Dec 05, 2013, 05:50 PM Aerial Photography Canada, BC, Vancouver Joined Jun 2012 101 Posts Building a lipo, how do I calculate mAH? I have built a few 3S lipos for my 130x from the turnigy nano 450mAH 2s lipos. However I am not sure how to properly calculate the capacity. As I understand it the capacity will be (450 / 2) x 3 = 675 mah, but something tells me that this is not correct. Please help me figure out my pack capacity. Thanks! PS I have a custom 130x with xp12a esc to take advantage of the lipos.
 Dec 05, 2013, 05:58 PM Registered User Joensuu, Finland Joined Mar 2002 1,294 Posts Just exactly how do you make a 3s pack from 2s batteries? A connection diagram would clarify what you are doing.
 Dec 05, 2013, 06:03 PM Registered User United States, MN, Minneapolis Joined Feb 2011 3,472 Posts Not sure I understand what or why you are doing it but the capacity won't change. Each cell is 450mah, the number of cells when in series is irrelevant to the capacity. If you put two cells in parallel then you would have 900mah, but in series (aka going from 2s to 3s) capacity stays the same, voltage just increases.
 Dec 05, 2013, 06:19 PM Registered User Staffs, UK Joined Nov 2003 9,398 Posts That's it...if you make a 3S pack out of 450mAh cells the pack will be 450mAh. Connecting cells in series changes voltage not capacity hence your pack VOLTAGE is now (7.4 / 2) x 3 = 11.1V . Steve
Dec 05, 2013, 06:40 PM
Aerial Photography
Joined Jun 2012
101 Posts
Quote:
 Originally Posted by grimbeaver Not sure I understand what or why you are doing it but the capacity won't change. Each cell is 450mah, the number of cells when in series is irrelevant to the capacity. If you put two cells in parallel then you would have 900mah, but in series (aka going from 2s to 3s) capacity stays the same, voltage just increases.
Excellent, that is exactly what I wanted to know. So instead of a 2s 450 mah battery i now have a 3s 450 mah battery. Just as a bonus, can you explain as to why that capacity does not increase with the number of cells in series? Thank you!

Quote:
 Originally Posted by slipstick That's it...if you make a 3S pack out of 450mAh cells the pack will be 450mAh. Connecting cells in series changes voltage not capacity hence your pack VOLTAGE is now (7.4 / 2) x 3 = 11.1V . Steve
Thank you!

Quote:
 Originally Posted by jkettu Just exactly how do you make a 3s pack from 2s batteries? A connection diagram would clarify what you are doing.
I took apart the 2s batteries and connected 3 single cells together in series, as a 3S pack. relatively simple to do.
Dec 05, 2013, 09:40 PM
Space Coast USA
Space Coast
Joined Oct 2000
19,502 Posts
Quote:
 Originally Posted by kdrcguy can you explain as to why that capacity does not increase with the number of cells in series? Thank you! .
Remember, the mah rating is at the packs voltage. 450mah@3.7V. It can't put out anymore than 450mah @ 3.7V. If you string them together, each cell will deliver 450mah @ 3.7V. 2 in series will deliver 450mah @ 7.4V, etc.

In parallel, you have 2 450mah cells @ 3.7V which gives you 900mah@3.7V.
Dec 05, 2013, 10:25 PM
Registered User
United States, MN, Minneapolis
Joined Feb 2011
3,472 Posts
Quote:
 Originally Posted by kdrcguy (450 / 2) x 3 = 675 mah
Let me try and turn your math into what you might have been somewhat headed for...

450mAh x 7.4V = 3.33 Wh
3.33 Wh / 2 = 1.665 Wh per cell
1.665 x 3 = 4.995 Wh per pack
4.995 Wh / 11.1V = .45 Ah = 450mAh

Really that's a complex way to get there though lol.
Dec 06, 2013, 03:40 AM
Registered User
Letchworth, Great Britain (UK)
Joined Jul 2004
10,729 Posts
Quote:
 Originally Posted by kdrcguy ... Just as a bonus, can you explain as to why that capacity does not increase with the number of cells in series? Thank you!...
Capacity, measured in mAh, is simply a measure of the number of milliamps the battery can put out for an hour.

What we're really interested in is power, which is measured in watts, which is amps x volts. So, when you go from 2S to 3S you're not increasing the number of milliamps it can deliver, but you are increasing its voltage by 50%, so the watt-hours increases by 50%.
Dec 06, 2013, 03:48 AM
222 km/hr Parkjet flyer
Latvia / UK visits
Joined Jan 2010
6,566 Posts
Quote:
 Originally Posted by jkettu Just exactly how do you make a 3s pack from 2s batteries? A connection diagram would clarify what you are doing.
I am curious also ... all the other posts are waxing on about capacity vs voltage ... series and parallel etc. but the main point is HOW IS 3S being made from 2S packs ?

Nigel
Dec 06, 2013, 04:04 AM
Registered User
Joensuu, Finland
Joined Mar 2002
1,294 Posts
Quote:
 Originally Posted by kdrcguy ...I took apart the 2s batteries and connected 3 single cells together in series, as a 3S pack. relatively simple to do.
Apparently he just mechanically disassembled the 2s packs into single cells and then connected three of those in series. Doable, but I'm not sure I would bother unless absolutely necessary.
Dec 06, 2013, 04:20 AM
222 km/hr Parkjet flyer
Latvia / UK visits
Joined Jan 2010
6,566 Posts
Quote:
 Originally Posted by jkettu Apparently he just mechanically disassembled the 2s packs into single cells and then connected three of those in series. Doable, but I'm not sure I would bother unless absolutely necessary.
Yeh .. sorry - missed that bit ...

I've thought about doing similar as I have a few packs that odd cell has failed .. but really - is it worth it ?

Nigel
Dec 06, 2013, 07:14 AM
Registered User
United States, MN, Minneapolis
Joined Feb 2011
3,472 Posts
Quote:
 Originally Posted by solentlife but really - is it worth it ?
Given that a 3s 460mah is only \$5.52 I would say no. Plus then unless you are watching the IR you'll end up with some poorly balancing batteries.
Dec 06, 2013, 01:35 PM
Aerial Photography