lipo testing

[josh h]

My lipo i had in my 450 went flat about 20 seconds into flight on satutday and fell to floor, only broke the skids so happy, a friend put in on his lipo tester an if i remember rightly said about 7 percent.
It was deffinatly fully charged, its on charge now but can I measure the internal resistance with a multi meter, don't think i can with my cell pro 4

[cjcj1949]

If you've only got one lipo I should buy another. See if it does the same. The internal resistance is really just a way of talking about the drop in voltage on load. If you attach some thinnish wires to your lipo battery say 3 to 5 metres long you can then measure the volts as you spool up the heli. You can work out the current the heli is taking from the legth of the flight. If its 6mins its 10C so you just a 0 to the pack capacity in amp hours. You should see a drop of less than 0.3V giving IR= 0.3(Volts)/20(Amps)=0.015 ohms or 15 milliohms. Usually it is just one cell and it will show up better if you measure individual cells. The thinnish wires don't have to be thin, but its easier if they are. Bell wire might be handy.

You might find that your lipo works better if you warm it up first. 30 Centigrade is a good figure, or you can let it warm up on its own for about 30 seconds in a gentle hover. Leave it a minute for the pack to equalise the temperature and then fly.

[josh h]

ive got another 7 so no big deal, it charged spot on, balanced fine, bit odd, i might just bin it but also tempted to try it again, just hover an see how it goes, just got to wait for some new skids,
was just curios as to why it dropped off so fast really.

cheers

[Cagey]

If that's the only one out of the 7 that does it, there's a little pointer there. Out of curiosity, what brand we talking about here Josh ?

[josh h]

6 are turnigy, other 2 are 1 flight old gens ace

[Cagey]

6 are turnigy, other 2 are 1 flight old gens ace

Gens Ace ? At that price I'd not mess with it, just send it back to them for a refund or replacement.

[chris jones2.4]

if i go at the weekend, ill take a look 4 ya josh

[cjcj1949]

The voltage drop is usually the killer. I can measure the internal resitasnce for you if you want to send it to me. The internal resitance will be measured at 17amps with a short pulse on each cell. Send me an email if you want me to do this. There might be other factors. Internal resistance does double at 10C compared to 20C. Resistances of around 5 to 10 milliohms oer cell will work with shorter fligghts to the same enf of flight voltage. I use 3.6V. If you end of flight valuse are high, around 3.9V or greater then your battery just can't use its capacity before its volts drop below what the motor will handle. There are lots of assumtions there though. I use a cheap 3 -8S monitor that Giantcod and epatchpark sell. It gives you a load alarm when the volts reach any figure you want in flight.

[matty-boy]

I've just got hold of a charger that measures IR. I've connected up some old 6S packs from my 2nd hand Trex 500 and the worst pack measures around 50 Milli ohms per cell. Should I bin this pack immediately and replace? Is there a figure people have in mind before replacing a pack?

Cheers,
Matt.

[cjcj1949]

Different ways of testing will give different results. It depends as well on the capacity of the cell and how you want to use it. I have Tipple packs 2300mah at 5miiliohms that are good for 6 or 7 minute flights that must be giving about 20 amps that I run to 3.6V on load. I have a Turnigy nanopack 2200mah at 10milliohms that works almost as well that will reach 6 minutes when its warm but not when its cold. Your 50 m ohm packs should drop so much volts that the esc will cut out before you can take off. That's if the charger is giving useful figures. I would test your bad pack on the heli and see what happens. You will then be able to work out in advance what will happen.