Separate Battery for VHF

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lpdsn

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What's the smallest, lightest 12V battery you know of?

I'm looking to replace my fixed VHF, and the idea crossed my mind that I could probably get a small extra battery in the space next to it, charged by some means from the main battery (prob a VSR). That way, in an emergency I would still be able to power the VHF even if the main batteries were flooded. At least until the water reaches the chart table level, by which time I wouldn't want to be still down below.

I know I could do a web search, but given the size of this forum there's probably somebody who's already found something smaller & lighter than I'd find starting from scratch.
 
A motorcycle battery would do. There was an article recently in PBO from somebody who did just what you are proposing (can't remember which issue).
 
The issue will be the power consumption, a fixed set uses 25w on transmit and through listening to lots of maydays you are on the set a lot - and a very small 12v battery may not last. But also your set below will be very restrictive in a mayday. Much better will be a decent handheld, YM tests showed that for coastal use the handheld on 4w was fine and you can take it with you if you decamp to a dinghy. There are 2 main causes of mayday - grounding and fire. In niether of those would I want to be below at the chart table.
 
Lazy Kipper said:
The issue will be the power consumption, a fixed set uses 25w on transmit and through listening to lots of maydays you are on the set a lot - and a very small 12v battery may not last. But also your set below will be very restrictive in a mayday. Much better will be a decent handheld, YM tests showed that for coastal use the handheld on 4w was fine and you can take it with you if you decamp to a dinghy. There are 2 main causes of mayday - grounding and fire. In niether of those would I want to be below at the chart table.
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Having been in a situation where I had to have the assistance of the RNLI I agree with the requirement of a hand held VHF. I had a sick terrified man on the helm who was not able to assist much and having to keep going down to the chart table to communicate with the lifeboat was a nightmare.
 
Lazy Kipper said:
The issue will be the power consumption, a fixed set uses 25w on transmit and through listening to lots of maydays you are on the set a lot - and a very small 12v battery may not last. But also your set below will be very restrictive in a mayday. Much better will be a decent handheld, YM tests showed that for coastal use the handheld on 4w was fine and you can take it with you if you decamp to a dinghy. There are 2 main causes of mayday - grounding and fire. In niether of those would I want to be below at the chart table.
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It's OK, I've got two handhelds too. One in a grab bag and one usually in the chart table and sometimes in the cockpit. The advantage with the fixed VHF is extra power and extra range.

I don't intend the small battery to be an independent power source. It will be powered itself by the main batteries but won't be drained if they are flooded by sea water.

Like everything to do with emergency planning, it won't be needed because it'll never happen to me, but just in case.
 
mdonnelly said:
Having been in a situation where I had to have the assistance of the RNLI I agree with the requirement of a hand held VHF. I had a sick terrified man on the helm who was not able to assist much and having to keep going down to the chart table to communicate with the lifeboat was a nightmare.
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Because we sail short handed, our VHF is mounted under the coachroof just inside the companionway, where it can be used from the cockpit.
 
This subject is covered in the NMEA0400 standards but it also includes nav lights and GPS in the emergency coms battery. The formula they give for choosing a suitable battery is a bit over the top in my view though as it calls for a 6 hour supply with 100% loads on the GPS and Nav lights and assumes (here's where I think it's a bit much) a 50% transmit time and 50% receive time for the VHF or other coms radio. I guess it depends on the distance involved in your cruising.
 
I'd have thought the type of battery used in exit signs and fire alarm systems would do the trick. Better still if you know anyone working on these kinds of equipment, often the batteries they change over as a matter of course have years left in them.

Rob.
 
Something like these from Yuasa
http://www.ebay.co.uk/bhp/yuasa-battery

The battery capacity required will depend on what sort of TX duty cycle you expect to need from the radio.

Be very careful in selecting a suitable charging system for the battery, dont be tempted in using a car battery charger. Use the correct charger for the battery with some form of safety cut off.
 
Thanks everyone, I've been doing a bit of research as a result of RogerShaw's post. I've seen that there's other posted along the same lines since.

It looks like I can get something like a 12V 4Ah Sealed Lead Acid battery for around £10 in the UK (Powersonic/Yuasa etc.). Weight about 1.5Kg and the dimensions look ok subject to measuring the gap where I'll be mounting it.

I think I'd then just use a diode with a Vf of about 0.4V to connect it to the main batteries. That will be convenient for the difference in charging voltages between SLA and normal Lead Acid.

Project definitely looks do-able without too much hassle & expense.

I wouldn't be intending to wade around in waist deep water for 6 hours after the main batteries were swamped, so I think I might skip NMEA0400 compliance.

I've used 0.5A for the VHF in quiescent state and 5A for transmitting at full power. Obviously 5A will not give 4Ah out of a 4Ah battery, but still it will give a number of transmission attempts over the course of a few tens of minutes. I reckon it would need 18" to two foot of water to take out the main batteries anyway, so I think the above will be sufficient before falling back on PLB, liferaft & H/H VHF.
 
Normal power use for a fixed VHF is 25w in high power and most sets default to 25w on Channel 16. That will give you a realistic transmit time of about 45 minutes on the battery which is probably enough.
 
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