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Split
Charge Diodes - Information |
All
boats have at least two battery banks (most motorhomes now also have two
batteries) - 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?
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 system tends to be very dated, and is 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 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 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.
2) 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 a 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, and 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.
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.. 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.
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 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 a advanced regulator can be used much better to
employ the lower cost diode with a advanced regulator, (see performance below).
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Conclusion:
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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 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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New Split charge Blocking Diodes
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Sterling has developed a range of low cost split charge diodes. These diodes
have enhanced performance over conventional diodes and at a lower cost.
The difference is in the devices. All other split charge diode manufacturers
use conventional alternator diodes, which at low current flow have about
a 0.93 voltage drop. When the full rated current of these diodes is approached,
the voltage drop increases to about 0.95 volts. This results in excessive
heat and power loss across the diode. For example: A conventional one
alternator in and two battery bank out, tested against a Sterling unit
had the following results:
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Conventional
Splitters |
Sterling
Splitter |
| Amps
Passed |
30A |
50A |
60A |
70A |
30A |
50A |
60A |
70A |
| Voltage
Drop |
0.93V |
0.95V |
0.97V |
1.1V |
0.78V |
0.75V |
0.74V |
0.74V |
| Power
Loss |
27.9W |
47.5W |
58.2W |
77W |
23.4W |
37.5W |
44.4W |
51.8W |
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Low
Voltage Drop Diodes |
| Alt
Input |
Battery
Banks |
Output |
Order
Code |
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One |
Two |
70A |
MLE00053 |
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One |
Two |
90A |
MLE00064 |
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One |
Two |
130A |
MLE00068 |
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One |
Three |
70A |
MLE00061 |
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One |
Three |
90A |
MLE00067 |
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One |
Three |
130A |
MLE00071 |
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1 IN 2 OUT SPLIT CHARGE DIODE
70A max. current
1 input
2 outputs
low voltage drop diodes
Order Code.: MLE00053
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1 IN 3 OUT SPLIT CHARGE DIODE
70A max. current
1 input
3 outputs
low voltage drop diodes
Order Code.: MLE00061
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1 IN 2 OUT SPLIT CHARGE DIODE
90A max. current
1 input
2 outputs
low voltage drop diodes
Order Code.: MLE00064
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1 IN 3 OUT SPLIT CHARGE DIODE
90A max. current
1 input
3 outputs
low voltage drop diodes
Order Code.: MLE00067
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1 IN 2 OUT SPLIT CHARGE DIODE
130A max. current
1 input
2 outputs
low voltage drop diodes
Order Code.: MLE00068
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1 IN 3 OUT SPLIT CHARGE DIODE
130A max. current
1 input
3 outputs
low voltage drop diodes
Order Code.: MLE00071
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Installation
tips here
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