Solar Battery Power

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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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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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• 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.

• 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.

SIXTH DETERMINE CHARGE CONTROLLER SIZE and TYPE

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)

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

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Flooded Batteries

** 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

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