HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jul 08, 2001, 01:32 AM
Registered User
Mikey_Flies's Avatar
Edmonton, Alberta, Canada
Joined Oct 2000
324 Posts
Small Battery Questions

How much difference would 4 mOhms of resistance make to a power pack for a small model. In particular I'm thinking of a model drawing between 2 and 3.5 amps and comparing either 8 110 nicads, or 8 120 TA Nicads. The 110's are listed as 30 mOhm resistance at 50% discharge, the 120's at 34 mOhm at 50% discharge. Now, will this mean that the 120's can only deliver ~88% of the current that the 110's can? (30/34, as v=ir -> i=r/v and v is the same thus i2=r1/r2 * i1) It looks like I can save 8 grams by going to the 120's vs the 110's (Doesn't sound like much, but a 110 cell weighs 7 gm and a 120 weighs 6, so that is really saving the weight of a whole cell!!) but I don't want to lose a lot of the 110's power.
Mikey_Flies is offline Find More Posts by Mikey_Flies
Reply With Quote
Sign up now
to remove ads between posts
Old Jul 08, 2001, 02:46 AM
Registered User
N. Staffs, UK
Joined Jan 1997
5,654 Posts
It's an interesting question but unfortunately you can't simply apply Ohms law to the effective resistance of a cell and get accurate answers. It only works over a limited range of loads. The only way to really find out is to try it.

The problem is that we are abusing these cells by dragging way too much current out of them so we are outside the manufacturers specs almost all of the time.

Steve
steve lewin is offline Find More Posts by steve lewin
Reply With Quote
Old Jul 08, 2001, 03:01 AM
Registered User
Mikey_Flies's Avatar
Edmonton, Alberta, Canada
Joined Oct 2000
324 Posts
Ya :/ at 3.5 amps, I'm exceeding 1C by aprox 32 times with a 110 cell. Fortunately this doesn't happen for long (although at 2 it is still 19 x 1C) I think that the weight savings will offset the thrust loss. If not, I can always add another cell and still be 2 grams ahead I suppose I'm not sure if the motor can take 9 cells for very long though :/ and field charging will be right out (8 you have to cheat a little)
Mikey_Flies is offline Find More Posts by Mikey_Flies
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Looking for a small battery connector. What do you use ? RonJ Parkflyers 18 Jul 12, 2002 10:04 PM
Speed 280 prop/battery questions russf Foamies (Kits) 19 Mar 16, 2002 09:15 AM
A few Battery Questions... TheIndigoJoker Power Systems 1 Oct 10, 2001 07:26 AM
Small batteries questions Mikey_Flies Power Systems 3 Jul 13, 2001 01:34 AM
Just Got my first look at the GWS Tiger Moth-Small battery compartment! mrebman Parkflyers 3 Jul 10, 2001 08:47 PM