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Split Charge Diodes Systems Facts and Figures

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.

Rotary switch

This system tends to be very dated and is not very common on boats. It is recognizable 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 it is easy to install. The bad side is that it needs a constant human intervention to ensure it works. Failure to operate it correctly will result in all batteries being discharger 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 and is manifested 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 of the switch – it should be cold.

Split charge relay

This system is both dated and extremely dangerous and more than likely will make your boat fall short on CE requirements, especially if an inverter is used or a bow thruster. The good side is that it is easy to fit and requires no alterations to the standard engine system, but 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 vibration faults and over loading. Say, for example, you have a 70 amp relay on your system and a 55 amp alternator, all seams great, but if you fit a 1500 watt inverter which can draw150 amps and one morning the domestic battery is flat. So, you start the engine to charge the domestic batteries, the 70 amp split charger relay will come on line to enable the alternator to charger the domestic battery bank. Then you load your inverter to 150 amps, the 150 amps will not be drawn from the domestic battery because it is flat but be drawn from the engine battery (which is full). That means you will draw 150 amps up the split charge cable and through the 70 amp 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. The system must be suitable for the purpose for which it is installed this is clearly not. Be warned about split charger systems using relays.

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 – 1.2 volts), which dramatically reduces the charge rate of the alternator on average by about 70%, however, do not forget the safety feature.

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 off 0.04 volts but no reverse current flow is permitted due to the operation of the mosfets. A good analogy is the safety of the split charge diode with the performance of a split relay. However, at a much higher financial cost, this system is ideal where a vehicle or engine is being used and the alternator cannot be altered for warranty or other reasons. However on standard marine engines where an advanced regulator can be used much better to employ the lower cost diode with an advanced regulator (see performance below).


Voltage across splitting systems

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.

Charge rate with different splitting systems

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 charger 4-step program increases the charge rate even further. The above tests were on a 300 amp hour battery bank, but can easily be extrapolated to 400 amp plus.

The ideal system is clearly a standard low cost split charge diode (for safety and cost) 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.

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.

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