Topics

Payload Power Sources

Joe WB9SBD
 

I am very confused on a battery pack.

Say you have a pack that can run 1 amp of current draw for 10 hours.
One would think that if you only drew 0.5 amps that it should run for 20 hours or close to it true?

Now many of you old timers here will remember these awesome surplus packs that looked like this.

These things were OLD like 20 years old when we used them yet the power they could provide was incredible!
They were rated at 7500mah, incredible! Sadly this supply of "NOS" dried up and new packs were very prohibitive in prices. Easily ending up being like $100 bucks a flight!

But I found a source for Brand new ones, that would make it like $15 to $30 bucks a flight! YES!

But it's performance curve makes absolutely no sense to me!

This makes zero sense at all. I can draw 2 amps  or 0.25 amps and it changes the lasting run time by only like 15 minutes?

That doesn't make any sense!

Joe WB9SBD
--

The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com

BASE_DePauw
 

Joe,

The graph that you have shared has the horizontal axis in units of Ah (Ampere*hours) not in hours.  It is fine.

This means that all current draws are about 7.5 Amp hours with the smaller draws performing better than the large draw.

Howard, KC9QBN
BASE_DePauw



On Fri, Mar 27, 2020 at 9:02 AM Joe WB9SBD <nss@...> wrote:
I am very confused on a battery pack.

Say you have a pack that can run 1 amp of current draw for 10 hours.
One would think that if you only drew 0.5 amps that it should run for 20 hours or close to it true?

Now many of you old timers here will remember these awesome surplus packs that looked like this.

These things were OLD like 20 years old when we used them yet the power they could provide was incredible!
They were rated at 7500mah, incredible! Sadly this supply of "NOS" dried up and new packs were very prohibitive in prices. Easily ending up being like $100 bucks a flight!

But I found a source for Brand new ones, that would make it like $15 to $30 bucks a flight! YES!

But it's performance curve makes absolutely no sense to me!

This makes zero sense at all. I can draw 2 amps  or 0.25 amps and it changes the lasting run time by only like 15 minutes?

That doesn't make any sense!

Joe WB9SBD
--

The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com



--
Howard L. Brooks
Professor of Physics and Astronomy
241 Julian Science and Mathematics Center
DePauw University
2 E. Hanna Street
Greencastle, IN 46135
hlbrooks@...
Office: (765) 658-4653
FAX: (765) 658-4732

David Akerman
 

The discharge capacity in Ah changes a little, due to internal I2R losses, so you get a bit more Ah from lower currents vs higher currents.  That's also why the voltage is a bit lower.  It's not showing you the run time directly - you need to divide Ah by A to get h, so your 0.25A load will run for more than 8 times the 2A load will.


On Fri, 27 Mar 2020 at 13:02, Joe WB9SBD <nss@...> wrote:
I am very confused on a battery pack.

Say you have a pack that can run 1 amp of current draw for 10 hours.
One would think that if you only drew 0.5 amps that it should run for 20 hours or close to it true?

Now many of you old timers here will remember these awesome surplus packs that looked like this.

These things were OLD like 20 years old when we used them yet the power they could provide was incredible!
They were rated at 7500mah, incredible! Sadly this supply of "NOS" dried up and new packs were very prohibitive in prices. Easily ending up being like $100 bucks a flight!

But I found a source for Brand new ones, that would make it like $15 to $30 bucks a flight! YES!

But it's performance curve makes absolutely no sense to me!

This makes zero sense at all. I can draw 2 amps  or 0.25 amps and it changes the lasting run time by only like 15 minutes?

That doesn't make any sense!

Joe WB9SBD
--

The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com

Hank Riley
 

Joe,

You're just reading or interpreting the curves the wrong way.  The X or horizontal axis is A-H, not run time.

The greater the current, the lower the A-H time.  So in your example of .5 amp versus 1 amp, the .5 amp run time will be a little over twice the hours.  And to continue, the .25 amp run time will be a little over twice what the .5 amp run time was.

I'll do the actual numbers in next email.

Hank
____________________________________________________________


On Friday, March 27, 2020, 09:02:24 AM EDT, Joe WB9SBD <nss@...> wrote:

I am very confused on a battery pack.

Say you have a pack that can run 1 amp of current draw for 10 hours.
One would think that if you only drew 0.5 amps that it should run for 20 hours or close to it true?


Hank Riley
 

Run time to a 24 volt cutoff because the effects on the capacity are clearer.

On the closeup graph view C1, C2 and C3 are the capacities for 2 amps, 1 amp, and .5 amp and lower.  That means at this cutoff voltage the ultimate capacity is reached at .5 amp and anything less for current drain does not lead to a higher A-H capacity.

C1(2)  is ~ 7.0   A-H  
C2(1)  is ~ 7.35  A-H       5.0% increase with respect to 2 amp capacity
C3(.5) is ~ 7.6   A-H       8.6% increase with respect to 2 amp capacity 

So runtime for 2 amps drain is 7/2 = 3.5 hours.

The other runtimes for 1 amp and .5 amp are 7.35 and 15.2 hours.

I've left off some of the units in the equation for 2 amps but as a check A-H divided by amps gives a result in hours so the units are dimensionally valid.


Inline image








Joe WB9SBD
 

DUH?!

I shouldn't be reading charts at like 3 AM. UG!

Thanks to all who answered my BrainFart!

Joe WB9SBD

The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com

On 3/27/2020 8:13 AM, BASE_DePauw wrote:
Joe,

The graph that you have shared has the horizontal axis in units of Ah (Ampere*hours) not in hours.  It is fine.

This means that all current draws are about 7.5 Amp hours with the smaller draws performing better than the large draw.

Howard, KC9QBN
BASE_DePauw



On Fri, Mar 27, 2020 at 9:02 AM Joe WB9SBD <nss@...> wrote:
I am very confused on a battery pack.

Say you have a pack that can run 1 amp of current draw for 10 hours.
One would think that if you only drew 0.5 amps that it should run for 20 hours or close to it true?

Now many of you old timers here will remember these awesome surplus packs that looked like this.

These things were OLD like 20 years old when we used them yet the power they could provide was incredible!
They were rated at 7500mah, incredible! Sadly this supply of "NOS" dried up and new packs were very prohibitive in prices. Easily ending up being like $100 bucks a flight!

But I found a source for Brand new ones, that would make it like $15 to $30 bucks a flight! YES!

But it's performance curve makes absolutely no sense to me!

This makes zero sense at all. I can draw 2 amps  or 0.25 amps and it changes the lasting run time by only like 15 minutes?

That doesn't make any sense!

Joe WB9SBD
--

The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com



--
Howard L. Brooks
Professor of Physics and Astronomy
241 Julian Science and Mathematics Center
DePauw University
2 E. Hanna Street
Greencastle, IN 46135
hlbrooks@...
Office: (765) 658-4653
FAX: (765) 658-4732