Dive Xtras Echo ESC question

[rjack321]

Original echo/sierra electronics were likely designed around 24v nominal NiMH pack (i.e. 30V at full charge). A 7s Liion pack is going to be around 29.4 to 30.8v fresh off the charger depending on exact cell and charger. If you don't plug the pack in straight off the charger and wait a little bit for the voltage to dip back below 30v you ought to be fine. (Ignoring the age and potential lifespan of components in an echo ESC). This is the same potential issue/resolution as folks blowing up cuda ESCs with 12s Liion packs.

Li-ion won't drop in voltage coming off the charger. But you can get a 7s charger that has an adjustable final voltage and on goes to 4.1V per cell which is 28.7V. The original packs had way more voltage sag when you started however. And the li-ion pack won't have a ~4V sag. That might be part of the reason for the intermittent failures.
But the reality is that the Echo ESC is OLD at this point. Expect them to fail and you won't be disappointed.

That was my original thinking as well. However, looking at the various 30-40 amp ESCs with similar form factors as the Echo's, they specify 6s for Li-ion batteries (if at all) which leads me to believe that rjack321's observations on reliability is likely to hold. As I use DPVs only on certain dives where reliability is important, I'll go with 6s and build a 30Ah pack.

If you can't swim back or out you need a backup scooter. Expect any scooter to fail, it can't be your sole life support.
6S is not necessarily a good idea. You will hit the low voltage cuttoff before using the majority of the pack energy

 

[bada3003]

Li-ion won't drop in voltage coming off the charger. But you can get a 7s charger that has an adjustable final voltage and on goes to 4.1V per cell which is 28.7V. The original packs had way more voltage sag when you started however. And the li-ion pack won't have a ~4V sag. That might be part of the reason for the intermittent failures.
But the reality is that the Echo ESC is OLD at this point. Expect them to fail and you won't be disappointed.

Li-ion voltage will also drop, just not as much as NiMH. I agree that the sustained higher voltage of Li-ion cells (at around 27-28V) is likely the cause for the intermittent ESC failures you noted. NiMH fully charged will also be in the mid-28V range, however, dropping down to 24-25V occurs rapidly and the ESC is far less exposed to higher voltage.

If you can't swim back or out you need a backup scooter. Expect any scooter to fail, it can't be your sole life support.

That goes without saying.

6S is not necessarily a good idea. You will hit the low voltage cuttoff before using the majority of the pack energy

Depends on battery capacity. Staying well away from ESC cutoff voltage must be part of dive planning.

 

[rjack321]

Depends on battery capacity. Staying well away from ESC cutoff voltage must be part of dive planning.

Cuttoff is around 20V so your 6S pack will be about 3.3V per cell under load. This is quite high to be cutting off the pack. Ideally you would have a ~16V cut off for a 6S pack (~2.6V under load). This is why Sierra conversions are almost all 7S

 

[bada3003]

Cuttoff is around 20V so your 6S pack will be about 3.3V per cell under load. This is quite high to be cutting off the pack. Ideally you would have a ~16V cut off for a 6S pack (~2.6V under load). This is why Sierra conversions are almost all 7S

The cutoff voltage of Echo's ESC is a concern as it's not programmable. However, 20V is low by my standards because I don't discharge 18650 Li-ion battery packs below 3.6V. Typically 3.7-8V is what I plan for in a dive. For me DPV is just a tool to reach a fixed distance with minimal effort and gas consumption. The interesting part, swimming, starts thereafter. And when swimming back with an intact DPV to practice for the event that it can fail at maximum penetration distance.

As to Sierra conversions being 7s, I'm new to Dive Xtras scooters so I could be missing something. In general, building a 7s pack to counter cutoff voltage is not recommended. Comparing, say, 7s6p vs. 6s7p their energy is the same. The main reason one goes with 7s6p is for speed. This is for both brushless and brushed motors. All else being equal, with the ability to go faster 7s6p is likely to result in reaching cutoff quicker than 6s7p.

 

[rjack321]

The cutoff voltage of Echo's ESC is a concern as it's not programmable. However, 20V is low by my standards because I don't discharge 18650 Li-ion battery packs below 3.6V. Typically 3.7-8V is what I plan for in a dive. For me DPV is just a tool to reach a fixed distance with minimal effort and gas consumption. The interesting part, swimming, starts thereafter. And when swimming back with an intact DPV to practice for the event that it can fail at maximum penetration distance.

As to Sierra conversions being 7s, I'm new to Dive Xtras scooters so I could be missing something. In general, building a 7s pack to counter cutoff voltage is not recommended. Comparing, say, 7s6p vs. 6s7p their energy is the same. The main reason one goes with 7s6p is for speed. This is for both brushless and brushed motors. All else being equal, with the ability to go faster 7s6p is likely to result in reaching cutoff quicker than 6s7p.

Your fully charged cells are going to drop from 4.1-4.2V down to 3.8V under load (or less) right off the bat. You need a ESC cutoff voltage sufficiently low enough to address the 15amp load the motor is applying. A 2.6V cuttoff under load (18v for a 7s pack) will be about 3.2V at rest. 3.6V (20.6v for a 6s pack) is far too high of a cutoff but you'll figure this out eventually.

6s isnt any faster or slower than a 7s pack

 

[bada3003]

Your fully charged cells are going to drop from 4.1-4.2V down to 3.8V under load (or less) right off the bat. You need a ESC cutoff voltage sufficiently low enough to address the 15amp load the motor is applying. A 2.6V cuttoff under load (18v for a 7s pack) will be about 3.2V at rest. 3.6V (20.6v for a 6s pack) is far too high of a cutoff but you'll figure this out eventually.

Depends on load. A 7s Li-ion pack charged to ~29.4V drops down to mid-27V range when light/moderate load is applied. Can be more depending on load. Your numbers and logic need to be couched in the specifics of the dives that you do to make sense. If 20V cutoff has been causing issues for you, all that you are conveying is that your battery capacity was undersized for the dives that you were doing.

6s isnt any faster or slower than a 7s pack

For a brushless motor of a given Kv rating, RPM is proportional to voltage. Most ESC's employ PWM control to modulate to what extent a motor is exposed to the 7s input voltage. Switch the battery to 6s, speed will decrease accordingly.

A brushed motor is typically modeled as variable resistance whose value is determined by back EMF which, in turn, is affected by load. All else being equal, the higher the applied voltage the faster the brushed motor spins.

I appreciate your comments on the empirical observation that Echo ESC's have experienced reliability issues when driven by 7s Li-ion batteries.

 

[Jarek Andrzejewski]

I built 7S7P pack for a Dive-Xtras Sierra (and put it inside the original NiMH enclosure) some time ago.
I saw freshly charged NiMH pack can have over 28 volts (NiMH can theoretically have 1.47 - 1.52V per cell, especially if fast-charged), so I assumed 29.4V maximum for li-ion will be handled well by motor and it's driver.

I wanted to keep it in original case (I built it originally for non-geek person ), but I think next battery can be 7S and more-than-7P - it still should fit in the nose and we still have some weight difference between heavy NiMH and much lighter Li-Ion. Of course not only weight matters - weight distribution is also important (pool tests showed that simple replacement NiMH with Li-Ion required additional 2kg near the nose, so I guess we could put few more Li-Ion cells inside instead) .

 

[bada3003]

I built 7S7P pack for a Dive-Xtras Sierra (and put it inside the original NiMH enclosure) some time ago.
I saw freshly charged NiMH pack can have over 28 volts (NiMH can theoretically have 1.47 - 1.52V per cell, especially if fast-charged), so I assumed 29.4V maximum for li-ion will be handled well by motor and it's driver.

I wanted to keep it in original case (I built it originally for non-geek person ), but I think next battery can be 7S and more-than-7P - it still should fit in the nose and we still have some weight difference between heavy NiMH and much lighter Li-Ion. Of course not only weight matters - weight distribution is also important (pool tests showed that simple replacement NiMH with Li-Ion required additional 2kg near the nose, so I guess we could put few more Li-Ion cells inside instead) .

Yes on the Sierra with the programmable ESC. Maybe on the non-programmable Echo ESC. The motor is fine, it's the electronics inside the ESC that may crap if sustained higher voltage outside the factory operating range is applied.

The DiveXtra Echo battery housing is neither waterproof nor well suited for the dimensions of 18650 cells. I tried various layouts and they are not efficient. Just build a custom housing using 9mm acrylic and threaded aluminum rods. As to weight, I agree, but don't push it too close. Having a pound or two for buoyancy and trim adjustment is useful.

 

[Jarek Andrzejewski]

Unfortunately I did not take pictures after assembling the battery, but I have few before, while planning layout:

a) there are six such sections connected together placet between the screws:

6x

• Jarek Andrzejewski
• Apr 2, 2021

b) and one such section in the middle:

1x

• Jarek Andrzejewski
• Apr 2, 2021

They look like this:

Layout

• Jarek Andrzejewski
• Apr 2, 2021

The PCB ( https://www.bto.pl/Obrazki/09591__MG_9537.JPG ) is above the cells (in the place where 20th NiMH cell was).
I also bought two rubber circles (about 172mm in diameter) and put it above and below the cells (to insulate and to get the flat surface on the cells ends) and kitchen silicone (it is temperature - proof) mat which I cut into the stripes and put between the "S" sections.

 

[bada3003]

Thank you for sharing the pictures. It's a nice 7s7p layout and leaves some gap between the 7 cell groups which is good.

I prefer to use spacers that provide gaps between neighboring cells when building larger battery packs to help dissipate heat. For smaller battery packs I use in dive lights and heated vests in cold waters, I just glue them which is standard practice. If a pack goes bad due to one or two bad cells, it's not a big deal. Not so for packs with many 18650 cells. Repairing a pack because a small number of 18650 batteries has gone bad is a major undertaking due to spot welding and soldering. And you have to be extremely careful not to short-circuit which is not difficult to do.

In my experience, the reason why Li-ion packs degrade prematurely (far below their recharge cycle count) is likely due to not considering heat dissipation, never mind managing, in battery pack design. Sometimes form factor leaves little wiggle room, in other cases it's cost and not sound engineering. The reason Tesla could building Model S production packs containing 7000+ 18650 Panasonic batteries is their active cooling system. Doing so in DPV packs is probably a bit much, but using passive cooling by having gaps between neighboring cells is the approach I have followed. 6s9p and 7s9p built 4 years back are still going strong.

My default layout for the DiveXtra Echo pack was 4s arranged vertically with airgaps provided by spacers and 2s arranged horizontally (one top, one bottom) for which there is ample room in the Echo/standard Sierra hull. But your layout with ample gaps between the 7 cell groups where air can circulate suggests another design where a fan could be attached to facilitate active ventilation. Although cells within a cell group are in contact, active cooling driven by a 5V fan may curtail excessive heat build-up. I have not used fans in earlier builds although it was a todo item for the future.