LiFePO4 alternative cells


#1

Hey guys,

as i have to rely on the battaries and experience the Trustfire phenomenal every now and again, i am thinking of buying new LiFePO4's.

I live in Germany and its always a little uncomfortable to receive it from overseas. I did some research on the german market and found an alternative.

What do you think about this cell?

https://www.akkuteile.de/a123-anr26650m1ba-2500mah-3-3v/a-100702/

its a 3.3V 2500mAh LiFePO4. Do you think it would also work fine like your suggested cell, which is not available here?

Can i use your adapter for these cells, too?

Thanks a lot

Tobias


#2

I have seen that battery also here in Denmark and think it should work


#3

Tobias,

I've ordered my cells here:

http://shop.lipopower.de/LiNANOZ-26650-3200-mAh-SL-FHE

As 26650 indicates a form factor (26x65mm) cells build to this standard should be ok, I understand.

Generally LiFePO4 cells should never see a voltage > 3.65V (if you want to be nice to the cell stop charging at 3.5V, i.e. 10.5V for a bank of three,this should settle the cell at about 3.3V) and never ever < 2.5V (stopping discharge at 3V will help to maintain a long life).

Different from other chemistries LiFePO4 cells show a very little decrease in voltage between 10% to 90% SOC (state of charge) and from there steep "knees". This implies that just monitoring voltage makes it hard to determine SOC and once you are <3V the battery voltage will decay quite quickly, especially in cases like ours (traction) where the discharge current is high.

Batteries that have seen voltages lower than 2.5V can suffer internal shorts in a future. In the case of LiFePO4 we should not see a thermal runaway (that's a nicer word than "fire"), but a melting cell is no fun neither.

After charging the cells should be balanced, i.e. ideally having a voltage that differs by few mV.

We have a LiFePO4 house bank (24V, 320Ah) on our boat since 2 years.

I'm curious to see how the small cells for the ROV will behave over time.

Hubert


#4

Tobias and Hubert- I'm very interested to hear how these batteries work out.

It seems like they should fit and perform well (I like LiFePO4 for the same lack of thermal runaway that Hubert described).

It would be great to know of some sources in Europe that people could use for batteries. If you end up testing any of these, please let me know how it goes!

Eric


#5

Eric,

as we will start to assemble the ROV in the next days (#1395) and have not played with it yet here a question:

will I see battery voltage and is there something like LowVoltageCut implemented?

We will come back with results once we have tested the unit.

Hubert


#6

Hey Hubert,

The OpenROV Cockpit display shows battery voltage (that is, the voltage of the input after the two parallel 3-cell packs are combined and go through the blocking diodes), so that can be used to monitor the voltage of the system.

From our experience so far, it seems that the ROV starts to shut down on its own when the voltage gets below around 7.5v (since the regulators supplying the computers, etc need more than that). At this pack voltage, each battery is right at around 2.5v, so the likelihood of going severely below that inadvertently is pretty low when everything else is acting nominally since the ROV won't even run properly.

Additionally, we've started including little inserts for battery tubes in our kits that allow slightly shorter cells that don't have built in protection circuits to fit in our battery tubes (which are long enough for batteries that do have those circuits). These battery adapters have 6A polyfuses in them which will cut off the battery tube if an unusually high amount of current is drawn (i.e. a short occurs somewhere).

We've also been talking about cool ways to build in alerts and housekeeping features into future versions of software. Brian Adams has done a great job making it possible for people to add plugins, so perhaps someone can make a battery monitoring plugin one day!

For now, the LiFePO4 26650s seem to work great. The only issue we sometimes have is that the "positive" terminal does not protrude out enough to make contact with the "negative" terminal of the next battery, so we've needed to add small solder globs to the top of each cell. If you have any easy/ low cost ideas for other ways to get around this issue, please let me know.

Anyway, there are many power management options out there, and I'm excited to see where it all goes.

Good luck with your build!

Eric


#7

Cheap batteries

Same batteries here in Denmark cost arround 13,23 Euro depending on Exchange rate AND you have to buy a minimum of 6pcs. If not then Price is 17,06 Euro's...

http://www.batteribyen.dk/hobby-og-rc-batterier/racing-packs/a123-systems-a26650m1a-3-3v-2-3ah-1-stk-4237

Of cause bit different as in Germany you pay 19% tax and here we pay 25% but still batteries are more than 6% more expencive here..