Boat ElectricsCTEK DC-DC Series Chargerby Kurt Kupper*


DC-DC Series Chargers

For many years, dual battery banks have been combined for charging by diode splitters, relays or voltage sensitive relays. While cheap and reasonably effective, there are inherent problems with all these options that make none of them ideal.
Diode splitters can lead to voltage drops. Relays can cause great inrush currents when a fully charged battery is connected to a depleted battery. Voltage sensitive relays can cycle, i.e. switch on and off repeatedly as the Voltage fluctuates when the batteries are connected and disconnected to each other.
Some improvements have been made, such as reducing the voltage drop in splitters and reducing VSR chatter, but none of these devices are able to regulate current, which can mean that the high currents could rush into a relatively small start battery and cause damage to the battery.
When using these devices, chances are that the starter battery will soon be fully charged, and the continued high charge current and voltage required for a regulated charging source to fully charge the house battery will cause an overcharge and damage to the starter battery. Alternatively, the house battery could never reach fully charged level. Charging the house and start batteries in parallel also means that the same type of battery should ideally be used for both.
For these reasons, charging the secondary battery by means of a smart, series regulator is an interesting alternative. The series regulator is connected to the house battery and siphons some of the current from any charge source feeding the house bank to the starting battery. It is possible to regulate charge in a way to optimally meet the requirements of the starting battery, while the requirements of the house battery can be the sole focus of the smart regulator of the primary charging source of the house battery.
In the past few years several products have been developed that significantly improve the prognosis for long battery life.
The Balmar Digital Duo Charge is a series charger that employs mosfet solid state technology to avoid any voltage losses. It limits charging current to the second battery to 30A. Four pre-set programs for different battery types are available to control cut-in voltage and upper voltage limits. In addition, it is possible to create a custom charging regime by manually selecting upper and lower voltage thresholds between which charging will take place. There is no current control other than the 30A upper limit.
This is not a two or three-stage charger, and the voltage will not be dropped to a float voltage once the battery is fully charged, but will follow the voltage of the charging source as long as it remains between the minimum and maximum limits, making it most suitable for use when the starting battery is of the same type as the house battery.
The Xantrex Digital echo-charge follows the voltage of the charging source up to a maximum of 14.4V. The current is limited to 15A, which it maintains if the voltage of the secondary battery is between one and two volts below that of the primary battery. For greater or lesser voltage differences the charging current is progressively reduced. The voltage is not regulated, it merely follows the voltage dictated by the charging device of the house battery, so it too is most suitable for charging similar battery types.
Redarc offer three series chargers with three-stage charging, i.e. boost, absorption and float. These chargers will charge from any input charge between 12.7V (9V under load) and 32V. Using a three-stage series charger is the best option for ensuring that both battery banks are optimally charged. Two of the models have selectable boost voltages, making them suitable for charging batteries of a different types.
The CTEK S250Dual is a series charger that also includes an MPPT controller that accepts an input from a solar panel. The unit feeds the charge from these sources to the secondary battery in a three-stage regime. When coupled with a CTEK Smartpass unit, it forms a complete battery management unit that maintains both battery banks, also routing charge from an AC battery charger and/or wind generator coupled to the house battery to the start battery.
With these new products significant improvements in battery management can easily be achieved with no manual effort at all.


*Kurt Küpper is director of Aquavolt Electric Boat Parts. Tel: 02 9417 8455