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Nov 28, 2016, 09:39 AM
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Charging time calculation and kWh calculation help


Hello,

I'm in trouble with the formula for calculating how many time my charger take to charge my lipo, following detail:

Battery LiPo
16000 mAh - 6s

Charger
SKYRC D100 DUO 100W

The battery need to be balanced.
I set up the charger to charge at 10A (it is the max output settable)

1 - What would be the formula to calculate now many time it take with the consideration of 20~40% efficiency loss?

2 - What would be the formula to calculate how many kWh it will consume start charging from 0% to fully charged 100%?

Many thanks in advance to who want assist =)
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Nov 28, 2016, 10:14 AM
Sleepy Hollow, IL
Two important things to understand:

First: A lipo battery is never discharged to 0% charge. Rule of thumb is to only discharge to 20% of rated capacity. At the 0% the battery would probably be ruined. That being said, you would only being putting back in 16000* .8 = 12800 mAh.

Second: Chargers are Wattage limited. Your charger is rated at 100 Watts and that is the limit to which it can charge. For a 6S battery, fully charged would be 25.2 volts. At 20% charge level the voltage is around 22.2 volts. Therefore, 100 W/22.2 V = 4.5 Amps.

Your charge would start out at about 4.5 Amps and slowly decrease until the battery reaches full charge. For a quick guess the charge time would be about 12800 mAh = 12.8 Ah/4.5 Amps = 2.8 hours.

If your charger were to draw 100 Watts for the whole charge time then worse case it would use 280 Wh. Now this doesn't happen due to the charging profile. That profile makes the Wh calculation a bit more complicated.
Nov 28, 2016, 10:58 AM
Registered User
Thread OP
Thanks very much ampmaker, almost all clear:

20% of 16000
16000/100*20 = 3200 > 16000-3200 = 12800 mAh > ok

20% of 25,2
25,2/100*20 = 5,04 > 25,2-5.04 = 20,16 V >>

is it right 20,16 or do you calculate 22,2 in another way

if my formula is ok:
100W/20,16V= 4,96 Amps

12800 mAh = 12,8Ah/4,96Amps = 2,58 Hour (with 22,2 V was 4,5 Amps) > Why if the battery is more discharged, take less time to charge

2,58H x 100W = 258Wh, as before, why if more discharged spend less Wh
Nov 28, 2016, 11:16 AM
Suspended Account
you can try to mathematically calculate charge times but that will never work with any battery or charger.. it will always take more time.. your 16,000 mah 6 cell will take a long time well over a hour..and with your 100 watt charger..id say 3 + hours you will never get close to 10 amps..
Nov 28, 2016, 11:22 AM
Registered User
Charge is not linear with voltage so you can't just say 80% of 25.6V = 80% charge.

80% charge for lipos is about 3.7V/per cell so 22.2V for 6 cells. If you ever get down to much under 3V per cell it will be completely empty (and also probably damaged).

Steve
Nov 28, 2016, 11:25 AM
Registered User
Thread OP
Hi rcdog69, in the real life the charger take about 4hour per battery, but i need a formula to do a rough calculation, that's why i need to right understood the formula explained by ampmaker


Hi slipstick umh, that's strange... the builder of my drone (12,000 € of drone) said to me to land at 21,7v, fast land at 21,4, die at 21,0.
I abitualy go to 21,7 before land.

Is it right what you say about goes under 22,2v damage battery?
Nov 28, 2016, 02:18 PM
Sleepy Hollow, IL
Here is a rough chart of battery voltage versus remaining charge per cell:

100% = 4.199 Volts
90% = 4.085 Volts
80% = 3.999 Volts
70% = 3.936 Volts
60% = 3.883 Volts
50% = 3.839 Volts
40% = 3.812 Volts
30% = 3.812 Volts
20% = 3.747 Volts
10% = 3.683 Volts
0% = <3.209 Volts

This the resting voltage after a flight when the battery has had some time to cool down and recover.

Very close to 20% you will hit 3.7 volts per cell which is 22..2 volts for a 6S battery.
Nov 28, 2016, 02:42 PM
Registered User
Thread OP
ampmaker do you where to find chart for every kind of battery?
Nov 28, 2016, 03:00 PM
Sleepy Hollow, IL
Not really. Lipo's are the only battery that I have been interested in.
Nov 28, 2016, 03:21 PM
Registered User
Quote:
Originally Posted by dixdylan
Hi slipstick umh, that's strange... the builder of my drone (12,000 of drone) said to me to land at 21,7v, fast land at 21,4, die at 21,0.
I abitualy go to 21,7 before land.

Is it right what you say about goes under 22,2v damage battery?
What I said was that going much under 3V per cell is likely to cause damage. 3V x 6 cells is 18V total.

Steve
Nov 28, 2016, 03:32 PM
Registered User
Quote:
Originally Posted by dixdylan
ampmaker do you where to find chart for every kind of battery?
Look up 'discharge curves' to find more. Note that Lipo has a very 'nice' discharge curve in that the voltage drops linearly over a fairly large voltage range for most of the discharge (voltage isn't linear to capacity starting from 0v but it's close from 3.6v-4.1v). If you look at Nimh or LiFe curves most of the discharge takes place over a much narrower range of voltage which gives a very flat curve. This makes measuring the remaining capacity from the voltage very tricky. You can at least tell if the battery is full or close to empty, but you'll struggle to get as neat a table/curve as you do for Lipo.
Nov 28, 2016, 04:47 PM
Registered User
Why do you need to calculate it? You know you're going to need more than 250W to do even a 1C slow charge so your 100W charger isn't adequate. You will want at least a 300W charger, but I would recommend a 1000W one.
Nov 28, 2016, 05:29 PM
Registered User
+1
100 W isn't even remotely adequate for a 16A 6S battery. Even an hour seems like an eternity for a 20 min ( or less flight). FMA PL6
Nov 28, 2016, 06:24 PM
Registered User
Quote:
Originally Posted by James_S
Why do you need to calculate it? You know you're going to need more than 250W to do even a 1C slow charge so your 100W charger isn't adequate. You will want at least a 300W charger, but I would recommend a 1000W one.
Maybe he was trying to figure out how many watt charger he needs to buy to get it charged in an hour or less? He Already said he tried it and it took four hours, I guess a few variables are missing in the maths..
Dec 01, 2016, 06:35 AM
Registered User
Thread OP
Thanks to all for contribution.

Well...

1st - I'd like to calculate the power consumption in charging (in kWh), in the worst condition or when the battery is fully discharged with a given charger and a given battery.

2nd - I'd like to calculate how long the battery is charged (time), in the worst condition, with the charger at my disposal (100W)

3rd - I'd like to have a formula to calculate what power must have a charger to calculate a given battery.

4th - Depending on the power available to the battery charger I would calculate the time required to charge the battery specification.

5th - I would like these formulas to put them all in an Excel file to automate various calculations

I understand clearly that these formulas will give a rough result, but it will be sufficient for the purpose that my original plan.


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