Rechargeable Batteries #1: NiCd vs NiMH

We all like the idea of recharging and reusing batteries.

The first batteries available in domestic sizes were Nickel Cadmium batteries, or NiCads as they became known. These were followed more recently by Nickel Metal Hydride batteries, or NiMH as they became known.

Each of these, at least in principle, can be used to replace the normal alkaline Eveready batteries in normal usage.

How well do they work? What are their pros and cons, and how do they compare with each other?

The first and obvious advantage of the rechargeables is that they can be recharged. They are also much lighter than the alkaline batteries.

On the negative side, the rechargeables do not possess the same energy density as the alkaline batteries. In other words, they go flatter quicker. The NiMH was worse than the NiCAd, as it would self-discharge; in other words, it’d go flat even if it wasn’t being used.

Some of you may have experienced this with mobile phone batteries. I once had an LG mobile phone with a NiMH battery that wouldn’t hold a charge for 12 hours, even if the phone was turned off.

The NiMH had a significant advantage over the NiCADs however – no memory effect. The NiCADs did have a memory effect, which meant that if the battery was not fully discharged before being recharged, it would lose capacity. So if, for example, the battery was only discharged by 10% before being recharged, this would become its new capacity – so when it was only 10% discharged it would be flat.

The advantage that the NiCADS have over the NiMH, however, is that they are capable of delivering higher currents, sometimes required for devices such as battery operated model cars.

Another significant advantage of the rechargeables is  their flat discharge curve. That is, if you were for example using a 1.5V battery, you want it to deliver 1.5V right up until the point where it goes flat, and both the NiMH and NiCADs were good in this regard.

Alkaline batteries, on the other hand, do not have flat curves, and the voltage will gradually drop with time.

The consequence of this is that if for example you were using rechargeables in a torch, the beam would be nice and bright, and then suddenly die when the batteries went flat. With alkaline batteries in the torch, however, the beam would gradually dim with time, and become dimmer and dimmer until it just wouldn’t work at all.

The next level of sophistication was the Li-ion battery, which became the battery of choice for laptops. Stay tuned.

Batteries and Jumper Leads

What’s the difference between these jumper leads and these ones?

Does it matter? Aren’t they all the same? And when you are jumping your car, why does the engine have to be running on the other car?

The answer to all of these lies in the voltage drop across the wires.  Voltage drop in wires is not something that we normally have to deal with, as copper is highly conducting and therefore by definition experiences little voltage drop.

In the case of jumper leads however the factor simply is the very massive current that is flowing.  With several hundred amps flowing the tiny resistance of the copper becomes significant and it can result in a significant voltage drop from one and of the cable to the other.  The other thing that determines the magnitude of the voltage drop is how thick the cables are and how long they are – the thicker the cables the lower the voltage drop and the longer the cables the greater the voltage drop.

In the example above the 750 amp cables would be noticeably thicker than the 100 amp cables.  And this is why the tables never seem long enough – the longer they are the greater the voltage drop.

So if they are connected to your car with your battery in good condition at say 13.4 volts, the voltage may have dropped down to say 12.2 V at the other end, which may not be  enough to start the other car.  But if you start your own car, the voltage from your car is now what the alternator is supplying, typically over 14 V.  This results in a healthy 13 V at the other end and increases the chance of a successful start.

Of course one way of knowing that your cables are too thin is if they melt, as they can get very hot if the cables are too thin.

One way around the problem of cables being too short is to make your own.  Go to a welding supply shop and buy some thick gauge welding cable, typically 10 mm in diameter, then get someone to braise on to the end some heavy duty welding clamps.

My jumper leads made this way are 5 m long, and I don’t have to start my engine when jump starting another car, as the heavy gauge cables mean there is negligible voltage drop from one end to the other, and as the other car starts it is able to draw power through the cables directly from my battery.  If you’ve ever had your car started by the RAC you will notice that they use the same type of cables.

How Car Batteries Work #2

Car batteries pack a wallop. They are capable of producing an extraordinary amount of power – about 3 times as much as is required to run a clothes dryer – in a couple of seconds when you turn the ignition key.

Several hundred amps blast through the thick cables coming off your battery into your starter motor, which engages a ring gear that turns your motor and starts it.

Now let’s look at some of the numbers on the battery. There are essentially two that are important – its capacity and starting power.

Capacity it is a measure of how much charge the battery can produce before it is completely flat, and is measured in Amp Hours (Ah).  A large battery for example may have a capacity of 60 Ah.  This means plant it could theoretically deliver 60 amps for one hour,  or one amp for 60 hours, or 2 amps for 30 hours and so on

But this is only a guide, and as you could never get anywhere near the theoretical amount. The reason for this is fact the voltage doesn’t have too drop much below 12 bolts and its essentially useless –an 11.5 V battery will not start your car.

The other number of interests is the Cold Cranking Amps. The CCA is a measure of how many amps the battery can deliver the moment you turn the key.  For small cars this is normally 300 or 400 and is part for larger diesel motors it can be as high as 700 or 800 Amps.



How Do Car Batteries Work #1?

The battery in your car is very old technology – dating back to 1830. The fact that they are still used in your car is a testament to what a good design they are.

They fulfill all the requirements of a rechargeable battery – they have a high energy density, they are cheap, and they may be cycled many times without loss of performance.

As with any rechargeable battery, however, there a few things we must consider and a few questions we must ask.

1.  Do they have a memory effect?

The answer is no.  This is a problem with nickel cadmium batteries, which I will discuss later, but not with car batteries.

2.  Should they be fully cycled on a regular basis?

This is connected with the first question and the answer again is no.  In fact, unlike most rechargeable batteries, you will prolong their life by keeping them in their fully charged state.  If a car battery is kept in its discharged state for very long, it will die.  This is important if, for example, you have a car that you aren’t using for a while.  If the battery goes flat, and stays in that state for very long, you will will find that you cannot recharge it.

3.  Can a battery be overcharged?

The answer is yes.  This won’t happen in your car unless your regulator is malfunctioning, but it may happen if you are using a car charger.  If you have a smart charger, it will detect when the battery is charged and turn itself off

But if you have an older style constant current charger, be careful.  If it’s left on too long, then once be battery is fully charged, the potential will ramp up high enough to begin electrolising the acid, producing hydrogen and oxygen gas.  This will kill your battery very quickly, as this process generates a lot of heat, and will result in the plates in the battery warping and short-circuiting.

So if your car isn’t being used regularly, invest in a smart charger, or perhaps a trickle charger, which you mount to the car, and then simply plug in to your 240 V outlet.

More on car batteries tomorrow.

How Do Batteries Work?

There are many different types of batteries around us.

From the lead acid batteries in our car, to be batteries in our torches, to the tiny batteries in our watches, we are daily surrounded by these remarkable energy storage devices.

And that is what they are – devices that contain energy that can be extracted, and in some cases put back.

The lead acid batteries in our car are one of the oldest batteries in existence, and the design is almost 200 years old. The only technology on cars that is older is the glass.

Any battery contains two electrodes – the anode and the cathode. Different reactions occur at each electrode, and the difference in energy of these two processes can be used to do things, like making a torch shine or a car start.

Imagine we had two water tanks, that were connected by a hose, with a tap between them. Suppose that one tank had more water in it than the other. If we opened the tap, the water woule flow from the full tank to the empty tank. This water flowing is now a source of energy that we could use – for example a little paddle wheel that would turn to make electricity (this is how hydroelectric power works).

In this example, when the water levels were the same, the paddle wheel would stop turning, and no more energy may now be extracted from the system.

This is exactly how batteries work – electrons flow from the anode to the cathode – when the energy of the two systems is the same, the electrons stop and the battery is “flat.”

There are many different designs, to serve many different purposes. Tomorrow we’ll start looking at them, starting with the battery in your car.