Electric blanket maths

WindermereColvic

WindermereColvic

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Apparently, sleeping bags dont cut it anymore, as I have just been told that tesco have a cheapy electric blanket going, and asked if it would work on the boat..hmm. Lets see what you think of my calculations.. 35W Electric blanket, P=IV so at 12V thats a draw of 2.91A. its running through an inverter so lets assume thats 80% efficient, 20% of 2.91 = 0.58, so with inefficency losses thats a draw 3.49A. Running from 11PM to 8AM is 9 hours so thats 31AH. running from a 60AH battery so considering the inverter cuts out at 10(ish) volts, about 40AH useable capacity. Hold on, this just might work. But being able to run an electric blanket for a night on 1 battery seems counter intuative, and its based on a few 'assumptions'. So feel free to pick apart my no doubt dodgy maths. Oh, and does anyone know if they cycle on and off or are they on permanently? Ta, Matt
 
Your maths and physics are sort of OK but you never want to let your battery fall below 50% charge and it takes a lot more time and power to charge a battery than is taken out.

If you had 600Ah, a generator, etc., then for one bed this might be a solution. On the other hand, a better solution might be a diesel heater or some decent thermals. You can be as snug as a bug in the right sleeping bag and kit.

Don't do it! Try buying the appropriate kit first, and the diesel heater second.
 
This would work fine if, instead of using your batteries and risking deeper than intended discharge, you simply connected the blanket, via an appropriate regulator, to one of the larger solar panels now on the market and did your sleeping by daylight.

Plan B is more appropriate to night time use and that is to connect it to a hand cranked generator.
 
change the sleeping companion for a younger model. Might be a tad more expensive option but a great deal more fun.

No your plan wouldnt work. Your blanket is more likely to be 50w even when turned down, and that would be per person. Your 60 ah battery would probably only ever yield that much when brand new and on the manufacturere test bench. Better base on 40 AH of which you should onle cycle maybe 50%. So you need much more battery power.

50w*2 = 100w *8hrs /12v = 66AH. Therefore battery capacity needed = 66*2/0.8 = 165AH
 
Your maths and your idea is just fine, ignore the jokes!

We had our boat on a swinging mooring for a year and had no eber then. We picked up a couple of single electric blankets and ran them off an inverter. The 35W you quote is on full setting (ours were 40W on full). We put them on the first setting and it was lovely, nice and warm and seemed quite decadent getting into a pre-heated bed on a mooring. The battery bank was only 200ah and each blanket used approx 2AH. Even with both on for 10 hours that's only 40amps (plus the inefficiency of the inverter, but ours is 90% efficient)and at a nice slow drain rate. Back then we had a Rutland 913 and with the engine on the next day everything was restored. Highly recommended.
 
why do we need a 12v one? the things work on resistance dont they? jus t put the 12 v to it direct surely?
stu
 
I don't know about modern electric blankets but the older style might be 52 watts or less but on medium setting they are half power one element on and on low a quarter power done by wiring the 2 elements in series. So 13 watts on low. I would not leave an EB on anything but low.. You should use a small inverter which runs nearer full power so is more efficient than large inverter running very low power. olewill
 
would you put 240v through a 50 watt car bulb and expect it to shine brightly for more than a few milliseconds ? /forums/images/graemlins/wink.gif

You would get about 3 watts out of putting 12v into a 240v electric blanket, I think, assuming the blanket normally produces 60 watts.
 
Well, we have done a lot of work with Eon photographing the testing of Electric blankets with Age Concern.... and after seeing the number of failures, and the types of problems, there is no way in hell that I would use a leccy blanket!!!

I would say that the majority of electric blankets that I saw tested failed and were potential firehazards....

/forums/images/graemlins/frown.gif
 
If you got hold of an old electric blanket and were able to access the elements such that you could divide the elements and connect in paralell you could make a 12v blanket.
So each element is 26 w (2 elements) = .1 amp R = 2400 ohms
If each element was divided into 4 at 600 ohms each and connected in parallel then 8 elements all in paralell would take 1.6 amps so 19 watts.
19 w is equivalent to the original blanket running somewhere between low and medium. Which is normally a good all night setting. This could be halved or quartered by using the original switch to connect to half the elements for medium or 2 sets in series for low. olewill
 
If you apply as much maths to buying a sleeping bag you will find you can get good ones that will keep you warm all year round. If you dont mind the bulk then it need not be too expensive.
I have one from Pro action (sold at Argos etc) that is good for 4 seasons. It is also rectangular, not mummy so you have room to move your legs.
 
When I proposed that we live on board a cruising yacht, my wife made 3 stipulations. I beleieve she thought she might scupper the idea. First was a bath-tub. Second was a certain length of book-shelf, and the third was an elec blanket. She lost! She got all of them.
This was a 40 ft LWL ketch we built ourselves and she had 24 volt circuits. I do not like electric blankets, but I found a "240 volt" version which supplied the blanket through a transformer "for extra safety". Chucked the transformer and fed it direct off the battery. It had a thermostat, and one could monitor the time it was ON by watching the lights. We agreed that the important time for el blankets was on first hitting the sack and for the first half-hour. We never had any problems and the blanket (which had a choice of three settings, lasted in that boat for 10 yeras and in the next one for a further five beffore the blanket part wore out and the wiring element stated to stick out.
Our battery was 400 ah Nife, a large bank of aircraft starting batteries that Her Majesty had no further use for.
They cost £50 from a dealer near Chelmsford and were still fine when we sold the boat.
Our next boat was bigger, with a diesel generator and a much bigger battery baank. No problems over 20 years of use.
Now, I have bought 2 blankets for the new boat as my old age has found them rather consoling. SSSH!

We have 800 ah at 24 volts provided by 40 Nife cells that Her Majesty once again (she presumably was sympathetic to my plight) decided her nuclear submarines could manage without, and I discovered them at a ship-breaker's at Lowestoft. I now use a Victron and run everything in the boat on 230 volts and it works well.
Motto, shop around and you can do anything.
 
[ QUOTE ]
You would get about 3 watts out of putting 12v into a 240v electric blanket, I think, assuming the blanket normally produces 60 watts.

[/ QUOTE ]

No, the power is given by the product of the current and the voltage: W = i x V. But the current also depends on the voltage (for a purely resistive load like a heater element): i = V/R. So W = V x V / R, and if you reduce the voltage by a factor of 20 (240/12) then the power will reduce by a factor of 20 x 20 = 400, and you'll only get 60 / 400 = 0.15 watts. Not even enough for a British summer!
 
NiFe batteries.

I often wonder why NiFe batteries are not more used on yachts. Yes, I know they're heavy, but on most cruising yachts the weight would not be unacceptable in view of the advantages. They are rugged, have negligible self-discharge, and they can be fully discharged without destroying them.

Back in 1940 my father built an Anderson shelter in our garden. To provide some lighting he used an old battery (NiFe) taken out of a lorry in 1914. Without further charging that battery provided light in our shelter fo three months before it was exhausted. And after charging it lasted for the rest of the Blitz, and was still usable in 1945. Superb devices, but heavy.
 
I thought I was the only one (crying in the wilderness).
I have 500Ah at 24V, half of them in wooden crates.

How to you manage to charge them? I have had two mains chargers tweaked supposedly for them, but they run out of puff at the top end (I only use one at a time). Now that I have admitted defeat and reduced them to 19 cells per bank, I can fully charge them.

Some of my equipment objects to anything more than 28V, so I have to limit the chargers to 29.2v.

What's your secret??
 
Matt, I dont understand why you would want to run the damn things for 9 hours each night. I thought the whole idea of lecky blankets was just to warm ya pit before ya got in it, and not to remain on whilst you were in it. Indeed it is my understanding that it is not recommended to sleep with one switched on. Aint seen one of these things since i was a kid so maybe technology has moved on in the lecky blanket world. /forums/images/graemlins/ooo.gif

If you do as your suggesting and only put it on for a couple of hours then i cant see it causing any problems with discharging your batts below a critical level.

Thats it from me today folks. Dinner is ready. I know this because the fire alarm has just gone off /forums/images/graemlins/grin.gif
 
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