# Battery Voltage Amp Question



## russfox (Apr 11, 2008)

Hi all,
I'm planning on running battery power with a QSI decoder and Gwire.

The question I have is if I use a 16.8 volt NiMH battery rated at 4200mAh and the locomotive draws 2.1 amps... I take this to mean that the battery would be dead in 2 hours of run time? Or does it mean that after 2 hours of run time or the voltage will no longer be 16.8 volts. I'm not very good at understanding how all of the volts and amps work.


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## Michael Glavin (Jan 2, 2009)

Typically batteries don't maintain rated voltage throughout their discharge curve. While 2.1A is an insignificant load on 4200mAh battery the average cell generally provides around 1.1V under load and finally @ .9V it’s reached its potential. More likely than not you'll see rated voltage hot off charge for a few minutes with diminished voltage thereafter. 

Michael


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## Greg Elmassian (Jan 3, 2008)

you have a 4.2 amp hour battery and a 2.1 amp load... yes, you could probably get 2 hours. Trying to discharge at a higher rate often does not give you a "linear" result. 

The capacity of a battery in amp hours is usually called "C"... discharging at 1/2 C (your example) typically would give 2 hours of run time. 

High performance batteries can be discharged at more than C, normal batteries are best at 1/2 C or less. 

Regards, Greg


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## TonyWalsham (Jan 2, 2008)

Actually, most batteries, with the notable exception of lead acid type, do maintain their voltage during the discharge cycle. 
This is certainly true of NiCd and NiMh which are slightly higher when freshly charged than the nominal voltage. This voltage drops quickly after starting discharge to the nominal voltage. This nominal voltage is then maintained until just before they are spent, when they will collapse in voltage quite quickly. 
The greater the mah capacity the longer the run time for a given current draw.


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## Michael Glavin (Jan 2, 2009)

Tony, 

Its been a while since I played with batteries, my recollection and testing support my assetion that typical cells do not maintain rated voltage under load. Load can be defined as rated capacity or C. Qualtiy cells with low internal resistance have trouble, typical cells perform worse. I'll see if I can locate the discharge curves I plotted several years ago. I performed tests with multiple batteries, capacity and chemistry with a PC and electronic load. NiMH maintains a flatter discharge curve than others but still average about 1.1V at the "C" rating, again if memory serves me. 

Michael


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## Greg Elmassian (Jan 3, 2008)

actually, nicad is flattest, nimh close, but more gentle discharge curve than the sharp discharge "knee" of nicads... 

Really the nominal voltage is not important, it's how the battery discharges... relatively flat and then goes quickly, or slowly declining... 

Regards, Greg


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## TonyWalsham (Jan 2, 2008)

I guess what we need to define is what is meant by a decline in voltage from fully charged to flat. 
It has been my experience that after the initial drop from fully charged to nominal voltage, and assuming the battery is capable of giving the current, and some batteries such as AA size cells cannot sustain much more than ½ - 1 amp without dropping voltage, the rate of decline in voltage is minimal until near the end of the charge. 
Certainly the voltage discharge graph is not dead flat. 
I agree with Greg that it is *relatively* flat. 
Certainly nowhere near as steep as the linear drop in voltage experienced during the discharge of non Deep Discharge lead acid batteries.


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## Torby (Jan 2, 2008)

Yup. It would run roughly 2 hours on a full charge. When the battery does run down, I find you have about 8ft of warning before the loco comes to a stop. If you're lucky. Usually Mr. Mallet crawls off to some inaccessible location to die in private.


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## Greg Elmassian (Jan 3, 2008)

Tom, what are you referring to with "it"? 

Your loco? What batteries? 

Greg


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## Torby (Jan 2, 2008)

His 2.1 amp loco and 4200MAH battery he asked about in the original post. 

Mr Mallet draws a bit less than that, going 2 hours or so on 3800MAH.


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## Michael Glavin (Jan 2, 2009)

Another variable with regard to run time is how hard we mash the pedal; full throttle operation for long running intervals is unlikely IMO. So yes 2 hours run time in this example should be doable, even with higher IR cells.

I was unable to locate my discharge curves without pulling my old server off the shelf. Looking at Sanyo's website it appears that their quality high discharge cells hold the nominal voltage line better than I recollect. I'll see if can figure out how to post the PDF files I saved. Sanyo's site appears to be down presently.
Michael


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## Greg Elmassian (Jan 3, 2008)

I don't think NiMh has quite that abrupt curve. You should be able to notice the slowing as they get discharged. Nicad will have a much more abrupt "end of capacity" effect. Of course nothing is as abrupt as a li-ion just shutting off. 

Regards, Greg


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## TonyWalsham (Jan 2, 2008)

I only ever use the high quality made in Japan Sanyo Sub C NiCd cells for most installations. Sure they are expensive but they don't crap out and they last longer in terms of the number of charge cycles. 
I use their ENELOOP Alkaline/NiMh hybrid AA size cells for smaller jobs. 

Those are the only sizes that I can get here that are relatively economical.


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## russfox (Apr 11, 2008)

Thanks everyone. 
This give me a better understanding on how these batteries work. 
My plan is to try and come up with a standard battery that would work in any of my locomotives using QSI and Gwire. 
I have a number of Aristo Dash-8s, SD-45s and e-8s and noticed I can remove the switch panel, under the removable roof section, and slide a battery into the locomotive. I don't need these switches or the smoke units and will install my own on/off switch. I noticed I can also make the roof section holding the smoke units on my USAT GP-38-2s into a removable access hatch by removing the inside screws. 

My thought is if I can find one type/style of battery that will easily fit in all my locomotives with stardard plugs... then all I need to do is lift off the roof hatch and swap out the battery in the field. All the batteries would be standard along with the chargers. The plan is to have all batteries onboard the locomotive... but are easily removed and no power cars. I don't want to be carrying the locomotive back to the house.


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## izzy0855 (Sep 30, 2008)

Question, how do you know if your train pulls 2.1amps per hour? Because you will not pull 2.1amps all the time, you may only pull 1 - 1.5 amps to start and stop your engine then another 500-800 milliamps per hour depending on your grade and the number of cars your pulling? Most of our 16.8V 4200mah HRD 20C Sub C Nimh cells with one engine, pulling 10 cars or less on a 3% grade or less, and depending if your blowing smoke or not you should get around 3 - 3 1/2 hours of run-time. But, if you're starting and stopping your engine every 10 minutes for a "beer break," then you'll probably get 3 hours of run-time. 

Rick Isard 
Cordless Renovations


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## russfox (Apr 11, 2008)

Hi Rick, 
I was using this as an example to get a better understanding on how these batteries work. 

I wanted to talk to you about your battery packs. I was looking for 16.8 volts but I'm now moving towards 18.5 volts based on Greg’s recommendation. If you look under my posting on wire sizes you will see a list of locos that I want to power by battery using QSI decoders and Gwire. I’m not planning on using any track power. 

I want to be able to drop the battery into the locomotive through the top of the roof. All of my engines have removable panels that would allow me to carry the battery onboard and be able to easily do a field swap. I want to standardize on the battery, connectors and charger so that I can put the same battery into any of my locomotives. 

The problem is that the 18.5 volt Lion Batteries are too wide to fit into the top. Can you build a battery pack that is longer and less high and wide? 

The smallest size would be 2-3/8” x 2-3/8” x any length. Thinking of the 18.5 volt Lion Batteries with at least 5200mah. Is this possible? 

I know you are working on a battery car with built in charge… but from the comments I read it sounded like people were looking for an easy swap out battery. I want to run long trains and have started to sell off all of my smaller engines for larger units where I can carry the batteries onboard. Maybe I’m in the minority but this is the direction I’m going. I also have space in my yard to put in 18’ (minimum) diameter curves to run larger engines. 

I would be interested in any thoughts you may have. 
Thanks so much, 
Russ


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## izzy0855 (Sep 30, 2008)

Hi Russ, it sound good so far and with the higher voltage you'll be able to go faster and quicker response, but let's see if I can explain the lithium battery size issue. We use the 18650 LG & Samsung lithium-ion cells, and the term "18650" means that the cells are 18mm in diameter by 65mm in length. Each cell is 3.77 volts each @ 2600mah and in order to get 18.5V, we weld 5 cells in series to reach 18.5V...and in the lithium world, welding cells in series constitutes voltage and welding additional cells in parallel adds more amps. So, to get to 18.5V 5200mah lithium-ion battery-pack we weld 5 cells in series and 5 cells in parallel. The size of these packs are 3-1/2 inches length, by 1-3/8 wide, by 2-3/4 inches in height...and the reason the pack is designed this way is because of the size of the PCB circuit board. We need to design the battery-pack around the size of the PCB circuit board and in most cases we are hampered by the size / millimaps / and voltage of these boards. Unfortuantely, this is as small as we can make this pack in the 18650 cells. BUT! I can purchase smaller lithium-ion cells "14500" 14mm diameter by 50mm length and this woudl decrease the size of the amps and voltage of the pack. 14500 cells come in 1800, 2000, & 2200 milimaps and if you add an additional row of cells to a 18.5V pack, the highest amps you would get is 4400mah. If voltage is not a problem you may want to consider our 22.2V @ 2600mah lithium-ion pack. The size of this pack is 2-1/4 inches in length by 1-3/8 wide by 2-3/4 inches high, and we can add any type of quick connector you choose.

Don't be discouraged about the size limitations on the lithium-ion packs, maybe what might work well in your case is a lithium-polmer battery. There long and very thin, for example: the polymer cells are kind a like you cell phone battery, the longer they are the more amp hours they will hold. Here are the measurements on a 14.8V 5700mah lithium-polymer battery-pack: 6" length by 1 inch wide and 2 inches high. Each individual cell is about 1/4 inch in width and 2 inches in height, but what changes to get more millimaps is the length. 

I hope I've answered some of your questions, and when you have time just give me a call, 
Rick Isard 
Cordless Renovations 
Office: 319-366-7294 
www.cordlessrenovations.com


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