Alternator Controllers

Also known as Alternator Regulators, they control the way the alternator charges the batteries.

Some specialist alternators do not contain any form of regulation. They simply will not work without an external regulator. Obviously if your alternator is of this type then you have no choice, you need an external regulator.

Most alternators, however, already contain their own, in built, regulator. Yet even with this type, for historical reasons, a large proportion of people are under the impression that an external alternator controller will vastly improve their charge performance.

This, quite simply, is an outdated fallacy. It was true perhaps 15 years ago. It is no longer always the case. Times have changed, alternators have changed, batteries have changed.

When external alternator controllers first started to appear on the market the public took a huge amount of convincing of their worth. The "oil lamp brigade" (an entirely affectionate trade term) simply refused to accept that there was anything wrong with their existing alternators or the way they charged their batteries. They believed alternator controllers were not required and were being sold by unscrupulous dealers who simply wanted something to sell. The manufacturers and suppliers of such equipment faced an enormous up-hill battle trying to convince people that their external controllers made such a huge difference.

Those people who took the suppliers at their word, and fitted such a device, took no more convincing. The end results of fitting an external alternator controller were nothing short of astonishing, reducing charge times to a third of the previously required time and increasing the resultant available battery power by up to four times from the same battery bank.

It is interesting to note that exactly the same thing happened when 3 stage battery chargers first appeared, and inverters, and battery monitors, and battery conditioners. The public takes a lot of convincing about anything new - now just about everyone has them because finally, they learned that they were actually very good ideas.

A typical alternator, from 10 years ago charged at 13.6 to 13.8 volts. They were built to recharge engine start batteries. They did this adequately without using too much water in the batteries. They got the battery to a sufficient state of charge to ensure engine starting the next time round and supplied power to other loads and equipment that was being used whilst the engine was running without using any of the precious battery power for such loads.

Remember that this is what these alternators were designed for. And therefore that is what they were good at. Alternators were designed and manufactured to supply the vehicle engine start battery market. There simply wasn't any other market big enough to specifically design a different alternator for. This is pure economics.

Now an alternator charging at 13.8 volts will do this. It will also charge a deep cycle battery that is used to run

lighting, TVs, Hi Fi etc. But it will do it awfully slowly. To reach full charge state at 13.8 volts is possible, but it takes a ridiculously long time. The manufacturers of the external alternator controllers often state that a standard alternator will not get the batteries much higher than about 65% charge state. This isn't entirely true. 13.8 volts will reach 100% charge state. But it will take about 72 hours.

What the manufacturers mean is that, under normal usage (say 4 or 5 hours cruising on a boat) the alternator will not get the batteries much higher than about 65% charge state. This is a perfectly fair comment, and entirely true.

If you now consider that the batteries should really only be run down to 50% charge state (see The 50% Rule) you will realise that if you only charge to 65% and run down to 50% you are actually only using 15% of your available battery capacity. Your 200 amp hour battery bank gives you 30 amp hours to play with.

On top of this is the problem that regularly charging batteries without reaching 100% charge state is one of the major causes of premature battery failure. It causes severe plate sulphation and can wreck a battery in less than 50 cycles.

What an external alternator controller does is override the internal regulator, and force the alternator to charge at a higher voltage. Usually in the region of 14.4 volts (remember this figure).

Many people think that an increase in charge voltage from 13.8 volts to 14.4 volts isn't much of a difference. It's an increase of about 5%, so it will increase the charge by 5%.

This is incorrect for the following reasons. Firstly a wet cell battery at about 50% charge state will not begin to accept any form of charge until the terminal voltage is raised to about 12.5 volts. After this point, the current starts to flow. So the starting point is actually 12.5 volts as opposed to zero volts. Now the difference between 12.5 volts and 14.4 volts, as opposed to the difference between 12.5 volts and 13.8 volts is an increase of 50%.

Somewhat higher than the initial guess of a 5% increase.

Secondly, you need to add on top of this the fact that the energy returned to the battery is (very roughly) proportional to the square of this voltage difference as opposed to just being proportional to the voltage difference and you find that the increase in performance is actually closer to 200%. That is 200% over what it started at. So that is already 3 times better.

So that is effectively a 300% increase in charge current by going from 13.8 volts to 14.4 volts charge voltage.

At certain charge states the difference is even greater. Up to a 400% increase. I know these figures sound ridiculous. Almost like salesman's talk but these are calculated mathematical figures. They have been backed up with numerous measurements over a 2 year period on development of the SmartGauge battery monitor. They back up the manufacturers' claims of increasing the charge rate, and therefore decreasing the charge time by a factor of 3.

99% of the external alternator controllers operate on this simple principle. After a set period of time at the higher charge voltage they drop down to a lower voltage in order to prevent excessive water usage in the battery (which would require continual topping up). Some of the external alternator controllers have a few extra bells and whistles. For instance some cycle the battery voltage around 14.4 volts, this has the effect of slightly reducing water consumption but keeping the overall charge rate the same. Others occasionally ramp up to a higher voltage of around 14.8 volts for a few minutes then back down again. This slightly increases the average charge rate (by a few percent overall) and also does a very mild desulphation.

These other bells and whistles merely increase performance by a few percent at most. They were seemingly incorporated for marketing reasons ("ours does this, theirs doesn't"). None of them have any effect that is even remotely as effective as the simple principle of increasing the charge voltage from 13.8 volts to 14.4 volts.

Remember that after the batteries are charged, these external controllers reduce the charge voltage to a lower voltage in order to prevent excessive water usage in the battery.

One last benefit of external controllers is that they convert the alternator to battery sensing as opposed to machine sensing. What this means is that the controller regulates the alternator output to maintain the correct voltage at the batteries. A machine sensing alternator (the normal type) regulates the voltage at the alternator. The 2 may not be the same. For instance insufficient cable size will cause voltage drops. And of course the worst cuplrit is the split charge diode. An external alternator controller will compensate for the voltage drop across the diode thus ensuring that the batteries receive the correct charge voltage (almost - see here for an explanation of why they don't quite do what is required).

Now then, I've sung the praises of alternator controllers and explained what they do, how they do it, and the benefits when installed on an old (13.8 volt) alternator with old batteries. i.e. from the period of 10 to 15 years ago when these devices became common. They were an excellent idea.

So why do I say they are not always needed?

Simple. Most modern alternators already charge at 14.2, 14.4 or even 14.6 volts. That is how they come from the factory.

For some reason that we cannot fathom there seem to be various people who (for whatever reason) will not accept (or admit) that modern alternators already charge at these higher voltages, as they come from the factory, without any modification. They simply refuse to believe it. All one has to do is measure one with a voltmeter!

Modern batteries do not use excess water if charged at these higher voltages for long periods of time. If they did, then all the truck and car batteries round the world would be dying very quickly because they are charged continuously at higher voltages. They aren't all dying. If anything, they are lasting longer than they used to.

Times have changed. Batteries have changed. Alternators have changed. Alternator controllers are not required on modern systems. They will achieve nothing.

So, we are left with a situation that the only installations that may benefit from an external alternator controller are systems with an old style alternator or.....

(and I find this interesting...)

Systems using split charge diodes. Which, as most people have now realised, are all but useless.

It is also interesting that the only people left advocating the use of split charge diodes are manufacturers and suppliers of external alternator controllers (or perhaps people who haven't caught up with developments in technology). Because, in most cases, split charge diodes are the only reason to fit one to a modern installation, in order to compensate for the voltage drop across the split charge diode.

It took a long time for the alternator regulator manufacturers to convince the public that they were a good thing (which they were), the public's ideas were stuck, ingrained. Now the same thing has happened again. The public's idea is stuck, ingrained, but this time that alternator controllers are required when in actual fact, most of the time, they are not.

Luckily this is changing. As evidenced by sales of the SmartBank split charge system. Many of our customers are fleet builders, they have all learnt that external controllers now do nothing. They make no difference whatsoever to the charge rate or battery life. They no longer fit them.

In actual fact, in many cases, entire fleets of operatonal vehicles are being recalled to the factories and having alternator controllers and split charge diodes removed, to be replaced with standard alternator regulators and SmartBank split charge systems. Because the performance is better and the reliability is better.

We are an engineering company and will not "put down" an item of equipment just because it doesn't fit in with our ideas of business. Alternators controllers are a good thing if you have an old alternator. They are a good thing if you already have a split charge diode installed. Though in this case replacing it with a SmartBank would still be cheaper and simpler than fitting an alternator controller and the end results would also be better.

If your exisitng alternator charges at 13.6 or 13.8 volts then an external controller will make a huge difference. If it charges at 14.0 volts then it will still make a difference, but less of a difference. Once you get to the stage where the existing alternator charges at 14.2 volts or above the arguments for fitting an external controller become very shakey indeed. The difference the controller will make is going to be marginal. In the case of alternators charging at 14.4 volts and above, the external controller will make no difference whatsoever.

Also bear in mind that many modern batteries have moved the goal posts quite drastically. Most gel cell batteries should not be charged higher than 14.2 volts. Likewise AGM batteries. These types of batteries also have much lower internal resisitance than wet cell batteries and therefore accept a much higher charge rate. These factors, even further, negate the arguments for installing alternator controllers. These types of batteries accept a very high charge rate more or less whatever the charge voltage is.

If you would like to take advantage of the simple process of running your alternator at a higher charge voltage (in order to greatly increase charge rates - but perhaps without some of the bells and whistles) then take a look here for details of a simple modification to increase your alternator charge voltage. A few hours hours work and almost no monetary outlay will achieve almost the same thing as an external alternator controller.

If alternator controllers are often useless other than to compensate for the voltage drop across split charge diodes (which have many other problems) then why not simply do without the alternator controller and diode (the controller in particular can be very time consuming to fit - not to mention invalidating the alternator warranty) and install a much better system in the first place?

i.e. one that is cheaper, easier to install, more reliable, doesn't affect factory warranties and doesn't suffer the many problems inherent with the other system. This better system being a standard modern alternator and a SmartBank.


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Page last updated 02/04/2008.
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