badaml

iTrader: (132)

The individual cell and overall pack voltage are not the same, is it a lipo issue or a charger issue? As an example, on a 2s lipo, I will get a reading of 3.95v per cell, but overall pack voltage will read 8.01v. I have noticed that the overall pack voltage will start to drop when nearing capacity (full charge), even though the individual cell voltage will stay the same.

Also, what would be the more accurate reading, should I go by overall pack voltage to stop charging or individual cell voltage? Thanks in advance for any help/assistance!

Starlite033

iTrader: (10)

I will try and answer your questions but if you don't mind me saying so, I think you should to read up on your batteries.

Here's a good place to start:

http://www.rchelicopterfun.com/rc-lipo-batteries.html

In the mean time, I will answer your question. The discrepancy you are seeing between cell and battery (pack) voltage is normal. Every lipo cell in the world has a nominal voltage of 3.7V (Depending on charge it will range from <3.0V to 4.2V). The 2S in a 2S battery means that the cells are connected with 2 cells in SERIES (3S would be 3 cells in series etc, etc...), this means their votages are added together.

Read more on Series and Parallel connections here

http://batteryuniversity.com/learn/a...configurations

Now if you looked at your individual cell voltages and found that 1 was at 3.4V and the other was at 3.9V, that would be a issue. That means you need to balance your packs.

This brings me to the issue of Charging, do you have a LiPo charger? If not please get one, they charge in a very different manner than NiMH's and NiCads. The simple solution is to find a reliable LiPo Balance Charger that will balance your individual cells while it charges your LiPos. The dangers of improperly charging LiPo's are well documented.

Star

badaml

iTrader: (132)

Yeah, I know that and you didnt really answer my question. Since EACH cell is at 3.95, in a 2s lipo, total pack voltage should be at 7.9, but what I am seeing is 8.10 - 8.25, which obviously doesnt add up. If each cell is at 4.20 on a full charge, total pack voltage should be at 8.40, but I will typically see the charger showing 8.46-8.50.

I have an icharger 208b. It would have been nice if you had read the post a little more carefully, because as far as I am aware of, there would be no way for me to get an individual cell reading from a non lipo charger. Sorry if I sound a little short, just tired of people responding without an answer to questions, and sounding condescending.

Dave H

That is not unusual, you probably are OK. (Although too large of a difference suggests it's possible the charge leads and connectors are higher resistance and/or longer than ideal, which will tend to increase charging time somewhat.)

The pack voltage reading comes off the main charge wires, due to the charge current going from the charger to the pack there is some voltage difference, the charger is at a slightly higher voltage than the pack.

For the balancer the current flow is in the opposite direction, current if any is flowing from the pack towards the charger. Thus the charger balance circuits may read slightly lower than the actual cells, or at least no higher if no balancing is going on.

As the charge current comes down during the Constant Voltage part of the charge, the voltages will converge.

The balancer readings should be more accurate, especially on a well balanced pack.

badaml

iTrader: (132)

Yes sir, thank you! I actually called one of the distributors, and he pretty much said the same thing. I was told that the main charge lead is actually giving the voltage going into the pack, and it will always read a little higher than the sum total of the individual cells, since it is necessary for current flow. I thought the main screen on the charger would show overall pack voltage, but I guess it only does that when not in a charging state.

Thanks again for the help and explanation!

Dave H

Does the pack voltage remain high, above 8.4V, even towards the end of charge as the charge current drops to low values?

badaml

iTrader: (132)

As it nears the end of charge, and ultimately finishes, the voltage does come down in line with the individual cells. All is good, had a Triton eq charger, 5A max 50w charger, was good , but needed better lol. Then had a Dynamite Passport Ultra, 8A max 100w charger, but balancing seemed to take forever if the cells were off by a little. Now I have the icharger 208b, 20A max 350w charger and so far, very happy with it. Nice to have data logging capability too.

Duster_360

iTrader: (21)

If all voltages you mention are from your charger, you need to check with a DVM/MM and see which is correct.



If the cell voltages are from DVM/MM, then it looks like its a charger prob.

badaml

iTrader: (132)

I was told that the main charge lead is actually giving the voltage going into the pack, and it will always read a little higher than the sum total of the individual cells, since it is necessary for current flow. I thought the main screen on the charger would show overall pack voltage, but I guess it only does that when not in a charging state.

I did however check the cell voltages with a few different multimeters, everything seems good. Nice to know I have the option of recalibrating if needed though

Duster_360

iTrader: (21)

The cal procedure is not easy, I looked at doing it on my 3010b, but bagged it. Mine seems to show less diff than your 208b does however.

badaml

iTrader: (132)

Easy or not, good to have the option. Typical voltage is .1 to .2 over sum of cell v, I guess that may vary depending on the charge current. Just seemed odd to me initially, but all is good. Thanks again to everyone for the help.

Starlite033

iTrader: (10)

Badaml

First off I am glad you got the answer to your questions.

I was not trying to condescend, just trying to help. I must admit I use my multimeter to get all my readings from battery voltage to individual cell voltage so I had no idea that by providing an individual cell reading it proved you had such a charger Either way I misunderstood the question as asked and I apologize.

I'll leave it that because I can understand your frustration considering the true nature of your question.

Star

badaml

iTrader: (132)

I appreciate the help, thanks for clearing things up. I apologize for being jerky, been a long week, but still no excuse. If you are ever in the TX Hill Country, look me up. A few places to run out here, and recently started running at a MotoX track!

Dave H

Just saying, it's a fairly straightforward concept, as much as you post on the subject somewhat surprised you haven't encountered or noticed this before.