Now you know that it’s not a good idea to leave a battery discharged, you need to know how to re-charge it safely, and without causing any damage. All batteries work with Direct Current – i.e. it is always of the same polarity. Alternating Current, such as you get from the mains cannot be used – unless you rectify it first. You can charge one battery from another, but there needs to be a difference in voltage in order to push current into discharged battery. This is what happens when you “Jump Start” a car. But without some additional source of power all you will (eventually) end up with is two partially charged batteries – more detail in a later post.
There are some small “boost” charge units on sale, mainly through newspaper adverts, which claim to start any car just by plugging into the cigarette lighter socket. However, on most modern vehicles this socket is only energised when the ignition is on, meaning you can’t get a charge into the main battery before trying to start the engine. Furthermore, the electrical load imposed by the multiplicity of computers and electronics will almost certainly mean the fuse protecting the socket will blow before any useful charge is transferred. I’m sorry, but I regard these things as little more than a gimmick – turning an engine over requires a lot of power, and the only way to do this is via a direct connection to the vehicle battery. Portable “Jump Start” units containing a sealed lead-acid battery with short leads and crocodile clips are the ones to go for – the better quality ones are used daily by breakdown recovery mechanics, and even the cheapest should be capable of getting you under way, but DO remember to keep it fully charged!
So, the two normal ways of charging a battery are 1) from the mains via a suitable charger, and 2) using an engine driven generator. As it happens, the mains is almost always derived from an engine driven generator – traditionally in a coal, gas or nuclear power station, where heat converts water into high pressure steam, this in turn drives large turbines connected to generators. Now of course, it could (if there’s actually any wind blowing) come from the army of bird mincers blotting the countryside, and, if the sun is shining, from fields covered with solar panels. In the latter case there are no moving parts, and the DC output from the panels is converted to AC with electronic inverters. The end result is the same – a “Sine Wave”, where the polarity and voltage are constantly changing:
This waveform is related to a basic principle of physics: moving a magnet past a wire will induce a voltage in the wire. Practical generators use rotating magnets – which have North & South poles, and as each passes the wire in turn, the polarity reverses. The voltage varies as the pole faces move towards and then away from the wire, and so the sine wave pattern results. A complete rotation results in one “Cycle” – the frequency is determined by the speed. Depending on the exact design this is usually 1500 rpm or 3000 rpm for UK “50 cycle” supplies. In the US and other countries which have 60 cycle mains it becomes 1800 rpm or 3600 rpm. Now you know why the ubiquitous Briggs & Stratton engines, such as the one I used for my charging set, are rated at 3600 rpm… If you see the term “Hz” or “Hertz” it is a way of describing cycles per second.
Mains alternating current needs to be changed before it can be used for battery charging. This involves firstly reducing the voltage. Most DIY type chargers employ a transformer, which is the heavy part. All transformers use the above principle of physics in reverse: Varying the current passing through a conductor creates a varying magnetic field around it. Now, if you place another conductor in close proximity, the magnetic field will induce a voltage in it. Most power transformers consist of a laminated metal core with two separate windings of copper wire wound round it. The first (the “primary” winding) consists of hundreds of turns of fairly thin, insulated wire, the second (guess what – the “secondary”) has fewer, thicker turns, since more current will flow on this side. The core acts to concentrate the magnetic field, and makes the whole process more efficient. This diagram shows the windings on separate sides of a metal “ring” for simplicity.
Some transformers do work this way, but the majority are formed from “E” sections interleaved, with the windings mounted together in the centre – this makes for a more compact layout:
The ratio of turns between primary and secondary determines the voltage at the output. 240:12 is a ratio of 20 to 1, although in practice a higher output is needed to push a charge into the battery.
O.K. so, we’ve got roughly the right voltage, but it’s still AC, and we need to convert the positive and negative cycles into all positive. This is done with a rectifier, which usually consists of four individual “Diodes”, although they can also be combined into a single package.
Diodes are effectively electronic one-way valves, and are arranged in a “Bridge” configuration, such that each half cycle is presented to the output in the same polarity. The next two diagrams show this: each diode is represented by a triangle facing a line – current will only flow towards the line.
Now you have both the positive and negative the half cycles working in the same direction:
In a “cheap and cheerful” charger these two components are probably all you will find inside – except perhaps for an ammeter and fuse. Some include a 6/12 volt selector switch, which changes the windings on the secondary side.
For use on an older style battery, with easily accessible caps for topping up, one of these is fine. Most of them only produce 4-8 amps output, and that is normally the maximum when first connected. As the battery reaches full charge the terminal voltage will rise, and the charging current then drops. However these “unregulated” chargers produce an output voltage proportional to the input mains voltage, which can vary from 210 to 245 volts (or worse, if you’re really unlucky). And in order to push that charge in the first place, the open circuit voltage at the output will typically be 15 volts or more, and means that the battery will start to “gas” when fully charged – the electrolyte will be converted into hydrogen and oxygen gas by electrolysis. This means 2 things:
1) There is a considerable risk of explosion if a spark occurs when you switch off or remove the crocodile clips, and:
2) The electrolyte level will begin to drop. Hence the need to have access to top up with water.
The water needs to be free of impurities, and “Distilled” is normally used, although these days it can be produced by another process, so the term “Deionized ” will often be seen on the container. Most batteries have a level marker inside each cell to give you a reference point.
The “Boost” or Engine Starting” chargers found in most garages require caution. They invariably give high output voltages, and if left unattended will destroy a battery – possibly violently! Fast charging generates heat, which can lead to the plates buckling, and rapid reduction of electrolyte level. They should have a temperature probe or sensor which needs to be used as directed. If the engine can be started, and the on-board generator is working properly, it is better to let that do the recharging, or use an overnight slow charge. With the latest generation of cars it’s questionable whether to use one at all – the risk of damaging very expensive computers and sensors is not worth it. Let a main dealer sort it out – they should know the correct procedure!
These basic unregulated chargers are also not suitable for any type of sealed battery – because of the lack of topping up, the need to avoid gassing, and also the high ripple currents involved. For these batteries a different approach is called for – regulated and smoothed outputs. Some DIY chargers of this type still use a transformer and rectifier, but they have extra circuitry to control the output voltage, and level out the abrupt peaks that a simple rectifier produces. However most newer types employ “Switch Mode” technology, which allows much smaller components, lighter weight and compact design, and for the “Eco” mob, improved electrical efficiency… They all supply smoothed, regulated outputs, and the better ones have multi stage control of the charging process. This is particularly important when dealing with large batteries which are regularly discharged. There is nothing to stop you using a fixed voltage for this, but it will always be a compromise. Too low and it will take a long time, too high and the battery will be damaged if left connected for long periods.
A 3 stage process is typically used. The more sophisticated units will first run diagnostic checks to confirm correct connection and ensure the battery is OK, then provide full output current. During this 1st “Bulk” phase the battery voltage will be monitored, and when it rises to a pre-set level the charger will switch to a “constant voltage” mode. It will typically be 14.5 volts for 12 volt circuits, and during this 2nd “Absorption” phase the current flowing is monitored. When it has dropped to a further pre-set value the battery is considered to be fully charged. At this point the 3rd stage kicks in, and the voltage drops to a long term “Float” value (usually 13.8 volts) which will maintain the battery without damage. Multi stage processes allows very high charging currents to be used without risk of damage, although larger chargers will come with a temperature sensor which needs to be attached to the battery case (or one of a multiple bank). These units are commonplace on boats, particularly sailing yachts if limited mooring facilities are available. The less time needed to re-charge batteries, the better! It is also normal for larger chargers to have a pair of “sense” terminals. These should be linked directly to the battery positive & negative terminals (via a small fuse) and allow the charger to compensate for any voltage drop in the main cable runs, which can often be significant. The same principle can also be applied to engine driven alternators, and involves bypassing or replacing the built in regulator, which I will discuss in a further post.
Hopefully you will now have a better understanding of what is required to charge batteries from a mains supply, and won’t end up destroying an otherwise perfectly good example.