Electrifying a Wot 4

[Chris Bott - Moderator]

Hi Timbo

I've finally got myself the A8 charger too (a proper charger) after charging A123's with various home made concoctions. Including a £15 laptop psu that gives 10A into 6S.

Proper round can A123's should not be taken higher than 3.6V 
As you go above that voltage the volts rise quite quickly but you don't put any "useable" charge in. Its almost like charging a small capacitor.
A123 say they can be taken to 4.2V but also state that this can reduce life expectancey. (LiFe expectancy? lol).  It's important to know that they are in balance, as the cell going high volts "deprives" the lowest cell from getting to 3.6V if you see what I mean?

It seems that once current has dropped off at 3.6V then all the active ingredient is used up. Overcharging cannot take place and is not useful,.This is a key reason that they are safe.

I'm really keen to see how the square (rectangular) ones perform so keep us informed.

By the way, on my A8, on fastest charge, 7A, into 6S, the charge is ended with a message "connection break" and I dont get the info telling me how much has been put in. This always happens so I guess its a software fault. Does yours do this at all?

[Danny Fenton]

The M1 A123 do not really self discharge. And if these behav like the M1 then I think if you should try on the 4s1p, give it 4 mins of reserved flying and see how much it takes out of them. I think you will find that because they give power right to the end you may have enough in 2000.

Can you do some eagle tree recording when you test fly?

[Tim Mackey]

Hi Guys. Funny you mention the connection break thing! Had it today for the first time when cooking these new cells - I was only set to 4A and it happened twice. After checking all leads and so on for any obvious breaks etc, I realised that I had the charger set to "charge" but had ( through habit ) plugged in the balancer lead also. I figured this may have upset the charger, so tried again with unit set correctly to "balance charge" and all went well.

I have not had this error before, and have charged 2P6S LiPo ( sorry for swearing ) at up to 6A without a problem, which equates to over 150 Watts.

Yes, the eagle tree unit will be fitted in the model after bench testing is complete. The sharp "knee" in the discharge curve is a good and bad I suspect - as I never deliberately fly to LVC, relying instead on noticing the motor revs and power drop off as the pack reaches the end of useful capacity...however I wonder whether there will be sufficient time / capacity remaining with these Lifes once they reach their fall -off point. How have you found this ?

[Danny Fenton]

the fall off is fairly sharp, but i have only deliberatly done it once with wot 4 and it wasn't a problem, the hurricane ran out of puff in the worst possible place, an overshoot on first flight over rough ground, i got lucky with only a loosened bulkhead and bent oleo leg. I like to fly on the timer to be sure, hence we rely on the eagle tree and checking the amounts we put back after flying. This was why I was so annoyed at the eagle tree calibration, it nearly cost me the model.

The watty will glide forever, so just keep it high. certainly never dare take a lipo to flat. Oh don't know if Chris Bott has mentioned it but make sure you set the LVC to be sily low, you don't want that cutting your power at lipo levels

[Tim Mackey]

Yeh already figured the low voltage point ( apparantley down as low as 1V is OK but not recommended) but dont think it will be any point in letting em get that low...2.8V is what I read somewhere, but that cant be right as I get me LiPos down below that under load. I think I shall aim for approx 2V per cell, so on a 4s pack thats 8V although the ESC I plan to use will not let me set that, but the 3s Lipo setting should be OK ( 7.95V ) What figure do you use ?

[Danny Fenton]

Hi Timbo, the smallest lipo cell count it will take, not had a problem. and yes watch the volts drop it is a little higher than lipo, you have seen eagle trees from the Hurri flight I believe?

[Tim Mackey]

Yes I have thanks.... The smallest LiPo setting is 5.30V ( 2 x cells ) which I think is a little low for a 4s pack, so I will stick to the 3s setting. Like you, I always use a timer to decide flight times and landing circuits.

Cheers...off to bed now, more tomorrow.

[Danny Fenton]

I didn't think you slept........... Just plugged yourself into a charger

[Tim Mackey]

Yawn..... see I do sleep !

[Chris Bott - Moderator]

For the LVC setting go as low as you can set it.  The volts stay so constant till empty LVC does not help in any way shape or form. Except if it cuts off while you still have charge left, because you are using so much current that the volts go that low.  By the way, I accidentally took my first pack (yes pack) to zero volts (yes zero) accidentally twice (yse twice) by leaving it on a fat test resistor and fogetting. What an absolute numpty I know! 

However I am still flying with that pack two years later and am still noticing very little difference. This of course is not something anyone would recommend, It just shows that  worrying about where to set LVC really isn't an issue. If you feel power dropping off at all, you are pretty much deadstick. It is almost like a fuel tank running out.

[Chris Bott - Moderator]

Of course we don't know about the "square" cells so we're looking to you to confirm the above. Best of luck and lets hope we get some weather soon.

[Former Member]

[This posting has been removed]

[Tim Mackey]

Well I have just about finished I think - just waiting on the second pack to arrive later this week, and of course for some decent weather. I think I might also modify the firewall with an opening as originally planned to accomodate my 4s LiPo packs, poked through from inside cos they are much longer than the LiFes as I reckon they will give more punch - a small mod and I should then be able to switch between the LiFes and LiPo ( ( remembering of course to reprogram the ESC accordingly ). The pictures show the final design of mount I ended up with - it forms a motor mount, and battery cradle and ESC fixing all in one, and with the lightening holes, adds nothing really. I have fitted a rubber sheet to the cradle floor just to add a little extra insulation protection to the battery cables - dont want 'em chaffing on the metal edge !

Final AUW is 5lb 3 oz which is pretty much what it was when IC ( .52 size engine )

I used the Jeti ESC 'cos I had it spare, but have gone with a 6V UBEC - mounted as can be seen in the airstream.

I will do static testing tomorrow to esatablish best prop ( probably 12 x 8 at a guess ) and do some thrust testing too

I will have the eagle tree running to monitor everything.... so will keep you informed of results.

As usual... a click will give the bigger picture

[Danny Fenton]

Timbo that looks awfully profeshaneal, you must stop that immediately  Looking great can't wait o see figures from these new cells, sad geek that I am

[FlyingAngel]

Good job Timbo! You are very inventive as I can see. The ESC will be out of the fuselage?

[Tim Mackey]

Yep, its staying where you see it in the picture - its only a 40A jobbie and will benefit from the airflow ( having said that, this particular brand is top notch so I wouldnt expect a problem had I chosen to fit it inside.

Figures tomorrow Mr geek

[Danny Fenton]

I am in Exeter tomorrow so wont see your figures until the evening

[Tim Mackey]

It will probably be that long before I get em posted up anyway

[Doug Ireland]

All fascinating stuff but you lost me some time ago! I've no idea what A123's are and an "Eagle Tree" has got me completely stumped!

[Tim Mackey]

Ah, then keep watching Mr Ireland..and prepare to be enlightened

[Andy Sayle]

I've just read through this thread, and i am looking forward to seeing it fly Tim!  You'll have to let me know when it is getting an airing at the field next (Open day on Sunday?).

I've been wondering about something .46 sized, but electric for throwing around recently.  I have a gap in my fleet now I got rid of my IC stuff

Cheers

Andy 

[Tim Mackey]

Just finished the mods to accomodate the 4s LiPo as an alternative to the Lifes - but today has been one of problems

First off I fried my eagle tree logger by careless waving of the battery positive terminal which momentarily touched a pin on the logger and its fried it - dead as a dead thing

Back to the trusty Whattmeter and optical tacho then - first run with the 4s LiPo on a gentle 11 X 7 prop gave a whopping 49A and an overloading 660 watts - quickly shut the throttle down as the motor is only rated to 500 watts and 45A - eeks ! Even allowing for unloading in the air, thats a bit much....especially on a 40A ESC!!

Somewhat annoyingly the data sheet with the motor claims 37A and 460W on the exact same prop and battery - and this is the sort of figure I was aiming for.....so somethings wrong there

Oh well, we will try these supa dupa new LiFePO4 cells then, with their lower voltage - only one pack arrived so far, and my plan is for 2 x parallel ( only 2000m/a each ). Hooked up to the new GT8 charger and got the same error as Chris "connection break" - which there isnt - managed to force a charge in eventually up to 15V ( should have been 14.4 but they are also good to 4V+ if really needed ). Opened the go stick, and voltage dropped immediately to 11.1V which was a bit naff - current showed 34-35 A and around 390 - 400 Watts.

Now admittedly this is 17C or so - although they are supposedly rated at 20 - 30C ! I was disappointed at the voltage depression but kept the tap open to see if they would recover as they warmed slightly.....Nah, stayed around 11V and soon dropped to 10+ after a minute or two at full chat. Varied the throttle abit to let em recover and thay never got past 11V. Considering they are supposed tio have a nominal of around 3V+ then to sag to 11V is not good. Obviously the 2 x parallel packs will hold up better...but thats not the point really.

To get to my target power I may have to go to a 12 X 8 or so, but then the current will be even higher and voltage depression more too - not a happy bunny with motor or battery at the present.

I will have to decide between a slightly larger prop on the LiFes or a slightly smaller one on the LiPOs  - the fast chargeability of the Lifes was a big attraction for this particular model, so it may be that option !

[Chris Bott - Moderator]

Hi Timbo

I was a little worried about your cell count. The round can A123 cells also drop in volts under load but I wouldn't expect quite this much.

On this 6S graph, picking a 35A point shows a voltage of about 17.5V which gives under 3V per cell, but not much under.

Maybe this is why, for 40-50 IC sized models, we tend to use 6S1P with something like a 670kV 600W motor. (which is what this graph is).

Chris

[Chris Bott - Moderator]

Timbo
I'm going to contradict myself w a little here. I've finally managed to overlay real time eagle tree playback onto a video. So you can watch Dannys Hurri fly and see the eagle tree numbers as it flies.

You can spot a 60A moment and check the volts if you are quick, and probably a 35A moment too. It looks like the Cylindrical A123s in this flight dropped a bit further than my guestimation from my graph.

Video graphically proves that they are useful though

Chris

[Tim Mackey]

Now thats cool stuff there Chris ...must let us know how you did that !

I will study it in a little while, but it sounds as though my 11.1V for a 4s pack of these "soft" LiFes is maybe about all to expect - it may of course improve slightly with a few runs under its belt. Shall probably go for a slightly larger prop on the Lifes and get the benefit of the fast charging.