Balancing Act...

[Simen]

... or; How to treat your LiFePO⁴ cells... 

I've used a number of LiFe cells with my e-bike the last couple of years without a BMS (Battery Management System) with no problem with cell balancing, since i use an iCharger 1010B+ whith a balancing port - but i thought it now was time to add a BMS...

Let me derail a bit now; Most of the Lithium battery packs on the market, meant for e-bikes, small to medium sized 12V batteries for solar/caravan/boats etc. does not have a BMS! They have a SBM board! (Simple Battery Management) It seems that the 'BMS' acronym has been worked into the jargon when people talk about the electronics that monitors and protect the Lithium cells (and the people).

Where a SBM just makes sure you does not overcharge or over-discharge the cells, and protects them against shorts, a BMS monitors and balances the cells all the time. Some SBM have balancing, but usually only a light balancing at end of the charge cycle.
And the overcharge and over-discharge limits in SBM are - in my opinion, too coarse. Manufacturers usually recommend a charge of 3.60V +/-0.05V for LiFe cells, but most SBM disconnects the cells from an overcharge at 3.90V! The same with over-discharge; recommended are 2.50V, but most SBM disconnects the cells at 2.00V...
This shortens the cells life! But alas, BMS are more expensive than a SBM, so my choice in this case are a SBM...

Ahh...
It was good to get that off my chest... 

Now, back to my e-bike LiFe pack...
I built this pack with 24pc of 3.3V, 10Ah LiFePO⁴ cells in a series/parallel to make a 36V, 20Ah pack. I connected the cells together with some 20mm² copper bars (2mm X 10mm).



My choice for a SBM was this one:


It's a 12-cell, 60A (cont. load, peak 100A) LiFe SBM. Now, my cells are rated max 20A cont. load/cell (40A for the whole pack), but that would not be a problem, since my motor draws max 21A. (750W motor) Besides, i'm thinking of adding 12 cells, making it a 30Ah battery...

Actually, it was a bit premature of me to start this thread, since i haven't connected the new SBM yet... But do not despair!
I got 4 more of those 10Ah cells, which i've made into a small 12V, 10Ah battery, with a small SBM board added.

Here it is, all connected:


The SBM are about the size of a box of matches, and fastened to the cells with some double-sided foam-tape (so it doesn't move around). The SBM can handle 10A, and disconnects the load @ 15A. The rated charging current for these cells are 1C, but they can be fast-charged with 2C, making them perfect for on-the-run battery. Though, there is a life-expectancy penalty when charging them at 2C...
The pieces of wood are to protect the terminals against shorts, and the surroundings.

I'm going to use this pack as a backpack battery; putting it at the bottom of my backpack, and connect a box with several usb-ports and cigarette-lighter sockets, to use it for charging, running laptops etc...

Here it is, all wrapped up.


Total weight are 1686 gram (3lb, 11.5oz) and it measures 19x16x7cm.

Just for fun, here is an inside shot of a 12V, 30Ah LiFe battery i bought a while back:


I'll update this story when i've figured out how i'm going to weather-proof and mount my revised bike-battery... 

[Bruce S]

Any new updates?
I prefer the ones with the nut/bolts on their ends, makes it easier to reconfigure if need be.
Cheers
Bruce S

[Simen]

I decided to put the battery pack in an old battery box (for FLA batteries), where i polstered the bottom, sides and top with some sitting pads (or whatever they're called ).



The box - now, too heavy and big to mount on the bike itself, has to go into the trailer. The box are not completely water tight, but it should withstand a light rainshower... I' probably would have to open the box and dry it out if i had to ride it in heavy rain though... (but you will not catch me in such a situation... )



The advantages to this, is that now i can actually pedal the bike without power (without breaking my back) when the trailer/battery are disconnected, and the bike behaves much better without the weight of the battery. (Not as top heavy anymore, even when the batteries was in the side saddlebags...)
The disadvantages is that now i always have to drag the trailer with me... But then again; i have more carrying space...

The performance of the bike are the same, but it's much simpler for me to charge the battery now without worrying about balancing, overcharging etc...

Edit;
The whole battery box ended at 11,3kg (25 lb.); not too bad for close to 800Wh...

[Mary B]

Look at sportsman dry boxes to put the batteries in. Many have gaskets and will survive dumping in the lake if a boat overturns.

http://www.mtmcase-gard.com/products/camping/dry-boxes.html

[birdhouse]

looking good! 

that trailer looks to be begging for a solar panel on top...   

adam

[boB]

That looks great !

What is that  Simple Battery Management  circuit board you are using ?  Is that your design or
something off the shelf ?

boB

[Simen]

Thanks all for comments.

Thanks MaryAlana, for the link; that kind of box would be perfect.

I've long considered mounting some solar panels as a lid on the trailer, but i have to consider the weight, and also the aerodynamics; the bike does over 40km/h (25mph), and any solar panels that would do any good (3x 20W) would make a nice sail close to 1m² Consider passing close to a large truck doing 55mph; the turbulent windblast from that truck would fling that trailer - and me, into the ditch... 
But it's still in the back of my mind... Maybe a custom-made, folding solar panel that can be rigged up when parked.

The SBM's are off the shelf from GWL Power. The ones i got, are SBM12/60 and SBM04/10.

I was now about to present to you a school example graph of a short discharge-charge cycle to demonstrate the balancing act of the smaller SBM, but was a bit surprised to see an unexpected behaviour of one of the cells...


I used my iCharger 1010b+ to monitor the cells.

The cells are at rest for the first 2 minutes in the graph, then i connected a 20W bulb, drawing around 1.7A for 20 minutes. Then i disconnected the load and let the cells rest for 5 minutes. Then i connected exactly 14.41V @ 2.67A charge (25 minutes into the graph).
During discharge the cell difference was at most 21mV; the difference at end of charge was 12mV.

As you can see, just when 3 of the cells reached 3.65V, cell 1 (the red graph (red pill?)) stopped being charged for some minutes, instead of continue charging...
After some (intense) brainfarting, i remembered that i've just replaced that cell with a new one, and that would probably explain the difference. The new cell have been charged/discharged just a couple of times, while the other 3 cells are over 2 years old, with many more cycles on them.
All these cells come from the same batch, so they're the same age, but the 'new' one are obviously not used before.

But - the SBM did it's job - it just used a bit more time to balance the cells at the end. Max. balancing current for these SBM's are just 47mA/cell, so batteries with cells much out of balance would need a separate Cell Balancing Board, or a full BMS.

[DamonHD]

Hmm, just the sort of thing I was thinking of being able to do with my pile of 100+ AA NiMH cells to keep them properly balanced without too much faff.

Rgds

Damon