Dynamo setup – can I buy or make anything that will output 40w?

marada

New Member
I will be spending a lot of time cycling very remote areas in Russia next year. I plan to go well off the beaten track where I will mainly camp and avoid towns. I want to take my laptop with me for blogging, pictures etc and wondered if there was any dynamo capable of charging it while on the bike? Most of the dynamos I have seen are only suitable for charging phones etc due to their low output. My laptop is an Asus Ul30 which is only 13 inches and has a huge 12 hour real life battery time. I notice on my AC charger it says: output 19v 2.1a. So this is about 40w which seems quite low as laptops go. Obviously 40w will power and charge the laptop in real time, so if I just wanted to charge the laptop when it was off over a few hours I think I would need far less power. Does anyone know what sort or output I would need from the dynamo, or if this is realistic? I am aware that you can get some 12 volt dynamos, so I was wondering if something like this would work with say a car charger to charge my laptop? I know I could go solar, however It really is not that efficient for higher powered devices, unless you have a huge solar blanket spread out. Obviously that is not an option when I am on the bike all day. Also in the winter you can’t always reply on solar, especially in Russia.


What do people suggest?
 

battered

Guru
40W is pushing your luck.

Having said that, you don't use your lappy constantly, do you? On that basis if you want an hour a day at 40W you need to add 40W for an hour or 4W for 10 hours, or 8W for 5 hours. 8W is doable.

Factor in that battery charging is only about 70% efficient so you *really* need 12W for 5 hours to run a lappy of 40W for 1 hour. That's feasible though.
 
OP
M

marada

New Member
Thanks for your reply battered
I just examined the laptop battery and it says 84 watt hour. As the laptop lasts around 10 hours before needing to charge, that means the average hourly consumption is only 8.4 watts. Does that mean I only need 8.4 watts per hour to charge the battery for each hour? I am a little confused as the power adapter is 40w, and obviously based on battered's figures I need to do a lot of peddling to charge for one hour.
 

zoxed

Senior Member
marada said:
... Does that mean I only need 8.4 watts per hour to charge the battery for each hour? I am a little confused as the power adapter is 40w, and obviously based on battered's figures I need to do a lot of peddling to charge for one hour.
I do not know the efficiency of laptop batteries, but I would guess you have to put a *lot* more than 8.4W in to get 8.4W out.
And I guess the 40W is the *max* power from that adapter, and is the same one supplied with a range of laptops, not just your, lower power, one.
 
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marada

New Member
zoxed said:
I do not know the efficiency of laptop batteries, but I would guess you have to put a *lot* more than 8.4W in to get 8.4W out.
And I guess the 40W is the *max* power from that adapter, and is the same one supplied with a range of laptops, not just your, lower power, one.
Anyone got any ideas how much juice I would have to put in?
 

arallsopp

Post of The Year 2009 winner
Location
Bromley, Kent
marada said:
Does that mean I only need 8.4 watts per hour to charge the battery for each hour
I doubt the battery and charging circuits are 100% efficient, and suspect you'll need to put 30% on top of whatever you take out. Also, if the laptop is expecting DC and you're supplying AC (or vice versa) all bets are off.

You might find the charging circuits won't event activate below a certain voltage. If you can get your hands on a multi function switchable AC power supply, you can check the cut off before hand, but otherwise assume it won't do anything below 9V 600mA.

The charge rate is not linear, and will almost certainly vary based upon the combination of voltage and current you can supply.

Best bet is to go googling for the OLPC project, and see if anyone's trying to power one of them with a dynamo / crank.
 

battered

Guru
Don't confuse watts (W), a measure of power, with watt-hours which is a measure of the *energy* in a battery.

The figure of 70% effciency for charging comes from an engineer I know who designed battery/charging circuits, so you can use that as a rule of thumb. In other words you have to put 100 in to get 70 out.

In terms of your *average* power usage of 8.4W, that's a reasonable assumption. Your battery capacity (total enegry stored) of 84 Wh can be used or replenished at any rate you please. You can have it running standby at 0.84W (say) for 100 hours, 8.4W for 10 hours or working flat out at 84W for 1 hour, assuming it draws these currents. Any figure between these figures will work too. (In practice higher power usage will flatten the battery quicker than you would hope as it derates, but we can gloss over that for now). Charging the thing is just the same as discharging it. Neglecting the 70% figure for now, you can charge it fully from totally flat at 8.4 watts for 10 hours, 0.84W/100h, 1W/84h, 2W/42h, 4W/21h, etc. This isn't important though, you don't need to go from flat to full. All you need is a topup. Consequently if you want 2 hours a day usage at 8.4W that's an *energy*use of 16.8 Wh and you can put that back anyhow you please. If you can deliver 20W charging then 20W x 70% = 14W, and you can make up 16Wh in little more than 1 hour. Half that work rate will take twice as long. It's like filling up a tank of water with a bucket, you can go like hell for short periods or take it easy for longer.
 

TheDoctor

Europe Endless
Moderator
Location
Stevenage
I notice you want to use the laptop for blogging.
If it's so remote that you can't get to a power socket, you may struggle to get an internet connection, no?
 
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marada

New Member
battered - Thanks for your clarification. So my next question is what dynamo on the market has the greatest efficiency for this task? Is there anything that will give me around 12 watts per hour? I have seen a few 12 volt dynamos around, so this would be very perfect if I could get a 12 volt dynamo that would output around 12 watt per hour. That way I could just hook up my laptop car charger to the 12 volt, this would convert to 19 volts which the laptop uses to charge. Any ideas?
 

andrew_s

Guru
Location
Gloucester
Back when dynamo lights were simple bulbs you could work out what the power pro9duction was quite easily.
with 1 3W bulb, you got full brightness at 6-7mph
with 2 3W bulbs you got full brightness at 12-14mph

So with a simple bike dynamo you would want to ride at 25mph or thereabouts to get 12W of electric power.
A good hub dynamo is about 60% efficient, so that's about an extra 20W of pedalling effort to put in over the top of what you would normally need to do 25mph.

So a simple dynamo is no good. You'll need some electrickery to get more power at lower speeds.

I'd suggest looking at the B+M E-Werk. This is quoted as giving up to 13.3V and up to 1.5A (provided you are going fast enough).
 
Location
Edinburgh
marada said:
... I want to take my laptop with me for blogging, pictures etc and wondered if there was any dynamo capable of charging it while on the bike? Most of the dynamos I have seen are only suitable for charging phones etc due to their low output. ...
Maybe I am missing something here, but do you really need to take a laptop? If you took a smart phone like the iPhone or the Google one you would be able to access the internet/blog, take photo's, etc. This would be a lot easier to charge off the dynamo.
 

mark barker

New Member
Location
Swindon, Wilts
TheDoctor said:
I notice you want to use the laptop for blogging.
If it's so remote that you can't get to a power socket, you may struggle to get an internet connection, no?
This is very true. I've driven through Russia a couple of times in the past, and outside of the cities & large towns the phone signal is dire. The second time I went I took a satellite phone, which worked fine but cost a fortune!

If you're just planning on blogging etc I'd forget the laptop, get a smart phone and use that when you get some signal. It'd be much easier to keep charged, much less likely to be stolen (Russia isn't the best place to be with shiny things:sad:) and one less thing to have to fill out on the customs forms!
 
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marada

New Member
I really hate using a smart phone for internet stuff. I can type at over 100 words per minute on a keyboard; however typing on a smart phone while squinting at the screen is just a joke. Also if I am uploading pictures and updating my own website with code etc I really don't want to mess about doing this on a tiny Smartphone. Am I the only one who does not get the iphone hype?
 
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M

marada

New Member
I know about the B+M E-Werk. The problem is most laptops charge at 19v, but the E Werk only puts out 13.3v. It still does not solve the problem of finding the most efficent dynamo that will put out the most power with the least amount of work. I was looking at the Busch and Muller Dymotec S12. This puts out 12v 6w. it is shown at the below link:

http://www.dotbike.com/productsP3027.aspx

Would this be the best thing to go for?
 
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