LiFePO4 3.2V 8AH - Other battery alternatives


#1

Hello all,

I am very excited to start building an OpenROV soon, I love the idea and can't stop thinking of all the cool things that I could do with it.

However I am trying to plan my build ahead and the battery options seems a bit unclear to me (I am considering a different battery type because the shipping price to europe of the recommended ones is very expensive -I think-)

I see that the currently recommended batteries are the 3.2V LiFePO4 3300mAH.

3 per tube = 9.6V connected in paralell with the other tube.

The older, TrustFire Li-Ionbattery are 3.7V

3 per tube = 11.1V connected in paralell with the other tube.

Does this voltage difference have any effect on the ROV? Don't the LEDs and motors need to run on 12V?

I am considering a LiFePO4 3.2V 8AH battery, since this battery is just a more powerful version than the recommended one, can I simply use 3 batteries in 1 tube and only add a counter-weight in the other tube?

Would that work?

Here is a link to the battery I'm considering:

http://www.accuswiss.ch/product_info.php?refID=1&info=p17102_Headway-38120P-3-2V-8000mAh-LiFePO4.html

PS: Are there any schematics to the electronics part? i can't seem to find any information and the Wiki page is down.


#2

Hi Achraf:

The LED modules are regulated and can take any voltage between 7 and 20 or so. The motors don't care what voltage they get- the higher the voltage, the more power. They still deliver plenty of power at the lower voltage of the LiFePO4 cells.

The schematic for the 2.6 Controller board is on the OpenROV Github site in the openrov-electronics folder, in pdf form. There is no current top-level drawing showing the hookups to the controller board, but it is not hard to figure out when you look at the wiring instructions for the DB-25 connector on Dozuki.

The battery you link will work just fine, but.......

Changes like this always have a ripple effect through the entire design. If you just want to build the ROV and get on with flying it, I would stick with the existing 26650 LiFePO4 batteries. If you like tinkering with your own designs, then it is an interesting exercise to design your own battery pack. A couple of issues if you want to use 38120 cells like the one you linked-

- These cells are BIG, so you'll need to design your own tube for them.- They are not designed to stack in series- they have electrical screw terminals on each end

- If you put all these on one side, you'll need a counterweight on the other side, and then your ROV will be way too negatively-buoyant, so you'll have to add a bunch of flotation somewhere.

- If you use only one battery pack, you should bring all the power into the ROV electronics tube using both sets of power wires- the individual power wires are not really rated to take the full ROV operating current for an extended period of time- the design is meant to share the current between the two battery packs.

This isn't meant to scare you off, just to warn you that swapping battery pack designs is not a trivial exercise. If you build your ROV with these cells, make sure you post some photos on the forum of how it turns out. We always want to see new variations on the design!

-W


#3

Hello, good day,
sorry for posting on this thread instead of new one, i’ve just receive my OROV 2.8 kit, but i dont purchase the battery together. been lookin inside forum since yesterday for compatible battery.
can i use a 3.7V li ion battery instead of LiFePO4? here in my place i could get the IFR battery but the suitable charger are very rare/hard to find/get, it seems to me that a li ion charger cannot be use for this battery since it charging v is 4.2V instead of 3.0v right?


#4

The ROV will run just fine using 3.7V batteries as opposed to 3.2V LiFePO4 batteries. However, most of the 3.7V batteries that you find in size 26650 (they are generally an “NMC” chemistry- Lithium Nickel Manganese Cobalt) don’t properly handle the current draw needed for the ROV. The cell you pick needs to be able to handle a 10A discharge rate.

There are a couple of different chargers available on the internet that will handle LiFePO4 batteries as well as lithium-ion. But you are correct, a charger that was specifically made only for Li-ion should not be used for LiFePO4 batteries, as the charging voltage is too high.

-W


#5

Couldn’t find the chargers in any local online store. guess i will have to repurchase from Orov store for convenience


#6

Hi Kyril,

Did you end up purchasing the Li ion batteries (3.7 V)? If so, how did they work for you? Which ROV model do you use?

Thank you,

Ellie


#7

Hi Walt,

Are Li ion batteries okay to use in the 2.8 model instead of the LiFePO4? Looking for clarity about the NMC, did you mean that Li Ion batteries might be Li + NMC?
Also, what is the tolerance of the stacked batteries in the 2.8 model as the design is intended? If I get batteries that are 67 mm long instead of 65.4 mm long, will it fit, or will I definitely have to remove some pieces inside the battery tube to fit?

Thank you,

Ellie


#8

Hi Ellie:

You can use Li-NMC batteries in your OpenROV, but that comment comes with a lot of caveats. We’ve only found one cell that works properly, and we sell them in our store now. The cells need to be able to source 10 amps continuously. Most 26650-size Li-NMC cells are only rated for about 5 amps or so, and if they have a protection circuit, they will cut out at this point. The problem is, under hard acceleration or deceleration, the ROV will need much more current than this, and the protection circuitry will cut out the battery pack.

If your cells are 67mm long, they probably have a protection circuit on the cells. You’ll need to test them to see if they can provide at least 10 amps or so without the protection circuit cutting out.
As for 67mm long cells fitting properly, I think with some modifications you’ll be able to get it to work, though I don’t beleive anybody at OpenROV has tried this. You could eliminate the 3mm plastic disc that holds the front battery tube electrode, and this would take care of most of the difference in length. You’ll need to figure out some alternative way of attaching the (+) wire to the forward electrode.

Hope this helps.

-W


#9

hello @elizabeth.eliades, yes i do…
and like what walt mention before it have some problem to draw higher current. during my dive i experience system freeze after some time on full thrust. i hv to disconnect the battery and wait a while before i can connect to cockpit again. it got some thing todo with those protection Ics inside these battery


edit…
but i think it will be okay if you are not using full power. thrust factor 1-3 might hv no problem.

btw. im using a 2.8 model.
and yes it harder to stack inside those tube, be careful not to break those tube cap like i did… and i did remove the acrylic holding the positive battery terminal to make it fit.


#10

If I happen to use Li-ion batteries that don’t have a protection circuit and can’t provide 10 amps, what is the worst that could happen if I use the ROV at full load?

Could it explode, or something like that?


#11

I doubt it would explode, but the cell will overheat, which can lead to rupture of the cell, and potentially a fire. Which is plenty bad enough.

-W


#12

Thanks for the reply!

I am considering buying some Li-ion batteries without a protection circuit. However, the fuses installed according to the guide of V2.8 should compensate the lack of the circuit, right? In that case, the fuses would prevent a cell overheat?