HPI Savage servo

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On selecting the best servo for an HPI Savage and powering it with a rechargeable battery pack instead of AA alkalines or recharegables

As usual these white papers begin with a user question and this one is no exception. This guy has the DS635BLHV servo in mind for steering the extremely durable HPI Savage 4x4 nitro rig. While this is a great choice for this rig and other similar-size 1/8 scale monster trucks, his real question relates to powering this servo whilst relying on the stock 6V type radio system, so let's jump in.

Q. Wanting to use the DS635BLHV servo and wondering if it's plug-and-play with my stock HPI savage X4.6 electronics? And if not what upgrades would be needed?

A. Yes, it's plug-and-play and yes, you should consider how to better use the stock electronics. Can you plug it in and it works? Sure? Works well? That's a different question. Here's why. The stock hpi-racing TF-50 2.4GHz FHSS Digital RC System uses 4x AA-alkaline battery cells. The math works out like this;

  • 1.5V/cell x 4 cell = 6.0V

. . . and the DS635BLHV will munch through those like a 4y/o with Skittles!

Fortunately, there's a solution, and it's inexpensive. In addition to the servo, add a LiFePO4 rechargeable pack along with the servo. These output 6.4V (6.6V fresh off charge but that's still OK). This, because as it turns out, 6.4-6.6V is actually within the acceptable range for stock-type 6V radio systems like yours. Easy peasy! So why not just use rechargeable AA batteries? It's to do with their current handling capabilities and in short, don't.

Note; reviewing the servo specs, they show higher voltage results in better performance from the servo (meaning the servo will operate faster and have a bit more power). So if you ever change radios (aftermarket systems work through 8.4V) 'and' feel a real need for more power and speed (as opposed to just in your head because the specs show this), then just upgrade the pack to a B2S2000 (different chemistry, LiIon, instead).

But honestly, I seriously doubt you'll ever be able to tell the difference between this servo on one pack vs. the other in your HPI rig. Plus, LiFePO4 packs are easier to live with than LiIon packs (I'll explain later).

So can we measure the difference between this servo on 6.4V vs. 8.4V with a test rig in the laboratory? Yes, absolutely. But in the field, e.g. in the real world? Hell no!

Are there guys that will brag they can tell the difference? Sure? But while i cannot prove it, I suspect it's pure BS on their part. Why do I say this? Experience with this servo in a Traxxas T-Maxx nitro powered monster truck. No, not the exact same (the Savage is a 4x4 and has a bigger engine) but it's close enough to form an opinion. More later.

Anyway a big time advantage of the LiFePO4 chemistry over the LiIon is you can put the pack on charge, then 'not' use it for a year and it'll a) not be damaged and b) retain the charge perfectly (note; we offer both so I don't really care which you prefer because we can supply you with either). However, while the LiIon technology outputs higher voltage, it also needs to be discharged to storage-voltage each time you put your rig away.

Yes, I know, this doesn't sound like a big deal - but - believe me, this works out to be a bit of pain in the hind end compared to leaving it to sit on the shelf. Especially because with LiFePO4 chemistry you can use a partially run down pack next week, and the week after, and the week after that until it finally needs recharging. Trust me, this is very convenient versus charging it every time before use 'and' once you're done for the day, running a storage mode program on it so that it's safe to put on the shelf.

Meanwhile, I am glad to see nitro rigs making a comeback. Why? Simple, it's because I still have two T-Maxx nitro rigs and and love bashing with them. That, and I love having a more powerful alternative from HPI. Why nitro rigs?

Simple, it's the smoke, the smell, the sound . . . I LOVE ENGINES!

Finally, we offer these LiFePO4 packs in four capacities ranging from 1500, to 2000, 3000, and 6000mAh. Obviously the 6000 will last 4X as long as the 1500, understand?

But as long as you have space for the installation, there's no real reason not to use the largest pack you can fit. Why? It's for extended run-time between charges. And for what it's worth, I use the B2S2500 in my own T-Maxx rigs and me and my grandsons can play for many days between charges.

What's more, I'm using an 8.4V capable Futaba aftermarket radio systems in these and while me and the boys are perfectly happy with the B2S5000 LiIon pack (higher voltage means the steering servo is a bit faster and more powerful), guess what? Yup, neither I nor my grandsons can tell the difference!

So because living with the LiFePO4 pack is just easier (because we can switch them off at the end of the day and not have the need to put it at storage voltage before storing the rig in the shop), then LifePO4 packs are just a lot easier to live with.

Last thing, you'll need a charger capable of charging the receiver pack (what they're called once installed - regardless of chemistry). These are readily available for about $55-70 and as long as it says it'll charge a LiFe or LiPo pack, it'll work fine. We recommend a couple different chargers.

Speaking of which, LiFe/LiFePO4 is almost the same chemistry but different construction just how LiPo/LiIon are almost the same, but with different construction. Thing is, plain LiFe packs are a lot cheaper than LifePO4 packs (just like LiPo are a lot cheaper than LiIon). So why not use the Life instead of the LiFePO$ if they're nearly the sames? It's to do with how the cells are built and LiFePO4 is better for powering receivers and servo. Why?

Basically it's because LiFe and LiPO are built within cheap plastic bags while LiFePO4 and LiIon cells are made within far more rugged metal shells. There's a huge difference in which is tougher and more durable, especially when being jostled in a rig that may hit curbs, be jumped, etc. Anyway, your money, do as you please but for our part, we long ago quit handling/recommending LiFe/LiPo type packs for control electronics due to the many failures. They just weren't worth the little bit of savings. But aren't LiPo used for propulsion? Yes, and in that application the LiPo can't be beat, but the packs are handled differently. Totally different purpose. Don't get confused.

Anyway, we offer inexpensive chargers but they're also easy enough to source on Amazon or eBay for about the same price. Either of these works:

ToolkitRC M8S - https://tinyurl.com/Toolkit-RC-M8S-charger

ISDT Q8 - https://tinyurl.com/ISDT-Q8-charger

Which is better? I have both and honestly, it doesn't matter in the least!

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