LRP SPX8 question

[sime46]

I have one of these and am very happy so far. My question is, although the manual says the BEC needs 7.4v, would 6v work? I've bought a 4s pack and was gonna plug the BEC into the reciever to give it a 6v supply to the speedo BEC. Otherwise the manual suggests picking my feed up from the balance plug on my 4s pack. Any help greatly appreciated. Cheers.

[Marvin]

No, I think it means 2S input max, for 6V output. Remember, the BEC can only take 2S, so if you're looking to run more than 7.4V then you'll need a separate BEC or a Rx pack.

[ojoloco]

I'm guessing it's 7.4v min, since the ESC is intended for 2-4S and all the info i could get about it didn't mention anything about external BEC's required.

Little off-topic, LRP SPX8 combo is light! I saved 123 grams from the MMM-2200Kv to the RX8-1900Kv, but this is even lighter! 100g less than the Tekin...I would be under 3000g now if i had the LRP...


PS: Yes, i'm a lightweight freak

[Tyson]

I have the SPX8 fitted in to my HB Ve8, If you run 2 normal stick packs to make 4s your ok. I do this using 2x Trakpower Dark 5000mAh 40c batteries.

There is a wire coming from the SPX8 with a switch that connects to the bridge wire between the 2 stick packs. Because you are taking a negative referance then the switch wire coming from the bridge wire is 7.4v.

When the system is connected in this way it does not unbalance either of the 2s packs (LRP designed it to work this way), if you are using a proper 4s single pack then it is not recommended that you connect the SPX8 to the balance port as it will unbalance the 4s pack, due to the reciever and all its devices draining one cell of the 4s pack unevenly.

Hope this explanation makes sence

You are right with the weight and size of this speedo, its so small it allowed me to make the HB Ve8 into a saddle pack like the caster. This turns it from a cool car to an awesome perfectly balanced car

[Marvin]

What? Yeah right, discharging one pack quicker than the other doesn't unbalance them, right on...

Also, that's a bit useless for anyone not using two stick packs. 7.4V max is rubbish - that's one of the reasons it's so light. By the time you add a BEC, it's not so light anymore, and you're limited to 4S max, unlike the Tekin, 150A Hobbywing, Losi and MMM, which don't need a separate BEC, and can run 6S (aside from the Losi, which'll do 5S).

[Tyson]

Perhaps i didnt explain right, tried to make it as simple as possible this time!

Connecting the BEC input wire from the SPX8 directly inbetween 2 saddle stick packs (4s) is apperently the best method as LRP only make stick packs no more than 2s. (They designed it around there product line not a 3rd party battery supplier)

The MANUAL states that if a single 3s or 4s battery is used, then you must correctly identify the the right pin on the balance plug of the 3 or 4s battery.
(This means that you are taking the supply voltage and current drain from particular CELLS within a 3 or 4s pack which also combined with other CELLS in the PACK supply the ESC)

*Perhaps this current drain from powering the RX, PT and servo on particular CELLS within a 3 or 4s PACK, could posibly leave those CELLS within the PACK at different voltages? i.e UNBALANCED CELLS in a PACK*

*Perhaps it might be better, if you use an external BEC when connecting a single 3 or 4s battery pack with the LRP SPX system?*

*IMO

I am also not telling anyone to go buy this system because it is better than MMM, Tekin, Losi etc, i was explaining the operation as dictated by the manual supplied with the SPX8. (I just own one)

[Marvin]

Right. Ok...

You seem to forget that one 4S pack is essentially two 2S packs wired in series.

As you correctly state for the 4S pack, plugging the BEC input leads into two of the 4 cells in the pack will discharge two of the cells at a higher rate, thus causing inbalance.

Utilising two 2S packs is exactly identical to the above. You plug the BEC input wire into one pack essentially (or as LRP say in their manual, between the two packs), which drains one pack, and not the other. To be able to discharge both at the same rate, and get 7.4V, they'd have to be connected in parallel. To get 14.8V for the drive, they'd have to be connected in series. There is no way to connect two battery packs in series and in parallel at the same time. I read LRP's manual, and there is nothing electronically special about a 'thin black wire'.

It is an SPX with a capacitor stuck on the side - simple as.

[offroadrc]

when you don't want to have questions about speedcontrollers for 1/8 cars get the ez run 150A

[Marvin]

Originally Posted by offroadrc

when you don't want to have questions about speedcontrollers for 1/8 cars get the ez run 150A

One of the reasons why I don't recommend the LRP, but do recommend the EZ Run, MMM, RX8 et al.

[sime46]

Wish I'd never asked. You are a grumpy bunch. I simply asked a question because of the gear I have. Its all a little different from the tenth stuff I'm used to. You lot bitch and moan at each other? Its not very inviting for newcomers to this class when everyone behave like smartarses and knobs. Think I'll wait until someone makes up thier mind as to what the hell this shambles class is gonna be. Remember girls, its only toy cars!!

[Marvin]

No, I was simply saying that the information given out by Tyson is incorrect - which it is based on all the evidence that we have.

I answered your question in the first post - I didn't get into any kind of argument about that.

I'll answer your question again shall I? The BEC's maximum input on the LRP SPX8 ESC is 7.4V (2S), and therefore, if you run more voltage than that, it is best to get an external BEC capable of at least 4S input.

[Tyson]

Originally Posted by sime46

Wish I'd never asked. You are a grumpy bunch. I simply asked a question because of the gear I have. Its all a little different from the tenth stuff I'm used to. You lot bitch and moan at each other? Its not very inviting for newcomers to this class when everyone behave like smartarses and knobs. Think I'll wait until someone makes up thier mind as to what the hell this shambles class is gonna be. Remember girls, its only toy cars!!

The funny thing is i am 100% 10th just bought a couple of 1/8th electrics a couple of weeks ago for me and my son to race at Bury, was only trying to explain how the manual reads regarding the LRP system.

Ok *perhaps* the second post was a little condescending

Sime i think we both have stumbled in to the "Marvin Zone" checked the posts out in the 1/8th section and now relise Marvin is god
Sorry god promise i wont come back

[Marvin]

I just like to post here .

Opinions and debate is all well and good - just some don't like it apparantly. I don't view anything said by either of us as grumpy...

You are most welcome, it's not my forum, I'm not admin, I'm not god, I can be wrong. However, from the evidence we have here, I can safely conclude (for now) that the SPX8 would be better off with an external BEC, and LRP haven't done anything to prevent faster discharge from one pack in a 2x2S set up.

[audriusv]

Originally Posted by Marvin

I just like to post here .

Opinions and debate is all well and good - just some don't like it apparantly. I don't view anything said by either of us as grumpy...

You are most welcome, it's not my forum, I'm not admin, I'm not god, I can be wrong. However, from the evidence we have here, I can safely conclude (for now) that the SPX8 would be better off with an external BEC, and LRP haven't done anything to prevent faster discharge from one pack in a 2x2S set up.

Hi,

well theoretically yes but in practice I get very similar discharge from both 2S packs. And I drive until LVC kicks in . Only modification I did was to install Deans. All battery I have use Deans not Bullet connectors...