Completing battery systems of single LiFe cells without BMS

[Gerhard Michel]

Hello all,

in this thread I’ll try to show how to complete a battery consisting of 8 LiFe cells for our tanks without using a BMS.

All we need is the following:

 8 LiFe cells
 A charger with balancer for up to 8 LiFe cells; otherwise a seperate balancer to combine with a charger (not recommended).
 A telemetry monitor for the RC equipment which allows to show the cell voltages at the transmitter’s display, combined with acoustic warning at a critical threshold of the cells. These monitors are available für several RC systems. I’m using the Graupner Electric Air module.
 A RC system with telemetry option.

The 8 cells can be mechanical combined in line, in two packs of 4 cells each and so on, depending on the available space. The positive pole of the first cell is connected to the negative pole oft he second and so on. Specific cell connectors can be used, but simple wires do the same job. At last we have one free positive and one free negative pole to be connected with the tank’s electric system, and we have seven electrical connections between the cells.
For easy loading and balancing management it is advisable to connect these nine points to a 9 pole connector, e.g. used at elder IBM PC mouse equipment, but any other connectors can also do it. Important: the connecting system must avoid any polarity reversal. The main wires to the tanks system (plus and minus) must be strong enough for the high driving currents; they must be wired additional to the balancing wiring. So in the end we have 2 main connections for loading and driving and 9 connections for balancing and monitoring.

(to be continued)

[Gerhard Michel]

Here we have 8 single LiFe cells of 40 Ah each; fitted by tape in two packs of 4 cells each:




Here a summary connector for balancing and monitoring. The plug remains in the tank:




Here one of two cell blocks with prepared wiring:




Here some parts for the balancing cable to the charger:




The completed connection cable to the charger; only for balancing. The loading wiring consists of two simple wires as usual:




Here my charger with balancer (idle state):




The charger display at charging load with 40 amps, the maximum current for these cells. The charger can charge with up to 70 amps when using both channels (!):





The cable for monitoring by the RC equipment when driving:




The transmitter's display for cell monitoring. It shows only the first 7 cells, another shows the leftover (up to further7 cells), This is not important to observe because an acoustical warning appears if one cell gets near to a critical state:

[Jerry Carducci]

Good information Gerhard! Will you show us the telemetry sensor used to interface with the battery cells within the model?

Jerry

[Gerhard Michel]

Hi Jerry,

here is the module. It was developed for electric airplanes and can show via telemetry several data like voltages, temperatures, currents, consumed charge (!) and so on. It can also handle warnings when a certain level is reached (e.g. voltages, temperatures..... ). I use it for an acoustic warning in my transmitter if a cell reaches less than 2.8 volts, if a motor gets too hot, if the model gets nearly out of range and so on. So I don't have to look continuously at my transmitter's display to receive important informations from my model. This method comes from model flight where a long look at the transmitter would cause the model to crash.

[Gerhard Michel]

As you see, using single LiFe cells without BMS is neither a sorcery nor a science. LiFes need monitoring only in two situations, driving the model and charging the battery. At a longer storage I can control my batteries with a little automatc monitor like this:



It shows the voltage of each cell one after the other automatically when plugged in the balancer connector of the tank.

[Jerry Carducci]

As you see, using single LiFe cells without BMS is neither a sorcery nor a science. LiFes need monitoring only in two situations, driving the model and charging the battery. At a longer storage I can control my batteries with a little automatc monitor like this:



It shows the voltage of each cell one after the other automatically when plugged in the balancer connector of the tank.

Thanks for this Gerhard..

I believe I have that very monitor in use with some of my battery packs. If triggered it emits a skin grating, blood curdling sound you simply can't
ignore!

I use telemetry in some of my gas models to monitor engine temperature and rpm. My new Turnigy and FlySky systems support all manner of telemetry sensors but aren't always easy to find...

I use Graupner Polaron chargers and they do a great job on smaller packs; not sure they have the ability to charge a larger LiFe pack...

Jerry

[Gerhard Michel]

Hi Jerry,

IMHO a Polaron charger can only charge 1 - 7 LiFe cells. But we use 8 cells.

[Christoffer Ahlfors]

Sorry to dig up an old thread, but this is the knowledge base, right?

The last time I bought LFP cells from China, I asked for specifications. They didn't have any but gave me this link, where I could read all about LFP cells.
Quite shocking, but it IS a good link with theory, measured performance and some practical do:s and don't:s. Luckily, we don't have to protect ours from water...

https://nordkyndesign.com/category/mari ... y-systems/

[Gerhard Michel]

Well, it's a little difficult for me to translate these items to German language. The automatic translator of my browser (Firefox) causes terrible results....