Battery Runtime Calculation 3 x SSC P7

[Goldigger]

Hi,
Apologies if i have posted in the wrong place.
I have put together 3 Seoul P7's wired in series driven by a Konlux 2800 driver + BOOM P7-S reflectors. I ran these of a 12volts mains adaptor with the p7's attached to a heat sink.
I think the Konlux 2800 driver drops 2 volts, so i get 10volts shared by the 3 leds 3.3 volts at 2800mah/8400mah? I know the leds require 3.7 - 4.2 volts max each.
They ran fine and didnt get to hot, the driver stays nice and cool. I plan to put this in a suitable host for a dive torch. The part that i am not to sure on is how much runtime i will get from a battery.
My options are a 12volt 4.5ah, 12volt 9ah or a 13.2 volt 4.5ah pack.
Please can somebody advise me how to work this out?

Is the following correct? is this total runtime? how much could i expect it to run at full brightness?
P led = 3 x 3.7V x 2.8A = 31.08W
Battery = 12.0V x 4.5Ah = 54Wh

Let's say efficiency is 95% for the driver.

P in = P led / Eff = 31.08W / 0.95 = 32.7W

Therefore Runtime = Battery / P in = 54Wh / 32.7W = 1.6 hours



Thanks in advance

 

[SilverFox]

Hello Goldigger,

Welcome to CPF.

Your calculations look good. The amount of "bright" time depends on the chemistry used in your battery pack.

Alkaline and lead acid will gradually taper off, where NiCd, NiMh, and Li-Ion will have an initial drop, then hold steady until the very end, then have a steep drop off.

I would tweak your final estimate by multiplying it by 80%. This would give you close to 77 estimated minutes of run time.

Tom

 

[wquiles]

Let's say efficiency is 95% for the driver.

I second SilverFox comments, except for the driver efficiency. At almost 3 amps, I think 95% efficiency is too optimistic a number. If you are needing this for diving, better to assume a more realistic 80 to 85% for the driver efficiency. If the actual efficiency is higher than 80/85%, then you got "free" extra runtime underwater

You can measure the actual efficiency by measuring the voltage and current at the battery output, and the current and voltage at the LED's. LED Power (V * I) at the LED's divided by Battery Power, will give you the true efficiency for that combination

Will

 

[Goldigger]

Hello Goldigger,

Welcome to CPF.

Your calculations look good. The amount of "bright" time depends on the chemistry used in your battery pack.

Alkaline and lead acid will gradually taper off, where NiCd, NiMh, and Li-Ion will have an initial drop, then hold steady until the very end, then have a steep drop off.

I would tweak your final estimate by multiplying it by 80%. This would give you close to 77 estimated minutes of run time.

Tom

Hi Tom,
Thanks for your reply, i'm thinking of using a 12volt 13ah nimh battery like this Green-Force one so i should get at around 2.5 hours burntime..
12volt 9ah
P led = 3 x 3.7V x 2.8A = 31.08W
Battery = 12.0V x 9Ah = 108Wh
Let's say efficiency is 80% for the driver.
P in = P led / Eff = 31.08W / 0.80 = 38.85W
Therefore Runtime = Battery / P in = 108Wh / 38.85W = 2.7 hours

Here's a image of the design i made for the light head, it uses 3 p7's wired in series driven by a konlux 2800 driver. How could i make this dimmable or even turn 1 led on then the 2nd and then finally the 3rd giving me max output?
I torch will be switched on by twisting the head into the battery pack..

Thanks again...

 

[VidPro]

not enough information on the driver spec sheet thing.
are you sure your going to have enough input voltage with 3 of the leds in series (has to be in series if your going to get the 2.8A out of it) then with the reverse polarity diodes in the curcuit you have a huge voltage drop.
do you have a 10.5 or 11v power source that does 3amps that you can test putting that into it as the input, and see what the actual current is on the drive?

the only reason i mention it, is BEFORE you buy or assemble a pack you might want to be sure of what it does when the pack is low, if it cant stay regulated with low voltage , you might get huge runtimes, but mabey not enough light. and (as usual) i am not saying that is bad or good, it could be just what would work great, just something you would want to know before hand.

 

[Goldigger]

not enough information on the driver spec sheet thing.
are you sure your going to have enough input voltage with 3 of the leds in series (has to be in series if your going to get the 2.8A out of it) then with the reverse polarity diodes in the curcuit you have a huge voltage drop.
do you have a 10.5 or 11v power source that does 3amps that you can test putting that into it as the input, and see what the actual current is on the drive?

the only reason i mention it, is BEFORE you buy or assemble a pack you might want to be sure of what it does when the pack is low, if it cant stay regulated with low voltage , you might get huge runtimes, but mabey not enough light. and (as usual) i am not saying that is bad or good, it could be just what would work great, just something you would want to know before hand.

Not to sure what you mean, but i have tested this with a 12volts mains supply which works fine. going to test at the weekend with a multimeter and see what the readings are accross the leds etc.

Im not using the reverse polarity protection on the driver...

 

[SilverFox]

Hello Goldigger,

Let's see now, 20 cells and 12 volts means that you are going to have 2 parallel strings of 10 cells. Charging parallel strings of cells can raise problems when the cells get out of balance. In fact, parallel charging NiMh cells is not recommended at all...

You can do it, but the times I have run into this, the solution has been to charge at 0.1C and charging took 16 hours. Charging can get a little strange with high numbers of cells in series, and you also have to pay attention to the low voltage cut off voltage during discharging too.

So, how do you plan to charge this battery pack?

Tom

 

[Goldigger]

Hello Goldigger,

Let's see now, 20 cells and 12 volts means that you are going to have 2 parallel strings of 10 cells. Charging parallel strings of cells can raise problems when the cells get out of balance. In fact, parallel charging NiMh cells is not recommended at all...

You can do it, but the times I have run into this, the solution has been to charge at 0.1C and charging took 16 hours. Charging can get a little strange with high numbers of cells in series, and you also have to pay attention to the low voltage cut off voltage during discharging too.

So, how do you plan to charge this battery pack?

Tom

Hi Tom,
With either the standard or fast charger that comes with the pack...