Notes and Comments from the 9 March Build Day


#1

Hi All:

I figured I'd start a thread where people could post what they saw and what they learned from yesterday's Build Day in Berkeley. I spent the day working mostly on motor and battery testing, so I'm just going to shamelessly copy the text I wrote in a reply on the thread about reconfiguring alkaline battery packs. But there was a lot of work done on re-architecting the software, lots of discussions about redesigns of the end-caps for the electronics tube, and I'm sure a lot of other things as well.

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Yesterday at the build day we got our first look at the performance of different battery pack configurations in running the motor. Colin made up a first cut at a motor thust test jig, and we put it in a small test tank and measured thrust vs. throttle position. We had an oscilloscope clipped across the ESC to watch the battery voltage drop during throttling.

So we didn't take a whole bunch of quantitative data, is was mostly to get an initial qualitative look at things. We ran Alkaline battery packs in an 8S1P and 4S2P configuration, NiMH packs in a 8S1P and 4S2P configuration, and Lithium 26650 cells in a 3S2P configuration. All of these fit into the two existing battery tubes.

The alkalines performed by far the worst, due to their high series resistance. The 4S2P configuration seemed a little better, but lifetime was still so poor that you likely couldn't get any real work done with the ROV. The NiMH packs were noticeably better. We didn't take any detailed measurements of whether the 4S2P configuration was better than the 8S configuration. It may have been marginally better but there did not seem to be a large difference. As expected, the 4S2P configurations for Alkaline and NiMH used the throttle range better than the 8S configuration, where driving the vehicle only uses a tiny throttle input.

The 3S2P lithium configuration was massively better, both in terms of peak thrust and battery lifetime, than any alkaline or NiMH battery setup. It has the same throttle issues as 8S Alkaline and NiMH, where you only need a small portion of the range.

We spent some time playing with the throttleability of the 11 to 12Volt configurations-- 8S alkaline, 8S NiMH, and 3S Lithium. With the lithiums, by the time you get to a servo throttle position of ~120 (with 90 as stop and 180 as peak), you've got way more power than you would likely ever need for the vehicle. So we looked at the minimum available thrust, By playing around with the programming of the ESCs, we were able to get a minimum thrust of 20 grams (less than an ounce). That seemed sufficiently low that driveability of the vehicle should not be a problem. As we refine the vehicle we may have to revisit this issue, but for now it seems OK, at least on the hardware side. BTW, improvements to throttling in the software are also currently in work.

So next month we're going to have a bigger test tank set up that will allow driving the entire vehicle, and Colin's going to make an improved version of the current motor thrust test rig. We'll probably be doing a lot more motor/prop/ESC/throttle software testing then.

-W


#2

I was planning on trying the 26650's in my design, but I haven't ordered anything yet. From your tests, should I proceed or is there another (untested) battery that would contribute more to the community knowledge, or would it be better to have someone with a working ROV running those?


#3

Well, I guess it depends upon whether you want to play with battery packs for a while, or play with a competed ROV :-)

I've got two kits on order in the next batch. I'm going to build one up as stock, with the existing battery tubes, and make the other one up in a much more experimental configuration. I'm expecting that the first one will get most of the driving time, while the second one will spend most of its life on my workbench.

If I were in the process of finishing up an OpenROV today, and wanted to be driving it next week, I would wire the existing battery tubes in parallel, and buy six protected LiMnNi 26650 cells and a good-quality balancing charger for the six cells. But with the extra power and energy of lithium cells comes the responsibility that you know more about how to handle the batteries. I want a six-place charger because accidentally making up the pack with cells at different charge levels will cause reduced performance, and potentially even damage to the cells. Using protected cells is even more important, so that really bad things don't happen if you do something stupid with the battery packs. Keep in mind that there is absolutely nothing in the current OpenROV design that would protect the cells from over-discharge, over-current, or reverse-polarity problems. The protected cells give you at least a basic level of protection.

I think there are lots of interesting experiments left to be run on battery packs, even if you build up your unit with the stock battery tubes. For instance, how does the performance of LiFePO batteries compare to LiMnNi? That's an experiment you could do at home just by swapping different cells into the existing battery tubes.

Hope all this helps.

-Walt


#4

LiMnNi it is. I built from scratch, and ordered slightly larger ID battery tubing for the specific purpose of such testing. I should be able to slip one of these PCBs http://www.batteryspace.com/PCB-for-11.1V-Li-Ion-Battery-Pack-4.0A-limit-----PCM-L03S04-559.aspx into each tube.


#5

The IMR 26650's that have been tested so far are the

- unprotected LiMnNi from battery space - from here

- protected Trustfire 4000mAh - from here

- protected Keygos- from here

Right now, we are leaning towards the Trustfires, since this dude has provided nice characterization of their actual discharge curves. see here

I've compiled up a sheet all about batteries that we've been looking at:

google doc on batteries

Hope this is useful!

Cheers,

Colin