Topic: 2s to 4s changeover switch?

Is there a simple way of converting 2 off 2s packs from 2s 2p to 4s 1p usin some sort of in flight changeover switch?

Why on earth would you want to? monitoring LiPo voltage wouldn't be easy the battery pair suffering unequal discharge, yes it would be possible.

J

You would need a fast break before make relay.  You'd have to break the parallel conenction before connecting in series.  That would give a temporary open circuit which might or might not cause a reboot of the esc (and even more problems if you are using 2.4 rx).

What is the problem / challenge you are trying to solve ?

S.

This is what I do on my bike:





Phil

Thanks for that.

I have sketched the power change over circuit and need 2 x normally open and 1 x normally closed contacts.  I was sorta hoping that some electronics expert would come up with a fairly simple multiple FET based system. Not running BEC so slow break  etc is not required.   Ideally signal driven off spare rx channel.   looking at 50w on 2S and 250-400w on 4s.

The reason? to allow an ESC to run flat out- at max efficiency?

You could do that with a 4 pole double throw relay and a microswitch on a servo.

What sort of efficiency is lost by running part throttle ? I know that's when ESCs can get hot - but that could be only a 1% drop in eff. (completely made up number from 400W total, 4W lost in ESC - that would start getting hot).  How does that compare to spending a bit extra on the ESC or motor.

Interesting idea though let us know how it goes.

And there is anther issue that John might have been hinting at ...

If in 4S mode there cells are not 100% matched and drain unequally (giving different voltages across the 2s packs) then there is a risk when you put them into 2s2p mode in one feeding the other to balance - how big this effect is and what the real consequences are I have no idea (always used to be stated as a big issue with creating new xP packs).

Actually the risk is the other way around. When in 4s mode the cells will drain at precisely the same rate because the same current is passing through all four cells. But in 2s2p mode the two parallel pairs could contribute unequal current (which is one of the reasons why the large series/parallel arrays of lipos that were around 3-4 years ago were phased out in favour of larger capacity cells).

There are a number of associated problems with this concept Simon - you'd have top arrange continuity of power to the Rx, and either reboot the ESC at each voltage changover (which might require specific throttle movements) or find an ESC that was "happy" switching voltages. Not sure how the ESC would respond to a "down-shift" where the input capacitors would be at a higher voltage than the incoming cell voltage etc etc.

There's also the minor concern that you'd need to be damned certain that there were no fault conditions which could cause a make-before-break event that would short the batteries...

I still think you'd be better off with a variable-pitch prop to achieve the same thing.

PDR

Actually the risk is the other way around. When in 4s mode the cells will drain at precisely the same rate because the same current is passing through all four cells. But in 2s2p mode the two parallel pairs could contribute unequal current (which is one of the reasons why the large series/parallel arrays of lipos that were around 3-4 years ago were phased out in favour of larger capacity cells).

Really ?

Yes - my bad in choice of the word drain, the current will of course be equal BUT unless the cells are matched it is the operation in series that will result in differing voltages at the cell terminals leading to the aforementioned issue when connecting them in parallel.

When in parallel some cells could, for various reasons, deliver more current than others but their terminal (and terminal) voltages must be the same.

For some (probably unfounded) reason I thought that arrays of xSyP got replaced due to balance charging issues when built as (xS)yP (which gave the flexibility to split packs back down to 1P) verses (yP)xS which meant packs could not be split into 1P and manufacturing costs were lower for 1 pack of twice capacity.

I could of course be very wrong.

S.

Thanks for your input.

This idea was based on duration and that’s not No1 on my list right now. I will leave this idea to simmer for a while, it seemed like it would allow a decent rate of climb for a short period at 4 S with a full or high throttle cruise at 2S for several hours.

Think the VP option is a good idea especially if it can be matched to suitably high / low gear reduction (probably two stage 12-24:1 to keep the motor running at max efficiency based on the pack voltage.  Motor would be tiny compared to the gearbox and might even need a flick to help get it started.   Lipo packs are a real pain here - 3,7v steps are really too large.

Not done any research on VP although I have a couple ov CP helis to look at.  Don’t suppose that a lightweight and low cost product is readily available somewhere? 

I have a test bed model ready to go with an AXI,  4s pack and cheap HK SS100A  opto ESC and will do some R&D.  Not concerned about powered climb but the duration might burn out the ESC that’s why its so massively oversized ( and a cheapo one!)  Never know, might even break a few records with this set up!

Si

For the VP option:

A couple of thoughts spring to mind. Firstly the "blue chip" ESCs have programmable ramp-times for throttle movements which might overcome the start-up problems, although decent transmitters have the ability to do slow-servo timing to achieve the same thing. Secondly a decent transmitter allows programming a pitch-throttle curve relationship which might be useful in fully optimising the setup as well. Most electric helis run the throttle close to flat-out all the time and just vary the collective pitch, allowing the motor to draw whatever current it needs. They might dip the throttle down to 85% or so at zero pitch to keep the rotor RPM constant, but this is for consistant control and gyro/tail-rotor issues that probably aren't needed in your application.

Secondly I would look seriously at the rotor-head assembly of a 450 or 500-sized helicopter as the basis of a VP prop hub. You could strip off most of the mixer/flybar stuff and just have direct links from the swashplate to the blade-holders. You'd need to make adaptors of some kind (or perhaps replacement blade-holders) to suit the available prop blades, as they blade pivots on a prop blade are oriented at right angles to those for rotor blades...

PDR

Got a couple of helis here, zoom 400 and Hirobo shuttle. will have a look at system with a view to modifying.

Actually now I think about it - many years ago I made a VP unit for a shockie using tyhe tailrotor assembly from a Kyosho Concept EP - it worked well enough, but was too heavy. I'm guessing a tailrotor unit wouldn't be up to the size blades you'd be using (even from a 90 size heli) but they're mechanically extremely simple and the design concept could easily be copied into something a bit stronger.

PDR

I've been searching but haven't found it again yet - a year or two back I saw advertised on the web an automatic variable pitch prop for electric use. IIRC it used the centripetal/centrifugal forces to change pitch with an offset blade mount sprung arrangement. I can't recall what size prop/wattage it was designed for but it was aluminium and other metals so should be reasonably strong.

May be worth a search.

HTH

Richard