# Digital Battery Capacity Checkers



## Homo Habilis (Jul 29, 2011)

I have just converted my first engine, a Bachmann Davenport to RC/battery. A big thank you to Kevin Strong for his Kaydee coupler modification instructions and to Del Tapparo for the RailBoss 4. I can't imagine being happier and the conversion being simpler.My question now is how to determine how much charge is left in the battery? While the RailBoss has a low battery voltage warning system, I was hoping for some type of gauge or meter that would allow me to test how much is left before I start running.I found digital battery capacity checkers that, conceptually at least, appear to do what I think I want. My plan would be to have something that plugs into the charging jack and gives me a reading of the amount of "juice" left. This is the type of meter, RCX Power Smart Guard, that I found, but it obviously won't directly plug into a charging jack. What are others doing or am I over thinking this and just let it run til it "dies"?

Thanks for any information that you can share.


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## Gary Armitstead (Jan 2, 2008)

That's a great product! I hope someone will step in here with an answer to how to hook it up to our existing batteries in our engines.


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## Dick413 (Jan 7, 2008)

**** habilis why can't you just buy a cheap volt/ohm meter $3 to $5 than go to R.S. and get a plug that fits your charging jack hook the two together 
set to low dc volts and plug in, should get volt reading. 
Dick


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## Homo Habilis (Jul 29, 2011)

Dick:


Thanks for your response.



The last electronics that I had was about 50 years ago and I wasn't a very good student. That being said, Li-ion battery chemistry didn't exist and I remember little of the lead-acid "theory".



I'm assuming that as the battery discharges it will continue to supply its rated voltage with a bit by bit reduction. As the battery is used, the voltage will drop lower and lower until the minimum is reached and then it will shutoff to prevent over-discharge.


I was assuming that a simple meter would not be good choice to show how much charge is left in the battery. If I am interpreting your suggestion correctly, as a simplistic example, a battery rated at 14.8V fully charged, would have a reading of, say 7V, and that would indicate that 50% has been used/remaining. Is that correct? This example ignores the realities of over-discharge and the protection circuits.


If your suggestion is truly how it works then as I stated I am over thinking this! I guess that I'll do some measurements and if it works as you indicate then I've got all that I need already in my parts box. Thanks.


By the way, I found this Fuel Gauge for PCB while searching around. Anybody think this would work as a simple indicator of how much is left?


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## Dick413 (Jan 7, 2008)

**** habilis 
if your using a Li-ion the cut off is around 12 volts you said " My plan would be to have something that plugs into the charging jack and gives me a reading of the amount of "juice" left." 
so the meter will do that for you. The fuel gauge you found is more money and can be off by 5% and is just lights. The first meter you found your bat. has to be wired for balancing. 
Dick


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## Homo Habilis (Jul 29, 2011)

Dick:

Thanks, then the meter it is!


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## Dick413 (Jan 7, 2008)

here is a meter you could hard wire http://www.hobbyking.com/hobbyking/store/__39148__In_Line_Voltage_and_Amperage_Meter_USA_warehouse_ 

, most people I know and myself just run for the day than chg. up. I get over 4 hrs run time. you can chg. over 500 times on most bat. now 
dick


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## Del Tapparo (Jan 4, 2008)

It is very simple. Just plug your battery pack/loco into the charger after each run. You then always have a full charge! Does't get any easier than that.

All of these "charge meters" are simple volt meters. The same thing built into the RailBoss. It just can't tell you the charge until you are near full discharge (well it could, but I choose not to make things more complicated than they have to be.


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## Michael Glavin (Jan 2, 2009)

Lithium-Ion cells typically discharge to 3.0V/cell. Discharging below 2.5V/cell will evoke the battery's protection circuit and put the battery to sleep. 

The RCX Power Smart Guard device works well for field testing batteries WITH balance pigtails (this gadget is labeled with different names and colors; mine is offered by Hesperian). Li-Ion batteries manufactured for consumers don't typically utilize balancing pigtails for monitoring/maintaining cells voltage, cell/battery balance and or charge/discharge regimens. Inexpensive Li-Poly hobby batteries typically are offered with the aforementioned balance pigtails, I add them to Li-Ion batteries... 

In simple words; it matters not whose or what device you use, measuring voltage to determine battery capacity is a poor excuse for a fuel gauge..... That said its a common practice. I use an expanded scale or loaded voltmeter to evaluate a batteries potential, these devices place a 'load' on the battery under test while displaying the voltage (some are fixed current devices while others start at 250mAh and scale up to 1 amp or better). 

OEM's suggest 80% depth of discharge for Lithium-Ion cells. Repetitive full (100%) discharges lowers the anticipated cycle count significantly. Best practice is to routinely charge Lithium chemistries, matters not what the state of charge is with Lithium! Worst thing to do is run them down regularly.

Michael


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## Greg Elmassian (Jan 3, 2008)

I was watching to see if anyone would bring up this point. Voltage tells you very little, and discharge curves (voltage vs. charge remaining) vary. It would be great to have a circuit recording 'charge used' but I don't know of anything like this both small and inexpensive. 

Greg


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## BigRedOne (Dec 13, 2012)

When I flew radio control I had a stopwatch I attached to my transmitter. This allowed an estimate for the fuel remaining in each flight, and the total time the radio's batteries were in use. Although not a direct measure of remaining battery capacity, it is at least a way to gauge use - and with a very low cost addition which served multiple purposes. 

Some type of resettable timer powered in parallel with the locomotive's drive may be another option. Boats have hour meters, and although intended to mount to an instrument panel, should readily fit in a large scale boxcar. It all seems too finicky compared to just charging up before running (or having a spare battery to swap out.) In radio control aviation I always charged prior to an afternoon's flying.


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