Articles: Power

Charging More Than One Battery Bank: The PYS Recommendation

Today, many recreational and leisure boats have more than one battery bank and proper charging systems can get fairly complicated. There is no one solution for every boater. It not only depends on the style of boat, but also on how the boat owner uses his or her boat.

One common piece of equipment that was very popular up to recently is the battery isolator. This simple device allows one charging source (an engine alternator or single output charger) to send a charge or current to two or more battery banks while stopping current from flowing back. These battery isolators use diodes, which can be thought of as one-way valves, configured in such a way so that current cannot flow from one battery to another. This inexpensive solution has one major drawback -- there is always a little bit of power loss going through the diodes. This is referred to as "voltage drop" and is typically 0.7 volts. Now 0.7 volts may not seem like it could cause any trouble, but it is something that should be taken into consideration. A typical 12 volt battery is considered "full" at 12.7 volts and at 50% capacity at 12.0 volts. This makes the diode voltage drop a big deal. Modern two and three stage chargers precisely regulate their voltage for optimum battery performance, so any voltage drop would seriously impact their performance. Another problem that the diode voltage drop causes is heat buildup, which requires large cooling fins and is a major limiting factor in battery isolator designs.

What we recommend now are "smart relays," which is like a switch with a brain. They are called AutomaticCharging Relays (ACR), Voltage Sensing Relays (VSR), or battery integrators, depending on the manufacturer, and are basically the same idea. The smart relay gets connected between two battery banks. When it senses either battery bank being charged is full (or almost full), around 13.3 volts and higher, it combines the two battery banks together. This allows the charge current to flow to the other battery bank, without any voltage loss. When the charging source is removed, or the battery voltage level drops, the smart relay disconnects the battery banks, isolating them again. Smart relays do have some drawbacks as well and should be installed according to their specific instructions or by a professional.


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