How big of an external battery for servo?

[dure16]

I'm considering a dedicated 2S servo battery for my DBXL and 5ive-T. My goal is to ensure the big servo always has the amperage it needs, instead of being limited by the ESC's BEC.

But I'm not sure how big of a battery to buy. 500mah? 1000mah? 2000mah? etc. I obviously need it to last as long as my battery packs but I also hope to keep weight down.

Does anyone use dedicated servo batteries in their big cars and have guidance they can share?

 

[Garcbomber]

Go as small as possible and you’ll immediately know when your steering runs out at the end

I’d hope Ext. BECs are smart enough to cut completely and not draw total voltage under set LVC

 

[dure16]

I’d hope Ext. BECs are smart enough to cut completely and not draw total voltage under set LVC

They are not that smart. No LVC built in.

 

[slick2500]

Don't the gas ones come with a 5000mah pack? I can't remember.


-edit- nope it is 4000mah.

 

[Chapinb]

Myself Id really rather a larger bec. It involves some custom wiring, but its not difficult. No extra lipo to charge, no lvc issues, no chance of forgetting it at home or in the rig, simpler to run once installed. Added bonus - can run it at a higher voltage to get the most of your big ol servo.
Otherwise a 2s, 4-5ah would run you a long time.

 

[dure16]

Myself Id really rather a larger bec. It involves some custom wiring, but its not difficult. No extra lipo to charge, no lvc issues, no chance of forgetting it at home or in the rig, simpler to run once installed. Added bonus - can run it at a higher voltage to get the most of your big ol servo.
Otherwise a 2s, 4-5ah would run you a long time.

I considered the large Castle BEC but decided on direct battery power instead. For these reasons:

• Not recommended for 12S cars
• Lower current output than a dedicated battery
• More expensive than a small 2S battery

 

[Chapinb]

Ok, got me there. I cant find any other bec that is 12s capable and has a decent amp rating. So if you need more power, lipo it is!
If your servo is HV (real HV, not the 8.4v HV small scale stuff), a 3s or even 4s isnt out of the question for a bit more speed.

 

[RCbuzz]

Myself Id really rather a larger bec. It involves some custom wiring, but its not difficult. No extra lipo to charge, no lvc issues, no chance of forgetting it at home or in the rig, simpler to run once installed. Added bonus - can run it at a higher voltage to get the most of your big ol servo.
Otherwise a 2s, 4-5ah would run you a long time.

I prefer this as well.

I considered the large Castle BEC but decided on direct battery power instead. For these reasons:

• Not recommended for 12S cars
• Lower current output than a dedicated battery
• More expensive than a small 2S battery

View attachment 399752

Is this just the input amps? Guessing the output can still sustain a higher current.

 

[Chapinb]

The way its worded with voltage limits, makes me think it's very sensitive to voltage spikes (like most cc products seem to be). A 12s motor backfeeding, or just the ripple from normal use might be enough to kill the bec. A plane motor, without braking, would cause relatively little ripple and almost no backfeed. Has 55 or 60v caps maybe?
The amps out from the castle would be enough to power most 5th servos in normal use, except maybe the 2,000 oz monsters. Even a crap lipo will still provide way more amps of course.
Interestingly, the much smaller cc bec 2.0 can handle 12s surface rigs at a reasonable amp output but might as well use the esc bec at that point.

Ah, the conundrums and the price you pay to be on the bleeding edge of performance.

 

[dure16]

The way its worded with voltage limits, makes me think it's very sensitive to voltage spikes (like most cc products seem to be). A 12s motor backfeeding, or just the ripple from normal use might be enough to kill the bec. A plane motor, without braking, would cause relatively little ripple and almost no backfeed. Has 55 or 60v caps maybe?
The amps out from the castle would be enough to power most 5th servos in normal use, except maybe the 2,000 oz monsters. Even a crap lipo will still provide way more amps of course.
Interestingly, the much smaller cc bec 2.0 can handle 12s surface rigs at a reasonable amp output but might as well use the esc bec at that point.

Ah, the conundrums and the price you pay to be on the bleeding edge of performance.

Good find. I didn't realize the smaller Castle BEC can handle 12S surface cars.