I thought I would explain here the different types of battery that you can put into Cybot, along with the pros and cons of each type.  As you will see, things aren’t as simple as they first look.  Inside all batteries, a chemical reaction takes place to provide the required voltage.  Different types of battery have different chemical reactions, as you will see.

Alkaline Batteries

These are also sometimes called Primary Cells.  Inside are two electrodes.  One is made of Zinc, and the other is made of Manganese Oxide.  These make up the positive and negative terminals respectively.  The electrodes are surrounded by an alkaline electrolyte, hence their name.  Basically what happens inside is the electrolyte causes electrons to move from the zinc electrode to the manganese oxide electrode.  This transfer of electrons causes a current to flow.  The zinc and Manganese have a resistance, which cause a voltage to develop across them both due to the current passing between them.  This voltage is 1.5V.  Now, unfortunately this chemical reaction does not go on for ever.  Eventually, the electrolyte will be weakened and there will be no more voltage at the electrodes.  This is when we decide to throw the battery away.

So, how strong is the alkaline battery?  Well, we already know that a single cell can produce 1.5V across its terminals.  However, this is only the case when the batteries are straight out of their packet.  Very shortly afterwards, their voltage drops down to 1.2V, where it stays for the majority of it’s life.  Current is anther matter.  Typically, a single 1.5V cell can provide 2,400mA/h.  That is, it will provide 2.4 Amps continuously for 1 hour.  Wow, that sounds impressive, doesn’t it.  Well, everything isn’t as it seems.  If you were to connect something to the alkaline cell that produced a high burst of current, then you might as well throw your battery in the bin straight away!  You have probably noticed this if you have a digital camera, as each time you take a picture there is a high surge of current which really drains the battery.

Nickel Cadmium Battery 

These are more commonly known as Ni-Cads, from their chemical formula of NiCd.  In a similar way to the alkaline battery, two electrodes, one of nickel hydroxide and one of cadmium, are surrounded by a potassium hydroxide electrolyte.  Very much the same thing happens inside a NiCd cell – a current passes between the two electrodes and this produces a voltage across them.  The thing that makes these batteries different to the alkaline batteries is that they can be recharged.  This is achieved by passing a current through the electrodes and the electrolyte to reverse the chemical reaction, and hence make it new again.  There is one drawback from NiCd batteries, though, and that is the ‘memory effect’.  If you recharge the battery when it is not fully discharged, say when it is only 50% discharged, then next time you use it you will only be able to use it until it reaches this 50% discharge state.  So, to re-charge NiCd batteries, make sure hey are fully discharged first. 

So, compared to the alkaline batteries, how strong are these?  Well, one single cell produces 1.2V across its terminals.  No big deal, as we have already seen that the alkaline batteries settle down to this level pretty quickly.  Current is another kettle of fish.  They are rated at 800mA/h, which again means that they can supply 0.8A continuously for 1 hour.  This doesn’t seem to be impressive when compared to the alkaline batteries.  However, they have an ace up their sleeve.  They are able to provide a high burst of current, without too much effect on their overall status!!

NiMH Battery 

This is another type of rechargeable battery.  The electrodes this time are Nickel and Mercury Hydride.  These have an advantage over the NiCd batteries in as much that they do not have a memory effect, so you can re-charge them whenever you want without the need to fully discharge them.  There is one very minor disadvantage compared to NiCd batteries though, and that is if you put them on a shelf for safe keeping they will discharge on their own quicker than their NiCd counterparts.  Not a big deal I admit, but worth mentioning anyway.

How strong are these batteries, then?  Well, typically they are rated at 1300mA/h.  That is they can provide 1.3A continuously for 1 hour.  This seems to meet the alkaline and NiCd batteries half way.  However, like the NiCd batteries, they are able to supply a very large current if demand is there – much more than the NiCd batteries.

So, that was a lot of theory there, and hopefully it made you think more about choosing the right batteries for your application.  But we must not forget that we are on a Cybot page, and our main focus is using batteries on Cybot.  Real Robots do not recommend using rechargeable batteries.  Why?  Well, quite simply, if the motors were to jam for any reason (for example if Cybot got stuck in a corner) then the motors will try and consume more power than they were designed for.  Three things can happen if you use rechargeable batteries.  First, you could blow up the motor control board, as the components are rated for a lot less current.  Secondly, you could burn out the motors, as the wire used inside them are only rated at a particular current.  Three, both the motor control board AND the motors could blow.

Solution?  Check back here frequently, as I intend to give circuit design modification on Cybot.  One of the first things is a current limiting circuit that will allow you to use rechargeable batteries.