Many of todays portable electronic devices require large amounts of power. Cameras, personal music players etc, all suck the juice out of battries at an alarming rate. While much equipment is provided with their own proprietary battery systems which should look after the batteries to ensure the longest possible life, many others utilise off shelf non-rechargeable battery types. Repeatedly buying these can be expensive especially if you use the equiment a lot.

So it is often a good idea to use readily available rechargable types and recharge them yourself. But it is not quite as simple as that.

Choosing the right battery.

First you need to make sure that the chosen type is suitable. The most common type is Lithium Metal Hydrides, (LiMH) types, but for some applications they do not supply enough volts, and for those applications Lithium Ion types may be a better choice. But beware, Lithuim Ion batteries have a higher voltage (3.7v) than the ordinary lithium battery, (3v) and this could damage your equipment. So always check this.

There are 'regulated' Lithium Ion types available which (at the cost of some storage capacity) provide the same voltage as a standard Lithium. They are also much more expensive. But be careful here, many suppliers do not seem to understand the difference and will advertise 'ordinary' Li-Ion rechargeables as 3v. The only answer to this problem is to mail the manufacturers directly to obtain precise techinical data. (Which might be difficult to understand!)  If they cannot provide exact information, I would avoid using them unless your equipment manual states that it is safe to use Li-Ion rechargables.

The lower capactiy of the regulated Li-Ion types might at first seem a disadvantage, but some equiment shuts off at a comparatively high voltage. Before some batteries are fully discharged.

The regulated Li-Ion type will sit at a healthy 3v right up until the last minute and then pack-up rather quickly. You don't get a lot of warning, but you do get all the charge out, so it may well run for longer!

Caring for your rechargables.

If you have found a suiable battery, then you need to make sure thay last as long as possible. Rechargeable batteies can only be recharged a certain number of times, and improper cherging can reduce the batteries useful life drastically.

Why there is a maximum number of charges.

 When you charge a battery it deposits a layer of material on one electrode. This does not happen perfectly evenly across the electrode surface usually depositing a thicker layer where the positive and negative electrodes are slightly closer together. Of course this makes them even closer than before, with the net result that after a large number of charges whiskers of the deposited material can grow accross the gap, effectively shorting it out. (This can sometimes be temporarily fixed with a short series of high current bursts which can 'vaporise' these whiskers, but this will only get a few more charge cycles!) 

This deterimines the maximun lifetime of the battery, but you can easily reduce the batteries useful life by improper charging.

Overcharging reduces the useful life.

Rechargeable batteries will only reach the maximum useful life if they are not overcharged.

When a battery is charged it changes the chemisty of the chemical substance between the electrodes. It is this chemical which provides the matrial which is depsited on the electrode when charging. Call it "Chemical A".

What is left behind is a different chemical (Chemical "B") which reacts with the electrode material to produce current in a reverse reaction.  As the amount of chemical "A" is fixed, the battery is charged when all of it is converted to "B". Disharging converts the material on deposited on the electode back into "A" by a  reaction with "B" which produces current when the electrodes are connected externally.  .... More or less.

If you provide too much charge, then what is left between the electrodes get's involved in some other chemistry. It gets hot, because the enegy suppied is not being stored as new material deposited on the electrodes, and  the chemical "B" which is required to react with the electrode deposits iself begins to be converted to something else. (Usually generating hydrogen gas in the process.) This is not a reversible process. Some of the chemical is lost. Lose too much and there is not enough left for the reverse process which produces the current. (There is some other chemistry which goes on too but it's quite complicated enough thank you!) 

If your batteries come off the charger hot. You are killing them!

Of course it is not easy to tell when exactly all of the chemical "A" ia used up in the charging process, so it is easy to overcharge. The battery manufacturers compensate for this by providing an extra reserve which should last as many charge cycles as the battery would otherwise last. (This does not give the battery any more capacity, as that is determined by the toatal surface area of the electrodes, as the layer of the deopsited material can only be so thick!) But they can only go so far. Too much and the battery won't work properly.

Getting the most out of out batteries.

So to get the most out of your battery you need to charge it only when you need to so as to cut the number of times

You also need a charger which stops charging when the battery is full. That means a "smart" charger.

In fact a smart charger will always slightly overcharge the battery on purpose! This is not so as to make sure you are always buying new batteries, but because that's how it detects full charge. It's also why smart charges are fast chargers.

As the battery charges, the battery voltage gradually increases. This is higher than the batteries discharge voltage, as some extra voltage is needed to push the charge current through the battery.  As the end of charge approaches, the voltage needed gets higher and higher, until all (or at least most of) our chemical "A" is gone. Now for most types the voltage starts to drop as the battery begins to overcharge.

That is what a good smart charger is looking for, and it stops charging as soon as it sees that. Of course the higher the rate of  charge the higher the voltage required and so the easier it is to detect this voltage drop.  

Simple timed chargers are simly not accurate enough, and only work roperly with a battery of exactly the right capacity, only if it is completely discharged and, only if the charger current is exactly right and the time the charge current is applied is exactly right. Of course as a battery becomes older the capacity is reduced, and so even if the charger got it right the first time, it will overcharge the second time and that will reduce the battery life even more. This builds up in a self stoking cycle, until the battery dies of chronic overcharge syndrome.

Smart chargers avoid this problem completely, and will always charge the battery just right.    

 So choosing the right charger is important.

Lithium Ion types must use only chargers designed for them. These chargers will always charge the battery properly, as incorrectly charging these can be quite dangerous. Do not use chargers designed for other types!

Both NiMH and the older  Nickel Cadmiums (Still available from some supliers) can use the same charger if it is a smart charger.

However that is not all, especially for NiMH batteries. 

Most chargers for  these types charge two at a time. This is because to further increase the voltage  the batteries are stacked one on top of the other. This means the same current is passing through both. If they are not both discharged by exactly the same amount, charging could end early leaving one battery with less charge, which can be mistaken for early battery death, or worse will overcharge the other battery. This will  lead to genuine early death of the other battery. Not good!

To avoid this mark your batteries in pairs, and only ever use them in pairs. This ensures that both batteries get the same use, and require charging pretty well at the same time. (The batteries will not be identical in the first place, so it won't be perfect.)

Also, never mix batteries from different manufacturers and of different capacities in the same equipment. One thing which will damage the cells quicker than overcharging is reverse charging, over discharge will not do them any good either!

If your batteries are stacked end to end as is the usual arrangement, then batteries with a lower capacity will run out sooner. They may still provide current, but once the voltage drops below a certian point, a large part of the energy is not going to your device but is heating the battery. To hot it can melt the battery compartment, or even burst. Also, one the battery is totally flat, the other batteries are forcing current through it in the opposite to the normal charge direction. This will not only exhaust the battery chemistry damaging it, but can prodice quite large amounts of hydrogen gas. (That's one reason for the exploding cellphones we have all heard about. Usually a fake battery showing a higher capacity than it really has is invloved!)

So not only does this increase the batery life, but it avoids possible equipment (or personal) damage!

Also if you have the sort of charger which takes four or more batteries try avoid placing the batteries in the same bay every time. The two bays may be controlled separatley, and leave your batteries at different end points. So to avoid the same pair of batteries always being the pair which get a little too much or too little, which could acellerate aging. (This is usually quite easy to do, just remember to forget which bay you last put them in!)  

Another tip here is to occasioinally 'charge balance' your batteries. What that means is geting your batteries to have the same voltage after charging. This is because the small difference between your batteries becones exagerated by using them and charging them stacked. (This is one reason that chargers do not stack more than two at a time.) What is needed is to equalise the vlotage between the two individual batteries.

This is easily achived by connecting them in parallel to each other, (positive to positive, negative to negative) and leaving them like like that overnight. This should allow the sligthy higher charged battery to slowly top-up the lesser, a bit so preventing any differences from becoming large enough to cause a problem. 

Rechargable batteries properly looked after can last a very long time.

So with the right battery, the right charger, and a little care you can greatly extend the useful life of your rechargeable batteries. Even if a smart charger costs a little more, the saving in the extension of battery life can exceed the differnce in price, which could be as little as the cost of a replacement set of rechargeables.

If that is not enough to convince you, just think about how irritating it is when you rechargeable batteries no longer hold charge for more than a day or so, or only run for half as long (or less) than you expect?

Digital camera users especially take note. How many spere batteries are you carring about in your kit bag!