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Improving charging performance

New devices overcome the problems associated with typical battery isolators and automatic charging relays

New devices overcome the problems associated with typical battery isolators and automatic charging relays

New battery isolation devices from Mastervolt and Blue Sea Systems improve the performance of charging systems on boats with two or more battery banks and one charging alternator.

I closely examined several new products and will explain why I chose the one I did for my Grand Banks 42, Maramor, which has a high-output alternator. I also will recommend a BlueSea product for boats with smaller alternators (60 amps).

First, some basics:

Cruising and fishing boats typically have two battery banks, a starting bank and a house bank. The house bank has heavy-duty deep cycle batteries to accommodate the electrical load when the charging system is off — for example, when you are at anchor overnight or fishing for the day. For single-engine boats with one alternator, the charging procedure is to connect in parallel both banks for charging, and disconnect them when the alternator is off to preserve the starting battery for getting under way. This can be done manually with a parallel switch. If you forget to turn off the parallel switch when you shut down the engine you will surely some day find yourself stranded with a dead starting battery after a day’s fishing or a night on the hook.

To automate this switching procedure you can install a battery isolator or an automatic charging relay (also known as a battery combiner). Each has its pros and cons, which I will discuss.

A little background

My Grand Banks 42, Maramor, has a single Caterpillar engine with a 165-amp Balmar alternator. The alternator charges the house bank (two 8D AGM batteries rated at 490 amp hours total) and the starting bank (one 4D AGM rated at 198 amp hours). Maramor is kept on a mooring, and I anchor while cruising, so she rarely sees shore power. My goal was to find and install a battery isolation device that doesn’t suffer from voltage drop, generate heat, or cause interference with on-board electronics.

The technology

A battery isolator usually is a semiconductor junction diode, a solid-state device that acts as a one-way gate, passing current in one direction and blocking it in the other. It allows current to flow from the alternator to the battery banks but prevents it from flowing between the banks, thus preventing the starting battery from discharging into the house batteries.

Battery isolators are readily available rated up to 300 amps for 12-volt systems with two diode legs to accommodate two battery banks, or three diode legs to accommodate three banks. More on diodes and their drawbacks shortly.

An alternative is an automatic charging relay — in effect, an automatic parallel switch that closes, connecting the battery banks when it senses a charging voltage from the alternator and allowing both battery banks to charge. When the alternator stops, the relay opens, separating the banks and isolating the starting battery.

One disadvantage of the automatic charging relay is that the battery bank with the higher voltage (usually the starting battery) can surge current into the battery bank with the lower voltage (usually the house bank) when the automatic relay closes and the battery banks are connected. All components and connecting wires must be rated to handle such an inrush of current to prevent dangerous overheating.

When we get under way after a night at anchor, the initial bulk charge from the alternator is in the 130-amp range because the house bank has run the lights, potable water pump, sanitation system, and other equipment on a modern cruising boat that requires electricity. With a relay, a large current surge between the starting bank and the house bank is likely, since the starting battery typically is fully charged. Due to the large capacity of Maramor’s batteries and the high currents involved (4,870 cranking amps), I favor the battery isolator over the automatic charging relay.

One other shortcoming worth noting: Efficient automatic charging relays use pulsating current to energize the solenoid coil, which can be “noisy” on communications equipment. A new offering from BlueSea for boats with smaller alternators (60 amps) has current-limiting capability that dampens surges between battery banks and doesn’t interfere with on-board electronics. More on that later.

That left me looking at battery isolators, which are solid-state devices with no moving parts. I should point out that a conventional battery isolator with silicone diodes also has serious drawbacks, mainly voltage drop across the diodes. And the ramifications of that on charging and battery life are not good.

The situation

To bring the junction diode to the state where the gate is open and the diode passes current from the alternator to the batteries, a small positive potential of at least 0.5 volts must exist across the diode to open the gate. The actual voltage drop when the alternator is producing a large current and heats up is about 0.7 volts at 50 percent load and 0.9 volts at full load. This may not sound like much, but consider that on Maramor when the alternator is producing 130 amps the power lost due to voltage drop is 117 watts — watts (117) = volts (0.9) x amps (130). This power loss is in the form of heat. In fact, the temperature rise in the battery isolator during charging can easily exceed 100 degrees F. The diodes in a battery isolator are imbedded in an anodized aluminum heat sink case to dissipate the heat.

The voltage drop across the battery isolator means that the batteries aren’t receiving the charging current at the voltage provided by the alternator and its voltage regulator — a real concern. A difference of 0.9 volts is significant and can, and probably will, result in undercharging and battery sulfation, a chemical condition that results in a loss of capacity.

What can be done about it?

The undercharging issue is overcome by fitting a microprocessor-based external voltage regulator — call it a “smart” regulator — to the system. (On Maramor I use a Balmar MaxCharge 612.) The external regulator, which has a battery sense wire that measures the voltage on the battery side of the isolator, adjusts the alternator field current to increase the voltage output from the alternator by the amount of the voltage drop across the diodes. The result is that the batteries receive the desired charging voltage. The disadvantage is that the alternator has to work harder, but the batteries are being charged correctly.

Is there a better solution?

I believe there is. Mastervolt’s Battery Mate greatly reduces the voltage drop by using field effect transistors (MOSFET)

instead of silicone diodes to provide the one-way gate for the charging current. The maximum current rating from an

alternator for this device is 160 amps. The specification states that the voltage drop is less than 0.4 volts and only 0.1 volt at 20 amps.

I measured 0.43 volts when the charging current was 130 amps, and the Battery Mate had passed this high current long enough to reach the highest temperature I observed. When the charging current decreased to 90 amps, but while the device was still hot from the 130-amp bulk charge, voltage drop was 0.26 volts; at 45 amps the voltage drop was only 0.11 volts. An impressive improvement over the silicone junction diode.

As batteries charge, their current-acceptance rate decreases rapidly and, as you can see, the voltage drop across the Battery Mate decreases rapidly, as well. Maramor was delivered with the internally regulated 110-amp alternator supplied by Caterpillar. As soon as any alternator heats up, its output decreases. This, combined with other factors, means that the bulk charge rate for the Caterpillar alternator is likely more in the range of 80 to 90 amps.

With this scenario — low maximum current combined with the rapid decrease in voltage drop as the batteries charge — the Battery Mate can be used with the standard internally regulated alternator that came with the engine (which can’t compensate for the voltage drop) without the battery isolator being a significant contributing factor to battery sulfation due to undercharging.

Voila. The combination of the Battery Mate and the external voltage regulator neatly solves the two most important objections to the use of a solid-state isolator.

Another good choice

For smaller installations (60-amp alternator) the Blue Sea CL-Series BatteryLink ACR (current-limiting) automatic charging relay should be considered. A charging relay senses the voltage from the alternator and automatically combines the house and starting battery banks during the charging cycle, and isolates them when charging stops.

The BatteryLink is a thermal device that heats up if there is a surge when the battery banks combine. As it heats, internal resistance increases, cutting the current flow and dampening the surge. A surge of 200 amps for 10 seconds will decrease the current across the BatteryLink to 10 amps. As the heat caused by the surge dissipates, the BatteryLink recovers its ability to pass its rated current of 60 amps.

The BatteryLink is an inexpensive analog device that is noise-free. It draws only 12 mA open and 175 mA closed, and has adjustable combining and disconnect voltages. It has indicator lights and provision for a manual switch, a remote indicator light and a remote sense for a second charging source.

The BatteryLink also can be used as a “load shedding” device — i.e., it can be used to disconnect non-critical loads to protect batteries from excessive discharge with an adjustable dropout voltage between 11.5 volts and 13 volts.

The Mastervolt Battery Mate and the Blue Sea Systems BatteryLink are significant technological advances that address serious shortcomings of the previous generations of these devices. But they are not the only solutions. The Balmar Digital Duo Charge, which provides regulated charging current from one battery bank to another, has many attractive features and should be considered, although its current rating of 30 amps is a limiting factor for larger systems.

Keep in mind you are dealing with high current in the marine environment. Gasoline-powered boats pose the additional risk of explosion from faulty installation.

All these devices must be installed in strict accordance with manufacturer instructions. Wire sizes and connections must be adequate, and programmable devices like the external voltage regulator must be set up with the correct values. For example, an East Penn gel battery calls for a charging voltage of at least 13.8 volts but no more than 14.1 volts at 68 degrees F, while an East Penn AGM battery’s charging voltage is 14.4 volts but no more than 14.6 volts at the same temperature — a significant difference.

Seek professional help if you are not absolutely certain you can achieve an installation that meets all regulatory and manufacturer requirements.

For more information:

• BlueSea Systems,

• Mastervolt,