Must know facts about Solar Batteries (Author: The Economist)
The very first thing everyone asks when we tell them that we have converted to off-grid Solar is “How many panels do you have?” Yes, panels are important but every off-grid solar system will have three main components:
1. The panels (electricity generation in DC).
2. The all in Inverter (incorporating battery charge control, conversion of DC to AC and supply control to the household).
3. The batteries (power reservoir).
The relative importance of batteries are often underestimated and underappreciated.
We use our solar system for water heating with a heat pump (daytime only), cooking with induction plates (daytime and at night) and use appliances such as the oven and tumble dryer (daytime only). A robust solar battery bank is important for both daytime and night time use. This discussion about the batteries is mostly not technical and is based on experience with regards to the basic research done before buying a solar system, research done to understand how best to look after batteries for an extended life and observation of our batteries in real life and real time.
• Your solar batteries must be deep cycle batteries. Unscrupulous salespersons often design solar systems with low quality batteries such as 12v car batteries. Household needs are to fill the battery bank with power during sunlight hours and draw the battery down at night and during overcast spells. Only deep cycle batteries are designed to cope with constant charging and discharging of the batteries. Unsuitable batteries will simply fail within 6-12 months and will degrade fast over its short life span. We have installed AGM (Absorbed Glass Mat) deep cycle batteries. AGM batteries are maintenance free and use gel rather than water.
• The maximum household overnight electricity demand for our battery bank must be less than 50% of the battery bank capacity for its model. We refer you here to the viability model at this link and “know what you use” at this link. We use about 7kWh overnight which means that we must have at least 14kWh stored in our batteries. Our battery bank was incomplete due to a shortage of batteries and for the first 3 months we had a total bank of only 14.4 kWh. It often ran out by morning particularly if the batteries were not fully charged the previous evening, or when we used a bit more overnight, or when the winter days got shorter and the nights longer, or when an appliance did a power spike in the morning. The battery bank is now at 28.8 kWh and provides sustainable electricity for about 95% of the time. It is insufficient to cope with our electricity needs during prolonged overcast weather even when we restrict usage of power hogs. We are still vulnerable to large power spikes when power supply from panels are non-existent or low. We will for now experiment with using a solar friendly generator to charge the batteries when they run low on overcast days.
• It is important for users to understand the battery bank’s vulnerability to power spikes. Every household has a few energy hogs which are prone to power spikes. The dishwasher is a main culprit. It uses little energy when washing but warms its own water. The power usage spikes strongly when the dishwasher engages its water heating system. It becomes problematic if it spikes during heavy electricity usage or when the batteries are at 75% charged or less as the fast increase in electricity demand cannot then be drawn equally fast from the battery bank. We are aware of this vulnerability and it is not particularly difficult to manage but it cannot be ignored unless we oversize our battery bank to compensate which we do not wish to do right now (though we may want to at a future date).
• Oversizing battery banks is better than “just sufficient” or under sizing it. An oversized battery bank will last longer, will be less likely to overheat, will be better able to cope with general and spike electricity demands from the household and will bridge overcast conditions for longer. Our battery bank is only slightly oversized and as a result we must manage our electricity use to protect the batteries against power demand spikes. The larger you battery bank the greater your usage freedom.
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| Solar 2v Battery bank. We have two banks of 24 batteries of 2 volts each for a 48 volt system. The volts may be 12, 24 or 48 and the choice will depend on how far the electricity must travel in your house. We had to use a 48 volts configuration. The batteries are 300ah in “strength” which translates to 14.4 kWh per battery bank or 28.8 kWh in total of which we can draw up to 50% (to ensure a longer life for the batteries). Note how the batteries are spaced apart to allow any heat to escape which helps avoid heat sharing among batteries, heat build-up and thermal runaway (see below). We would need to add one more battery bank of 24 batteries to well oversize our total battery bank. |
• Our AGM deep cycle batteries are valve regulated lead acid (VRLA) batteries which still need some room ventilation as they may release very small quantities of hydrogen which can collect in unventilated areas and auto-ignite (explode) if trapped in those unventilated areas. It is generally considered an extremely low risk for solar batteries but it is important to be informed and aware of the risk irrespective how small. Additional technical information can be obtained at www.cdtecno.com , and from this document http://www.cdtechno.com/pdf/ref/41_6739_0112.pdf .
The fundamental principle is that gassing occurs when the batteries are overcharged thus the owner and householder must at the very least watch for the symptoms of over-charging which is over-heating of the batteries. Also make sure that the batteries are not stacked together but have at least 5mm ventilation gaps between them on all sides (discuss with your installer); don’t have direct sunshine on the battery bank which can over-heat it; and make sure batteries are not in close proximity to any heat source which can influence the ambient temperature of the battery bank. Batteries must not exceed 50 degrees Celsius. A simple test is if your batteries are warm to the touch then you would be well advised to call out your installer to investigate.
• Most solar batteries presently are AGM VRLA batteries and as such home owners must take basic care together with the installer to ensure that the batteries do not over-charge and do not over-heat. Not only do the batteries gas when they over-heat, they also can go into a state called “Thermal Runaway”. The inverter will be set to charge batteries in a safe manner but it would not normally be able to detect a faulty cell or a faulty battery. The inverter will continue to supply charge as if all the batteries are fully functional and as a result of a single cell failure a chain reaction can occur in the whole battery bank where the faulty cell’s charge gets distributed to the other batteries and cause over-charging and over-heating. In turn more cells and batteries fail and eventually the whole bank will collapse and may even burst the batteries. Again the homeowner risk is relatively low but battery ambient temperatures still need basic awareness and care from the home owner. More information on thermal runaway can be obtained at www.cdtecno.com , and from this document http://www.cdtechno.com/pdf/ref/41_7944_0712.pdf .
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| AGM VRLA Solar Battery valve. In the event that the battery is over charged or over heated, hydrogen gas will form inside the battery and to prevent the battery from exploding, the hydrogen will be released through this one way (from inside to outside) escape valve. |
All households are exposed to a myriad of risks, sharp objects, changes in elevation, glass breakage, slippery floors, etc. Solar systems requires the same care that we take to manage all these household risks and will serve the responsible home owner well. Another obvious requirement is that the battery bank must be enclosed in a ventilated space, which will not allow pets or children to gain access or to touch the live wires.
The final take-away on batteries are:
Ö Use only batteries that are specifically
designed for solar systems;
Ö Oversizing the battery bank is the best, do not economise
on batteries;
Our solar system was designed and installed with the assistance of Jurie Venter, cellphone 083 557 6031 and email jurie@sunor.co.za . For details on the whole system, see the post How to go off grid permanently.
Related Posts:
Part 1: Solar Power in South Africa - How to go off grid permanently (The System Set-up)
Part 3: Solar Power in South Africa - Solar & Seasonality (Solar in winter)
Part 1: Solar Power in South Africa - How to go off grid permanently (The System Set-up)
Part 3: Solar Power in South Africa - Solar & Seasonality (Solar in winter)
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