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9.4v nimh worth it or straight to lipo?

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Fitted my goolrc brushless 3650 4300kv motor and S120a esc to my dual ridge yesterday, have it geared with a modified TT01 61t spur and a 23t pinion. Yet to try it on grass but on the road it really flies so hoping I’ve found a good spot for the gearing already. 
 

Naturally I’d like some more power anyway, would a 9.4 or maybe even higher nimh pack make a significant difference? Only ask as they are less fuss than a lipo. 

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In my experience of building custom 8.4v packs to try and remain off lipo, i don't regret going lipo for a second. The problem with nimh packs - however high voltage they are - is that they don't flow as much current as lipo. You might have seen lipo batteries with a C rating? 30C,60C etc? Well, i don't fully understand the physics of it, but nimh typically max out at 12C, so even a fairly tame lipo 7.4v pack will comfortably out punch even a fully charged 9.6 nimh. Power = volts x current. If we just go with the C rating as the arbitrary current value, your 9.6 nimh will flow a maximum of 9.6 x 12 = 115.2 power. A 30C rated lipo will flow 7.4 x 30 = 222 power. And my 30C lipo is just the average. Its peak theoretical flow is 60C which is 7.4 x 60 = 444 power. That's nearly 4 times the electrical oomph the lipo can send to the esc, which is why you need to ditch the tamiya connectors when going lipo. They will melt because they aren't designed for that current level. 

Allied to all this is that lipos are available in stock pack size and are way lighter than even 7.2 nimh it's worth the bother. I'll admit i was super scared and cautious of lipo because of all the horror stories, but they are reay not that bad. For a start they store at 7.6v, and don't just lose charge over a few days like nimh, so if you just want a 10 min spin in the street, i just chuck a lipo in at storage charge and run in until the low voltage cutoff is reached. For more than that, i often just charge them for 15 minutes to get a bit of juice in them and run them. My touring cars with 17.5 brushless with run easily 30 mins and probably be barely bekow storage voltage when i am finished. Just throw them on storage charhe when you get back, and charge them somewhere you can spot (like in the kitchen) so you remember to unplug them before bed. 

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The only thing I don't like is how light lipos are!

What's the best way to add weight, stick on car weight? Or has anyone found a "sleeve" that tops up the weight to a nimh????

Edited by taffer
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3 minutes ago, taffer said:

The only thing I don't like is how light looks are!

What's the best way to add weight, stick on car weight? Or has anyone found a "sleeve" that tops up the weight to a nimh????

A sleeve would make the pack bigger so it would not fit. LiPO means you can add weight where you want/need it. Alloy wheel weights work well and are cheap

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14 minutes ago, taffer said:

The only thing I don't like is how light looks are!

What's the best way to add weight, stick on car weight? Or has anyone found a "sleeve" that tops up the weight to a nimh????

I guess for vintage stuff that could be an issue. Given that i like tinkering i have always just put softer springs on the cars. Stick on wheel weights like @Nobbi1977 says are good. I use them on the front of my 2wd cars. 

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40 minutes ago, ThunderDragonCy said:

If we just go with the C rating as the arbitrary current value, your 9.6 nimh will flow a maximum of 9.6 x 12 = 115.2 power. A 30C rated lipo will flow 7.4 x 30 = 222 power. And my 30C lipo is just the average. Its peak theoretical flow is 60C which is 7.4 x 60 = 444 power. That's nearly 4 times the electrical oomph the lipo can send to the esc, which is why you need to ditch the tamiya connectors when going lipo. They will melt because they aren't designed for that current level. 

I would just like to point out that the  calculated values (W / Watt) are the theoretical maximum values the different batteries can deliver. The actual current is limited to the actual current draw of the motor at a given voltage. So, if the motor doesn't draw more than 10A at 9.6V, the 9.6V NiMH will deliver 96W, whereas the 7.4V Lipo will deliver  approximately (10A x 7.4V / 9.6V ) x 7.4V = 57W (but the lower weight of the Lipo will of course have a positive impact on the model's performance).   So, a higher C-rate of the battery doesn't necessarily mean that the actual power is higher too.

Furthermore, the voltage values are just nominal voltages. Actual voltage under load will always be lower than the resting voltage at the same state of charge. However, if the stated C-rate of a battery is honest and correct, the drop in voltage under load will normally be lower the higher the C-rate.  Also, the C-rate isn't constant during the whole discharge cycle.

Also, the load profile (varying current draw during discharge) will have an impact on the voltage curve during discharge, thus having an impact on the resulting power (W) and voltage (V, which roughly relates to rpm).

As a rule of thumb:

Maximum available power:  Voltage of fully charged battery right after complete the charge  X   C-rate

Minimum available power: Voltage at the point of cut-off   X    Value lower than C-rate

 

Actual power consumed by motor: Voltage "seen"* by motor   X   maximum current draw of the motor at that voltage (but limited to the battery's C-rate at the actual state of  charge)

Actual power delivered by motor: Actual power consumed by motor minus losses**

*(battery voltage at any time minus voltage drop of cables, connectors and ESC)  

**(Losses vary with temperature, rpm, voltage and current, and generally, the losses will increase exponentially with increasing voltage and current)

 

 

 

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Mmm so a strip of Velcro on the battery and then stick on some wheel weights afterwards if possible.....I want the weight to be in the same position!

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Some great info there thanks, will read and digest. 
 

The motor specs say a max of 900w at a max of 12v but how believable that is may be another matter. I went for the 120a esc so I have some headroom. 
 

Specifications:
KV(RPM/Volt): 4300KV
Max Voltage: <12V
Max Amps: 75A
Max RPM: 50000
Max watts: 900W
Resistance: 0.0075
Rotor poles: 4
Diameter: 36mm
Length: 50mm
Shaft length: 15mm
Shaft diameter: 3.175mm
Plug type: 4mm golden banana plug
Item weight: 181g
 

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It looks like this represents decent value for money, if I’m right using the above replies I’ll comfortably be getting the full potential of the motor and be well within the spec of the esc. 
 

https://www.modelsport.co.uk/absima-greenhorn-lipo-11-1v-45c-4000-hardcase-t-plug-/rc-car-products/440265

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On 5/24/2020 at 8:21 AM, taffer said:

What's the best way to add weight

For racing, you get ,under Lipo weights , (only for shortys though) helps keep the weight low down.

https://www.modelsport.co.uk/core-rc-under-lipo-weight-35g-brass-1s-shorty/rc-car-products/404375

 

Switch to Lipo, you'll wonder why you clung onto nimh for so long! , if you can get a 3s lipo nailed in there, you'll have more power than you can shake a stick at (but still want more ,once you've got used to it...😁

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Shame they can't make a stick weight

For the moment just needs some inventive thinking where to put weight depending on chassis 

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It looks like that absima one will fit the tt02b exactly, I can always break out the dremel  if I need any more clearance. I did break a plastic dogbone two or three minutes into playing the other day, not sure if it was the way it landed from a roll on that wheel or if I still had power on. Either way I have some spares now, am of the opinion them being the weak link in the drivetrain might be a sensible move for now. 

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Decided to order this, their lipo bag and another rigid type bag so I can double up. I have a steel box in the garage too so may store it out there. 

https://www.modelsport.co.uk/overlander-4500mah-3s-11-1v-55c-low-profile-lipo-hard-case-sport-track/rc-car-products/396373

Chose overlander on the back of very good experiences of the two 3300mah nimh pack we currently have.

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