Maxxing a servo's performance requires 8.4V, right?

by | |

Q. What batteries are you using for 8.4V? I’m been looking for a regulator that outputs 8.4V to use with A123/LiFe packs because I despise LiPos for receiver packs (I like to build airplanes that require burying packs in the nether regions, e.g. never to be seen again, as an aid to balance). Unfortunately my search has brought up nothing.

A. There are three typical ways of making 8.4V to obtain maximum power from our servos.

1. The old school way is with a 7S NiCd pack (7-cells in series). The math is simple; it's 7x1.2V/cell=8.4V (nominal). Use 2000-2400mAh cells (and 3200mAh/cell are available in sub-C NiM chemistry, which are a great alternative). niCd and NiMh are both very rugged and reliable. Bit heavy but for a large model, who cares? Especially as many scale models can stand to use the mass for balancing (note the question above mentioned burying the pack for balancing purposes - this is quite common especially with scale models).

2. Alternatively, albeit with more complication, a standalone BEC (basically a voltage regulator using FETs to boost the voltage). Example of these include a nice 20A unit from Castle Creations. Power it with a 2S LiPo (or 2S A123 for greater safety), and set to boost the output voltage, e.g. set it to deliver 8.4V.

3. A third way - even more complicated - is via a power box (there are various flavors (brands). Some include gyros and all kinds of goodies for the ultimate in complicating your life. These are often fed via one or even two LiPos, and have regulators (just like in a BEC) and thus, you can set them to output whatever voltage you please, once again 8.4V.

Me? Honestly, rather than these methods I'd rather keep things simple and just use a simpler battery - a bit less than 8.4V - and then size the servos to suit the chemistry (remember, servo performance varies based on input voltage and since different chemistries have different voltages, the same servo will deliver higher performance on a 2S LiPo vs. on a 2S A123 pack). Thus, if the model would call for using a DS270DLHV and you didn't want to use LiPos, then in the alternative start off with a more powerful servo and feed it lower voltage. E.g. use DS360DLHV and power them with A123 cells (lower voltage, but you start with a more powerful servo). So at 7.4V the DS270DLHV outputs 235oz-in while the DS360DLHV on 6.6V is still outputting 290oz-in. So there are many different ways to skin the cat and the answer to which servo and battery is best is . . . it depends!

ProTips: in order of increasing safety but decreasing voltage . . .
1. I'll use a 7.4V LiPo if I can retrieve it easily for charging because I don't trust this chemistry to charge unsupervised within a flammable object like a model airplane. Thus, I ALWAYS remove LiPo packs for charging. No exceptions - unless you fancy taking the risk of waking up to a fire in your workshop because the batteries you were charging overnight caught fire! me? I don't think the difference in performance between 8.4V and 7.4V is such that I'm a sufficiently good pilot to discern. Common sizes are a 2S2300mAh.

2. I also like 7.2V Lion packs (built within cylindrical metal shells) because they are significantly safer than LiPos, which are built in a plastic bag. We offer them as do many others (Relion is a name brand). Common sizes are a 2S1500mAh and a 2S2P 3000mAh capacity.

3. Safest of all the lithium chemistries, in my opinion, are packs made up of A123 cells. Again, these are built within cylindrical metal shells similar to the LiIon cells (as well as the old school NiCds and NiMh cells and AA Alkalines for that matter). At 3.3V/cell, they are the safest of lithium cells - but - also the lowest voltage. Basically, reducing the voltage results in ever increasing safety (reduced fire hazard). So hand in hand with more safety comes reduced voltage. And if I want 8.4V and I want to do it with a battery, I'd opt to use a 7SniCd or 7SNiMh packs. Heavy? Deal with it.

Anyway, I am sufficiently confident in A123 cells to be willing to bury a pack within a model (to aid in balance) and leave in situ for charging unsupervised - like I've done historically with NiCd or NiMh chemistry. Not so with the other Lithium flavors . . . but everybody has their own tolerance for risk. Note; A123 packs are the most pricey.

Regarding packs; we offer them, so do lots of other companies. Buy them from us, buy them from someone else, we don't really care. Honestly, there are so many competitors they've become subject to commoditization (meaning there's very little money to be made selling this kind of product). The point being; we principally offer batteries as a convenience more than anything, e.g. add a pack to an order while you're getting servos and save money versus ordering a pack from someone else later. Ditto extensions, switches, servo arms, screws, etc.

4. What's missing? LiFe packs made in plastic bags to form bricks (similar to how LiPos are made). Many swear by them. And consequently, many companies offer them. We swear at them. Long story but largely it's a matter of miserable experience with longevity - they crap out sitting on the shelf brand new! us? I don't want any part of that story, e.g. any responsibility for selling them . . . it's why we don't!

Summary of achieving 8.4 volts:

The smart thing is first decide on chemistry, then decide on the servo that will offer the performance you need at that voltage and avoid 8.4V altogether. This is our best opinion because we've looked at the voltage being output by FETs using an oscilloscope and there's no way 'I' feed that to sensitive electronics like the receiver and/or gyros. Servos don't really care so pick your poison. Also note; there's information on calculating voltage drop within servo extensions - another factor to consider. This, within yet another article in the askJohn section of our website.

Fortunately large scale models are relatively easy to select for the right servo because they're not overly demanding. E.g. a servo suitable for 144" wingspan B-29 bomber may only cost $50 each while servos for a 144" wingspan Extra 300 powered by a 210cc engine - I've owned motorcycles with less displacement than that - yikes! Anyway, these models easily require servos costing 3-4X that amount, capice?


John Beech - GM (and janitor)

This entry was posted in no categories.