False gas alarm when battery charging in SO379

[pkempthorne]

I have a SO379 which is now three years old. When the house batteries have got down to 50% and the charger is working hard. ( In the last event it was charging at 24 amps.)the charger was hot. The gas alarm went off. The sensor is situated under the stove near the stop cock and only about a metre from the charger which is in the compartment under the bunk in the starboard quarter. I have read that hydrogen given off by charging batteries can lead to a carbon monoxide alarm going off falsely. Has anyone else had this happen? I am thinking of putting more ventilation in the front of the batter and charger compartment and if this doesn't lead to an improvement install a computer fan which activates if the temperature goes up in the compartment. I am interested in suggestions. It seems a very poorly ventilated compartment and not far from the gas sensor.

[jdl01]

The limited air circulation in both 379 battery compartments was one of the reasons we went with AGM batteries from the get go. They are sealed units giving off essentially no gases and enjoy a broader and longer charging life - worth the higher initial price. If you do go this route, remember to switch over the charging pattern on both the shore power charger and your alternator.

[rxc]

I have always had golf cart batteries on my boats, and on my Catalina 34 the fire alarm went off because of overcharging of the batteries due to a failed sensing line. The batteries boiled nearly dry, and were VERY hot. The sensing line from the batteries had an inline fuse in the battery compartment, and I think that off-gassing of the batteries caused it to corrode and fail. The code requires these fuses, "as close to the batteries as possible", but I have concluded that "inside the battery compartment" is a bit too close.

My 43DS has a small blower for the battery compartment that only runs when the battery voltage is greater than 13v - ie, only when something is charging the batteries. The separate compartment for the charger itself used to have a fan, but no longer. I need to fix that, this winter.

[Mistroma]

I also fitted a fan and ducting on my 42DS. Jeanneau didn't allow for any ventilation and the underside of the ply covering the batteries was very discoloured.

I bought some cheap 100mm round pipe, a couple of 125 > 100mm reducers and flexible hose. I already had a PC fan and spare Ebersbacher outlet.

£1.50 each www.toolstation.com/100-round-pipe/p29322
£1.42 each www.toolstation.com/125-round-ducting-reducer/p99431
£3.32 each www.toolstation.com/pvc-flexible-ducting-hose/p32740 (p30076 is 6m if 3m is too short)

The fan fitted neatly inside the 2 125mm reducers which were then joined together with duct tape to try it out. The 100mm end of the reducers were connected to the 100mm pipe to join up the battery box and the louvred duct. Another section of 100mm pipe was inserted on the other side of the battery box and connected to flexible ducting leading to the stern.

I put this together to see how well it worked and meant to improve on duct tape and cheap ducting. However, it worked perfectly, sits out of site and appears to be quite robust. I don't expect to make many changes.

I might have some pictures somewhere. I was making the box slightly wider and deeper to handle Trojan T105s. It seemed sensible to add ventilation when I'd be charging them at 14.8V.

The fan is off when discharging, 50% power when charging starts and switches to full power when voltage is high enough to start gassing.

Trickiest part was drilling oval holes either side of the battery box to just fit the pipes at the correct angle. I cut the pipe to the correct angle and used the end to mark the cut. A combination of hole cutters and padsaw did the rest.

Yes, it is pretty well known that CO alarm sensors are sensitive to Hydrogen. No particular surprise to get an alarm when batteries are gassing.

[pkempthorne]

Thanks everyone for the advice. All was very valuable and reassuring since alarms are always a little concerning. I have added more ventilation into the battery compartment and now that I understand the problem I have found opening the cupboard which houses the gas sensor during periods when the charging rate is high has completely removed the problem. This cupboard is low down under the stove where the gas pipe connections are and I can see exactly how gas from the battery would creep along the bilge and into that cupboard. The extra ventilation seems to be keeping things cooler but I will add a fan if I detect any sign that it is still too warm in there.