Batteries & Inverters

If you can accept some of their limitations, deep cycle lead acid batteries often are the best method of providing alternative power to your home for limited periods, particularly for low energy lighting. Their main benefits are:

  • Low maintenance cost and effort required if correctly selected, installed and operated
  • Silent or at least low noise operation
  • Relatively low capital cost

Their main limitations are:

  • They store a finite, relatively small amount of energy in relation to the unlimited amount of power most people are used to. Consequently they are best suited to devices which don’t consume large amounts of power.
  • Batteries have a finite life and need to be replaced periodically.  Depending on the quality of the battery, as well as the other circumstances, they can last anywhere between a few months constant daily use, to 3-15 yrs.  Buying the cheapest is definitely not necessarily a good idea in this case.  In addition, modern battery technology is developing rapidly.  You need to do your homework on this subject before placing any orders!

However the use of batteries in standby power systems is well established and the above limitations have been successfully overcome in many industrial applications such as uninterruptible power supply (UPS) systems designed for large computer centres. The technology is well established and can be used with confidence.

Lead Acid Batteries

Many people are familiar with the battery in their car. The vast majority of car batteries are lead acid batteries (basically they consist of plates of lead suspended in a solution of sulphuric acid). Lead acid batteries are currently the most cost effective form of battery for storing large amounts of energy.

Battery Capacity

A battery’s capacity is defined in “amp hours” (Ah). If for example its capacity is 100Ah, that means that it can supply 1 amp for 100 hours, or say 10amps for 10 hours before being completely discharged.

To convert the Ah capacity rating into an energy storage rating, multiply by the nominal voltage of the battery.  So a 100 Ah 12V battery has an energy storage rating of 100 x 12 = 1200 Wh = 1.2 kWh.

Depth of Discharge (DoD)

Lead acid batteries should never be completely discharged as they usually suffer damage when that happens, which leads to a short life. Most systems have an automatic low battery cut off to protect the battery to avoid this.

Normal car batteries are termed “high cycle” batteries because they are designed to provide a large current for a very short period of time to your starter motor. They are not designed to be regularly discharged by more than 25% of their capacity. This means that for a 60 amp hour (Ah) battery, you should not use more than 15amp hours at a time before recharging it. Car batteries are thus not suited to applications where one wants to extract as much of the stored energy as possible before re-charging.

“Deep cycle” lead acid batteries are designed to be repeatedly discharged to at least 50% of their capacity, which makes them suitable for home power use. A deep cycle 100Ah battery thus has a “design” capacity of at least 50Ah. Some deep cycle batteries can be repeatedly discharged to 80% of their capacity or more. You should check with your supplier regarding the different options available.

Battery Life

Most batteries are very sensitive to the way they are treated.  For optimum life they should not be discharged below their recommended level, they need to charged correctly as well as kept nice and cool.

It is very important that your system charges the batteries correctly and that the batteries are maintained within the recommended operating temperature range.

You should follow the battery suppliers specifications precisely in that regard and ensure that you use a charging system, and operating environment that suites your batteries.

Charging

Lead acid batteries need to be charged in specific ways in order to maximise their life and to minimise the release of gasses. Simplistically, they need a higher current to begin with which is then gradually reduced as the battery voltage increases with charge. Most systems have intelligent chargers which do this automatically.

This means that while a deeply discharged battery will quickly reach say 80% capacity, it can take very much longer for it to reach 100%.

You need to make sure that the charger included in your system is the correct intelligent charger that is suitable for your type of batteries. You also need to ensure that it is capable of charging at a high enough rate to ensure the battery is fully charged in time for its next potential use. Typically one would want the battery to charge fully within 12-18 hrs – in time for the next day/night cycle.

As an example, if one discharged a 100Ah deep cycle battery down to 50% capacity, 50 amp hours would need to be “replaced” during charging. Clearly a 2 amp charger will not be able to do that in 12 hrs (2 x 12 = 24 Ah which would be a bit more than half of what was required)!

Inverters

Batteries deliver electricity in what us known as “direct current” (DC). This means that the electrical current flows directly from the positive terminal of the battery to the negative terminal, in one direction only. However the electricity that comes from Eskom is what is known as “alternating current” (AC) which flows backwards and forwards constantly in a wave. This wave is known as a sine wave which has a specific shape and frequency (50 cycles a second).

The role of an inverter is to convert the direct current produced by the battery into alternating current required by all your house hold devices.

Less expensive inverters do not produce a perfect sine wave, but what’s called a “modified sine wave”.

Some devices, such as “neon tube” type fluorescent lighting and microwave ovens will not work when supplied with modified sine wave power. However many devices, such as the new low-energy fluorescent bulbs, most TV’s and computers, printers, laptops will work fine. If you are unsure whether you can use a modified sine wave inverter as a power source, you should check with the suppliers of your appliances. If in doubt or if you have devices that will not accept modified sine wave power, a pure sine wave inverter will be required.

Inverters also have a range of efficiencies, with poor designs being no more than 50-60% efficient, while good designs can reach 85-98% efficiency. Low efficiency means that a very large proportion of the battery’s energy is wasted by the inverter. It’s easy to tell if it is inefficient by simply feeling how warm it gets when it is operating. If it gets hot while supplying less than full power it means its turning most of the battery energy into heat, not electricity! Clearly, given the limited storage capacity of batteries, it’s no use using an inefficient inverter.

For home standby use one normally needs an inverter with a built-in battery charger.

Choosing the Right Size System

Most suppliers/installers should be able to advise you properly with regard to the most appropriate system. The basic approach is as follows:

Battery Selection

Be aware that even lead acid batteries come in many different forms.  Battery technology is constantly improving and you should investigate the various options very thoroughly, because batteries are likely to be the biggest on-going expense in this system.  It will pay to do your home work properly here!

To work out how many batteries you will need, you need to first estimate the total energy required.  A good way to do this is to use the new PowerProphet electricity consumption modelling tool.  You can use the tool to predict energy consumption and maximum demand over any period given a list of appliances as well as information about how those devices are normally used.  For example, if you want to provide standby power to your home for a period of two days; use PowerProphet to predict the energy consumption (kWh) over that period, as well as the maximum demand (kVA).

You can convert the energy consumption kWh value predicted by PowerProphet into Wh (Watt-hours) by multiplying by 1000.

Then you need to establish the energy storage capacity of a battery by multiplying the battery amp-hour rating by its nominal voltage (e.g. 12V).  That will give you the maximum energy storage capacity of the battery in Wh (Watt-hours).  Because its good practice to never drain a battery completely, you need to reduce that battery energy storage value according to the desired depth of discharge (DoD).  For example if you have decided on a maximum DoD value of 50%, then you should only use 50% of the batteries energy storage value in this calculation.

Lastly, divide the predicted energy consumption by the reduced battery energy storage value, to calculate the required number of batteries needed.

 So the calculation would look like this:

If your predicted energy consumption is say 10 kWh, and your inverter/system efficiency is 90%, then the energy required from the batteries is 10 / 0.9 = 11.111kWh = 11,111 Wh.

If you are using 100 Ah batteries then their equivalent energy storage rating is 100 x 12 = 1200 Wh.

If you decide the maximum depth of discharge should be 50%, then the available stored energy in each battery is then only 1200 x 0.5 = 600 Wh.

Thus the number of batteries needed will be the required energy divided by the available battery capacity i.e. 11,111 / 600 = 18.5

So in this case you would need at least 19 batteries.

If you are connecting your batteries in series to get higher voltages such as 24V (two 12V batteries in series), or 48V (four 12V batteries in series), or even 60V (five 12V batteries in series), then you would want to choose a total number of batteries that has the required multiples that can be connected in series (e.g. in the example above, 20 batteries would be a good choice because then you could have two, four or five batteries in series, whatever your system voltage).  Higher voltages are normally preferable because you can use thinner cables and thus run longer cables economically.

Please note that the real cost of batteries lies in the need to replace them periodically (every 3-15 years depending on the quality of the battery and the way they are treated).

Inverter Selection

Decide whether you need a pure sine wave inverter or whether a modified sine wave inverter will do.

In most cases a pure sine wave inverter is necessary.

Select an appropriate inverter that has a power rating at least 20% larger than the maximum demand (kVA) you have calculated or measured for your application, after taking the inverter’s efficiency into account. Bear in mind manufacturers may be slightly optimistic with their power ratings, so check with the supplier about what they would recommend if their rating is close to your target rating. Choose the highest efficiency inverter possible.

Also be aware that some appliances like fridges, air conditioners, electric motors etc. have very large instantaneous start-up currents.  If you are going to power these via an inverter you need to ensure that its design is sufficiently robust to withstand those very large short duration currents.  Please inquire from your supplier about this.  Many inverters cannot handle those sorts of loads.

Installation

The thing to remember about all alternative sources of electricity is that you need to provide the same levels of protection to your house wiring and appliances as you would for Eskom power. All alternative sources of power can be just as lethal or hazardous as Eskom power, simply because they are designed to replicate Eskom power. Please ensure you conform to all applicable regulations and authorities as well as ensure that competent people do any installation work and that they issue you with a certificate of compliance on completion.

6 Comments

  1. mycal hoffman October 21, 2014 Reply
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