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All boats have at least two battery banks, some have three. These tend to be the engine start battery, the domestic battery bank (please note that if you join three or four batteries together in your domestic battery bank it is still one battery), and the bow thruster battery. Having introduced 2-3 battery banks onto your boat, the problem then is how do you charge them from one alternator source (or two alternators which I will discuss later).
There are four various options employed by boat builders, below are the options with a short explanation giving both the positive and negative aspects.
1) Rotary switch. This method is very dated and not very common on boats. It is recognisable as a large circular switch with four marked positions on the switch. It is marked, off, 1, 2 and both. The good side of this system is that it is easy to install. The bad side is that it needs constant human intervention to ensure it works. Failure to operate it correctly will result in all batteries being discharged or not being charged correctly and possible damage to the alternator. They also tend to suffer failure if large prolonged current is passed through them. The spring in the switch can over-heat and loses its tension; this leads to an exponential break down of the switch that manifests in heat. When these switches fail they tend to melt the plastic case (if you are lucky). Simply check the temperature of the switch every so often by touching the back - it should be cold.
2) Split charge relay. This system is both dated and extremely dangerous, unless understood and the correct relay used for the correct job, ie current limiting relays may be required for safety reasons. The good side is, that it is easy to fit and requires no alterations to the standard engine system, but, it merely connects the domestic battery bank to the engine battery via a relay, which is energised when the engine starts.
The bad side (and the very dangerous side) is that a relay is prone to over loading. Say, for example, you have a 70A relay on your system and a 55A alternator, all seems great, but if you fit a 1500W inverter which can draw 150A and one morning the domestic battery is flat. So, you start the engine to charge the domestic batteries, the 70A split charger relay will come online to enable the alternator to charge the domestic battery bank. Then you load your inverter to 150A, the 150A will not be drawn from the domestic battery because it is flat but can be drawn from the engine battery (which is full). That means you will draw 150A up the split charge cable and through the 70A relay. If you are lucky you will destroy the relay, if you are not so lucky then you will set fire to the cross over cables, hence the dangerous aspect, A Sterling Currint limiting relay prevents this problem. (see later) The system must be suitable for the purpose for which it is installed and this is clearly not.
3) Split charge diodes: By using a set of diodes on a heat sink, one can ensure no back feed through the diode, thus ensuring that high currents from other battery banks do not flow up the charge lines and cause a fire. This is the most common method by far employed round the world and is the standard in the USA, for 3 reasons, safety, safety and safety, by the way did I say safety? However, all is far from perfect. The big down side with a split diode system is the voltage drop across the diode (in the order of 0.8-1.2V). This dramatically reduces the charge rate of the alternator on average by about 70%, however, this can easily be over come using products such as the Advanced Alternator regulator in conjunction with the Split Diode.
4) 0 volt-splitting systems: These are electronic devices using a control circuit and driving mosfets. The end result is a very low voltage drop across the splitting system (in the order of 0.04 -0.6V) but no reverse current flow is permitted due to the operation of the mosfets. However, on standard marine engines it is much more effective to employ the lower cost diode where an advanced regulator is fitted, (see performance).
5) 0.0 volt splitting system, The new Pro Split R from Sterling has a voltage drop about 1/10 that of a split charge diode and 1/5 that of a 0 volt drop mosfet system. See Pro Split graph below.
Conclusion: Test 1: From fig1 we can see the voltage drop across different splitting systems. This directly relates to the ability to charge the batteries, the larger the voltage drop across the device, the less effective the batteries charge.
Test 2 shows the clear advantage of using advanced regulators in conjunction with a conventional split charge diode. The advanced regulator automatically compensates for the voltage drop across the diode, plus the high charge 4-step program further increases the charge rate. The illustrated tests were on a 300 amp hour battery bank, but can easily be extrapolated to 400 amps plus.
The best low cost system clearly is a standard low cost split charge diode (for safety and cost) or the new Pro Split R and an advanced regulator on the alternator to compensate for the diode faults and charge at the constant current charging curves. This, not only charges 2-3 times faster (on a good installation, but much higher on a bad one) but puts about 100% more useful power into the batteries.
The best system but a bit more expensive is the new Pro Split.
For a twin alternator system, the ideal system is: on the largest alternator, fit direct to the domestic battery bank and attach an Advanced Regulator to that alternator. On the smallest alternator split this with a split charge diode between the engine battery and the domestic (and any other battery bank) and add another advanced regulator to it. This gives maximum charge rate to the domestic batteries.