7.4V battery reading 8.6V

[jayhoskins]

Hey guys, just started noticing that when I charge the battery for my cx2 the red and green light has started blinking. I checked the voltage and it's reading 8.6 volts. Tried it out and it flew for a while but when I started having to use more throttle to hover I took it out and it's reading 7.3V. In the book it says you can use it as long as it doesn't go below 6 volts... Any suggestions why It's overcharging and losing power at 7 volts??

[miami6]

it sound like the cells are out of balance

[jayhoskins]

I used the supplied charger which is suppose to balance them... If you put the battery back on before it's low could it overcharge?

[jayhoskins]

I tried the battery tonight and it worked for a bit then I started losing altitiude so I stopped, took the batt out and checked the voltage, it read 7.2 volts. I decided to let it cool down and try it again. I tried it again later and it flew great for a while then the same thing, losing altitude. Took the batt out and this time it read 6.6 volts. I was reading on a website about a new feature on the cx2's here is the quote:
"The newer version of the Blade Cx2 also comes with a aluminum head/hub and over-current protection to protect the ESC and motors in the event of a blade strike with PTC resettable fuse devices. Positive Temperature Coefficient (PTC) thermistors work by heat, as it heats up, its electrical resistance increases and eventually the resistance of the device is so high that the current through the circuit virtually stops flowing, once the PTC is cooled normal operation returns. "
My CX2 came with the aluminum head/hub, could the motors just be getting hot and this PTC be making the bird stop flying?? If so should I get the heatsink, or upgrade motors?? I dont' know!

[carlo_the_wonder_frog]

When it starts losing power, thats your sign that the battery is DONE. Flying it until it reads 6v is KILLING that LiPo. A fully charged 2 cell LiPo will read 8.2-8.6 v (8.4 nom) and a discharged one will read 7.0-7.4v (7.2 nom) a dead lipo that won't take a charge or will only last a few flights will read 6.6v or less. Good Luck.

[jayhoskins]

Thanks for the info, I thought because it said 7.4 volt on the battery that it meant that was the fully charged rating. Still learning here I guess. I've ordered a good charger, balancer and some new batteries so now i'll know what to expect.

[rds0811]

Sorry to dig up a slightly old thread, but I was just having these thoughts when I put a voltmeter on a used batt and then my other, fully charged, batt. The used one was about 7.3V and had reduced flight power (i.e. I stopped flying as soon as I noticed a power drop), and the charged one was 8.5V. How do these values tie into the rated voltage of the batteries (7.4V)?

Note: My new CX2 has the resettable fuses built-in. What surprised me is that, being thermally activated devices, the fuses are crammed flush against the motors. Considering how hot these motors run, I wonder if the heat from the motors causes the fuses to limit current (as opposed to the heat generated by an overcurrent through the fuses). This could give false indication of the end of flight time. It doesn't make a lot of difference for me, since my new 180SE's just showed up, and I bought the separate fuses. The new fuses won't be in contact with the motors anymore.

[TowPilot]

I used the supplied charger which is suppose to balance them... If you put the battery back on before it's low could it overcharge?

It shouldn't overcharge the battery pack. It is supposed to balance and stop charging when full.

When you get your new battery charger and balancer, you will be easily able to see the individual cell voltages.

It is difficult to use a VOM to measure cell voltages unless you make up a plug that fits the balance plug on the packs.

The stock stuff works well enough but if you are in the hobby for the long run, better charging and balancing is simply one of the toolsets you will acquire along the way. And this better equipment will benefit the first helis you bought like the CX2.

To be able to check the individual cell voltages, a jack is needed to give easy access to the balance plug on the pack for the voltmeter.

And then you could manually balance the two cell pack by draining the higher voltage cell through a resistor until the higher voltage cell matches the lower cell voltage. Balancers usually drain the high voltage cells at 150 mAh or 300 mAh for higher capacity cells.



Or an external balancer like the Cell Pro by Common Sense RC (http://www.commonsenserc.com/product_info.php?cPath=38_29&products_id=549)
The stock CX2 charger is inexpensive but that matches the package as a whole. It works well enough for the price.

A $200 Triton charger and a $40 Common Sense RC balancer is better but then that exceeds the cost of the entire CX2 package as just one example of a good charger and a balancer.

An inexpensive cure is to buy just a balancer. You can use the balancer to check the cell voltage difference, add the two cells to find the pack voltage. Time the flights so that you stop a minute before detecting you have lost power. About halfway into the charge on teh stock charger, you can pull the pack off and put it on the balancer to see if the stock charger is balancing the pack. If there is an imbalance, let the external balancer fix it and then put the pack back on the stock charger. Check the pack after the charge and let the external balancer tweak the pack if necessary.

It is better if you have a charger that tells how much was put back into the pack. For instance, it is good not to have to put more than 640 mAh back into a pack with a capacity of 800 mAh. This means the pack is not being used past 80% of the capacity and is easier on the pack.

Without spending a bunch of money now, I'd just time the flights, and end early. How much does someone want to spend to get the last 30 seconds from a $25 2S pack if you have no other electric models to use the chargers and balancers on, that is up to the person of course.

I bought a nice charger and balancer because I'll now have $100 packs to care for.