Wiring for a battery compartment fan

[Rafiki]

Hi,

I have often read of the need to provide ventilation for the battery compartment, which I fully understand. Suggestions ahve been made that this is best achieved by fitting a small fan (computer ventilation fan) such that it is always on when either the shore power or engine alternator is on and always off when both are off!

Seems a good idea but I can't work out how the circuit would work.Obviously easy enough to do this on a switched circuit but if you aren't relying on a switch to turn it off where do I connect the positive lead ? ( I assume negative goes direct to the negative circuit).

My thoughts are;
1)For the engine circuit perhaps the positive is connected to the alternator side of the split charge diode. Presumably the current can't flow from teh battery the "wrong way" through the diode and therefor there will be no power unless the alternator is working.
2)As regards the shore power battery charger I'm stumped since connecting the positive to a terminal of the battery charger would appear to have the same effect as connecting to the battery itself - hence always on! Or do I need some sort of blocking diode? if so can anyone recommendd a suitable component at a low cost?

Hoping some of you electric experts can help.

PS I do understand the basics and we have virtually re-wired the boat in the last few years.

Regards

Andrew

 

[Manuel]

You say you are good with electrics in which case your best bet would be to build a small voltage sensitive relay WITH FUSE wired across your batteries, so when the volts are greater than 13.5 the fan comes on. Try googling for a circuit. You should be able to get the parts from Maplin and build it on Veroboard, mounted in a small plastic enclosure. Don't forget the fuse. Anything connected across your batteries becomes dangerous unless fused. Go for a very low drain design using FETs or CMOS ICs.

If your batteries are flooded (open) then remember that the fumes are acidic and can cause damage.

 

[halcyon]

Hi,

I have often read of the need to provide ventilation for the battery compartment, which I fully understand. Suggestions ahve been made that this is best achieved by fitting a small fan (computer ventilation fan) such that it is always on when either the shore power or engine alternator is on and always off when both are off!

If your boat was still fitted with the original mains charger that we supplied Westerly, it had a terminal to drive a extractor fan or blower when the charger was on high charge. When it dropped to float charge, it turned off, not many manufactures have carried on the feature.

Simplest solution would be a small VSR, would then cover both charge sources.

Brian

 

[Rafiki]

Battery compartment fan.

It seesm a voltage sensitive relay is the way to go - so only on when voltage is above X volts. I don't think I'm up to actually building one myself. I've not done anything like taht before so will need to buy one. Any recomendations?

No the boat didn't have an original Westerly charger. When I bought it there was no charger at all.

Regards

 

[Captn D]

Unless the fan, and switch if near the batteries, can be guaranteed not to produce any chance of a spark, I would be very careful using a fan for battery ventilation. Under charge batteries give off hydrogen which if ignited could be disastrous. Our battery compartment is ventilated via a 1 inch pipe to a vent in the topsides. Such passive ventilation should be adequate for most battery installations.

 

[Manuel]

I don't think I'm up to actually building one myself. I've not done anything like taht before so will need to buy one. Any recomendations?

There are plenty on the market but they generally drive big relays to switch battery banks, not computer fans. They are very expensive and an overkill for what you want. Hobby electronics is not that difficult. Have a look at the Maplin site and read a few of the beginners' electronics magazines. Do you have a friend, or schoolboy, whose hobby is electronics? A simple VSR for the fan should take no more than five components, a bit of board, a small box and cost in parts five pounds. A lad doing A level Physics would be happy to do that for you for a bit of sailing instruction.

As for the hydrogen problem, presumably there is already a vertical path from the battery box to the outside? In which case even if the fan is not running the H2 will pass through and out.

 

[Chris_Robb]

Brian - I presume the reason for fitting a fan to the battery compartment is to reduce heat build up, and disperse hydrogen. In reference to the latter, being ever so slightly explosive, I presume you will need special fans and switches that do not spark. Surely the best way is to allow the gas to vent naturally upwards?

As to heat build up, I would not have though that in the UK this was necessary - unless that is we get a BBQ summer.

So does the poster need to bother to do anything, other than to ensure natural ventilation - (which Westerly never seemed to put in) by putting a louvre or two on the ply panel side of the battery box.

 

[John100156]

Unless the fan, and switch if near the batteries, can be guaranteed not to produce any chance of a spark, I would be very careful using a fan for battery ventilation. Under charge batteries give off hydrogen which if ignited could be disastrous. Our battery compartment is ventilated via a 1 inch pipe to a vent in the topsides. Such passive ventilation should be adequate for most battery installations.

I would agree, go passive if you can to avoid build-up. If you really wanted to, you could install a fan, but a bit OTT if you ask me. If its not a bifurcated fan (fan motor out of airstream, unlikely if a small fan) and it breaks down or sparks, you could be creating the very problem you are trying to avoid! I think you are actually increasing the risk not reducing it!

Alternatively, if you do decide to have a fan run when engine or charger is on, two simple 12v single pole relays (cheap from car parts and more reliable than VSR?) would do the trick, one coil energised off the ignition switch circuit and one from the the 12v charger output (if charger output is permanently +ve 12v then use a 230v coil single pole relay on input side). Then from battery +ve: fuse, out of fuse into common of both sets of contacts, out of both sets of contacts to fan motor, -ve on fan motor to battery...... But I would not fit a fan!

 

[emsworthy]

Help!

Sorry to hijack this thread slightly, hopefully it may be of use to the OP, but on reading this I am concerned by a couple of things on my boat.

Firstly the battery compartment is immediately below the quarter berth cushion (the head end), my bunk! and secondly there is no apparent means for charge gases to escape.

The set up is 2 x 85Ah leisure batteries charged by a solar panel and engine alternator with a shore power charge circuit. Obviously it is unlikely I would be sleeping much if the engine were running or during the day when the sun is out, but what are the risks of sleeping right on top of a couple of batteries that are drawing a hefty charge whilst connected to shore power at the end of a long day's sail?

Tim

 

[halcyon]

Brian - I presume the reason for fitting a fan to the battery compartment is to reduce heat build up, and disperse hydrogen. In reference to the latter, being ever so slightly explosive, I presume you will need special fans and switches that do not spark. Surely the best way is to allow the gas to vent naturally upwards?.

When we introduced these chargers, high charge voltages to 14.3 were not common, they were still only charging to a fixed 13,6 volt. So gassing was now a possible danger, so we incorperated a drive circuit to vent any gas that was given off. We did not specify what with, but most used builders used a bilge blowewr suitable for petrol engined boats. Once the charger dropped down to 13.6 volt charge, the blower would be turned off. So if the boat blow up, we were covered, or at least I did not feel guilty.

One thing that does worry me the Westerly chargers we supplied them, they were set for Delco Freedom batteries. These charged to about 15.5 volt, and floated at 14 5 volt, so if normal flooded batteries were fitted later, they could be destroyed.

In the 80's we were not into massive charge rates, we used a 10 hour charge, so a 200 amphr battery bank would be a 20 amp charger, so tempreture was not so much a factor as to-day. The other side, with the lower charge rate, absortion phase was not required, so the time spent on a high gassing rate was much lower, as was water usage.

Brian

 

[John100156]

Sorry to hijack this thread slightly, hopefully it may be of use to the OP, but on reading this I am concerned by a couple of things on my boat.

Firstly the battery compartment is immediately below the quarter berth cushion (the head end), my bunk! and secondly there is no apparent means for charge gases to escape.

The set up is 2 x 85Ah leisure batteries charged by a solar panel and engine alternator with a shore power charge circuit. Obviously it is unlikely I would be sleeping much if the engine were running or during the day when the sun is out, but what are the risks of sleeping right on top of a couple of batteries that are drawing a hefty charge whilst connected to shore power at the end of a long day's sail?

Tim

Tim,

To be safe I would definately look to vent the battery compartment, this can be easily achieved using say a 25mm dia. plastic pipe!

 

[William_H]

Battery veentilation

Small aircraft often use 2 pieces of 8mm beverage tube going from the battery compartment out to the outside. One from the front of the box one from the back. The tubes are shamfered at 45 degrees so front facing forward aft facing backwards. This provides a stream of air flow across the top of the batteries.
You could do something similar with 2, 25mm tubes perhaps to the side of the cabin or hull side. However you would worry about water getting in.

If you want a fan driven by VSR. You need an op amp ua741 is OK. But there are plenty of other types available. 12v to pin 7 earth to pin 4.
A 1000 ohm resistor from +ve to a 6v zener diode to ground the junction of the 2 goes to pin 2 and provides a stabilised 6v on the -ve input. The positive pin 3 input goes to the centre of a potentiometer from +ve to ground. This is adjusted so that when the 12v supply (as divided by the potentiometer to become 6v) exceeds the 6 v of the zener diode then the output of the op amp on pin 6 will go nearly to 12v from normally zero. This 12v o/p can switch a transistor to switch on the fan.
It is quite simple and a nice liitle project to fiddle with at home.
PM me for more details if you want. olewill

 

[Rafiki]

Battery ventilation

Thanks for all your advice guys.
Some good points about not bothering with a fan as any fault could itself cause an explosion.
The battery compartment doesn't currently have any ventilation ( a common problem in westerlies apparently) and like another poster said the batteries are right under the head end of the quarter berth. Putting a small vent in shouldn't be a problem but there is virtually no space high up and I think the gas does rise so I may vent it into the next locker (foot end of quarter berth), which is empty and from there may vent it upwards.

Thank you again for your help.

Regards