How to determine Mah for separate lipo?

[Sandracer_NL]

If I want to run two fans, each on 8.4v and 0,6A, by using a Y- splitter for 90 minutes to 120 minutes I assume I need a 2s lipo but how much Mah I have no idea.
Anyone an idea and if possible let me know how you calculated it?

 

[Ari33]

See https://convert-formula.com/mah-hours#:~:text=The formula is (mAh)/(,)/(200)= 15 Hours.

 

[Turbobrick]

You have 3 options if available and

1, If you are using a high hv servo and receiver you can change the esc to output 8.4v and plug the fans into a spare channel on the receiver. No need for an additional lipo.

2, connect the fans to the ballance lead with a jst plug at pin 1 and 3 this will give you 8.4v. No need for additional lipo.

3, you already have the working for the answer 0.6amp is 600mah x 2 = 1200mah of current draw would take 1 hour to fully discharge a 1200mah battery its not linear like this on a fan as the voltage drops so does the current to a point.

So a 2s 1800mah would last 1.5 hours 90mins.

 

[Yonic]

0.6A is 600mA, batteries are rated as mAh (milli amper per hour), so for 120 minutes you need at least 1200mAh.

you probably want to add 20% do be on the safe side, so something like a 1500mAh would be fine.

Edit - I forgot you said that you have 2 fans, so double that to a 3000mAh battery for 120 minutes.

 

[nino]

In addition, you can run them on 4s connected in series. In case you run 4s that's the easiest choice. (edit after having my coffee: this may or may not work as fans are not linear loads so the voltage across them may vary!).
Otherwise, the guys already told you how it's calculated.

Personally I dislike the idea of multiple batteries and would probably get a BEC (a cheap one would do, as long as it's adjustable or appropriate fixed output V) and hard wire it to the ESC input leads. More work upfront but simpler down the line. Can also be used to run a HV servo.

 

[Ari33]

This ^

I tried the seperate 2s Lipo idea but it proved to be more hassle than it was worth. It was difficult to secure due to placement, ejected easily in crashes which could have damaged receiver pins... and ensuring it was charged each run was another avoidable task.

I like to keep things as simple and as light as possible. An Esc with a HV Bec is obviously the best solution but if that wasnt an option I'd get a seperate BEC.

There is a good BEC test thread on RcGroups , I'll see if I can find it.

 

[Sandracer_NL]

Thanks for the physics 101 class guys!
Those ‘little’ fans must really have some serious fire power then now I know what if 0,6A means for battery capacity.
I just unscrewed the current 50mm fans I have now to see what they use. No info on the fan themselves, but on the website I found 0,18A. Quite a difference...
Now I have to rethink everything because I don’t want to take 2x0,6A off my battery (FYI I was actually thinking of installing 4 of those 30mm fans, which would really seriously cut runtime!
Have to agree with you that trying to run it without a separate lipo is better. My manual says the ESC (HW WP-8bl150-rtr) has a BEC output of B6v@5A, so it can’t do the 8.4v.
I really like the look of the fans (nice colored aluminium) and the thought of 28.000rpm. Would have been a nice Sunday purchase...
Back to the drawing board Unfortunately I have more imagination than the required handyman abilities.

 

[Ari33]

If I have picked you up wrong.. my apologies but it sounds like you might be comparing 2s lipo runtime directly with 6s powered Bec ouput runtime?

A switching bec switches on/off at a given frequency to achieve an average voltage output and this pulsed output is smoothed out by capacitors.

What that means is.. for 6s ouput going through a BEC to output 2S voltage, it is only on aprox one 3rd of the time compared to a 2S lipo which would be on constantly.

Ie; a 6s lipo going through a bec to ouput 8.4v to 2 x 0.6Ah fans alone... it could power them for over 12 hours... which is fairly insignificant in my book.

Or... If your 6S Lipo lasts around 30 mins runtime. You average 10A over that period and your new highspeed fans only account for aprox 5% of that.

 

[Sandracer_NL]

Ari, this forum is great, tons of knowledge here! I think I’m understanding it now. Although I could run on 6s, I’m running on 4s for now. So the 4s is sending 14.8v-16.8v to the ESC and my BEC reduces that to 6v max; so let’s say 40% from the input it gets.
So running two 7.4v-8.4v 0.6A fans via my receiver and linked to my current 4s lipo/ESC would result in the BEC limiting 6v to each fan and a battery usage of 2x0,6A x 40% = 480mAh?

 

[Ari33]

Yes, that sounds about right...

Double Edit- it will still draw aprox 1.2A but using a 4s Lipo going through a Bec it will be the equivalent of drawing 480mA from a 2s Lipo of the same capacity, in terms of runtime.

 

[nino]

That's not how the math works, but I don't have time to elaborate
Anyway. On 6V fan current will be even lower. The impact on your 4s packs will be negligible.

 

[Turbobrick]

This video I made about the fans I use might be of use it to you shows voltage v current draw with 4 fans

 

[Ari33]

This video I made about the fans I use might be of use it to you shows voltage v current draw with 4 fans

Cool test, take it they are different fans... do your results tie in with the current draw in the specs?