Would extra capacitor on esc help with voltage sag?


Is ‘upgrading’ the capacitors something that could help with voltage sag?

Been reading about capacitors and inductance and the capacitors role, and thinking about changing my current ESC’s 63v 470uF single capacitor to two quality 63v 1000uF in parallel (like the vesc).

I have a 10s2p 18650 battery and I think in seeing voltage-sag sometimes, mainly uphill.

Caps on your ESC leads help with battery wire induced inductance spikes… they will help with smoothing out spikes, you going WOT, your motors drawing peak and it taking time for the current to hit your motors from your batteries and you adjusting…

As I understand it

SAG on the other hand comes from the “c” rating of you batteries not being able to provide the amps your motors want.


Agree with @Cobber. The smoothing ability of caps is not going to help with voltage sag.

You will see sag if your setup current capacity (delivery of amps) in C is insufficient.

I guess theoretically you could do a huge cap bank, but not very feasible or practical. Caps are good at delivery power in milliseconds - not seconds long delivery under heavy load… As I recall with my limited understanding (not an EE).

I would instead work on a better battery solution - add more in parallel to reduce sag.



Ok thanks!

Panning a bigger battery as well, but that will take time.

Deciding between LG HG2 (3000nah) or Sayno (3500 mah) to make a 10s3p, but that probably won’t happen until vinter.

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Seems like the 10A continous discharge Version? If so it is much too less for eboard ing a 10S3P configuration. Then i would recommend the HG2!

something that might help with sag is a smaller 1P pack in parallel and in front (as in closer to the ESC, not in series) of the main pack, using cells with a more explosive discharge curve.

it would have the advantage of providing the immediate current for acceleration when needed and just sitting there generally contributing when cruising, ideally…

wired parrallel with the main side’s balance leads may also keep it balanced with the main packs’s voltages.

just a crazy idea.


A crazier idea is supercapacitors. Would take too much space at the moment, though…

yeah that is a crazy idea :wink:

So do you mean a extra pair of wires back to the battery or do you mean a 10s1p pack at the front with positive to mainpack-positive and negative to mainpacck-negative?

what cells would be good for that?

@longhairedboy have you tried this?

no i have not. It just came to me in a blur of delerium.

@leven i don’t know… something that can handle tons of abuse and has a high discharge rate.

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Unless you need monster range - i’d instead recommend a higher C cell (more current/amp delivery capacity), with smaller mAh capacity.

I’d recommend the 2600mAh VTC5a (25A capacity tested), or 30q (20A tested iirc).

7.8Ah * 42v = 327Wh = ~32km/20mi 9Ah * 42v = 378Wh = ~37km/23mi

Rought estimate of 10Wh=1km.

30Q is 15, you want the 25R for the 20 amp rated.

The 25R is a great cell for mid-large, high discharge packs. ITs basically perfect for a 4P at 20amps continuous and 100amp pulse. I went with 30Qs on my 5P because more cells means each one can do less for longer and the range dramatically increases because of it.

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Ok, thanks, but isnt LG HG2 a high discharge cell as well? It has 20amp maximum and 3000mah.

VTC5a is interesting, i have been looking att Sonys VTC6 3000mah but they are a bit expensive.

Hey LHB - i thought it was “rated” at 15, but actually ran much higher output?

Its true. I feel like all the samsungs perform higher than specced in a lot of cases.

Technically you could use a super capacitor between the battery and the ESC that has the ability to expel a much higher current than your batteries, in much the same way as the lipo/li-ion based spot welder does.

The super cap will need time (not that much) to reach full capacity, then when you need a burst of power it can supply it, all while getting topped back up by the slower, less current providing cells. This could even out the voltage/current supply from your battery and ultimately stop voltage sag. But it could only be sustained for short bursts.

This is something we’ve been thinking about doing actually. It could potentially eliminate the effects of sag completely. Finding the right form factor has been difficult though.

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Well, we’d need flat and long and at the moment they’re all round and very fat! :confused:

that is correct. We need one in the form factor of either an 18650, a 26650, a prismatic, or one of those new 207xx cells and i don’t think materials science is quite there yet, and if it is, the demand hasn’t been recognized. I have about 4-6 cubic inches of space for it, so they need to get on it.

I’ve tried the big capacitor thing… it helps, but only slightly.

Those were the two biggest capacitors I could find at my local hackspace.