# Capacitor Voltage to Replace Battery in a Sierra?



## toddalin (Jan 4, 2008)

I was getting the engines ready for the season and as per usual, the batteries in the engines with Sierras have gone south. Rather than replace them, _again_, I was thinking of replacing them with some super-caps. Of couse I would have to use a couple in series to get a sufficient voltage.

The original battery is 6 volts, but obviously when it is charging from the rails, the voltage at the leads would be higher. Does anyone know what the maximum voltage is that the battery leads gets up to? Then I could size the caps a bit larger.

Thanks


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## beavercreek (Dec 12, 2008)

The lead acid gel batteries can get to nearly 7 volts. If you are charging up the caps from the sound card then you will not need a regulator (as the card regulates the charging voltage), but if you are charging it from the rail voltage then you will need to have the supply regulated and also rectified before it gets to the caps.


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## toddalin (Jan 4, 2008)

They would simply replace the batteries being wired off the sound card and I would disconnect the charge jack to avoid any "accidents" (not that it's necessary). I'm concerned about any peak voltage spikes and wouldn't want to have a cap explode within the engine


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## beavercreek (Dec 12, 2008)

The caps would be charging at the card output rate and it only does this when the track voltage is above a certain voltage (I think that the Sierra is about 7.5 to 8 volts). The charging supply should be perfectly fine for the caps.
Have a look here at Georges tips: It is only for replacement batteries but it does show the output etc

http://www.trainweb.org/girr/tips/tips6/sierra_tips.html 

And also here
http://members.shaw.ca/SaskRailMirror/dcc/Sierra-battery-charger/index.html


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## toddalin (Jan 4, 2008)

Posted By beavercreek on 02 May 2012 12:12 PM 
The caps would be charging at the card output rate and it only does this when the track voltage is above a certain voltage (I think that the Sierra is about 7.5 to 8 volts). The charging supply should be perfectly fine for the caps.
Have a look here at Georges tips: It is only for replacement batteries but it does show the output etc

http://www.trainweb.org/girr/tips/tips6/sierra_tips.html 

And also here
http://members.shaw.ca/SaskRailMirr...index.html 



Thanks. I should have known that George has covered this.

The NiMH look like a nice alternative. I would think that I could also charge these off of the 6 volt, 100 mA wall wart provided with the Sierra. I'm wondering how long the board would run from ~1.6 farad of super cap (@ 7.5 volts), which would probablty be a bit cheaper (~$11), but may not maintain the sound at low speeds.


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## toddalin (Jan 4, 2008)

I decided to try the supercaps and ordered a lot of 10 from the Bay for $18.25. That should do three systems with one cap left over. These are 6 Farads at 2.7 volts, so three together will still give me 2 farads at 8.1 volts which should provide some "cushion" at about $6 for each system.

I didn't like the idea that it takes a day to charge up the NiMH cells.


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## Dan Pierce (Jan 2, 2008)

I have used the super caps on LGB sound cards for several years, and I used a 8 volt regulator to limit the charge to 8 volts. 
I like not having to charge batteries or swap out ones that go bad. 20 seconds of power is more than enough for me.


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## Greg Elmassian (Jan 3, 2008)

Isn't the "battery output" voltage on the Sierra already regulated/limited? 

Greg


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## toddalin (Jan 4, 2008)

Actually I thought about it and I will have 6 Farads at 8.1 volts. That's a lot of Farads, and I think that the ~30 second idle period after the rails cut voltage should be no problem to maintain. I just wish the lead time out of the Far East wasn't so long (2 weeks).

Because I will leave the existing charge jack in place, I should be able to pre-charge the supercap above the rating output of the Sierra board. I think the existing charger (for the old gell pack) is 100 mA. I could even have a bank of supercaps precharged and instantaneously charge the caps in the engine from the bank through the jack, and would probably include a high wattage, low resistance resistor to limit the current transfer.


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## toddalin (Jan 4, 2008)

An update.

The supercaps arrived from the Far East today. These are rated at *6 FARADS* at 2.7 volts (2.85 volts surge). But BION, they are only about the size of a _*20 microfarad*_, 50 volt cap!

I wanted to be able to "pre-charge" the system through the original 6 volt, 100 ma wall wart that was supplied with the Sierra system. I figured that three caps in series would give an 8.1 volt capacity (8.55 volt surge). I know that an "unweighted" power supply will give more than the rated voltage, so figured that I had better check the wall wart unloaded before proceeding and possibly having the caps explode. Well the unloaded voltage from the wart was 8.75-8.8 volts. Figures!

So I added a fourth cap in series to see what the voltage would come up to when actually charging the caps. The fourth cap should raise the working voltage rating to 10.8 volts (11.4 volts surge) and provide plenty of reserve to test the wall wart under the load of the caps.

Under the load of the caps, the voltage quickly came up to about 7 volts then the rate slowed. After a couple minutes, I terminated it and the voltage was 8.0 volts..., barely creeping up reallllly slowly. Nothing got warm. So, I figure that the three caps with a rating of 8.1 volts should do the trick.

I discharged the cap through the motors of an USA 44-tonner that was having its wheels cleaned and under no load (upside down), the motors actually ran for a couple seconds before the caps discharged! Should be plenty of juice to let the Sierra idle for 30 seconds.


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## Dan Pierce (Jan 2, 2008)

You could use a 12 volt power pack and charge the caps via the LM7808, or use a LM317 and adjust it to 8 volts or lower.


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## Dennis Cherry (Feb 16, 2008)

Do you realize that your 6 farad caps connected in series is now only 2 farads?


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## Greg Elmassian (Jan 3, 2008)

Actually, he added a fourth cap, so he has 6/4 farad, i.e. 1.5 farad.... 

Greg


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## toddalin (Jan 4, 2008)

Posted By Dennis Cherry on 20 May 2012 07:56 AM 
Do you realize that your 6 farad caps connected in series is now only 2 farads? 

Yeah, if you go back and reread the post, that's what I originally figured. But even 1.5 (or even better 2 Farads) will be plenty for the Sierra to idle for half a minute. So for $6 per system, I'll be done with replacing those gel cells every year or two. And these are so small, that space (and weight) is not a consideration as it was with the gel cell.


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## Greg Elmassian (Jan 3, 2008)

Yep, I think you can take Dennis' and my comments literally, as a simple question... nothing more. 

Regards, Greg


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## Dennis Cherry (Feb 16, 2008)

Posted By Greg Elmassian on 20 May 2012 01:23 PM 
Yep, I think you can take Dennis' and my comments literally, as a simple question... nothing more. 

Regards, Greg 
I posted my comment for others who want to use the Super Caps the same way for them to understand what happens in wiring any capacitors in series.


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## toddalin (Jan 4, 2008)

I tried out the system using the three supercaps in series. This is definitely the way to do it! I put a 12 volt battery to the wheels and measured ~7.1 volts at the Sierra battery terminals, so tried the supercap.

Without any pre-charge on the cap, I turned off the motors, and attached a 12 volt battery to the wheels and let the engine (USA GP-9) "run" _*for just 30 seconds at 12 volts*_ with the sound system on and front and rear Mars lights and strobes running, then disconnected the battery from the wheels. The sound system continued to run for *75 seconds* going through the regular routine of having the bell sound then the idle, lubricating of the journals, etc., before finally sputtering out. The Mars and strobe lights continued to run for an additional ~10 seconds before finally extinguishing.

I then tried a 1 minute "charge" and this extended the idle by only an additional ~10 seconds, so at 30 seconds of track charge, the cap is pretty much near full charge.

For me, 85-90 seconds of "unpowered" sound is fine because I let my diesels stop for service and to take on fuel for 30-60 seconds before moving on.

There are bigger supercaps available that are only slightly larger and pennies more if one needs more time than this.


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