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Apr 27, 2008, 01:32 AM
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How far down should I take my Li-Po?


I'm certainly no expert on Li-Po batteries, don't profess to be, and don't really care to be...

With that said, however, I do try to keep up with the latest information on the care and feeding of Li-Po batteries, and how far down they should be taken without doing them damage and/or shortening their lives.

It seems there are two measured voltages we should be concerned with. The 'working' voltage, or the voltage that our battery is at when it's supplying voltage to our motor, and the more accurate 'resting' voltage. The resting voltage is what we measure after our Li-Po's have had a chance to cool down and stabilize after being subjected to the stresses of powering our motors.

It also seems that we will maximize our Li-Po life by taking them down to no lower than 20% of their capacity. Here is the latest and greatest information I have managed to gather from extensive reading on this subject.

"Resting" Voltage per cell

4.20v = 100% of our battery's amp capacity remains.

4.03v = 76% of our battery's amp capacity remains.

3.86v = 52% of our battery's amp capacity remains. (A good voltage to store our Li-Pos at)

3.83v = 42% of our battery's amp capacity remains.

3.79v = 30% of our battery's capacity remains.

3.75v = 20% of our battery's amp capacity remains. (Where we want to take our Li-Pos to for long life)

3.70v = 11% of our battery's amp capacity remains. (Detrimental 'battery voltage dump' begins)

3.6?v = 0%

In line with the above, we do NOT want our Li-Po's resting voltage to be less than 3.75v per cell, which would mean we had used about 80% of their capacity.

Oddly enough, this 20% figure is exactly the percentage I have always used, even without knowing the above data. This might explain why I seem to have phenomenal life out of my Li-Pos!

Chuck
Last edited by NoFlyZone; May 10, 2009 at 10:33 PM.
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May 02, 2008, 01:03 PM
BD Flyer's Avatar
Chuck,

I just got my first LiPo batteries a couple months ago, and have not used them yet. I have a watt meter and a CellPro 4s LiPo charger, so I can take care of them. But what is the voltage per cell that should keep them at during storage? Would it be the 80% (3.75v per cell) you're talking about? I know you said that you're not an expert, but i'd figure I would ask you since you're a pretty smart guy...

Thanks,

Bill
May 02, 2008, 01:06 PM
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Originally Posted by BD Flyer
Chuck,

I just got my first LiPo batteries a couple months ago, and have not used them yet. I have a watt meter and a CellPro 4s LiPo charger, so I can take care of them. But what is the voltage per cell that should keep them at during storage? Would it be the 80% (3.75v per cell) you're talking about? I know you said that you're not an expert, but i'd figure I would ask you since you're a pretty smart guy...

Thanks,

Bill
Hi Bill,

The consensus from the guys who know way more than I do is to store them at about 3.85 volts per cell (about 50%), the reasoning being that it's better not to store the Li-Po's with a full charge.

Chuck
May 02, 2008, 01:21 PM
BD Flyer's Avatar
Thanks Chuck. So... I just plugged in one of my 3 cell LiPo's into my watt meter (Astro Flight 101), and it popped up with what it says in the picture below. Now, does the 12.10v mean the total voltage that the battery is putting out? Also, if I divide that by three since the battery has three cells, is that number (which is about 4.03) the voltage for each cell? I'm just learning about all of this battery stuff, and I find it a bit confusing.

Thanks,

Bill
May 02, 2008, 03:08 PM
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Quote:
Originally Posted by BD Flyer
Thanks Chuck. So... I just plugged in one of my 3 cell LiPo's into my watt meter (Astro Flight 101), and it popped up with what it says in the picture below. Now, does the 12.10v mean the total voltage that the battery is putting out? Also, if I divide that by three since the battery has three cells, is that number (which is about 4.03) the voltage for each cell? I'm just learning about all of this battery stuff, and I find it a bit confusing.

Thanks,

Bill
Hi Bill,

If your battery is 'balanced', then yes, the 4.03 volts per cell your reading means that the Li-Po is charged to about 76% of it's capacity.

Now, here's the thing to remember. A balanced Li-Po means that each cell in the Li-Po has the same voltage. This is why you hear about people using a 'balance' charger, which makes sure that each cell gets charged to the same voltage.

Fot those of us who don't use a balancing charger, there are Gizmos like the TP 205 balancer, which can be hooked up along with the regular charger to do the same thing... simply make sure that each cell is charged equally.

Let's look at this example:

We have a 3 cell Li-Po, and we measure it's voltage at 12 volts. We look at our chart, and we find that 12 volts corresponds to a 3s Li-Po that is charged to something just under 75%, and we think we're doing good.

BUT, let's look at what MIGHT be going on under the hood, so to speak.

Cell 1 is fully charged and has 4.2 volts
Cell 2 is fully charged and has 4.2 volts
Cell 3 is stone empty, and has 3.6 volts

We add up the voltages to 12.0 volts and think we are okay, but we're NOT!!!!

In fact, there's a "chance" that when we go to charge this battery, that #3 cell just might erupt and burst into flames!

So what we do is either use a balancing charger, OR the gizmo that hooks up to our battery (The TP 205) and balances our pack for us. And THEN we take our reading.

Does this begin to make a little sense?

Chuck
May 06, 2008, 09:26 PM
BD Flyer's Avatar
Chuck, that makes PLENTY of sense. I just need to order some FMA pigtails for my LiPo batteries so I can check if the cells are balanced using my FMA charger. I will try to do that as soon as possible because I sure don't want my batteries exploding just because the cells weren't balanced.
May 06, 2008, 09:31 PM
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Quote:
Originally Posted by BD Flyer
Chuck, that makes PLENTY of sense. I just need to order some FMA pigtails for my LiPo batteries so I can check if the cells are balanced using my FMA charger. I will try to do that as soon as possible because I sure don't want my batteries exploding just because the cells weren't balanced.
Hi Bill,

Keep in mind that Li-Po's are great tools. And with just a little bit of care and maintenance, they will last you for many hundreds of safe and fun flights...

Chuck
May 08, 2008, 06:45 AM
tree scavenger
Chuck my apologies, just realized this was your Blog,
but.....im desperate sir lol

im still havin trouble w/the math
whats the formula?
how do i figure 80% of 4.2
4.2 x 0.80 = 3.36v ??
May 08, 2008, 10:46 AM
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Originally Posted by silentsurfer
Chuck my apologies, just realized this was your Blog,
but.....im desperate sir lol

im still havin trouble w/the math
whats the formula?
how do i figure 80% of 4.2
4.2 x 0.80 = 3.36v ??
Hi silentsurfer,

First of all, don't apologize for asking! Heck, this blog was put up for just this kind of thing; as a way of sharing information and asking questions...

Actually, I'm glad you asked this, because I can see now where it's a little confusing to the casual reader. Thank you for bringing this to my attention... So let me put it this way....

We are really concerned with the amount of amps we are using from our battery. That's usually what people mean when they say they have used "so many amps", or are "pulling so many amps", or are "over-amping" their system.

Let's take an imaginary, typical 3s Li-Po that is rated at 1000 mAh (milli-Amp-hours). We use it in our plane, and then when we're done, we put it on the charger and charge it back up. We can sometimes think of it as putting back the 'milli-amps' we used from our battery.

Some chargers like the Triton, will actually tell you how many milli-amps that it put back into your battery.

So my Triton charges my 1000 mAh battery, and when it's done, it tells me that it put 950 mA back into the battery. YIKES !!!!!!!! THIS IS NOT GOOD!!!

This means I used 95% of my batteries capacity, and actually had it down into the danger level, where battery destruction sets in!!!! Wouldn't it be nice to have a way of knowing how much capacity I had used BEFORE I drained it down this far? And there is such a way...

What we can actually do is measure the voltage of our battery, and using the voltage reading, calculate how many amps we pulled from our battery. The 80% figure means that we don't want to pull any more than 80% of our batteries 'amps'. So in our imaginary scenario above, we would like to read the voltage as being 3.75v per cell.

We're not shooting for 80% of the voltage... we're shooting for 80% of the amp capacity. In other words, we want to use no more than 80% of the amp capacity of our pack. We want to drain it down to no lower than 20% remaining. It's like having a 100 gallon fuel tank on a truck, and we only want to use 80 gallons of fuel before we stop and fill our tanks again. We want to keep 20 gallons (20%) in our tank as a minimum. In fact, the fuel gauge on the dashboard of our truck or car actually uses voltage to calculate and show you how much fuel is in your tank! Voltage readings are just our way of calculating this amp capacity.

Please let me know if this 'sort of' makes more sense, ok? And thanks to your wonderful question, I'm going to go back and edit the original chart to make it a little easier to understand. See if the new chart makes more sense.

Chuck
Last edited by NoFlyZone; May 08, 2008 at 10:58 AM.
May 08, 2008, 06:11 PM
tree scavenger

..dangerously hovering near Chucks 'ignore' list hehe


Chuck
1st thank you so much for taking the time to explain this.

ok, i understand were talkin 'capacity' Amps (milli-amps)
in our scenario (1000mah) is full 100% capacity.
using 80% means using 800mah, leaving 200mah (20%) remaining.

i guess the nearest i can accurately query this is:
where are (we) getting the ....(relationship?) between the voltage drops and our current capacity values?

can i not figure it mathmatically?
'calculate' implies i can run a .... (formula?)

Quote:
"What we can actually do is measure the voltage of our battery, and using the voltage reading, calculate how many amps we pulled from our battery."
HOW ?
..so we're calculating using our actual data (voltage) readings from the charger and using this 'chart' then??

is this 'chart' pre-rendered by someone
who actually 'logged' this data from their meter/charger then??
thus considered an reliable reference of sorts?

man i feel so thick-headed lol

btw ive an DN/CSRC balance Chrgr., and the Multiplex LN5014 which i believe Does display readings for volatge level and current...(replaced?) ...need to find that dang manual.

finally, is there (then) a direct and definite relationship between voltage drop and current capacity, im guessing?

i'll re-read this after dinner, maybe im hindered by hunger? lol
thanks again
ss (Joe)
May 08, 2008, 07:02 PM
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Hi Joe,

I just asked the guy who knows tons of info about Li-Po's (and whom I got the chart from) to come and answer our questions. I can not answer your questions, and I always make it a point to not attempt to give answers to things I don't know about....

Chuck
May 08, 2008, 07:28 PM
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everydayflyer's Avatar
The numbers in post #1 are correct but as most things in life not 100% accurate. They are like your fuel gauge in your car. Just because it says the tank is 1/2 full and your owners manual ststes you have a 18 gallon tank does not mean you have exactly 9 gallons of gas left.

The main thing is that within ten minutes after a flight that the cells are above 3.7 volts each and 3.8 is much kinder on them.

Most everyone tried to use the lowest capacity LiPoly they can a squeze every bit out of it then they complain about poor performance and short life. If you need 1000 mAh to fly for 10 min. and you feel you must fly for the full ten minutes then use a 1300 or 1500.

Charles
May 08, 2008, 07:37 PM
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Hi Charles,

Thank you for dropping in! Can you tell us if those numbers are gathered empirically? Or is there some kind of formula?

Thanks again,

Chuck
May 09, 2008, 02:23 AM
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Rats...

I was hoping everydayflyer would have hung around a bit more, because his knowledge of batteries, etc., is so far above and beyond my own.

Be that as it may, I'll just continue to use 3.75 volts resting voltage as the point I bring my planes in at. It's worked for me exceptionally well so far, as all my Li-Po's are still performing like the day I bought them...

Chuck
Jun 02, 2008, 04:59 AM
Prop me up
cloud-9's Avatar
Hello Chuck, your handling of questions and responses is as generous and graceful as ever.

Three questions...

Is the TP 205 what you use to balance your packs after discharging/charging to storage voltage? Do you use it while or afterwards?

The voltages you cited are resting voltages. If one is discharging, with the light bulb technique, then the battery is under load, and you mention that there is 1) cutoff (under load, I presume) voltage, and 2) resting voltage. Do you have a sense of how the two relate, with respect to the under load voltage that corresponds to 3.75v resting? Or should one just watch the watt meter until 3.75v/cell is achieved, and not worry about it?

I'd like to not have to watch the watt meter. Is there a linear relationship between amps drawn and voltage remaining? I bought two tail/stop light sockets and bulbs. Connecting both brake and tail lights draws just under 2.5 amps. I'm thinking about a second one in parallel to draw a total of near 5 amps. A plane drawing 5 amps might use 50% of a 2500mah lipo in 30 minutes. Can I assume that when 50% of mah are gone, voltage has dropped to the 50% mark too? This would allow one to set a timer to ring when .5 mah was gone (if I know amp draw), rather than stare at the volts. Probably not a linear relationships, but have you run across anything like this? Voltage graphs show an initial drop, followed by a fairly shallow drop for a long time, then a steeper drop off towards the end; but this does not rule out a linear relationship.

Thanks,
Jim


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