Lithium Lipo + LifePo4 batteries in parallel

[Christophe]

That's it and yes we will not quibble ... but it would be interesting to have precise information on this subject (theory, equations?) ... because it concerns all batteries put in //

Besides, I have a mysterious element on the LifePo4 (not mounted on the battery) whose voltage oscillates (?!?) I will make it a subject!

Mystery quickly solved: after re-verification, it only oscillated in auto range multimeter mode ...
The lifepo4 is at 0,2V. (so in PLS mode limit of the infarction, although I have 0.0V too!) .. in auto range it oscillated between 0,2 and 1,2 ... It is all the same strange it must all similarly this to pass a trick. I checked dozens of cells (li-ion, lifepo4, lipo ..) I have never seen this kind of oscillation ... it is undoubtedly a sensitivity of this cell which makes that the measurement influences its voltage in auto range mode.

[izentrop]

I tried on one of my electro road prototypes the Lipo 12 Ah with LifePo4 10 Ah in parallel ... fairly close capacities so I was confident but in fact not!
LifePo4 simply did not discharge at all or almost not, 99% of the current was drawn on the Lipo!

lipo
Nominal voltage: 20 * 3,7 = 74 V
Max voltage 20 * 4,2 = 84 V
Min voltage (cut-out by configurable BMS) 20 * 3.2 = 64 V

LiFePO4

Nominal voltage: 24 * 3,2 = 76,8 V (the mounted BMS is sold for a 72V)
Max voltage 24 * 3,6 = 86,4 V (can be increased to 3,65 V)
Voltage min 24 * 2,8 = 67,2 V

Both were charged at 2V with the same charger but not at the same time

It is not a problem of internal resistance, because it is in general negligible compared to that of the load and depends on the capacity of the elements https://www.masolise.com/cellule-lithiu ... 02x3340858
If they are 18650, 60 mOhm * 24 = 1.44 Ohm, which authorizes a theoretical maximum current of 53 A

The LiFe should only start to discharge when its voltage drops below its nominal voltage 76.8 V.

It all depends on the discharge current, but the LiFe element quickly drops below 3.2 V.

While Lipo is still at 3.5 V at the end of discharge at 2C
http://velion.fr/index.php?page=bonne-g ... terie-lipo

[sicetaitsimple]

This is not a problem of internal resistance, because it is generally negligible compared to that of the load

Load resistance is what it is. Here we are talking about the internal resistance of two branches of a circuit of different batteries in //, and therefore of the distribution of the current between the two branches of this parallel circuit which have the same DDP between the two connection points upstream and downstream .

[izentrop]

It would take the datasheet of each battery to know if the internal resistance is so different

[izentrop]

I am thinking rather of an electromotive force opposing an EMF.

[Christophe]

The fem, the valves, the bars, the pipes ... full of valid analogies ...

I would like an equation ... if possible?

[Christophe]

It would take the datasheet of each battery to know if the internal resistance is so different

I must be able to find this kind of information in my archives ...

One thing is certain, this resistance varies with A ... and with the number of cycles

[izentrop]

It also varies a lot with the temperature, the more it heats the more it can draw amps, to burn in case of short juice.

[izentrop]

It would take the datasheet of each battery to know if the internal resistance is so different

I must be able to find this kind of information in my archives ...

Otherwise give the references written on the elements.

[Christophe]

Not much usable in direct information: these are LifePo4 10Ah elements in 32650. I will see if I can find something with the barcode.

Here is the 1/2 LifePo4 battery (which I currently convert to 12S because I no longer need the 24S: before there were 2 identical suitcases in series):

LifePO4_12S.jpg (170.43 KB) Viewed 5276 times

So you can see what the wiring of a BMS looks like.

Note that the BMS smoked on circuit 14 ... no problem, I don't use it anymore