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Hairdryer to discharge LiPos

Old 12-24-2014, 08:33 AM
  #1  
JetPlaneFlyer
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Default Hairdryer to discharge LiPos

I saw a thead elswhere about using a hairdryer as a resistive load to discharge LiPos for storage. This seemed like great idea so I think I'll give it a try. It could be the ideal thing for those occasions when I have some big 6s LiPos to take down to storage and it has the potential to discharge much faster than other methods like resistors or lamps.

My Charger (icharger 4010 Duo) allows a resistive load to be connected, I'm sure other chargers are capable of this too. The 4010 Duo has the capability to discharge into a suitable resistive load at up to 40v and 40A. So that's a whoping 1600W of discharge power This gives the potential to discharge even big 6s batteries in a few minutes rather than in hours.

The big advantage of using the charger to manage the discharge rather than just connecting the LiPo direct to the hairdryer is that the charger should discharge any cell count battery without having to adjust the resistance of the load, plus it will automatically cut-off once the optimum storage voltage is reached without any risk of over discharge.

I picked up a 2000W Revlon hairdryer for 9 (about $15) which I hope to be able to modify. More to follow along with photos.....
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Old 12-24-2014, 09:12 AM
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Can't you just wire the Wife up !!



Cheers
Nigel
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Old 12-24-2014, 09:20 AM
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Originally Posted by solentlife View Post
Can't you just wire the Wife up !!



Cheers
Nigel
I did look at 're-purposing' her hairdryer, but it was only 1200W so not ideal (plus I'd have been in the dog house for weeks).
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Old 12-24-2014, 07:19 PM
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I thought discharging LiPos would ruin them. I keep mine topped up all the time.
This past week - I needed emergency lighting and hooked up a 12V lamp to a 3S Lipo
I had. It eventually ran the LiPo totally down and the battery pack puffed out - so much
so that it does not take or hold a charge anymore.

Gerry
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Old 12-24-2014, 07:36 PM
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Here is the hairdryer fresh out of it's box and about to get ripped apart:


This is what's inside:


This is the business end of the dryer, the heating element and the fan:


First all the wiring needs to be trimmed away as none of it is needed:


It's critical that the resistance of the unit as about 1 Ohm, this is to make the most of the 40v and 40A rating of the chargers discharge circuit. Too low resistance means the amps will be high but the voltage would stay low. Too high a resistance and the situation is reversed.

To get the correct resistance I took a couple of lengths of 14AWG wire and stripped back the insulation from 6" or so. The bare wire is then braided along the length of the heating element frame, wrapping around each of the resistor wires. Soldering is out of the question as the heat would melt solder.
It's not especially neat but will do the job:

On the opposite side of the frame the opposing wire is braided in the same way, looking end on the wires are in the 12 and 6 o'clock positions. I was in luck and the resistance came out spot on 1 Ohm. Other models of hairdryer would need most likely treating differently to get the correct resistance, the design of this one turns out to be ideal for the job.

To power the fan the voltage has to be reduced down otherwise with a 40 v input the motor would burn out. To do this i tapped the motor positive wire into the resistor wire element one third was between the positive and negative wires. This seemed to give a fan speed that sounded 'about right'. It's easy to adjust speed by altering the position that the motor wire connects to the coil.

This is what it looks like all back together, handle was cut off as it's no longer needed.


I tested it out on my iCharger and it works great. Discharge power is actually limited not by the hairdryer but by the 40A limit on the battery side. It easily discharged a 6s battery at 40 Amps, this was about 1000W load on the hairdryer, which it coped with no problem at all.

So the experiment is a complete success, i don't think any other discharger design comes anywhere close for sheer brute power. It is quite noisy of course but at least the noise doesn't last long. The only thing that would improve the discharger is some sort of stand rather than just laying it on it's side, but that's just aesthetics.
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Old 12-24-2014, 08:38 PM
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I guess "necessity is the mother of invention"......seems like if you've got time to kill and a spare dryer, have at it.......as for me, my lipo charger works quite well, fairly quick and the screen monitor helps with a much safer monitoring aspect........
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Old 12-24-2014, 08:47 PM
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The internal discharger in the iCharger 4010 Duo is the most powerful of any charger available (200W total or 130W per channel). But even so if i end up with a few 5000mAh 6s do take down to storage then it's still rather slow. The hairdryer is almost 10x faster plus it doesn't cause the charger to get hot like using the internal discharger does.

The hair dryer is controlled by the 'regenerative discharge' function on the iCharger, so it is safe. It stops when it reaches target storage voltage, just like if using the charger's internal discharger.
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Old 12-24-2014, 09:57 PM
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The heating element in the hair drier is a RESISTOR. Problem is the resistor is designed for 120 volts. So if the wattage is, then Resistance is::

1500 watts R = 9.6 Ohms
1200 watts R = 12.0 Ohms

So no chance of burning the heating element up, but not much of a load at your battery voltages. For example if the pack is 3S is only going to draw .925 amps @ 10.2 watts up to 1.15 amps @ 12.7 watts.

To be a bit more useful you need to get down to 1 to 2 Ohms @ 100 watts. I use two different things. I have a 1 Ohm 200 watt resistor I use on my 3S packs and a car 110 watt car head light. Takes no more than a few minutes.

Hair dryer will work, but it is going to take a while depending on what voltage your batteries are.
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Old 12-24-2014, 10:57 PM
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Originally Posted by gerrynj View Post
I thought discharging LiPos would ruin them. I keep mine topped up all the time.

Gerry
Actually, keeping them fully charged, is one of the things that kills them. Most of the LiPo manufactures talk about this and how important it is to keep them at storage charge levels...

Things that kill our LiPo's before their time:
  • Over-discharge
  • Keeping them stored while fully charged
  • Heat
  • High discharge rates
  • Non-use

Mike
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Old 12-25-2014, 09:29 AM
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Originally Posted by dereckbc View Post
The heating element in the hair drier is a RESISTOR. Problem is the resistor is designed for 120 volts. So if the wattage is, then Resistance is::

1500 watts R = 9.6 Ohms
1200 watts R = 12.0 Ohms
If you read my previous post you will see that's why you need to connect them up differently. The input and output wire is braided along the length of the frame rather than an input at one end and output at the other and would be the case for 120 or 240V AC input.

After re-wiring the resistance of my unit was 1 Ohm. I've tested and it allows me to discharge at 32V and 32A for 1000W of discharge power... no way you would get this with a wire wound resistor or any sensible number of lamps! It would do more power but the limit I'm hitting is the discharge channel on the battery side of the charger which has a 40A limit and my batteries are 6s which means about 1000W out of the batteries.

Last edited by JetPlaneFlyer; 12-25-2014 at 11:13 AM.
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Old 12-25-2014, 09:35 AM
  #11  
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Originally Posted by gerrynj View Post
I thought discharging LiPos would ruin them. I keep mine topped up all the time.
This past week - I needed emergency lighting and hooked up a 12V lamp to a 3S Lipo
I had. It eventually ran the LiPo totally down and the battery pack puffed out - so much
so that it does not take or hold a charge anymore.

Gerry
No, as per previous post, keeping LiPos fully charged when stored is very bad for them and will shorten their life considerably. But you are right that fully discharging a LiPo below 3v per cell will kill them.

When storing a LiPo its best to keep them in half charged state. Something in the 3.75-3.95v range is good. All decent chargers have a 'storage' function that discharges (or charges) a LiPo to about 3.85v.

The idea with this hairdryer discharger is to make discharging down to 3.85v much quicker. It would only be used to fully discharge a battery if the battery was at the end of it's life and being disposed of.
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Old 12-26-2014, 07:56 AM
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Nicely done JPF!!
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Old 12-26-2014, 07:18 PM
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I used the discharger for the first time in anger today. I was out flying my helis, it was very cold and i lost feeling in my digits before getting all the packs flown. i was left with two 6s 5000mAh and two 6s 3000mAh packs at full charge.

For comparison an Accucell 6 (5W discharge)would take at least 36.8 hours to get these down to storage. My iCharger 4010 Duo using the internal 200W discharger would take over an hour.

Using the new hairdryer discharger they were down to storage voltage in about 15 minutes! The charger itself also gets an easier time as it doesn't get hot like it does when discharging internally. The hairdryer warmed the room up a bit too
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Old 12-26-2014, 07:51 PM
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Originally Posted by JetPlaneFlyer View Post
After re-wiring the resistance of my unit was 1 Ohm. I've tested and it allows me to discharge at 32V and 32A for 1000W of discharge power...
Not saying that will not work, but safety comes to my mind. The nichrome wire made for that hair dryer is only rated for around 10 amps max. I would worry about the wire fusing and catching something on fire.

Originally Posted by JetPlaneFlyer View Post
no way you would get this with a wire wound resistor or any sensible number of lamps!
I would not say that, but at your price point hard to beat. I have a battery load box used at work to test battery capacity can easily do that. But more to my point I have another expensive hobby, racing electric golf carts and DIY Electric Vehicles. They make very high power resistors for Dynamic Braking up to 65 Kw like those from Vishay and others
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Old 12-26-2014, 08:39 PM
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Originally Posted by dereckbc View Post
Not saying that will not work, but safety comes to my mind. The nichrome wire made for that hair dryer is only rated for around 10 amps max. I would worry about the wire fusing and catching something on fire.
But due to the way that the dryer is re-wired the current is split between 14 resistor wires, 7 resistor wires each side of the frame. That means that even at 40A discharge rate each wire is only seeing on average about 3 Amps. The wires don't even get as hot as they would in regular hairdryer duty.

In any case the elements are contained in a ceramic frame and ceramic surround. If the nichrome wires burn out all current stops flowing, no harm done. Of course like any LiPo charging or discharging you should not leave it unattended. At least 'not leaving unattended' is realistic because it's all done in 15 minutes. So on that basis I'd argue that's it's safer than other forms of discharger which could take hours to do the job.


Originally Posted by dereckbc View Post
I would not say that, but at your price point hard to beat. I have a battery load box used at work to test battery capacity can easily do that. .....
Fair point, but as you say, at the price point and considering practicality and physical size nothing that i know of comes remotely close.
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Old 12-26-2014, 09:56 PM
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Originally Posted by JetPlaneFlyer View Post
But due to the way that the dryer is re-wired
My bad did not catch that part.
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Old 12-26-2014, 11:41 PM
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I must be missing all the fun. I never discharge batts , plus my hair is still wet.
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Old 12-27-2014, 01:36 AM
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Originally Posted by JetPlaneFlyer View Post
Fair point, but as you say, at the price point and considering practicality and physical size nothing that i know of comes remotely close.
Agreed:

You can buy 1000 Watt one ohm resistors, at a far higher price: Digikey is down to their last 130 of them, so buy one soon

http://www.digikey.com/product-detai...574-ND/2367679

This power resistor would be rated at 31 Amps continuous duty.
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Old 12-27-2014, 02:13 AM
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Originally Posted by kyleservicetech View Post
Agreed:

You can buy 1000 Watt one ohm resistors, at a far higher price: Digikey is down to their last 130 of them, so buy one soon

http://www.digikey.com/product-detai...574-ND/2367679

This power resistor would be rated at 31 Amps continuous duty.
And then you would have to build a fireproof container or it would set your bench on fire! It might also unsolder the connections if you didnt have a fan running in it full time
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Old 12-27-2014, 02:27 AM
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Originally Posted by Larry3215 View Post
And then you would have to build a fireproof container or it would set your bench on fire! It might also unsolder the connections if you didnt have a fan running in it full time

LOL
These power resistors are designed to handle their rated power with normal convection air cooling, fans are not required. This thing is massive, at 2.3 inches in diameter, and 11.5 inches long.

Power resistors do run hot at rated power though. Touching one at full rated power will brand your finger tips.

(I've got a few power resistors with even higher wattage ratings)
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Old 12-31-2014, 04:35 AM
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Originally Posted by solentlife View Post
Can't you just wire the Wife up !!



Cheers
Nigel
You have no idea of the picture that put into my head!

(or maybe you do... )
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Old 12-31-2014, 06:43 AM
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I built my discharger using the headlight bulb style and connectors / LVA etc. Works a treat ... but this hairdryer jobbie ... mmmm interesting ... using the same Nichrome wire basically that is in electric fires and the added fan cooling.

I have a bag full of wire-wound resistors at hime - bought to replace the bulbs on my discharger. This hairdryer idea has made me wonder about using a computer fan fitted to a case of my resistors in a discharger ....

Nigel
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Old 12-31-2014, 06:44 AM
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Originally Posted by FlyWheel View Post
You have no idea of the picture that put into my head!

(or maybe you do... )
I discounted the idea of 'wiring up the wife' in the end ... too much noise and mess !

Nigel
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Old 01-02-2015, 01:18 AM
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kyleservicetech
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Originally Posted by solentlife View Post
I built my discharger using the headlight bulb style and connectors / LVA etc. Works a treat ... but this hairdryer jobbie ... mmmm interesting ... using the same Nichrome wire basically that is in electric fires and the added fan cooling.

I have a bag full of wire-wound resistors at hime - bought to replace the bulbs on my discharger. This hairdryer idea has made me wonder about using a computer fan fitted to a case of my resistors in a discharger ....

Nigel
Adding a decent fan to a resistor bank will notably increase its ability to absorb more watts.
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Old 01-02-2015, 03:06 AM
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Dennis, I have another expensive hobby, EV's. Nothing big or too fancy, even a little silly, racing golf cart. Well I could race it as not many golf carts can do what mine will like go 50 mph. May not sound fast to you, but trust me - to 50 mph in 5 seconds in a golf cart will scare the pi$$ out of you.

My point here is the cart uses lithium batteries, AC motor, controller just like planes. Just on a whole lot larger scale. Also a professional engineer who has worked with battery plants all my professional life.

What rate are you discharging the batteries? I would be a little concerned going more than 1 to 2C. Not because of fear of fire or safety, but battery life.

Granted these batteries are made to discharge at high rates does not mean it is good for them.
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