Some PowerFunctions insights: The LEGO LiPo rechargeable battery and L

[Toastie]

Dear All,

for some time now, I have carefully followed the wonderful discussions in the various EuroBricks forums on the usage of power systems for electrification of LEGO models, particular the ones in the TrainTech section. Every now and then, there appear to be serious “How-to” questions with regard to 9 V/PF train “conversions”. Here are some comments about this issue, focusing on the LEGO Lithium polymer rechargeable battery (#8878) for application in electrified LEGO train models (Note: Sorry for using BrickShelf deeplinks, just uploaded the stuff ...)

Generally speaking, TLC appears to push the various PowerFunction elements as a more or less global approach for electrification of LEGO models, at least in the current “9 V domains” (Technic, Mindstorms, RC-Train), right? Recently a PF-type train motor appeared and in fact, the prefix “PF” appears to be well justified – as Philipe Hourbain has very recently demonstrated. After reading this article by Brian Williams in RailBricks, Issue 6, page 17, I was finally convinced that $50 may be worth spent for some LiPo “research”: Is the LEGO LiPo really that, a full blown real world “LiPo” or does it need to be purchased in pair with the LEGO AC charger (#8887) and be carefully treated like a raw 10 V egg? In other words: How does it behave electrically? What can we really do with it?

In short: The LiPo meets expectation in every regard! Here are the results from some measurements. To really benefit from the superb LiPo performance however, DC track power-pickup is necessary. This has been amply discussed in EuroBricks; I prefer modified 9 V train motors, where the original “9 V motor only” function is fully restored when a classical LEGO 9 V cable (with two 2x2 electrical plates) is connected to the terminal of the motor. Two more custom cables (DC pickup to LiPo with bridge rectifier, and PF receiver to modified 9 V train motor) are required as well.

There will be a full article about the LiPo performance in an upcoming issue of RailBricks – with all the details on cables, “test-bed” set-up, etc. (Basically, the approach is completely analog to my RCX stuff shown here (page 53, figure 9. Here, the RCX charging cable does not need to have a bridge rectifier though, TLC already built that into the RCX, very smart!).

Here we go:

1) The 7.4 V/1.1 A LEGO PF Lithium polymer rechargeable battery can be charged with a broad range of DC input voltages ranging from 9 V to 18 V, see figure below. 10 V or above is recommended, since the then available full charging current is minimizing charging time. 12 V seem to represent the best price-performance ratio, as judged from the availability of cheap “wall wart” type power supplies. In other words: You don’t need to buy the way too expensive 10 V LEGO charger! I believe, 10 V is simply a good marketing joke (who on earth else is using 10 V? Oh yes, sorry I forgot: Safety, safety, safety. Well, at least you can’t swallow cheap wall warts that easily, and the power cord can probably be viewed as a safety pull-back mechanism. Ha, another one: Google for “10v dc power”. Hit #1: LEGO S&H … they are smart at TLC, aren’t they).

2) The maximum LiPo charging current was calculated in this MBFR forum entry, replies #3 and #4 (you would need to polish up your German though ..., but I bet there are many relevant forum entries at EuroBricks as well) before to be around 750 mA average, which was based on information from the LEGO service department and the fine print on the LiPo (1 ½ hours charging time, 1.1 Ah LiPo capacity). The present measurements however suggest a maximum charging current of 550 mA, even with two stalled XL motors present either directly on the output of the LiPo or a connected PF receiver output, respectively, set to full forward, see figure below. So when your cheap wall wart can lift that, you should be fine.

Figure 1: LiPo DC input voltage vs. LiPo EDIToutputINPUT (arghh, sorry!!!) current, with a stalled(!) PF XL motor present. That thing should draw more than 1 A ...

At 20 V DC-in the LiPo "turns off", and can't be re-animated anymore. Well, I did some measuremenst on the PCB, see Philo's photos here, everything seemd ok, but it still played dead. I waited 4 days, and then: Bingo, back alive, however with fully discharged battery. Charged it at 12 V, did it again, it "died" again at 20 V, same procedure - the LiPo is doing well again. A-m-a-z-i-n-g!

3) This suggests that for electrification of large train layouts with constant DC new or used computer power supplies are perfectly well suited; they deliver easily well above 10 A on the 12 V line. 12 V car lead batteries would also work well for many, many hours at high total track currents and they can be recharged with relatively cheap power supplies, e.g. overnight. There are many more options.

4) The LiPo works well as uninterruptible power supply (UPS); the output characteristics remain pretty much constant, regardless of charging voltage. The transition from DC to battery power is smooth and without any problems. There are some reports on issues with the interruption of the charging voltage (i.e., LiPo turns off). This has been observed in the present measurements only, when the charging plug was forcefully removed from the LiPo, but never when the charging voltage was simply turned off (e.g., by driving a DC pickup train from powered to non-powered track). The figure below shows the performance of a test-bed train on a layout with two reversing loops (short DC blackout, polarity change), some RC tracks (power outage), and mostly powered 9 V track present.

Photo of priciple of operation (the charging line has an SMD-type bridge rectifier):

Photo of train test-bed:

Figure 2: Round time vs. total run time of the test-bed shown above (Hybrid 9V/RC train, power pickup, PF LiPo, PF receiver, 10 cars, speed level "3", both motors on one PF receiver output. Red curve: Fully charged LiPo, no track power. Run time slightly above 4 hours. Blue curve: Starting with entirely flat LiPo from red experiment, with 12 V track power. LiPo seems to net-charge over time, since round time eventually goes down. After 7 hours, 12 V DC was removed: Indeed the LiPo was almost fully recharged, it ran for another 3 hours

Summary:

The LiPo is a PF element which readily combines the best of both worlds, PF and 9 V train. DC power pickup is required in some way though.

Due to the relatively stable output of the LiPo, train driving characteristics are virtually constant when going over powered 9 V or RC plastic track.

More than one train can be operated on one stretch of powered 9 V track (I know, we all knew that already ...)

Charging of the LiPo is automatically enabled on powered stretches of 9 V track; no access to the charging jack is necessary. This means that your train design is less restricted; just completely hide the LiPo somewhere inside and stop the train on powered track.

What about using DC power from you 12 V train layout for feeding a LiPo equipped PF train as well? I have no 12 V tracks, so this is pure speculation, but should work well: The 12 V train system features a beautiful power pickup brick (#x552). If you run the 12 V via a bridge rectifier into a PF LiPo, you can use all your PF elements on the 12 V system as well … which then of course means that you may want to cross over to 4.5 V tracks without a glitch due to the USP function of the LiPo battery.

This is what I’d call “Building across multiple themes”. As far as I am concerned, TLC did it again.

There is more to come – the PF receiver is another very nifty piece of equipment …

Have fun + Play Well

Thorsten

Edited September 3, 2010 by Toastie

[hoeij]

What happens in this setup when the train is placed on the track facing the wrong direction, so that the LiPO battery received -12 volt instead of +12? Is there some protection built in, or is it RIP?

[Siegfried]

What happens in this setup when the train is placed on the track facing the wrong direction, so that the LiPO battery received -12 volt instead of +12? Is there some protection built in, or is it RIP?

He said there was a bridge rectifier; that should prevent any reverse voltage.

Man... I'm impressed. I really want one of those batteries now!

[talltim]

A very interesting post, I'll have to come back and read in more depth later

[Carefree_Dude]

A very interesting post, I'll have to come back and read in more depth later

This opens up many possibilities. I have two of the batteries, and I would be happy to try this out myself some time. I have more 9V track than I know what to do with. Would you use the old speed regulator to get power to the track still though?

[Toastie]

What happens in this setup when the train is placed on the track facing the wrong direction, so that the LiPO battery received -12 volt instead of +12? Is there some protection built in, or is it RIP?

Hi there,

nothing happens at all, the LiPo has a built-in diode labeled "A" on the copy of Philo's photo below:

However, the LiPo would also not charge, which is not effective. This is why you should add a bridge rectifier into the charging line - in this case there is always + 12 V connected to the center and 0 V to the outer part of the DC plug (BTW the plugs frequently come with cheap wall warts, just cut of the plastic base of the 3.5 x 1.35 mm plug. I did that as shown in the photo below:

I am prefering the small SMD types, as shown here - the rectifier should rate about 1 A:

Once installed, you will always have charging voltage with the correct polarity at the LiPo DC-in jack.

Regards,

Thorsten

[roamingstop]

What is the supply voltage loss over the rectifier?

[Toastie]

This opens up many possibilities. I have two of the batteries, and I would be happy to try this out myself some time. I have more 9V track than I know what to do with. Would you use the old speed regulator to get power to the track still though?

Hi,

well, you could, just set it to full forward or backward all the time - the speed regulator puts voltages close to 9 V out. However, I'd rather go with a 12 V supply for the LiPo - max. loading current is less, which is alwys good for longer stretches of track, since the voltage drop will be smaller.

Regards,

Thorsten

[Toastie]

What is the supply voltage loss over the rectifier?

Well, I should have mentioned that, sorry:

There is of course a voltage drop across the rectifier. The SMD type I am using leads to a measured 1.4 V drop as expected (12.0 V DC at power supply, 10.6 V DC at LiPo in) since two diodes are always part of the conducting branch.

If you'd go with Schottky type diodes, the drop could be reduced to about 0.6 V.

The voltages in the figure shown are nominal voltages at the LiPo DC-in jack.

Another good thing when using the LiPo as UPS on a PF receiver conrolled train on DC powered track: The max. current it draws from the DC line is 550 mA (10V) and less at higher voltages, regardless of it's charging state. Should the train motors need more, they simply would draw that from the LiPo battery, which can deliver much more than 550 mA. This way, you are pretty much safe regarding max. total current on your layout, should you run more than one tain on a "demanding" powered stretch.

BTW, since about 3 months I am now using a €20 13V DC 65W (5 A!) switching power supply

, which I got from "Lidl" last summer - the sticker says the thing is good for 12V applicances such as small fridges. 13 V is perfect!

Regards,

Thorsten

[broomhandle]

If this can be done, what about a custom battery pack? creating our own smaller, cheaper recharge packs?

[Toastie]

If this can be done, what about a custom battery pack? creating our own smaller, cheaper recharge packs?

It can absolutely be done. LiPos are all over the place, the charging circuits as well. However, they did a good job on the LiPo part!

Cost reduction:

My guess is it may be rather tough to go dramtaically down in cost, at least if you want to stay with:

1) capacity of LiPo >= 1000mAh

2) UPS functionality incl. primary charging current restriction

3) Pulse width modulated "extra" output (the PF C1/C2 lines) of the LiPo

Size reduction:

Could work, but again not dramatically. You could get the LEGO LiPo PCB + battery out, put the top part back on , there is a considerable amount of "air" in the bottom part of the LiPo box. This may be a safety things again, fast charging LiPos may get - well - warm. But the electronics does not allow that, so you'd be safe.

I guess the $50 bill is not that far out of the real world as the $25 for the LEGO wall wart. That's a joke.

Regards,

Thorsten

[Lighti]

I digged out this old thread, as I seem to have a problem charging the 8878 Box.

I just got it last week, and search function brought me here when I was looking for a charger other than the 10V Lego one.

From what I understood in the initial post, the 12V 500mA powersupply that came with my eneloop charger should do the job.

It charges for over 8 hours now, and is still blinking. The Box itself came from bricklink, but it was bought as new and came sealed, so it should not be broken.

Anyone else had trouble with the initial charge? or with a 12V charger? what should the normal time be with 12V and 500mA?

thanks in advance...

[Rick]

I digged out this old thread, as I seem to have a problem charging the 8878 Box.

It's good that you search for existing information on a topic, but please don't bump multiple topics with the same question.

[JopieK]

@Lighti: This might be a good solution for you...

[Lighti]

It's good that you search for existing information on a topic, but please don't bump multiple topics with the same question.

ok sorry, since the topics are in different subforums, I thought asking multiple parts of the community would be ok.

Wont happen again.

But there is indeed different information about that topic, since someone in the technic forum told, below 9,5V is no charging, and some guys here say don't go above 10V. But it seems that 9V and 12V work perfectly fine if the charger delivers enough mA, and my 500mA are not enough.

Is it ok to reply again in the thread in the technic forum, and post a link to this thread here? Since the information in the initial post is pretty helpful.

@JopieK: that is indeed helpful, but doesn't solve my actual problem.

The charger I used has the proper plug, but not enough current. I will buy a new one with multiple adapters and 1000mA.

[Rick]

Is it ok to reply again in the thread in the technic forum, and post a link to this thread here? Since the information in the initial post is pretty helpful.

Yes, it is, as long as the discussion does not become scattered across two topics.

[JopieK]

@Lighti, it needs to be able to draw enough current indeed, that is why I used a 2A version! I have not tested what a AC adapter does with the LiPo, I would not recommend it, but I might try to see how the LiPo behaves (for practical reasons in my school :))

[Toastie]

Good evening, has been some time since I've been here ...

Don't worry about the blinking charging light. The LiPo is most probably fully charged. I have the same issues - sometimes - sometimes not. Take the charger off, plug it back in: Same blinking? Or do you get a steady light?

As you can see in the chart (initial post) it takes for ever to charge the LiPo with 8V. 10 V is OK, but more is even better.

Best regards,

Thorsten

[LEGO Train 12 Volts]

Welcome back Herr Professor!!!

I like to see your avatar back again!

Edited October 16, 2013 by LEGO Train 12 Volts

[Toastie]

Well, thank you very much for your kind "welcome", LT12V, it's good to be here again!

[elicend]

Hi

this is a great solution.

but sorry, my english is a little too bad, and i missed a detail. In summary :

you pick up 9v (or 13v) from 9v track

It is drive by a bridge, in order to have + and - in the same place

You use a standard battery box (rechargable one)

and you mix RC track (non electric track) and 9v track

Is it possible to have just a little part of 9v track, for example, 1meter, an 9 or 10 m rc track ? and just stand the train on the 9v track after using for charging, then, drive on both track? in my mind, i think there will have no problem to stand on the 9v track for charging (nothing is needed to stop charging when full????), but my question is more when you drive and pass 1 9V track, then 1 rc, then 1 9V (caricatural, but, possible?)

thanks for your informations ;)

[JopieK]

of course a charge section is very possible. the bridge is short for bridge rectifier, so it indeed rectifies the polarity so that you always have the correct polarity but could even use alternating current instead of direct current (but will need capacitors to flatten the output current then). you could of course make a solution with RC, 9V, RC, 9V but that will cost you a lot of cables and probably your batteries will wear out very fast so, maybe not such a good idea.

[elicend]

Thanks,

the batterie box have a recharge module added isn't ?

I am agree, a RC 9V RC 9V track is not a good idea, but in my mind, i think i'll do 1/4 in 9V and 3/4 in RC, and stand in the 9V to recharge...

And if i understood, it is possible to let 9V in the 9V area when train runing, it will charge a little each time it will go in?

Edited December 21, 2013 by elicend

[JopieK]

yups that would be possible I think!

[elicend]

And the battery box will stop charging when battery is full or do you have to manage it manually?