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Monday, July 09, 2012

Distributed Power Trick: Battery Box Ventilation

Here is a trick one can do with a distributed power system that otherwise would be difficult, if not impossible, with a traditionally wired breaker system.

Battery Off-Gassing
A regular lead-acid wet cell battery can generate some gas during charging. Even under normal charging conditions (a good battery and proper operational charger) the batteries can give off a bit of hydrogen gas. If the batteries are in poor condition, or worse, the charger is malfunctioning and attempting to "overcharge", large amounts of hydrogen and oxygen gas can be released (electrolysis of the sulfuric acid electrolyte: why the addition of water to batteries is occasionally needed). A battery can also emit hydrogen sulfide gas (rotten egg smell) if overcharged.

In a well ventilated boat, such off-gassing is not a problem. Even volatile hydrogen, being much lighter than air, will dissipate quickly and is not likely a danger. But if the battery box is confined and sealed, the build-up of such gasses within the box can be a concern. Also, batteries tend to heat up when being charged. So, to keep things cool and to avoid any kind of Hindenburg-like event, forced air ventilation of the battery box is a good idea.
Well sealed battery box lid
Draft Assist Fan
This is the case with the Westsail: the battery box, located under the floor in the main salon, has a pretty tight fitting lid. Under normal conditions the box and lid will be bolted down snug to minimize shifting and movement with the boat. The box lid has two vent holes cut into the top. I want to add a draft assist fan to help draw air through the box. Ok, easy enough. But, the fan only needs to switch on during charging, and I don't want to have to remember to turn the fan on/off when the charger switches on.
24volt 0.1amp ignition protected draft assist fan
Distributed Power Control
Here is where the Mastervolt distributed power system can help. Control of the draft assist fan (on/off), can be switched by the power system in response to certain events during the battery charge cycle. Chance of off-gassing and temperature increase is highest when the charger is in high voltage "bulk" or "absorption" modes. The ChargeMaster 24-30 battery charger, signals such events so the power system can be programmed to switch the draft assist fan in response. When the charger enters "float" mode (a maintenance mode that keeps the batteries topped up) the voltages are normal, there is no off-gassing so the fan can be switched off. The event programming goes something like:
Programming the Battery box vent fan to respond to events from the battery charger.
Turn Draft Assist Fan ON when:
  • charger is turned on
  • charger enters "bulk" charge mode
  • charger enters "absorption" charge mode
Turn Draft Assist Fan OFF when:
  • charger enters "float" charge mode:
Other Charge Sources
But what about other charge sources? In the future, there will be an engine alternator, maybe solar panels, maybe a wind generator. The same hazards exists when charging from those devices. Well, the MasterShunt device, which holds the main circuit fuse and measures electrical current, also measures voltage. The MasterShunt generates events similar to the battery charger, except that there are no charger specific events like "bulk charge" or "absorption charge". It does generate an event when voltage increases to charge levels ("charging"). This can be used to turn the fan on. For turning off the fan, there is a "battery full" event generated when no more charge can be supplied to the batteries, or is '100% charged'. 

Turn Draft Assist Fan ON when:
  • shunt detects 'charging'
Turn Draft Assist Fan OFF when:
  • shunt detects 'battery full'

Programming the Battery box vent fan to respond to events from the master shunt.
Configuring the fan control this way allows it to respond to ALL charge sources, not just the battery charger.
As far as which configuration to use, the latter is best to accommodate all charging devices (alternator, solar, wind). But there is no reason we cannot have both, the system allows it.

And there you have it. A "smart controlled" battery box fan requiring no human intervention. Try that with your traditionally wired boat!

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