Daily Care of Home Power Systems.


This article is more for those who are thinking about going to an off-grid home power supply, so they can get some idea of what is required.

Those with  home grown power systems, will have learnt one way or the other, or given up after destroying the batteries, or installed a fully automated system.


The heart of the system.

Actually the system has two hearts - the inverter and the batteries - and they both have to work all the time.


Inverter.

It is too big a topic to go into all the inverters that are available - you need to investigate it well as to what type is best for you. The more "bullet proof" the better - that is it should turn off if overloaded or the battery is under voltage.

But two comments from my experience.

If you can at all afford it, buy bigger than you think you need. The two main reasons for this are - if the inverter rarely reaches its capacity, it should be less stressed and therefore should last longer, and in Summer, they can run hot. Again, less stress should equal less heat.

The type - such as "OutBack" - that can convert to be the main charger when power is available, simplifies the running of things. It also makes it easier if you want to run the generator (to save the batteries) while you use grunty things like vacuum cleaners or washing machines. The inverter switches the power straight through, and if excess is available, it will charge the batteries at the same time. When the generator is stopped, the inverter senses the drop in voltage and switches back to the batteries in an instant.


Batteries.

This is the expensive part, and needs to be looked after.

There are many different types of deep cycle batteries (and they must be deep cycle, not car batteries), so I can only talk about principles. You need to find - use the Internet to search on the battery code - the specifications for your particular battery type. You need to know what voltage you can charge it to. For example, a lead/acid 12 volt bank can be charged  at 13.5 volts, and should not go over 13.8 volts. The gell cells that I have are to be charged at 14 volts, and are not to exceed 14.2 volts.

Physically, the batteries need to be up off the floor (you don't want the bottom getting colder than the top) and mounted on wood or other insulating material. If it is a wet cell, the electrolyte needs to topped up with good quality soft drinking water. If in doubt about the water, use bottled,distilled or demineralized water.

The most important thing about batteries, from all I have read, is the smaller the daily drop in voltage, the longer the battery will last. Hence my comments about being able to run a generator for higher wattage short run items.


Two meters.

There are two meters that are required - to know the state of the system.

First the ampmeter - this measures the current in amps. For my system, I have a 100 amp meter with the zero in the middle (measures in both directions). Putting it in one of the leads to the inverter, it measures the current being drawn from the batteries, and when the inverter is charging, measure what current is being sent back to the batteries. The maximum voltage is set in the inverter via a control panel.

If you have a separate inverter and charger, then you will need an ampmeter on both. The ampmeter on the inverter will tell you when you have too many electrical items switched on (the current draw is too high), and what your residual power draw is (TV, computers, and so on, not switched off when not being used.) Also, when installing new equipment, you can note the amps, turn the new equipment on, and see what the increase in amps is to know what power it uses.

The ampmeter on the charger will tell you when the charger can be switched off. If the charge to the batteries drops to - say - 10 amps, then it is inefficient to continue to run the generator. The charger needs to be the type that you can set the voltage, or bought to be the correct voltage for the batteries. Batteries can be charged at a higher rate when the voltage is low, but then the rate should drop off as the voltage comes up. A set maximum voltage will do this automatically, and allows the batteries to absorb the current at their own rate.

The second meter is the volt meter. With batteries, 0.2 of a volt can be significant, so you need a meter that you can read down to that level. A cheap digital multi-meter will do, if you don't mind turning it off and on. A large (easy to read) analogue is ideal - as it is always on. It should be connected across the positive and negative of the battery bank supplying the inverter.


Bank voltage.

The voltage of the bank, tells the percentage charge of the bank.

There are published figures for a battery that has been at rest for 12 hours, but it is very rare for that condition to exist in a home power system - especially if you have an electric fridge as we do.

[ 12 volt lead/acid:  100% = 12.65 volts, 75% = 12.45 volts, 50 % = 12.24 volts, 25 % = 12.06 volts]

However, there is an easy way to know the state of the batteries as they are working. It may not result in an accurate percentage, but it will give you the feedback you need.

Simply start the generator and look at the amps and voltage of the charger.

If the voltage is low compared to the set voltage (mine is 28 volts with gell cells), then the batteries are down on charge - and you should keep charging. The amps will be the maximum of the charger.

If the voltage is the set voltage - then look at the amps going in. If the batteries are pulling - say - 50+ amps at the set voltage, then they are reasonable, but need to be topped up - keep charging but note the time, and come back and check how things are going from time to time. If the amps going in are below - say - 20, then the batteries are close to being fully charged. You can leave the generator going and give the batteries what they call a "float" charge, but you also need to weigh up the cost of fuel to do so.

If you have a small battery charger, or are using a small petrol engine with a DC generator - say putting out 20 amps - then things are a bit different. You then need to monitor the voltage as the current will only drop slightly as the voltage goes up. Keep charging until the voltage reaches the recommended charging voltage. If you don't quite get to the charging voltage then, the batteries won't be fully charged, but charged enough to get you through the night. With luck, the solar panels will do the topping up during the next day. 

I advise doing this voltage test every day you don't have a "burn off" situation (excess power from the wind generator being dumped into a resistor, or the solar panels being shut down) at around 3:00pm, and run the generator till you have the input down to 15 - 10 amps. Then check the batteries first thing in the morning before you start using anything (yeah, I go out with a torch). If my 24 volt bank is 24.4 volts or better, then I know we are in good shape. Remember, we have used a lot of electricity during the night, so you can't expect a full charge of 25+ volts. If your bank is consistently below its nominal voltage (12, 24, or 48) first thing in the morning with no large power usage at the time (fridge not running), then your battery capacity is too small, or your usage too high. Sure, the system will run, but you will shorten the life of the batteries.

Let me expand on "no large power usage". The voltage of a fully charged battery will be about 12.65 volts for 12 volts or 25.3 volts for 24 volts battery banks - but only after they have "rested" for 12 hours. Also remember, temperature does have an effect, so the actual voltage can vary with the seasons. However, it is impracticable to shut down the system for 12 hours just to find out if you are OK. So - you have to "adjust" - in your mind - the present voltage, by what is going on. A couple of examples. If nothing is charging the batteries, and the inverter is drawing 20 amps, and the voltage is showing 25 volts, then if you shut the system down for 12 hours, the voltage in the batteries would rise . In other words, the voltmeter is showing lower than actual. The reverse is true if the batteries are being charged but not used - the voltmeter is showing higher than actual. Over time, by checking the battery bank voltage every morning, you come to "sense" the correct voltage the night before, based on the maths of what is going out verses what is going in. In my case, 26 volts with 10 amps charging is OK, or 25 volts with the fridge and computer running, is also OK. Both situations will give over 24.4 volts next morning.

I do this charging/testing around 3:00pm and I am always finished by 6:00pm. I find this is least upsetting to any neighbours within earshot. You will have to work out your own best time. The other reason why  3:00pm suits me, is that the solar cells are starting to finish their input for the day, so it is a good time to get ready for the high usage time coming up at night.


Summary.

Each day you need to check the voltage of the batteries while being charged. Sometimes the solar panels or the wind generator does this for you without you starting the generator. This is where experience comes in. If my bank of batteries is showing 26+ volts with just the solar panel, then I know they will show up as charged if I was to start the generator, so I don't bother.

The less voltage drop in the batteries, the longer they will last. Nickel/iron batteries are an exception to this, but have other maintenance needs.

At least once a year, you need to do an audit of the system and find out where the power is being used. You can buy usage meters the go between the equipment and the power point. Laptops use less than desktops, LED less than CF, digital TV less than analogue, and so on. Also, a couple of times a year, you need to check the voltage of each individual battery under load. Just as a chain is only as good as its weakest link, so too a battery bank is only as good as its weakest battery.

My suggestion is that over time you need to convert all the lights that are on for more than just a few minutes to LED. At the moment I am trialling using two 120 volt LED's (cheap from the USA) in parallel, but there are cheaper 240 volt AC LED's becoming available. We are also using 12 volt DC LED's  over the computers


Contact.

You can contact me via email, if you have any questions or suggestions.

rso-dot-54321@gmail-dot-com


Bob Orchard Jan 2013