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Revisiting 1-Cell Versus 2-Cell Li-Po
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 Posted: Fri Apr 1st, 2016 05:33 pm
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davecttr
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Sorry about starting a new topic but multiple searches have failed to find the original one where the subject was discussed. I conducted some tests yesterday and think i now have the answer!

Briefly my problem was some of my large locomotives draw about 0.4A which is marginal for a 9V Pololu regulator fed from a single cell lipo. Using 2 cells in series vastly increases the efficiency of the Pololu which can handle up to 0.8A or more, however do you still get the same run time? For example a large loco with a 1S 220mAh battery may run for 30 minutes. Will the run time be the same for 2 batteries in series?. Generally the last discussion reckoned that the extra battery gave more efficiency but no longer run time as you were just getting double the voltage.

The tests.
TEST 1 - I fitted a loco with a 9v Pololu and a single 70mAh lipo, set the Rx LVC cutoff to 3.4V and ran a timed test hauling 8 coaches at scale 60mph.

Result - the LVC stopped the loco after 21 minutes.

TEST 2 - 2 x 70mAh batteries were used in series with the LVC cutoff set to 6.8V. Again the loco hauled 8 coaches at a scale 60mph

Result - I stopped the loco after 47 minutes as I was worried the LVC had not worked. I need not have worried.

The test 1 battery had a residual voltage of 3.68V and the test 2 batteries 3.70V and 3.72V

so it seems that adding a second battery in series not only allows the Pololu to power a demanding loco plus you get at least twice the run time.

Happy Days!



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 Posted: Fri Apr 1st, 2016 09:45 pm
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fallen
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Very useful info Dave, thanks for taking the trouble!

The point about the voltage step-up circuits is that the more they step up the voltage the more they also step up the current. So roughly if they double the voltage to the receiver then they also double the current drawn from the battery.

So the same physical volume of battery could be either a big 1S or two cells wired as a 2S with twice the voltage but half the current capacity. Or as I think we concluded last time that two cells could be wired in series as 2S or in parallel as a 1S battery with twice the capacity, and it would make no difference to the duration, give or take the chages due to the efficiency of the convertor.

What you have done is slightly different. When you add the second cell, the voltage from the cells doubles so the convertor is not having to do so much work. It is not having to push the voltage up by so much. As a result it does not increase the current draw so much either. The current drawn from the two cells will be roughly half that drawn from the one cell. So the two cells should last roughly twice as long, which is what you have.

Good to know how it works out in practice though - many thanks for the info.

Frank

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 Posted: Sat Apr 2nd, 2016 11:41 am
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davecttr
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Thanks for describing the physics behind all this. I admit I can't get my brain around the concepts but at least it seems to work.

I can now start converting my big locomotives to 2S operation. I expect to get an hours use out of a pair of 220mAh batteries which is ample for my 15-20 minutes average run time in an operating session.



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 Posted: Sat Apr 2nd, 2016 11:59 pm
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d10ng
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Dave,
Do your locos need the step up still with having 2 cells? Would 7.4v not be enough to run them at scale speeds?
I'm still trying to determine my HO diesel needs, but will go either 9v step-up with a 1s or just a 2s if I can find the right sizes.



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Thanks for your time
Dave

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 Posted: Sun Apr 3rd, 2016 03:29 am
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davecttr
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With testing I found that 7.4V was not enough to give me speeds over a scale 60mph when hauling 12 coaches. The 9V Pololu allows all my express locos to have top speeds of 60 to 70 mph.

Another advantage of using a voltage regulator is you get the same throttle response and top speed throughout the battery discharge cycle. A pair of unregulated lipos will deliver 8.4V when fully charged but this will then drop to 6V when the batteries need recharging.

I have decided all my locos will have voltage regulators fitted, most with 9V Pololus but maybe one or 2 will have the 5V version



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 Posted: Sun Apr 3rd, 2016 03:38 am
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Rod Hutchinson
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That's interesting Dave. I am in the throes of setting up a 2S. However my locos are geared locos so whether to add a 9v stepup is something for me to mull over.

My other two geared locos have 1S with a 9v step up.



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 Posted: Mon Apr 4th, 2016 03:07 am
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johnhu
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Talk about timing. :-)

I had been having an issue with some of my larger locomotives (On3 MMI loco's) that were originally configured with 3 800mAH batteries wired in parallel (so a 1S pack) supplying power to a 12v step-up which feeds a CVP airwire (convrtr) receiver.

Every so often when running, they would just come to a sudden stop for a fraction of a second, and then start running again.

Looking at the airwire receiver - which has a power LED lit when it has power - I could see that this power led blinked off when the loco stopped, so obviously the issue was power related. I checked the batteries (all ok), and also replaced the step-up with another just in case, and still it happened.

I then plugged in a spare 3s battery pack to see what might be going on, and the issue stopped happening. So I came to the conclusion that there was an issue with the step-up and the current being drawn by the larger motors in these models.

I have since reconfigured the batteries from 3 in parallel, to a 3S pack (with a wired in balance connector), and all has been fine.

In smaller locomotives (with smaller more efficient motors), the step-up converters have worked ok, but the ones I were using did not handle the larger constant current loads. Of course I could have purchased a different step-up that has a larger current capacity to solve this issue also.

As Dave noticed though, due to the in-efficiencies of the step-ups, I've noticed that I now get a longer run time from the 3 batteries wired as a 3S pack, than I did when the same batteries were wired as a large 1S pack with the step-up.

So for now on the larger loco's, I'll stick with them wired as a 3S pack.

John

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 Posted: Mon Apr 4th, 2016 03:17 am
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Rod Hutchinson
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The stepup experiences presented are providing clarity in their performance.

The issue of current draw for different size motors seems to have an impact on the ability of the stepup to perform.

Changing motors to low current draw but highly geared, such as the locos by Bernard Snoodyk, is an option but complex mechanically.

Last edited on Mon Apr 4th, 2016 06:14 am by Rod Hutchinson



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 Posted: Mon Apr 4th, 2016 05:34 am
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Bob R
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Johnhu

I have had similar experience with the step up regulators when used in higher amp draw engines. The Pololu regulators are current protected. When too much current is drawn they shut down for a moment and then reset automatically. Keep in mind that the more the voltage is stepped up the more the current draw. I use 1S lipos in my small engines with minimal amp draw but use 2S in larger engines.

Bob R



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 Posted: Mon Apr 4th, 2016 07:46 pm
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mwiz64
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If you raise the voltage, you lower the current draw for a given work load...at the motor but sometimes a different winding on the motor is necessary to take advantage of the higher voltage, do to the KV of the motor. Of course, the opposite is also true. If you run a lower voltage, the current draw will be higher for a given work load..... and your battery will run dead quicker. Voltage boosters, as you've seen, use up energy. There ain't no such thing as a free lunch. Run the voltage you need from the battery and skip the booster, if you can.... even if it means running two really small cells. Or at least be open to giving that a try. You might just be surprised at the duration of the charge. Higher voltage is more efficient.



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