ptxman
Posts: 351
Joined: 7/31/2002
From: Calgary, AB, CANADA
Status: offline
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Remember, these are 'nominal' pack voltages based on manufacturers individual cell specs. Vendors are targeting to achieve 1.2v/cell over the rated maH capacity of the cell for NimH & Nicd chemistry. (Alkalines are I think 1.5 but another story). In reality, the pack voltage is the sum of N cells in series and that can vary a bit based on charge state. For example, its not uncommon to see a 4-cell NimH pack come off the charger at +5.5 volts (=1.375v/cell). Similarly Ive seen a 5-cell packs come off the charger at close to 7 volts. The 5-cell pack may settle down to 6.5v after a few minutes of use as packs typically deplete in kind of an S-curve. But the larger the maH capacity of the pack, the longer it will sustain these elevated voltages.
Most manufacturers spec their servos at 4.8 & 6.0v and you get more torque & faster transit time at 6.0v vs 4.8v. But if the pack is much higher, say 6.5-7.0 v, certain servos (especially the earlier digis) can experience buzzing & even magic smoke emanation issues. Thats why we cant run 3.7v/cell based lipos, a 2S pack would be 7.4v & they actually have the same 'fresh-off-the charger' elevated voltage issue as Nimhs.
You can buy voltage regulators which is probably the best solution from the servo's standpoint as they can be set to a specific voltage level, ex 5.0 or 5.2 volts. That ensures consistency to your servos over consectuve flights regardless of pack voltage state. But... they cost money too, are about the size of a unboxed Rx to fit, it introduces one more link in the chain to fail, there are good ones & bad ones in terms of how efficiently they perform their regulation duty, they too have max amp duty so they need to be sized for expected max servo amp draw.
Some guys feel a 5-cell pack is insurance against losing a cell, especially for Rx's which need (is it min 4.0v?). In reality thats not quite true, or I guess it all depends on what is meant buy a cell failure. If a cell becomes damaged & acting like an in-line resistr, the remaining cells might well overcome it *for a while*. If its moe severley damaged/vented it could act more like a direct short & the remaining cells wont help. Whats worse, a flakey pack may appear to have good initial voltage but it may rapidly decay to under-voltage (IOW it has very limited capacity to sustain that voltage) & its game over.
For that reason Im of the plain old 4-cell, 4.8v camp, but with caveats:
- buy quality cells of high maH capacity.
- buy/make packs with good cell interconnections. Spot welded tabs are fine but insulate the pack so vibration doesnt loosen them because the physical weld is actually very tiny. Iv eseen some vendor with 3 spots, others with just one.
- buy a good quality charger & regularly C/D cycle the packs to verify close to nomnal maH capacity & no degradation over time
- give your nimh pack a couple C/D cycles after a long dormant storage period, it wakes up the chemistry
- if you are really serious, get a CBA which usb's into your pc & it graphically shows the discharge plot at your prescribed amp load. You can save this plot & even overlay this on previous ones so see any degradation over time. Not only doesit validate the maH capacity to terminal voltage, but any visual humps or bumps in the discharge curve are indicating individual cells are giving up early & time to replace the pack.
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All Forums >> RC Airplanes >> Pylon Universe - RC Pylon Racing >> QM-40 Racing >> 4.8 or 6 volts 
4.8 or 6 volts - 
