Troubling Battery Behaviour

[DVDdoug]

I'm not a battery expert, but I'd "be careful" about putting LiPo batteries in parallel.    And, you probably can't get "optimum" charging in parallel. 



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Battery Resistance:

An "ideal battery" has zero resistance.   Real batteries have a (low) internal resistance, but of course it's not a "resistor" and it may not be linear and it may change with the state of charge, etc. 

The internal resistance limits the current, it makes the voltage drop when you draw "excess current" (or when you short the battery), and it's the internal resistance that causes a battery to warm-up (when you draw lots of current from it or when it's charged.) 

You measure the internal resistance by measuring how much the voltage drops with a known load resistance.   You've got a voltage divider and if the voltage drops in half the internal resistance is equal to the load resistance.   (But, I wouldn't recommend loading a LiPo battery to the point where the voltage drops in half...   You might kill it or start it on fire!)

I've seen cheap LED flashlights that have the LEDs wired directly to the battery (through a switch of course) with only  the battery's internal resistance used for current limiting.  (The ones I've seen use regular non-rechargeable alkaline batteries.)

[DoggerDaze]

It looks like you have a bms intended for use with a cellphone battery bank. It has a stepdown converter to charge the battery, and a step up converter to supply 5v2.1a to a usb port. The only thing you should have connected to the battery on both terminals should be the bms. I didn't look at the bms or its specs. But you should be powering your load from the output side of the bms. Your load should not be directly connected to the battery which it looks like it is from the diagram. You will over-discharge the batteries and damage them if there is nothing between the load and the battery. That is what the bms does.

If you are using lipo batteries charge them with the bms and watch the voltage. If it exceeds 4.25v STOP. And put in a voltage divider to cut the max voltage to 4.17v or so, maybe this can be a resistor mod on the bms itself. Then put a load on the BMS and watch volatge. If it goes below 3v maybe modify the bms to cut off at 3.05v . this is likely a simple risistor change.

Don't mess with lithium chemistry batteries. They can go from 0-300 really quickly.

[raschemmel]

Measuring Lipo battery internal resistance

[MarkT]

The nominal voltage for a lipo cell is 4.2V

The nominal voltage for a LiPo cell is 3.6V or 3.7V, depending on precise chemistry, not 4.2.

4.2V is the maximum charging voltage above which incendiary behaviour is a possibility.

Lithium ion cells of most types have quite a change in voltage between full charge and discharged, so nominal
really means nominal here!

Time to go to university!  >> batteryuniversity.com <<

[DoggerDaze]

I've noticed that when fully charged, and being maximally loaded the voltage droop goes somewhere near nominal. With Lithium Ion batteries. That is to be expected. Once the load is removed the voltage will increase some. If the batteries warmed up enough from the load (basically overloaded) they may exceed the voltage they were charged to until they cool off. This is real world experience. Ive built several packs and done extensive testing. This is how I got up to 7s8p of 18650's for my 100watt flashlight.

I've settled on using pouch cells with 200amp contonuous draw for the flashlight. 7s2p, maybe 3p.

[raschemmel]

The nominal voltage for a lipo cell is 4.2V

Thanks for catching that Mark. That was actually a typo. I couldn't make up my mind if I was talking about
the maximum charged voltage or the nominal voltage. So I wrote about one while thinking of the other.

I actually flew RC fixed wing aircraft for almost ten years so do know what the nominal voltage is.

[VielenDank]

Thanks for all the replies, I really appreciate it.

Is there any link you can offer me for the setup I want, if it's clear enough from the picture? Or maybe should I put a more detailed one?
Usually these sim800 modules are supposed to be connected to LiPo batteries directly, as their input should be 3.4V-4.4V so I guessed that fitting it to LiIon could also work.
Since the sim800 takes a lot more power than arduino I figured out it would be better to boost arduino from battery voltage than to step sim800 down to 4.2V. Also these powerbank controllers seem to cut off power if they think they're not outputing energy, which basically means that my arduino can work 30seconds at most when powered from the USB port. This led me to believe that the only option would be to connect the parts directly.


Also, I'm charging the batteries from the module, and not the 5V directly.
The batteries are LiIon (18650) and the controller is also for LiIon batteries.

I also believe there's a 477A capacitor on the sim800 . Could this have any effect on the batteries?

[raschemmel]

I also believe there's a 477A capacitor

no such thing


Look up the current rating for the Sim800  and get a 1S Lipo large enough.

[DoggerDaze]

Use a buck converter from the output of the bms for the sim800. Never hook things directly to Li chemistry batteries unless you are sure that each cell has its own protection circuitry. This and the arduino will load the output enough that it should stay on.

[srnet]

Now, the behaviour I'm was getting, which rather scared me was after usage the battery started increasing the voltage (at the begining was 4.1 but rose to 4.3)

So based on the circuit diagram in your first post, did you ever find out why the battery voltage was rising apparently on its own ?

It does sound like the batteries are being subject to uncontrolled charging ...........

[slipstick]

Never hook things directly to Li chemistry batteries unless you are sure that each cell has its own protection circuitry.

Now that's advice that millions of people doing RC are ignoring with great success. If you look on the hobby sites like Hobbyking you won't find a Li-ion or Lipo battery that has any protection circuitry. You just need to be a little careful with them. You don't need to rely on electronics to do your thinking for you.

Steve

[DoggerDaze]

Now that's advice that millions of people doing RC are ignoring with great success. If you look on the hobby sites like Hobbyking you won't find a Li-ion or Lipo battery that has any protection circuitry. You just need to be a little careful with them. You don't need to rely on electronics to do your thinking for you.

Steve

One small detail. The rc things that use batteries(all of them) usually shut themselves off once the battery is depleted. In that case, the protection circuit is in the the rc device.

This guy is running without one period.

[VielenDank]

So based on the circuit diagram in your first post, did you ever find out why the battery voltage was rising apparently on its own ?

It does sound like the batteries are being subject to uncontrolled charging ...........

I haven't found it yet unfortunately.

My plan now is to buy some charge discharge protectors and put them on each cell individually, connect those in parallel, and only then connect the powerbankmodule and sim800+arduinoboost.

I'll let you know how that goes.

[MarkT]

Now that's advice that millions of people doing RC are ignoring with great success. If you look on the hobby sites like Hobbyking you won't find a Li-ion or Lipo battery that has any protection circuitry. You just need to be a little careful with them. You don't need to rely on electronics to do your thinking for you.

Steve

Yes but they also sell fireproof battery pouches for storing the batteries in when not in use, and RC folks know to keep batteries out of the house and never to charge them unattended.

If you don't use protection circuitry and keep LiPo's in a domestic environment you are risking life and property, and no insurance company would pay out.  LiPo's without protection should be treated like petrol - which also needs one to be "a little careful"

I tend to use LiFePO4 batteries where possible for this reason, much safer (not 100% safe, just safer than other LiIon chemistries).

[slipstick]

Yes RC people generally know how to handle batteries but it wasn't born in them. They learned. Other people can too.

Steve