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(how “big is this battery)

Everyone these days is considering the impact of Eskom’s ongoing problems, and how to provide for our own power using other options. We’re all considering batteries and Solar and Wind Power and generators to provide our own power for our homes and businesses. To achieve freedom from dependence on the centralised electricity supplier grid you’ll probably want to know a little about batteries.  But what do you need to know in order to move to a battery-powered home or business? This article will cover what I’ve learned in the last couple of months from the engineers and electricians at Revov, and how I got it all to make sense for myself.

Battery specification sheets are full of the measurements that electricians, installers and grade 8 science pupils use every day. If you’re not one of those three or don’t have one of them nearby to whisper answers in your ear as you read a spec sheet, I thought you might like a refresher. Here it is.

This article will cover measurements of how much a battery can store and how much it is able to deliver. Other articles in the Revov battery measurement series will cover the specifications governing the output from, and the refilling or charging of a battery. We’ll cover in another article advantages of Revov Lithium Iron batteries over generators and traditional lead-acid batteries and how to work out the amount of electricity your home or business needs to run. We’ll also do one on how to choose the right Revov battery to turn your places in a Revov powered home or business. 

But first, let’s talk water tanks, cause using them as an analogy is how my brain made sense of the science. You can skip over my analogies if you prefer and can jump around in this information to find exactly the measurement you may be looking for. There’s a paragraph or two about each unit of measurement and what it means.

Capacity

How big is a tank, how much can it hold? In the case of our tank capacity is the size of the space inside that we can fill with water. We’d measure by how many litres of water it could safely hold. Obviously, we get tanks of differing sizes some holding more than others. In the battery world batteries also come in different sizes – How much charge can a battery hold.

When talking water tanks capacity is about the amount of water it can hold, when talking batteries capacity is the amount of electrical energy or the charge it can hold. 

Battery capacity gives an idea of how much electricity can be stored inside the battery. Where tank capacity is measured in litres, battery capacity is measured in ampere/hours or Ah. While you are using a battery to run a bunch of electrical items like laptops and lights and TV’s it runs down until it can no longer run those items. The capacity of the battery is really how long it can steadily supply the necessary current. Battery spec sheets often have a section labelled Nominal or Nameplate capacity. In stationary storage systems for homes and businesses and factories, battery capacity is usually measured in Amp-hours (Ah). I’m sure we’ve all had experience with our cell phones that relates to the capacity of the batteries they use. Usually the bigger the capacity of the battery the longer they last and the greater the load (the number of working electrical items) they can support.

Usable Capacity (recommended maximum Depth of Discharge (DOD))

You have your water tank installed and it’s filled and you have a reason to use the water you’ve stored. There are questions to ask when you want to get the water back out.

For example – Can you get all the water out again? Let’s examine this below and see how it relates to what you can expect your battery can deliver.

Right, you’ve filled your water tank (fully charged your battery). Now that it’s filled how much water can you get out of it? If you look at the tank in the illustration above you will probably notice there are two taps on the tank. It’s obvious that the top tap will not allow you to drain as much water out of the tank as using the bottom tap would allow.  The top tap may only allow you to get ½ the water back out. That would mean you could not use ½ the water because the tap is too high to let it out. That would mean the top tap only allows you to drain the tank to a level of about 50% of the tank capacity. Perhaps the bottom tap would allow you to drain the tap to a depth of 98% of the tank capacity, allowing you to use much more of the water the tank can store.

So you could have two tanks of the same capacity(say both hold 5000 litres), but depending on the placement of their taps you would get differing amounts of water out of them. In our example of the two same size tanks each holding 5000 litres the one with the lower tap would allow us to use 4,900 litres while the one with the tap halfway up would only allow us to use 2,500 litres. They both have the same internal volume, the same storage capacity, but different usable capacity.

I think we would agree that in most cases we would prefer to be able to have the tap on the tank positioned to allow us to take the maximum amount of water out. 

Depth of Discharge (DOD)

Right, so you have fully charged your battery, now how much can you safely take back out of it without damaging the battery? Different batteries with the same nominal capacity have different USABLE capacities. Two batteries may have the same nominal capacity (like two water storage tanks having exactly the same internal volume) but differ in the amount of electricity you can get out of them, much like the two water tanks with different tap placements allow you to drain your tank to a different level. 

In the battery world, the proportion of the charge inside a battery that is actually safely usable is talked about as the battery’s recommended Maximum Depth of Discharge (DOD). Depth of Discharge is a measure of the % of the capacity of a battery that is safely usable without “damaging” the battery.  Every time you use the battery you will be discharging it and the percentage of its capacity that your use will be the depth to which you have discharge the battery while using it that day. 

To keep your battery in tip-top shape and keep it working for as long as possible you need to start recharging it when or before it reaches this recommended Maximum Depth of Discharge. If you use more than the recommended percentage then the battery will begin to decrease in the amount of charge(electricity) it can store i.e. its capacity will decrease.  Some batteries you have to constantly monitor and ensure that you never (or very seldom) use more than 40-50% of their charge. That means you can never really use half of the capacity you paid for.

The big advantage of Lithium batteries like Revov’s range of batteries over Lead-Acid batteries like car batteries is that they are much less sensitive to this and so deliver much more and their service life is not as easily cut short due to this kind of degradation. (We’ll examine the difference between Revov’s Lithium Iron Phosphate batteries vs Lead -Acid batteries in another article)

Voltage

Residential Water Pressure Explained

Voltage is like water pressure. The more you increase the pressure in your water tank the faster the water will flow out of any hose pipe you connect to it. The higher the pressure the faster the water will flow. 

Source: plumbing supply.com

In an electrical system, increasing the voltage will make more current flow. Battery spec sheets often talk about different Voltage categories. These are explained below.

Nominal Voltage: 

This is the normal/usual voltage at which a piece of equipment or an electrical system operates.

Nominal means ‘named’. So a nominal voltage is the ‘named’ voltage. For example, the nominal voltage of the Eskom supply to domestic plug points in South Africa is 230V (In the USA it is 110V in UK 230V) and they manage the actual voltage on the grid to keep it in a range within +/- 6% of that 230V.

With batteries it is the same thing. The nominal voltage is the voltage the battery usually puts out steadily till it is “flat” or “empty”.

Rated voltage:

Rated Voltage is the range of voltage that will not damage the equipment 

This is usually seen in the label on a device or appliance eg. Your cell phone charger or your kettle, or your Revov battery. The range will usually indicate the lowest and the highest voltage the equipment can operate at without being damaged.

Image result for filling water tank

Float Voltage:

Float voltage is the recommended voltage that is required to maintain a battery fully charged. The idea is to keep the battery fully charged when you are not using it. All batteries “self-discharge” to a certain extent. It’s like having a small hole in your water tank that is always letting a little water leak out. Maintaining charge level of a battery at 100% is also called Trickle Charging

REVOV Life Range

REVOV 2nd Life

REVOV 2nd Life

Conclusion

We’ve had a quick chat in this article about some practical impacts of the measurements of batteries. 1. Usable capacity depends on the amount of energy a battery is rated to store AND the depth of discharge it supports.