RunRyder - Question About Measuring Voltage/Mah's ETC. And Charging

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e-Electric Batteries & Chargers > Question About Measuring Voltage/Mah's ETC. And Charging

para57
New Heliman
Location: Ohio, USA

Hi guys,

Ok, I have a 3 cell (11.1 V), 1800 mAh battery. I understand that equates to 1.8 amps, and I should charge it at 1C (which is 1.8 Amps).

My question is more about capacity and how to measure it. I hear that you should fly a fully charged pack (100%) until you hit about 80% capacity. Is that true? And why?

Ok, so my charger does not tell me how many mAh's I take out or put back in... Would I need to buy a voltmeter or something from radio-shack so that I can check to see how much of my batteries im using after flights? How do you do it?

Also, for the 3s battery I shouldn't let it discharge to less than 2.8 volts correct? And charged at a maximum 4.2 volts correct? That would mean 3.7 is where I would want them for storage right? (50% charge)?

So, I'm guessing 4.2 volts is where the pack is at 1800 mAh? but what about 2.8 volts and 3.7 volts?

Can you please help me out and recommend something that can tell me my voltages and mAh? (voltmeter?) Thank you!

06-11-2008 09:20 PM

Micro-Maniac
Elite Veteran
Location: Pasco,Washington Formerly: Captain Chaos

The more you use a battery the shorter it will live and this use vs life curve takes a steeper turn during the last 20% of capacity

With my helis it seems 80% comes right at nominal resting voltage
(3.7V/cell - Measured after flight with no load)
With my planes it seems I can run the voltage down lower for the same amount of Ah usage

I would use nominal resting voltage until you get a better charger to know for certain what your Ah usage is

The Cellpro 4S is a good and fair priced balance-charger that will take good care of your packs and give you the accurate info you need

You shouldn't discharge lipos below 3V/cell under load so if you're hitting your ESC's low voltage cut-off then you're pulling them too deep

06-11-2008 09:58 PM

para57
New Heliman
Location: Ohio, USA

Buying a new charger is out of option for me (just got a TP-535 thunder power charger, with a balancer). So, there isnt really a way to check mah then? But if i bought a voltmeter I can check after my flights that i'm at the nominal resting voltage? Is that a good way to go?

06-12-2008 12:10 AM

Micro-Maniac
Elite Veteran
Location: Pasco,Washington Formerly: Captain Chaos

Well that's certainly the easiest - Actually calculating Ah is a little difficult with CC/CV (constant current/constant voltage) charging - The charger will charge the pack at a CC until the pack reaches full voltage then will switch to CV and ramp down the current until it reaches about 1/10C or 1/20C - You could keep track of the voltage during charging and measure the time it takes for the pack to reach full voltage at the set CC (which may take a few minutes to ramp up to) then the time it takes to finish the charge at CV and try to extrapolate an Ah from the CV time and add it to the calculated CC Ah to get the total Ah - The higher the CC the faster the pack will reach full voltage and the longer it will spend in CV - So the lower the CC you charge at the less time will be spent in CV making it easier to calculate/guesstimate Ah

06-12-2008 01:30 AM

"Cam"
Veteran
Location: Asia

DO NOT

Discharge your pack below 3.1 volts per cell - that's 9.3 Volts for the pack (terminal voltage).

The best way to do what you want is to get a Chart of a discharge curve for those cells, that will show Voltage and Capacity in mAh, then read the voltage with a voltmeter, then look on the chart to find out how many mili-amp-hours are left.

Yes, when the cells are at 4.2 volts, they will be at 1800mAh.
When at 3.9 v/cell, they will be at 60% Capacity (1080mAh).
When at 3.1 you should have landed already!

The relationship between volts and mAh is not linear, that is to say you cannot work out one value of mAh by knowing two voltage values:
If 4.2v/cell = 1800mAh and 3.9v/cell = 1080mAh, you cannot use numerical interpolation to work out the mAh for a voltage between these values! - You have to look at the Discharge Curve! (It's too easy).

DSX9
TRex 600, 500, 450
Hurricane 550
Yak 180, 40% Extra - 15S
Custom eRaptor 700

06-17-2008 02:42 PM

"Cam"
Veteran
Location: Asia

PLEASE DON'T USE CC/CV on LiPo Cells

These things can burn very quickly and they make their own oxygen!

I've never met you, but if you can't afford a good charger you certainly can't afford a fire.

Use a LiPo setting for your Lithium Polymer Batteries and use a balancer.

I know you said you just bought a new charger but do yourself a favour and SELL IT. Buy one that will really look after your batteries, one that shows you the individual cells voltages, has a mAh display, has built-in balancers, a charger that won't let you connect your battery wrongly or use the incorrect settings!

DSX9
TRex 600, 500, 450
Hurricane 550
Yak 180, 40% Extra - 15S
Custom eRaptor 700

06-17-2008 02:48 PM

Pistol_Pete
Elite Veteran
Location: Tampa Bay non-Buccaneer

Quote
Ok, I have a 3 cell (11.1 V), 1800 mAh battery. I understand that equates to 1.8 amps, and I should charge it at 1C (which is 1.8 Amps).

correct

Quote
My question is more about capacity and how to measure it. I hear that you should fly a fully charged pack (100%) until you hit about 80% capacity. Is that true? And why?

not the way you worded it...

you fly consuming 80% of capacity
OR
until you hit 20% of capacity left.

so given your 1800mA...using 80% of that leaves you with only 360mA left on batts.

why? it started with extending life to packs rather than risking the chance of going below cut off which is 9.3V as stated for your 3S pack. That plus it gives you time to execute landing rather than a forced auto.

Quote
Ok, so my charger does not tell me how many mAh's I take out or put back in... Would I need to buy a voltmeter or something from radio-shack so that I can check to see how much of my batteries im using after flights? How do you do it?

For starters your ESC should have a cut off setting set for 3S or 9V according to intructions.

An important option is to TIME your flights.

Start with 5 minutes then land or come to hover close to ground ready to auto and fly until you can tell that batt is weaker or cut off occurs...good practice none the less.

Once this cut off or weak batt takes place you have a range of flight time.

deduct at least one minute and this should be safe range to land when timer goes off.

Quote
Also, for the 3s battery I shouldn't let it discharge to less than 2.8 volts correct? And charged at a maximum 4.2 volts correct? That would mean 3.7 is where I would want them for storage right? (50% charge)?

correct...storage is meant as in long term...if only a few weeks then not an issue. store in a safe container of some kind...research in here for many options.

Quote
So, I'm guessing 4.2 volts is where the pack is at 1800 mAh? but what about 2.8 volts and 3.7 volts?

It has been my experience when measuringt 3S packs at rest (a minute or two after disconnected and used) I get 10.8V.

Quote
Can you please help me out and recommend something that can tell me my voltages and mAh? (voltmeter?) Thank you!

An volt/ammeter will only display actual mA going in as in 1C or 1.8mA and not offer an actual break down as you think.

For that you will need somethin like THIS...

a must have piece in the electrics arsenal in my opinion.

<><>...the lunatic is in my head...<><>

06-17-2008 03:14 PM

HOMEPAGE

para57
New Heliman
Location: Ohio, USA

Thank you all very much for your help! You have really helped my understanding of this issue!

06-17-2008 04:42 PM

Micro-Maniac
Elite Veteran
Location: Pasco,Washington Formerly: Captain Chaos

Um, "CAM" I believe CC/CV is the only practical way to fully charge lipos - That is afterall the method all chargers use to charge lipos unless it's a cheap one that doesn't fully charge lipos by only doing the CC portion of the charge - So telling someone not to use CC/CV on lipos is telling them not to use any of the better lipo chargers on the market

06-17-2008 05:26 PM

"Cam"
Veteran
Location: Asia

I'm telling them to use the "LiPo' Setting.

DSX9
TRex 600, 500, 450
Hurricane 550
Yak 180, 40% Extra - 15S
Custom eRaptor 700

06-18-2008 11:19 AM

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e-Electric Batteries & Chargers > Question About Measuring Voltage/Mah's ETC. And Charging

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