level of discharge will be lower. Just as in your car, the less your batteries have to work, the longer they will last. For the best overall performance in the long run, choose the largest, best quality battery bank that you can afford.

Application Example

The batteries you see above are two 6 volt deep cycle batteries hooked in series to give 12 volt output. They are rated at 240 AH.

I like to convert to watt/ hours ( 12  volts times 240 ) = 2880 watt/hours

Which means you could run your 100 watt light bulb for 28.8 hours from a fully charged battery

The design process is a fairly simple and straight forward process that doesn’t require a lot of technical knowledge, just some simple grade school math skills. The initial steps are:

  • Determine the maximum energy required in a 24 hour period. Electrical energy is measured in Watt Hours and the formula is; Watt Hours = Watts x Hours.
  • Determine the size of the solar array required based on Charge Controller type that will be used.
  • Determine Charge Controller size.
  • Determine the battery capacity and voltage based on panel wattage.

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The following are two example systems to determine major components, cost, and objectives. Once the preliminary design is completed, the system can be fine-tuned when actual components and cable distances are known.
Our basic objective in this simple example is to provide power to a 250 watt load (light bulb) for 24 hours per day in two different cities, Tucson AZ and Seattle WS, to demonstrate how critical location is to the overall expense. This design will provide power for about 90 to 93% availability for each calendar year. To achieve 99% availability, what a utility averages, would require quite a bit more expense in the way of materials and equipment like generators.


In this example the worst case is simple to determine because the load is continuous 24 x 7 x 365 of a 250 watt light bulb. So the worst case is the month of December and January when the Solar Insolation is at its lowest point. In some instance, the worst case is in the summer. This requires you to perform two designs and then to select the larger of the two designs.

So in this example we need to determine the energy needed in a 24 hour period. This is done with watt-hours. To determine the watt-hours is straight forward of Watts x Time (in hours). So 250 watts x 24 hours = 6000 watt-hours or 6 Kwh in 24 hours. Make note of this number as it will be needed latter.
NOTE: I cannot stress enough how important it is to determine the real number of watt hours needed. If you get it wrong, 3 things can happen and 2 are very bad. If you underestimate you will go dark and likely destroy your batteries. If you over estimate you will empty your bank account as you will see it is very expensive.


OK next we need to account for system losses. This can be done for each step of the process, and is done when we have real equipment specifications and cable distances. But for preliminary design we just lump it all together right up to get a good idea to pursue the project or not because it will be within a couple of percent of the final design.
This is a 2-step process; 1 adjustment factor if using MPPT controller, and/or another for a PWM controller. In most cases if the panel wattage is going to be more than 200 to 300 watts, it is economically justified to use a MPPT controller. PWM controllers are generally only used on very small systems.
  • Adjustment factor for a MPPT controller is 1.6, and 2.1 for PWM. So if using a MPPT controller: 1.6 x Daily Watt Hours = 1.6 x 6000 wh = 9600 watt hours.
  • PWM Adjustment = 2.1 x Daily Watt Hours = 2.1 x 6000 wh = 12,600 wh.
Now take note of the 2 figures as one will be used to determine panel wattageis figure.


This is where it gets fun and educational, we now find the panel wattage required. Most solar map data are given in terms of energy per surface area per day. No matter the original unit used, it can be converted into kWh/m2/day. Because of a few convenient factors, this can be read directly as "Sun Hour Day” The number you want to use in this example is for December since December days are the shortest. Tucson AZ receives 5.6 kWh/m2/day in December, and Seattle is 1.2 Kwh/m2/day. So we need to note 5.6 and 1.2 Sun Hours as it will be used to determine the solar panel array wattage.


The size of the array is determined by using the adjusted daily energy in Watt Hours and Sun Hours. The formula is Watts = Watt Hours / Sun Hours. All we are doing is factoring out the time element Sun Hours to leave us with Watts. In this example I am only using the MPPT adjusted daily watt hours because it is obvious the panel wattages required are larger than 300 watts. Ready for the learning fun?
Tucson AZ Panel Wattage = 9600 wh / 5.6 h = 1714 watts. This needs to be rounded up to some whole number like 1720 to 1800.
Seattle WS Panel Wattage = 9600 wh / 1.2 h = 8000 watts OUCH!

Battery voltage selection is based on panel wattages:
  • 1000 watts and less = 12 volts or higher
  • 1001 to 2000 watts = 24 volts or higher
  • 2001 to 4000 watts = 48 volts or higher
  • 4001 to 8000 = 96 volts or higher.

Any lead acid battery should never ever be discharged more than 50% at any time and must immediately be fully recharged. Otherwise you will shorten battery life considerable and batteries are very expensive on the order of $200/Kwh of capacity. On also must account that there will be cloudy days, and you will need extra capacity to carry you through a couple of cloudy days.

So with this in mind to provide 90% availability we design for a minimum 5-day reserve capacity. This gives us a real 2.5 day reserve capacity without going below the 50% Depth of Discharge. So to determine Battery Amp Hour Capacity is very simple, the formula is; [5 days x daily Watt Hours] / Battery Voltage. I will only demonstrate the Tucson example because Seattle is not really feasible, but the same method is used. So our daily watt hour requirement is 6000 watt hours, and we are using 24 volt batteries so [ 5 x 6000 wh] / 24 volts = 1250 Amp Hours @ 24 volts.

More battery economic math fun. For each 1000 watt hours of battery capacity the battery will weigh approximately 70 pounds and cost $200. With the above example we need a 30,000 wh or 30 Kwh battery. Go figure that one out on your own. Keep in mind it needs replaced about every 5 years.


For PWM is real simple. The charge controller current rating must be equal or greater than the Panel Isc (current short circuit) rating. This is because input current = output current on PWM controllers. So let’s say you are using 3-100 watt solar panels made for 12 volt battery systems. A typical 100 watt 12 volt panel has a Isc rating of 7 amps. Since you are using a PWM controller you have no choice but to wire the panels in parallel. In Parallel circuits current adds, and you have 3 panels of a total of 7+7+7= 21 amps. So you would need at least a 21 amp PWM controller. They do not make 21 amp controllers so you would look for 25 amps.

For MPPT is even simpler. Controller Amps = Panel Wattage / Battery Nominal Voltage. So for a Tucson we selected a 1800 Watt solar panel and 24 volts. 1800 watts / 24 volts = 75 amps. We would need an 80 amp MPPT controller.
Well that is about it to get you started to determine what it will take to take you off grid. Dig real deep in your pockets.

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Discover EV series

Advanced AGM and Gel batteries

  • 100% maintenance free. Never need to check electrolyte levels.
  • Sealed recombinant construction eliminates spills, gases and terminal corrosion
  • Operates from -60F to 160F
  • Can be used in any orientation (upside down not recommended)

download brochure

    Amp-Hr Terminals Dimensions Weight  
Part No Volts 20 HR STD OPT L W H TH KG List Price
EV185A-A 12 234 AM   386 180 346 367 67 Call for pricing*
EV22A-A 12 55 Q X 230 138 206 224 16 Call for pricing*
EV24A-A 12 84 AM   272 172 206 226 24 Call for pricing*
EV250A-A 6 260 AM   295 180 274 296 37 Call for pricing*
EV27A-A 12 100 AM   323 172 206 226 28 Call for pricing*
EV305A-A 6 312 AM   295 180 347 368 48.9 Call for pricing*
EV31A-A 12 114 AM   330 180 216 236 33 Call for pricing*
EV34A-A 12 70 M6   260 159 178 183 21 Call for pricing*
EV4DA-A 12 245 AT   528 222 229 250 64 Call for pricing*
EV8DA-A 12 290 AT   528 282 229 250 82 Call for pricing*
EVGC6A-A 6 213 AM   260 180 254 274 31.4 Call for pricing*
EVGC8A-A 8 170 AM   260 180 260 287 30 Call for pricing*
EVL16A-A 6 390 AM   295 180 405 426 56 Call for pricing*
EVU1A-A 12 33 Q X 198 132 164 182 10.1 Call for pricing*

* lead prices have been fluctuating. Please call us for pricing.

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Flooded Batteries

      Dimensions (Inches)  
Part No Volts Amp-hour L W H List Price
G24DC 12 87 8.9 6.7 8.7


G27DC 12 105 12.8 6.7 9.8 $182.99**
G2200 6 220 10.4 7.1 11.2 $185.99**
G2300 6 235 10.4 7.1 11.2 $208.99**
US145 6 244 10.4 7.1 11.6 $290.99**
US305 6 315 12.2 7 14 $360.99**
USL16 6 375 12.2 7 16.5 $435.99**
US1850 12 200 15.5 7 14.2 $430.99**

** these prices are more stable but pricing is subject to change without notice.

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The Charging Process

Early charge controllers were crude:
generally little more than voltage-actuatedon/off relays. Modern charge controllers have substantially improved battery charging and longevity.
The charging process for flooded cells involves four steps: bulk, absorption, float, and equalization. During the bulk phase, which generally fills the cell to around 85% of its capacity, the charge controller (from a PV, wind, or hydro source) or the inverter(from a generator or AC source) allows all available charge current to flow into the cells. As the current is absorbed by the plates in the cell, the cells' voltage steadily increases.

When the voltage reaches the bulk voltage set point (typically about 2.45 V per cell), the controller moves into the absorption stage. During absorption, the voltage is held at the bulk set point, and the charge is regulated to the current necessary to maintain that voltage (plus power any loads that are on).
When any flooded lead-acid battery approaches full-charge voltage, the cells begin to "gas." The cells are no longer able to absorb all of the energy, and the excess energy separates water in the
cells into hydrogen and oxygen gases. Gassing is an important part of the charging process: the process brings weaker cells closer to the charge level of stronger cells, and the bubbling action destratifies the electrolyte.
As the cell approaches 100% SOC, the amount of current necessary to maintain this voltage steadily drops. When either a preset time duration (typically 2 to 4 hours) is reached, or the charge current drops below a set threshold (typically a C/50 rate, or about 2% of a healthy cell's capacity), the cell is considered fully charged, and the controller

battery Charge

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