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Batteries, the heart of the system

We will start with the batteries, as they are the heart of the system, and frequently, the most neglected and misunderstood part, thus a long page to explain things so that you can make a wise and informed choice. The batteries also represent a substantial portion of the investment in a Solar System, so you will want to make sure that you get good batteries and take care of them properly.

Which technology of Battery is best for my "Off Grid" Solar Power System?

The most common and suitable type of battery used in "Off Grid" Solar Power Systems is the flooded lead acid battery type. This battery technology has a long proven track record and generally speaking provides the longest life while tolerating a certain amount of abuse. Battery manufacturers generally recommend this technology for "Off Grid" Solar Power Systems.

Advantages of Flooded Lead Acid (FLA) batteries

  • Generally will have a longer life and more available "cycles"
  • Lower cost for the same capacity
  • Tolerate a certain amount of abuse
  • Can be equalized if they have been undercharged and sulfation has occurred.

Disadvantages of FLA batteries

  • Require ventilation
  • Require more maintenance than sealed batteries

Sealed Batteries come in the Absorbed Glass Mat AGM and Gel variety. These batteries have a sales appeal in that "theoretically" they do not require maintenance. They do in fact require maintenance and are more likely to be neglected because they are marketed as "maintenance free" batteries. Maintenance free batteries are great (for the dealer), you do not maintain them, you just keep replacing them.

Advantages of sealed batteries

  • Can be operated in different orientations
  • Do not require water to be added
  • Many are not classified as dangerous goods and therefore can be transported more easily, especially by air or boat (wet cells are subject to Transportation of Dangerous Goods regulations)
  • Generally have a lower self-discharge rate
  • Are generally well suited for remote seasonal applications, since they will not require servicing and will fare better through the winter without being attended

Disadvantages of sealed batteries

  • Higher cost for the same capacity as FLA batteries
  • Do not tolerate overcharging
  • Cannot be equalized to reverse the effects of sulfation
  • Generally will not last as long as FLA batteries or deliver as many cycles

A high quality FLA battery will provide 5000 cycles when discharged 20% (80% remaining) or 3200 cycles at 50% Depth of Discharge (DOD) (Rolls 5000 series batteries)

 



(Click on graph for a larger view)

A medium quality FLA battery will provide 2000 cycles at 20% DOD or 1280 cycles at 50% (Rolls 4000 series batteries)



(Click on graph for a larger view)

AGM batteries vary greatly depending on the quality, about 300 to 1800 cycles at 50% DOD or 900 to 2700 at 20% DOD.


Be sure to check the specific make and model for the projected cycle life of the desired battery. For example, the Trojan L16RE-B has a cycle life of 4000 at 20% DOD; the more common Trojan L16 H, even though it has a higher rated capacity, has a rated cycle life of only 3000 cycles at 20% DOD. Cheaper L16s would have an even lower cycle life. Boyd Solar packages use batteries that have high rated life cycles for the type of battery being used. This provides for the best value! Generally, when we are looking at the DOD to use for determining which battery to use, the 20% DOD is a good value since, in an Off Grid home, in the summer the DOD may be 10% or less and in the winter you would only occasionally get to 50% DOD.

Remember also to consider not only the initial cost of the batteries, but also the cost to replace the batteries. It is generally more cost effective to use the best FLA batteries, as they will provide the best cycle life and will not require replacement as often as cheaper or sealed batteries.

The Rolls 5000 series and the Trojan Industrial series both have dual wall construction for strength and safety.

Remember to follow the manufacturer’s instructions and regularly check and maintain your batteries as they make up a substantial amount of the investment in your "Off Grid" renewable energy Solar Power system.

Batteries are to be installed in either a battery room or compartment. All conductors leaving this compartment must be installed in conduit.
Battery banks must be sized carefully, under sizing a bank can lead to heavy cycling, higher operating temperatures, shorter life and insufficient capacity for poor weather days. Over sizing a bank on the other hand can lead to sulfated batteries if there is not sufficient charging capacity.

The generally accepted sizing of a battery bank allows for about 3 to 5 days of autonomy. (The length of time that the batteries will supply your "Off Grid" home before running out of power). For systems without a generator, the autonomy would be more in the 8 to 14 day range.
Generally speaking it is best to use one or 2 strings of batteries. The more strings of batteries, the more connections and unequal charging and discharging of the banks. 3 strings is the maximum recommended number of strings that are connected in parallel.

It is important to be careful when using manufacturer’s specifications for designing a system. New batteries generally do not come up to specification until they have been cycled many times. As they age, they also start to lose capacity. If a system is designed just based on the specification and the rated capacity is used, (remembering that the rated capacity is based on the battery being totally discharged) then there are some things that will occur.

First and foremost, you do not have the rated capacity available to be used. For example, let’s say you have a battery that is rated at 400 AH and you have a 48 volt system. This would theoretically give you 19.2 KW to use. Battery manufacturers do not recommend discharging more than 50%, so now you are left with 9.6 KWH of energy if the batteries are operating at 100% of the rated capacity. If you have sized this system for 8 KWH a day, then you will barely get 1 day of autonomy. This is working the battery really hard and would require the generator to basically be run every time you have a bad day or two. When the batteries are at the beginning of their life and the end, the capacity will be less, so you not even have one day of autonomy!

Secondly, in this type of scenario, it is unlikely that the batteries will actually be fully charged as they require the completion of the absorb cycle. This will lead to sulfation and therefore even less capacity. This is one of the most common problems that we have seen in systems that were not designed by us. A generator is very inefficient at doing this and people usually turn off the generator before the batteries are actually fully charged. The following slide illustrates this.

As you look at the actual amount of current going into the battery, once absorb voltage is achieved, and the absorb cycle starts, the current drops fairly steeply over a time period of about 2 to 2 ½ hours. The end current is frequently given as 1% or 2% of the 6 or 20 hour rate. My observations here show the value to actually be less than 1% for new batteries. When you use a generator this way, you actually waste a lot of fuel for very little actual output and pollute the air while you are at it. If you just figure generator run time and solar output based on what the batteries require, without accounting for the cutting back of the battery charging current, then you end up with some very skewed numbers, both in terms of required solar capacity, the cost per KW generated by the Solar Array and the cost for the use of the generator. Add to this the amount of time and money spent getting fuel for the generator and servicing it, the actual cost of running a generator rises dramatically.

When an undersized battery is used, the life cycles drop dramatically. For example, if a standard L16 size battery has an average DOD of 40%, then the cycle life is only about 1100 cycles. That translates to about 3 years, and that is without the shortened life due to sulfation. The question then becomes "how often do you want to change your batteries?" It is much more cost effective to get the proper size and type to start with.

 

An important note here regarding the choice of batteries.

These charts represent different aspects of a life cycle cost analysis.

The price is strictly based on the cost of the batteries, additional costs are actually incurred with the L16 size of batteries; since the industrial batteries include the battery interconnection cables and will generally be a single string. To get the required capacity, L16 size batteries will normally require multiple strings, therefore extra wiring, time and maintenance. The interconnects and extra wiring would likely be an extra 7% or so. (While the actual price may change, the relative cost will likely remain)

Obviously, the best value choice is the industrial series of batteries, and these costs do not even include the labour and travel cost of replacing the L 16 size of batteries more often.

Another note on the Rolls L16 size of batteries. If they are properly charged and maintained, experience has shown that they will likely last longer than specified.

As we can see, L 16 size batteries, though good for the smaller systems, should not really be considered for a full time Off Grid residence.

Be sure to get the "short circuit ampacity" for the batteries so that the circuit breakers or fuses can be selected properly. Many systems are not designed properly here as the short circuit ampacity of the batteries is greater than the rating of circuit breakers or fuses. For example, 1 string of Rolls S1380 batteries has a short circuit ampacity higher than 10,000 amps. This exceeds the 10,000 amp rating of most of the smaller breakers, and if no separate battery disconnect (with a large interrupting capacity is installed) you can have some very bad things happen. Direct Current (the type of power that the solar modules and batteries put out is much more difficult to interrupt than Alternating Current, which is what we have in our homes). Ideally, the brand, model, voltage, capacity and short circuit ampacity should all be put on a label by the batteries for reference.


Rolls Manual


Trojan Manual

Battery Disconnect

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