... or; How to treat your LiFePO
4 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 LiFePO
4 cells in a series/parallel to make a 36V, 20Ah pack. I connected the cells together with some 20mm
2 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...