Mixed voltage loads and charging

What do you do when you’ve got mixed voltage loads in your system?  Such as a main drive battery of 48v and smaller loads such as radio, chart plotter and so on at 12 volts?

The usual solution is to use a dc-dc converter to step down the voltage from 48v to 12v. You have to be careful to make sure that the dc converter can deliver enough power for the combined load of all the 12 volt devices.  And if you have a big load, such as a 12v anchor winch, it can be expensive or difficult to find an dc-dc converter powerful enough to handle the load.

Solar Converters Inc of Canada have an innovative solution to this problem—and they call it a battery equaliser.  Here’s how it works…

What if you tapped 12 volts of one of the batteries in the string that makes up the 48v pack—then you’d be able to take as much current that the battery can supply.  But the difficulty then is to recharge this battery to correct level to match the others in the string. And this is where the battery equaliser plays its part.

A regular dc-dc converter produces a set voltage.  But in this case the voltage of the battery being charged is moving target.  It needs to be matched to the other batteries in the string. The battery equaliser, instead of producing a fixed voltage, produces a voltage of exactly 1/4 of the input voltage.  Thinking of a 48v pack, its voltage when fully charged will be about 51.2  (that is four batteries at 12.8 volts), and when partly discharged could be around 48 volts so the converter needs to account for this. If the pack voltage is 50 volts, the equaliser would keep the tapped battery at one quarter of this, which is 12.5 volts.

When you run a big load, say an inverter, it can take whatever current it needs from the tapped battery, and then the equaliser can balance the string in its own good time.  In a way it doesn’t matter much if it takes a minute or an hour to balance the string again.

Even without a single large load such as an inverter, a few small loads all on at the same time can easily overload a regular dc converter.  You could easily have a load of 20 amps made up of smaller loads all on together.  Its needs a substantial dc-dc converter to can deliver this power.  By using an equaliser you can meet these transient peaks from the battery and then later the equaliser tops up the battery from this usage.

Sometimes a dedicated 12v battery is added, together with its own dc-dc converter to keep it topped up.  This is a similar solution but requires the installation of another battery.

Another advantage of an equaliser is that it works both ways.  So it allows you to have a mix of charging sources. For example you may have a grid connect charger operating at 48 volts to charge the main pack when your boat is attached to a jetty with power.  But when untethered you could use a 12v solar panel to charge the tapped battery and its charge would be distributed to the other batteries in the string by the equaliser.

This setup requires a common negative, that is the negative of the 12v is the same negative as the 48v.  And this can cause a possible galvanic corrosion problem if there are devices in contact with the water and bound by this common negative.  Most 12v devices on board will not be in contact with the water.  But if you use an anchor winch, it could create a return path by way of the chain and the common negative.  In this situation where an anchor winch is used, and the 48v side is used for electrical propulsion, it is recommended to switch the negative supply for both the anchor winch and the propulsion motor. (That is, its normally just the positive that’s switched, and the negative is usually connected to the frame of the device and remains connected even when the device is siwtched off. By switching the negative as well, the device is totally isolated from the rest of the wiring loom)  This keeps the 12v and 48v wiring looms from being connected through the water.  The remaining connection between them through a dc converter is ok.

And come to think of it, the idea of switching the negative of the anchor winch could be useful for any steel or alloy boat as well.

And closely related to this topic is charging from lower voltage solar panels to higher voltage batteries, or vice versa.  Both of these tasks can be easily handled and I’ll talk more about that in another post.

We have a few products available from Solar Converters, for various conversion. There’s equalisers, regular dc-dc converters, and solar chargers to allow charging from lower voltage panels to higher voltage systems.  If you need a converter for a particular task, let me know, I’m sure we can get one for you.

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