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Solar Batteries:
Q1: Do I really need a battery bank?
A1: It is advisable, but not essential for domestic homes already on the Grid. However, if you do not have a battery bank and rely solely on the PV yield for some, or all, of your daily power requirements; then you will be limiting the size of your system to whatever you consume between solar charging hours (say 8am-5pm)). Having a battery bank will enable you to cater for more of your daily consumption as most houses with a ‘working’ family, consume most of their power outside of solar charging hours and employ a battery bank to support them during those hours. Most homes are not completely Off-Grid but between 30-80% as an average. Also of Eskom fails you (unlikely, I know!) you have something to fall back on in the evenings, by having a battery bank as a back-up.
For businesses, it makes sense to go for a Grid Tie solution as most of your power is consumed during solar charging hours anyway. You can then employ Mains or a Genset as a back-up facility in case of non-sun days or grid failure.
Q2: How deeply should I cycle my batteries?
A2: That depends upon many factors. Firstly the type of batteries you decide upon. Normal lead acid and Gel batteries are cycled down to 20-50% in most solar systems in S.A.; whereas Lithium Ion type can comfortably be cycled down to 80% and still give the same, if not greater, life expectancy, than lead acid and Gel batteries at that Depth of Discharge(DoD).
Secondly, some people don’t like to cycle them below 20% to cater for power out situations when they don’t want to be caught with inadequate power reserves. Most common durations of power-outs are between 2-6 hours these days. If they happen during solar charging hours, you have PV yield to cater for power anyway. If they happen in the evening, then you can cycle your battery bank, in emergencies, down to 80%.
We consider 40% to be a good average for Lead Acid and Gel batteries. 70% for Li-Ion batteries.
Q4: What is the difference between a solar battery and a normal deep cycle battery?
A4: A normal deep cycle battery usually has thinner plates and is thus not designed to cycle as frequently as a solar battery. Normal deep cycle batteries are used for leisure (E.G. 4x4 Off Road) and Back-Up power applications, and will only usually give some 250-350 cycles. Solar batteries, by our definition anyway, should give a minimum of 1.200 cycles at 40% DoD.
Q5: Can you mix batteries of different ages?
A5: It is not recommended to mix batteries of the same type with units that have more than a 6 month age difference. It also depends upon how frequently these batteries have been cycled, and down to what level, of course. People do it all the time, but there is little benefit in so doing because the older batteries tend to pull down the performance of the new ones and it can also lead to over-charging of the new ones - if their performance characteristics differ excessively.
Q6: Can I mix different types of batteries in my bank?
A6: This is a definite ‘No-No’ and will lead to issues regarding the charging requirements for the different batteries. Avoid this at all costs!
Q7. What sort of life can I expect out of my solar batteries if I discharge them by 40% (i.e. 40% DoD)?
A7: This is a bit like defining beauty?!! It depends upon the type and quality of the batteries (refer web article on different types of batteries). A good solar battery can give you up to 10,000 cycles (Li-Ion and flooded 2V cell types) and the latest Lead acid and Gels from leading manufacturers will also give you as much as 4,000 cycles at 40% DoD. Specifications are improving all the time – thanks to our friends in the electric car industry, as well as extensive R&D being undertaken with energy storage for the alternative energy industry. Watch this space…!!!
Q8: How much does temperature affect battery performance?
A8: A great deal, is the simple answer!. Battery performance drops off rapidly once temperatures exceed an ambient of 25 degrees Celsius, but is even worse when temperatures start to plummet into single figures. The Li-Ion batteries will take higher temperatures better than lead acid, but suffer more from lower temperatures. Most solar charging components such as MPPTs and Inverters have temperature sensors connected to the battery bank to compensate for swings in temperature as different charging rates are required for varying temperatures. Generally, lower temperatures tend to affect performance, and higher temperatures, life expectancy.
Q9: How important are charging rates for solar batteries?
A9: Very, and greatly misunderstood. A solar battery usually requires to be bulk-charged at a minimum of 10% of its rated capacity – so 10A for a 100Ah battery. Thus if you have an 800Ahbattery bank (12V say), then you will need to charge it at 80Ah minimum, to be effective. Some batteries like the Li-Ion prefer higher amperages, and even slightly higher voltages, over Lead Acid batteries. Most MPPTs and Inverters have settings to suit the battery type into which they are charging.
Q10: Should I cycle my batteries deeper, or use Mains?
A10: That depends upon how deeply you are already cycling your batteries - and how much spare yield you have available to recharge them. If you are cycling them to only 20%, and have spare PV yield capacity, then yes cycle them deeper. However this is only if you can’t power up other appliances directly with the surplus yield, because that will be more efficient. If you can’t, then you can either add more batteries, or cycle them deeper, or switch to Mains. Remember though, that there must be a good balance between your MPPT output and the battery bank (refer above question).Undercharging can also damage batteries and reduce life expectancy.
There is always a big debate about whether it is cheaper to use Mains power than to cycle your batteries deeper. However, as Eskom prices continue to rise, and batteries get cheaper and more efficient, so this argument will fall away.
Q11: How long will my battery bank last:
A11: This depends upon many variables: Battery type; depth of Discharge to which the battery is generally subjected; number of cycles per day. On average a good solar lead acid battery will give you some 3-5 years, with flooded and Li-Ion batteries capable of double that at the same DoD.
Q12: What are the best type of solar batteries
A12: It depends upon the application and climatic conditions under which the battery has to operate. Flooded batteries were the most popular due to their longer life expectancy – but maintenance in our continent is a bit of an issue, so they fell out of favour. Hot climatic conditions were also an issue for them. Lead acid batteries, although maintenance-free, did not give the life that many users expected of them over the past 5 years. The jury is still out on the more modern improved versions from leading manufacturers. Li-Ion batteries are becoming more popular – although their initial high prices tended not to favour them despite their lower cost per cycle over time. If Li-Ion batteries continue to reduce in price over the next few years, then I think that many people will move over to this technology.
Q13: What is a good ratio of battery amperage to PV yield
A13: Most batteries require a charge of at least 5% of their rated capacity, but many industry specialists tend to prefer 10%. Thus if you have an 800Ah battery bank then you should at least have 80Ah of charge available from your PV yield. Under-charging od solar battery banks is a common practice with badly designed systems and frequently leads to premature failure of a bank.
Q14: What is the most amount of batteries I can put in a string
A14: General opinion appears to be between 8-10 but it also depends upon whether your system is a 12, 24 or 48 volt system, of course.
Q15: How deeply should I cycle my Batteries
A16: This depends upon the type of battery and whether you are concerned with possible power-outs in your area. Battery banks store power, usually for use overnight and periods out of solar charging hours. Thus they are usually at their lowest just before the sun starts charging the battery bank. Most people in SA cycle down to a Depth of Discharge of between 25-50%. 40% is a good mean average. People with concern about power-outs often only cycle down to 20% - to ensure adequate reserve power at any time.
Q27: What is a ‘cycle’ when referring to battery performance capabilities
A27: A cycle of a battery usually refers to one discharge and recharge and, with a normal solar system, only occurs once a day. Thus most people equate cycles to days of use but this is not always the case if your battery is used to support power during the day because there is no Mains, then it is possible that there is more than a single cycle during a day.
Q28: What should the spacing distance be between batteries
A28: Spacing between batteries is more critical in hotter climates where ambient temperatures can require better bank cooling. Spacing is an important part of this, but keeping the battery room cool with vented bricks, or even fans to improve airflow and reduce ambient temperature by as much as possible. A distance of 6cm between batteries is minimal.
Q29: Are flooded batteries still the best solar option
A: Flooded batteries are still a good option, but not necessarily the best option any longer. The latest sealed Li-Ion batteries are proving a better option in hot climates and require no maintenance which is an important aspect in Africa in general.
Q30: What is the best way of monitoring battery banks
A: Monitoring a battery bank – your biggest investment component in any solar system., is essential in keeping your system operating optimally. To do this accurately, you should pass current through a ‘shunt’ and there are several good units available these days.
Q31: Does the solar regulator have the same quality charger as the Inverter
A31: That depends upon the quality of either. The latest chargers from the leading manufacturers are very sophisticated and accommodate all types of batteries.
Q32: If a regulator is charging the same time as the Inverter – is there a ‘conflict’
A32: If the two components emanate from the same manufacturer, then the chargers usually would be of the same technology and would work in unison. However, with different manufacturers, you might get one charger shutting down before the other – in which case it would be preferable if the Inverter switches off before the MPPT or solar regulator – saving Mains power and optimising yield.
Q33: What happens if I have a ‘Rogue’ battery in a string
A33: This is more of an occurrence in battery banks than one might suppose. A ‘Rogue’ and under-performing battery will pull down the other batteries in the same string, as well as affect the overall bank’s performance. Monitoring your battery bank performance is thus an essential factor in ensuring optimal solar system performance. String equaliser can be employed to ascertain early signs of failure in a string.
Q34: Are Battery equalisers really worth the cost
A34: Some say yes, and others say the jury is still out on this topic. They can definitely assist in highlighting rogue batteries in a string and purport to improve charge distribution and thus extending battery life.
Q35: Do I need to keep my battery room cooled with a vent or fan
A34: It is always preferable to have moving air – especially in hot climates with high ambient temperatures. Spacing between batteries is also an important factor (refer Q28) but if the batteries are in a fairly cool place, then it is not so critical. Check manufacturer’s specifications regarding heat and performance to see how critical heat might be in your performance.
Q36: Can my battery bank be left on a concrete floor
A36: No, that is another ‘No-No’ !! Batteries should be located on racks, or elevated off the floor, to allow good airflow around them. Some people use wooden slats – although this could become a fire hazard in some people’s opinion.
Q37: Do modern batteries still give off toxic fumes
A37: The modern sealed and Li-Ion batteries do not give of toxic fumes. This was more common on older flooded batteries where chemical action created such fumes when being charged. However, it is always better to locate your batteries away from living areas as a safety precaution.
Q38: A BMS, MPPT and Inverter-Charger all monitor the battery bank – which one is correct
A38: Good question! Although the accuracy of all 3 varies according to the quality of the monitoring method employed, a battery monitor employing a ‘shunt’ is always your best bet for accuracy.