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Functional battery boxes
I have now shared my battery box design.

I want to design functional battery boxes go with my hub drive system. This will leave more space in the body for 2-3-2.

On an initial check it looks fairly easy to get at least 48 18650 cells per battery box. In a 6S 8P configuration this will give around 24V (fully charged) and up to 24Ah (if you get expensive 3ah cells) per foot. This is way more power than I believe I need, so I could either go cheaper cells, or reduce the number. 

[Image: attachment.php?aid=647]

I also need room in the battery box for a BMS for battery protection, so the layout above is just a proof of concept.

I might need to redesign the battery box with a removable end to make servicing easier.

I have also been looking into cheap Chinese battery spot welders like the Sunkko 788h which looks like it would make making the battery packs fairly easy.

Attached Files Thumbnail(s)
Having a BMS installed is what I would call, Mandatory.
If there is room at the top or the bottom, then definitely mount it in there.
The BMS will maintain the cell balance during charging and cut off the output when the batteries are discharge, but not to low for recovery.

I would suggest a 3 or 4 battery pack arrangement for the whole of the R2D2:
One for each leg in the battery box as you have proposed.
One in the body, larger if possible to coordinate the power distribution to the other battery banks.
One smaller one in the dome, this can then be trickle charged from the main bank and supplies the power for all the gear in the dome.

This allows for a single charge point on the body and the potential for a removable battery pack in the body.
The body pack would need to be 6S while the other should be smaller, 5S for the leg banks and 2S or 3S for the dome.
A small step down SMPS can then be used to supply the correct voltage for charging from the main battery pack.
A secondary Pack in the body could also be charged from the main pack to provide power for the electronics and servo in the body.
This allows the main power storage pack in the body to be removable without the R2D2 being shutdown and also allow the R2D2 to have sufficient power once the main pack is depleted to return to a charge point for auto recharge.

It's just a thought
3 or 4 battery packs sounds a bit overly complicated for my taste. I think a couple of battery packs in the feet will do me fine.

Actually, now that I am looking at the price of 18650 cells, I think I will probably scale the size of them back a bit. I am now leaning towards 7S 4P packs in each foot.

[Image: attachment.php?aid=650]

It looks to me that you could fit this many batteries in the battery boxes and still have room for the standard drive motor.

Bringing the number of batteries down to 28 in each foot and using 2600mAh cells should bring the battery cost down to around $200AUD plus BMS modules.
[Image: attachment.php?aid=651]

Attached Files Thumbnail(s)
Have you found a 7S BMS?
maybe a 4S7P battery pack?
4S BMS are readily available, and so are the motor drivers both for brushed DC and 3 phase or brush-less motors.

In either case, you have the same number of batteries and the same energy density.

At 7S your looking around the 26 Volt mark
At 4S your looking around the 14.8 Volt mark

The pros of the higher voltage is lower current for the same power
The cons are components become more expensive, you need to allow a little over head voltage for charging, if you top over the 30 Volt mark for even an instant, then components will fail and all the stored energy in the pack will fuel the bang and fire.

As you drop the voltage, you increase the current for the same power, that means thicker wires to prevent voltage drop and over heating.

Voltage drop is is parasitic power loss, and yes we want to avoid that as much as possible.
You seem to be questioning my choice for going higher voltage packs and then supplying a whole bunch of reasoning for why higher voltage is better... No idea what components you are talking about that suddenly explode at 30v?

The choice has been made to suit the components I already own, and comes with the side advantage of lower currents and voltage drops. My hub motors are rated at 24v, the Odrives controllers for 48V. The Odrive can limit current through the motors so going a little over 24v is no issue.

All of the electronics in my build will be running off buck converters (many of which can handle well over 24V). So they don't really care what voltage packs I run.

The reason I have decided to move from 6S to 7S is that there seems to be a wider range of BMS modules available for it. The majority of the 6S BMS modules I could find all tended to have the same max current cutout.
For example:
My advice is based on the available components i'm used to being able to source in industry.
If the Odrive is rated to 48v, then by all mean go with the higher voltage.

I hadn't seen the 7S BMS, I had found a large range of 2 to 6S BMS unit using eBay.

Just a side note, one of the reasons hybrid cars such as the Prius have 48V battery packs and not higher voltage ones, is to do with the electrical laws in most country's. Once you reach 50Vdc or 32Vac, you need to be a licence electrician to work on the equipment. Since most motor mechanics aren't licenced electricians, the kept the voltage of the battery packs below that limit.

In industry, the most common low voltage dc supply, is 24Vdc, and most of the industrially hardened equipment is rated to only 30Vdc max.
But in the Prius they series up the battery packs to over 200v, which makes the whole thing a bit pointless.

On Aliexpress there seem to be as many really large BMS units as there are small ones. They must be intended for industrial type uses.. Golf carts?

For example 24S 200A:

That is over 14KW! Think how fast you could get R2 going with that! Smile
(06-17-2018, 10:25 AM)Cyber_One Wrote: ..

At 7S your looking around the 26 Volt mark
At 4S your looking around the 14.8 Volt mark


12V becomes an issue if you have to drive R2 on any type of carpet - people using 24V don't seem have any problems with this

But the whole flaming R2 thing is why I would use removable battery packs. I've seen too many RC batteries go up in flames during charge to trust charging in my droid.
I really like where you're going with the battery boxes. I'm not smart enough electrically to comment on battery types, and was originally going to go with a couple SLA, but the 2-3-2 mod will likely force me to reconsider the SLAs for something that can fit in the battery boxes.
(06-17-2018, 03:11 PM)LarryJ Wrote: But the whole flaming R2 thing is why I would use removable battery packs. I've seen too many RC batteries go up in flames during charge to trust charging in my droid.

I think the cylindrical Lithium cells are more robust that hobby lipos, but if possible I would like to come up with a way to charge them outside of R2 to totally remove any chance of fire.

I was thinking I could remove the battery boxes from the feet for charging, but then they would have to be held on with magnets or something instead of screws. Also the battery hoses would need to quick release (magnets again?).

The other way to go would be to design the battery pack so that it inserts into the back of the battery box through the back plate. Given enough design and iteration I reckon I could come up with a design that works like a lithium powertool battery, and could even have an external dock that you plug them onto to charge. The downside of this would be that it is going to visibly change the look of the back of the battery box, at least with extra seam lines, if not visible release clips.

I wonder if I could come up with a press to insert, press to release design like an SD card slot?

The width of the inside of the battery box is limiting. With the cells in the most convenient orientation (like my second render above) there is under a millimeter of space. I would probably have to thin down the walls of the box a little.

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