RX18 Max Voltage

[Tankbear]

I was having a bit of a tidy up the other day and found an old 9.6v NiCd 650mAh battery.

Having recently read in an ElMod post that it likes higher voltage, I thought what will the RX18 do with an extra 2.4v?

So I grab my plastic tiger which has DIY 3:1 gearbox mod (which makes the tank very slow), plugged in the battery (it even fits the battery compartment!) and switched it on.

Headlights flashing, good sign . Engine started and all functions worked! Obviously due to increased voltage all the motors turn quicker, HL flash unit works, smoker works a billion times better, had to open the window and hoped nobody called the fire brigade.

So to compare.

7.2v (6cell)
Tank covered 2.5m in 13 seconds
Voltage at motor leads 6.5v

9.6v (8cell)
Tank covered 2.5m in 8 seconds
Voltage at motor leads 8.5v

Overall about 60% increase in speed. So that would make it about half the speed of the standard gearbox version.

So suitable to slower tanks with low ratio gearboxes.

I’ve still to test the long term effects of the higher voltage. To see if burns out the RX18, ESC, Motors, etc.

[caddyshack]

How are you getting on with the testing? Hope all is going well. Waiting for TB's next installment.

[Tankbear]

So far so good.

Ran tank on full battery to discharged twice and all appears to be ok. Gave it some serious stick with tight turns on grass, up steep gradients etc.

Even got about 20 minutes run time on the 650mAh battery.

[Tankbear]

Still testing and still working

Had a small set back which delayed testing for a week or so. Decided to change the cooling fan from a 5v one to a 12v just incase the extra voltage damaged it and then the heat effect the rx18. Problem was whilst I was in there managed to knock off one of the motor leads, so had to remove to resolder, when I put the gearbox back in I must of over tightened the screws in the plastic because the gearbox came loose so needed to fix that. Once I fixed that the loose gearbox also managed to damage the plastic gears. So out it came again to change the gears to metal ones.

What a chain of events.

Anyway everything still appears fine, only thing i've noticed is the HL flash unit now makes a high pitch noise (even with normal 7.2v battery) so maybe the 9.6v battery has done that. Also running the smoker and tank shortens the performance/run time but that is more probably down to the battery pack only been 650mAh

[Tankbear]

I was speaking to a work colleague about impact of increasing the voltage of a DC motor etc. And we came to conclusion that increasing the voltage will reduce the current drain, although not as simple as voltage up by 10% current down by 10% as the motor actually spins faster and therefore using more power. So current drain should be less but how much is hard to determine.

So as the current through the ESC creates the heat in theory these should be cooler although controlling more power.

Moved on with the testing, now trying 8 cell battery in my KV-1 which has the Asiatam 3:1 gearbox and motors (not the new 18000rpm motors). Did a quick speed check and with 9.6v battery I get it to a full size speed of about 19-20mph, which isn’t far off the 22mph of the original . I’ve also cut open a couple of my 7.2v batteries to make a large capacity 9.6v battery for some long term testing.

[Dietrich]

Hi Tankbear,
VERY INTERESTING....I bought a 4 wheel drive r/c car from ebay for a whole 99p. The 6 volt rechargable battery was knackered so I put in my old Heng Long 1700Mah 7.'2 Volt and it has worked perfectly, runs longer and much faster....although I did add a couple of cooling fans as the electronics were VERY enclosed.
Have you added any other additional cooling of have you just stuck with the one fan Did you cut the usual holes in the RX-18 casing .....I have been concidering adding some metal cooling plates to the 'FETS' ?? ( Is that the right name) to try and help with more cooling.....BUT...this strange electrical phenomenon...of more voltage drawing less current....and so I guess creating less heat......looks like it might be a real winner
The 9.6V 3800mAh Rechargable Battery + Charger Car Tamiya are on Ebay for, about 16 pounds.....The 9.6 volt unit is 174mm long and the 7.2 volt unit is 130 mm long, but a few cuts to the battery tray will 'squeeze' in the larger battery assuming the Ebay vendors are right, we need to find an extra 44mm of space, to get them in.....OR just cut the 7.2 pack and stack the extra 2 cells on top like tamiya used to do in the early days.
You were a brave man to risk an RX-18 unit....but it looks like it may pay off for everyone if they want to try it.......Do we have any 'Electrical Wizards' who can explain this 'Effect' in simple terms
Alb

[Tankbear]

Hi Dietrich,

Sorry for short answer but on my mobile.

No didn't add extra cooling apart from the one fan on top of the rx18.

Battery size I had to cut the battery box to fit in the now longer 8cell battries.

The science bit. Power is voltage times current, therefore if power iz constant voltage increases then current must reduce. As you know it isn't that simple as the motors run faster therefore using more power so I recon the current must be same or less than before because the run time also increases.

As for the electronics handling the voltage the way I see it, the components wont be specifically made to operate at 7.2v but just generic ones which are probably made for 12v hence why good even on a fully charged 8cell which is near 11.2v.

Probabily not best explanation but all I have.

Ian

[greentank1]

Does anyone have a rx13 to try this out? Don't want to risk mine lol...

[Tankbear]

Does anyone have a rx13 to try this out? Don't want to risk mine lol...

Yes I have but not in a tank. I dig it out of the safe place I stored it (where ever it is) and give a try. It be a bench test so no load on the motors but hopefully should prove can handle voltage.

[greentank1]

Ok, thinks for taking a risk, let us know how it work out. (and if it was on fire or smoking.)