Split Charge Diodes are dated and do not work properly.
The case against normal diode splitters
In simple terms diodes operate by only allowing current flow in one direction. A sort of one-way valve for electricity. Therefore 2 diodes could be connected from an alternator output terminal, one to each of 2 battery banks. The alternator can thus charge each battery bank but no current can flow in the opposite direction so loads connected to one battery cannot affect the other.
Seems like a simple solution?
If diodes really were a simple "one way valve for electricity" then, for many installations (not all), they would be an almost perfect solution to split charging multiple battery banks. However, real world diodes are not perfect "one way valves", they are anything but perfect .......
One of the side effects of diodes is that there is a voltage drop associated with them. This drop is not constant and varies from around 0.6 volts to 1.1 volts depending upon how much current is flowing through them. At very high charge currents the voltage drop can actually be as much as 1.6 volts. The higher the current, the higher the voltage drop. This means that the output voltage of your alternator at say 14 volts is not being presented to the batteries. The batteries will only be "seeing" between 12.9 and 13.4 volts depending upon the charge rate. Obviously this drastically reduces charging capability. The charge voltage on the batteries is simply too low.
Anyone who tells you that the voltage drop across a diode is fixed at 0.6 volts and does not change does not understand the subject and possibly read "beginners guide to electricity" and then stopped studying. Sorry, but that's the reality.
One solution is an external alternator regulator which senses the voltage at the battery instead of at the alternator (as per a normal alternator). This has the effect of compensating for the voltage drop across the diode by increasing the alternator output voltage.
Even this solution has it's problems. Firstly one has to decide which battery to sense. The domestic bank or the engine battery ? Let's put that to one side for the moment.....
Remember the voltage drop across the diode depends upon the current through it so the engine battery (which will usually be drawing much less current than the house bank) diode will have a lower voltage drop. Let's say it's 0.6 volts. The voltage drop across the diode to the domestic bank will be much higher (due to the higher charge current). Let's say it's 1.1 volt. We now have 0.5 volts difference between the battery terminal voltages.
Back to which bank to sense.... If the engine battery is sensed then we find that the domestic bank is being charged 0.5 volts too low. Obviously not what we are aiming for. If the Domestic bank is sensed we will find
that the engine battery is being charged 0.5 volts too high. This of course will damage the battery.
There is no solution to this problem using diodes. The only way round the problem is to fit 2 alternators, one for each bank. This isn't always practical for mechanical or space reasons. It also isn't always something the boat owner can do him/herself, perhaps involving welding up brackets or other engineering techniques that they do not have access to. Obviously if it is possible then it's a reasonable workable solution
Another problem with diodes is the wasted energy. Imagine a 70 amp alternator charging a battery through a splitting diode running at full charge capacity. That is 70 amps X 1.4 volts = almost 100 watts of power that is wasted in heating up the diode. That's power that could have gone into your batteries, another 7 amps at the normal 12 volt charge voltage of 14.4 volts. Some diode suppliers and manufacturers deny the existence of this wasted power (in the form of heat), others simply forget to mention it. Ask them why they have such huge cooling fins on them.
Further, if 2 alternators are fitted, it becomes apparent that the alternator charging the engine battery is not being worked very hard. It's only working for the first 15 minutes or so. After this time there appears to be spare alternator capacity that isn't being utilised. So the boat owner installs a diode splitter, to utilise this otherwise wasted capability, thus reintroducing the voltage drop/difference problem that has just been cured!
And so far we haven't even touched on the subject of AC powered chargers and/or combined inverter/chargers, wind generators, solar panels etc!
Take the case of a combination inverter/charger. This has to be connected directly to the domestic bank. No diode can be fitted (even assuming one large enough could be found). At a first glance there appears to be no way to allow the charger to charge the engine battery. Well, it can be done, by fitting a single diode from the domestic bank to the engine battery. It's far from ideal. We are still suffering from the large voltage drop associated with diodes.
More serious is that if the engine is cranked with a very low or flat engine battery the starter motor will attempt to draw it's current through this diode from the domestic bank. This, of course, will blow the diode almost immediately.
The same problem arises with single output chargers, wind generators or solar panels. Each extra charge source requires yet more diodes or yet more compromises.
So what exactly is the solution?
Simple. Fit a SmartBank and suffer NONE of these problems.
Fact: A SmartBank with or without an external alternator controller is the best charging system available in that it....
A) Suffers no voltage drop problems.
B) Utilises the FULL charge capability of the alternator or other charge source.
C) Is far simpler and quicker to install than another alternator.
D) Is compatible with every charge source that is installed. No need to add more equipment when a new charger/wind generator/solar panel is installed.
E) Wastes far less power. In the example given above almost 100 watts of (what could have been available charge) power is being wasted by heating up the diode. In exactly the same situation a SmartBank with a typical 200 amp relay would be using less than 4 watts (25 times less wasted power).
F) In a twin alternator installation SmartBank ensures that the spare capacity from the engine battery alternator is not wasted once the engine battery is fully charged. Instead the spare capacity will be diverted to the domestic bank. This means that a twin 70 amp alternator installation would effectively be charging the domestic bank at 140 amps once the engine battery was fully charged. This installation also gives inherent backup to the system in that any one component can fail and the entire system will continue to operate (albeit with reduced capacity). Two failures are required before the system starts to malfunction.
G) The alternators being fitted to modern engines (both marine and vehicle) are now charging at a much higher voltage than previously - this is due to the way the market has developed and battery technology has changed. Typically between 14.2 and 14.6 volts. These do a FAR better job of charging domestic battery banks. So good infact that many engineers (myself included) are now considering alternator controllers to be redundant and un-necessary on newer engines, and perhaps even "yet one more thing to go wrong". The only reason for fitting them would be to compensate for the voltage drop across a diode splitter. See here for a more detailed explanation of why alternator controllers may be a complete waste of money.
So the options are A) fit a diode splitter and alternator controller or B) fit a SmartBank. The answer is obvious, A SmartBank is MUCH simpler and quicker to install than the other 2 and also costs about half as much.
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Page last updated 02/04/2008.
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