IOMICA Forum

Hi, there everyone,

I've just discovered how the price of Lithium Polymer batteries and chargers have dropped recently. As you probably know, they are used for electric flight since they are light and deliver a high current. I am concerned about using them since lithium is a very reactive metal which reacts rapidly with water giving off hydrogen. Manufacturers plaster the cells with dire warnings of widespread devastation if they are abused. I don't want any model of mine to explode!

Another possible problem is that each cell has a high voltage of 3.7 so two in series would give 7.4V. Is this a disadvantage?

I don't recall the specific source, but I inquired of this some time ago and the response was that the battery was completly sealed and could be immersed.

Personally, I would mount them a bit higher than the bildge so as to keep them from getting immersed.

But as I see it, the only reason to use these batteries was if your boat was quite a bit overweight. Most boats are under 4 kg and need batteries to bring them up to the 4 kg minimum.

Jamestj wrote:Hi, there everyone,
Another possible problem is that each cell has a high voltage of 3.7 so two in series would give 7.4V. Is this a disadvantage?

the winches that connect straigth to the battery (eg. RMG) should have no problem. The winch just goes faster !

With a winch connected directly to your Rx you should check that the Rx BEC circuit can handle the higher voltage.

btw. does anyone know why some li-polys have two different sets of leads. one for charging and one for discharging (I'm looking at the towerhobbies list of batteries)

EDIT: before going out and buying lipo batteries and chargers I would take a look at this....
http://www.rcgroups.com/forums/showthread.php?t=209187
scary stuff.

to charge each element individually, with perfect control of voltage applied on it ?????

Awallin

Many thanks for the link. It was most ..... interesting. However, the aircraft guys do give their equipment quite a hammering and use lots of cells in series with the associated problems with overheating of the innermost cells.

We only need two cells to drive a sail winch so they will be less likely to overheat.

If I were to use them, then I would charge them well away from the car or buildings and fit a micro switch to cut the power if a cell started to balloon.

Its comforting to know that our sport is taken by the side of a lake with plenty of water to put out burning batteries!

Jamestj, just a quick thought on that. Water is no great help if Lithium is on fire

I have had a bit of a look at some of the micro RC helicopters who are also switching to Lipos. Besides the charging and loads on the batteries I understand that you also have to monitor how deep you discharge these cells. That is why a growing number of packs now come with a LED to check the charge level.

From what I have read and heard so far I am not going to be using these in a long time as they seem to dangerous for use in our boats where you can not really keep an eye on them like you seem to have to.

Oh dear!

I have just spent the pocket money on a lithium polymer battery 7.4V 800mahr.

I took advice and the expert used these cells for electric flight. He had never had any problems with them and they have no memory effect and do not self-discharge. They are dead easy to charge - just keep the current below the manufacturers maximum and do not exceed 8.4V.

the site that Anders mentioned said something about throwing them into salty water if they balloon out but the instructions which came with the battery said "Keep away from water"

I'll have to experiment a bit cos they are so small and light. I don't feel too threatened by the wee beastie since I once dropped my cell phone down the loo and that didn't explode.

I could live with that bit about not letting them discharge below 6V because I could change the battery between races as we do when racing cars.

I tried a car setup but the winch moved quicker in one direction than the other and there was a pause while it thought about it. Obvious, really cos thats what cars do when you need to reverse. I did not think this one through.

Presumably I don't want a speed controller on a yacht but something is needed to drop the 7.4 from the battery so that it does not wreck the radio.

Can anyone point me in the right direction?

Thanks.

My understanding from talking to some lithium battery suppliers is that the packs are designed for use for electric motors, not to power servos and receivers. Apparently, lithium packs like a constant current draw. I was told that in order to use them for receivers and servos you need to buy a voltage regulator. Check with the manufactuer that sold you the battery and charger. And I also have heard the rumors that if they get wet they can catch on fire.

Hi RoyL

I've done it now so if boat blows up I'll tell everyone here.
I've ordered a lithium polymer battery charger chip from Radiospares and if the battery explodes then I'll have to go back to the drawing board!

Here is a rough idea of the energy in watt hours stored per kg

lead acid, 40
Lithium polymer, 170

My calculatios show that I am on a tight weight budget. As you can see, lithium polymer batteries do have a large energy stored for a small weight so I would like to see if this battery is suitable

It's only a small battery so it will only be a small explosion!

If you have a dmaged lithium polymer pack the procedure to render them inert is to place them in salt water and open the case. The salt water will prevent the reaction from occuring.

Most likely you won't have a problem. But Lipo's aren't to be taken lightly. Then can and have cuased very serious injuries and loss of property (houses). I'm using Lipos in an aircraft and store the batteries in a high temp ceramic vault when not in flight.

Rob

RoyL wrote: ...Apparently, lithium packs like a constant current draw. I was told that in order to use them for receivers and servos you need to buy a voltage regulator. ....

Hey RoyL
One Servo is an electric motor. Digital servos are working all the time to KEEP INFORMATION AT THE POINT TRANSMITTER SEND. Because of this is wellknown that they consume a lot of energy. For receiver you have to check max volt and have a voltage regulator as you said
XMAS
JohnM

Keep this subject alive. I think this is the future as prices on Li-Po batteries, charges and so on are dropping as consumers growing

Nigel

Jamestj, just a quick thought on that. Water is no great help if Lithium is on fire

This might be a stupid post but ... for when... Solar Energy ???


I don't think anyone sail at night!!!

Hi Signatus

Unfortunately solar cells aren't good at delivering high currents in bursts. Some sort of energy storage is needed and batteries are usually used. I have thought of using a solar array to charge the batteries between races since carrying a lead acid motorcycle battery around to charge batteries would stretch my arms.

Thanks for the thought, though.


Hi RoyL

Lithium polymer batteries are now being used for electric aerobatic planes and are giving petrol engines a run for their money. I would have thought that powering servos should not be a problem. A capacitor in parallel might be needed, but I will tell everyone here when I have performed experiments. This is a new technology which is improving rapidly.

I'll keep you informed.


Hi Rob

Thanks for the tip. Do you know the reaction which neutralises the reactants?

Many thanks.

happy new year guys! here's some interesting info on LiPo batteries

http://www.rcgroups.com/forums/showthre ... did=151687

extra care must be taken when charging or you'll say goodness gracious great balls of fire


The video can be seen at http://rchomepage.com/~dna/LipoFire.wmv

happy new year
ed

Very interesting pictures Ed.

It is gratifying that lipo cells have to be severely mistreated before they explode - but what an explosion! Don't you think that his is bound to be a consequence of having such a high energy density? Even the same energy stored chemically in say, petrol, would have produced a similar explosion.

I am undaunted by the tales of doom and I intend to go the lipo route . If it does not work, this forum should, I hope, be the first to hear.

I have ordered a Graupener Regatta sail winch which should run at 8.4V . I understand that this winch is a little slow so a higher voltage will speed it up somewhat! I understand that this winch has a battery elimination circuit for running the radio and steering servo. I see from Marine Modelling International January 2005 which published the IOM Class European Championships that the third placed boat had this winch, the second placed boat had a Whirlwind which is no longer manufactured. The winch in the winning boat was not disclosed. Anyone help on this?

I already have a dedicated chip which is used to charge lipos. I am in the process of breadboarding a circuit which should charge a two cell lipo battery. If it works I will publish my results.

If anyone out there has done this already, please let me know - not much point in 're-inventing the wheel'

Happy 2005 everyone!

Jamestj wrote: I see from Marine Modelling International January 2005 which published the IOM Class European Championships that the third placed boat had this winch, the second placed boat had a Whirlwind which is no longer manufactured. The winch in the winning boat was not disclosed. Anyone help on this?

Graham used a Hitec digital arm servo with a ca 100mm wheel mounted off center on the servo, see
http://www.onemetre.net/Reports/US2004/US2004.htm

I think hitec has discontinued the model(hs-5745mg) that most people used as an arm winch and the "robot servo" (hsr-5995tg) with smaller size and weight but better performace has replaced it !

Many thanks, Anders.

That is a most ingenious arangement in Grahams boat. It would be interesting to see if similar advantages could be engineered in a drum winch arangement - without resorting to expensive machining.


I am having problems soldering the lipo charging chip. The pins are too fine for me to solder to. Lipo chargers are advertised in RC plane mags such as "Fly RC" from www.magazinecafe.co.uk

Hey Jamestj

As Anders said, Servo Hitec (ultimate hobby robotics servo - in their words) HSR 5995TG (Titanium Gear) is now on the run. Weights only 62gr rotates 180º and have a power supply from 6.0 to 7.5 volts, with a torque of 24/30Kg (HS 5735MG has a torque of 19kg at 6.0Volts and HS 5745MG only 18Kg at 6.0Volts). We remember Whirlwind winch that weights +-85gr and has a sufficient torque for IOM and even Marblehead Class boats. You may see this at my page http://rcsailor.tripod.com/what_s_on_th ... arm_servos
or at Hitec web site
As I am interested in everything regarding Li-po batteries I asked Multiplex “if I may use Li-Po batteries with servo Hitec HS 5745MG and they answered me – NO
They don’t gave me any more explanation, but that’s it.
I will try to find their explanation with another mail

Thanks
JohnM

Many thanks JohnM

Your help is most appreciated