|
From time to time I field enquiries from people who believe they have problems with their solar and electrical systems. Surprisingly often, the system is working well. The ‘problem’ is a simple misconception about how it is supposed to behave. It seems timely, therefore, to summarise a few of the more common concerns – and hopefully enable a few more people to sleep better at night.
Q. My original deep cycle batteries only lasted 18 months and I’m now worried about how long their six month old replacements will last. The solar brings them up to well over 14 volts during the day but this drops off to only 12.8 volts within less than an hour when night falls, and as low as 12 volts under load for a time.
A. Instant voltage checks of deep cycle batteries mean little. The charge is best seen as being held in the electrolyte (the water/acid mix) and there is a long time lag between charging the battery, or applying a load before the charge is evenly distributed. That’s why high quality chargers have a boost cycle, an absorption cycle, and finally a float cycle.
When you measure the voltage whilst the battery is charging or supplying a load, all you are checking is what’s happening on and very close to the surface of its plates at that moment. It tells you next to nothing about the general state of the electrolyte. A close-to fully charged battery may thus present as close-to flat for some time after applying a load like a microwave oven. Likewise, a close to flat battery may (for a short time) present as well-charged, after it has been on a boost charge for some minutes.
A ‘rapid drop from over 14 volts to 12.8 volts’ is absolutely normal. What you are measuring there is mostly the charging voltage. The battery’s voltage will drop way below 14 volts when charging ceases, and will fall progressively to under 13 volts within a few minutes.
The 12.8 volts you are seeing shows that the battery is 100% charged. Switching on even a light load is then likely to cause the voltage to drop to 12.5 – 12.6 volts. Yet that is typically the voltage when a well-rested battery is 75-80% charged. It is quite common for a battery to register 12.0-12.2 volts whilst on load, and yet still be at least 50% charged. (Voltage readings across non-rested AGM batteries are closer to reality, but there is still some time lag). I suspect your original batteries were just fine, as seem to be your present ones. The only way to be 100% sure is to add an energy meter to your system. This notes how much comes in, how much goes out, and shows what’s left allowing for internal losses. These meters may be bought as stand-alone items. The facilities are also included in all up-market solar regulators and it is often worth upgrading an existing regulator as the cost is much the same.
Q. My solar set up was designed following the advice in one of your books. It works very well most of the time but when there’s a lot more than average sunlight the daily current input does not increase accordingly. What’s wrong?
A. Probably nothing! As with any well designed system there is some excess solar capacity to cater for periods with less sun. On days when there is a fair bit more than average the batteries reach full charge earlier in the day and the solar regulator cuts back the charge accordingly. It’s a sign that the system is working well!
With domestic solar systems this otherwise unused energy availability may be diverted to pump water to a high-located tank or, as in our case, to increase irrigation to our mango trees (we have ample bore and stored rainwater supplies). Most of the more up-market solar regulators have an auxiliary terminal that can be programmed for all sorts of uses – including that one.
Q. I check my solar system’s input and load readings every day. There’s always 10% more coming in than going out. I put in new batteries a few months ago and that helped a bit, but there’s still some sort of leak. My local auto sparky knows a bit about solar and reckons there’s nothing much wrong. Can I do anything to fix this loss?
A. Someone once said that a good interpretation of the Second Law of Thermodynamics was Malcolm Fraser’s famous (infamous?) statement: ‘There’s no such thing as a free lunch’.
Your system is working fine! What’s happening is that the process of storing and recovering energy from a battery is not 100% efficient. Depending on battery type, and how heavily you charge and discharge it, this inefficiency causes a loss of 10-15% and, as with people, batteries get progressively worse as they age. It is generally considered that a battery should be replaced when this loss exceeds 20%. In this regard, gel cell and AGM batteries have the edge. I fear you may have tossed out some perfectly good batteries.
Q. I’m getting a bit too old to safely climb onto the roof of my coach to clean the solar modules. Is there any way to clean them safely without climbing up there? I’m really concerned about the modules getting too dirty to deliver power.
A. A lot depends on whether or not the modules are subject to industrial grime and/or bird droppings. If they are not, there’s probably no need for you to clean them at all. Occasional rain does the job well enough. Our now eight year old array up in the Kimberley was initially washed two or three times a year, but we then left it for a year to measure the loss. It was less than 1.5% and we’ve never cleaned it since. It does of course get a good natural cleaning during our short wet season but the rest of each year is normally all but dry. As far as I’m aware none of Telstra’s big outback solar arrays are regularly cleaned.
Q. I’m worried about my batteries overcharging. The solar runs through a really good solar regulator but the batteries begin to boil before the regulator cuts back the charge.
A. Good. They only seem as though they are boiling, however, what’s happening is that once a battery’s charge exceeds about 70% the electrolyte begins to give off oxygen and hydrogen gas. That’s the ‘boiling’ you hear. It’s meant to happen and does nothing but good as long as you top up with water (never with acid) every second month or so.
When our previous 24 by two-volt 630 Ah batteries went over about 90% charge you could hear them bubbling from 10 metres away. They gassed away happily like that for close to 10 years before needing replacement. It’s when a battery does not gas that one needs to worry. It is possible to wreck a battery through overcharging, but far more are damaged by undercharging and over discharging.
Q. My solar regulator is allowing my Trojan deep cycle batteries to go well over 14.7 volts before dropping back to 14.2 volts. My mates all say 14.7 volts is way too high. Are they right?
A. Only if you live somewhere like Darwin. Charging is temperature-related but 14.7 volts is just fine in most places for conventional batteries like yours – as long as they don’t stay at that voltage for more than an hour or so. Even 15.0 volts is OK in temperate climates. PL series regulators typically run batteries up 14.9-15.0 volts before dropping back. If I were in Darwin I’d cut the boost back to 14.7-14.8 volts, but I ran my earlier batteries up to 14.9 volts - in Brome.
|
