Hi,
since this is my first post just want to say kudos to the community.
So, I am getting a really strange battery behaviour in my current project.
I am trying to build a project that will send me data online for a few days so I've decided to use the arduino and the sim800. Since the arduino would need to work on 5v (because of some sensors) I also put the step-up boost module before it. The sim800 is connected directly to the battery. There is also the power-bank-ish controller to recharge the battery when it gets empty.
You can see the whole setup on the picture, it's rather simple. The batteries are Li-Ion connected in parallel. 6 of them.
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) and when I checked the resistance of the battery pack it basically said it short circuited itself. So the resistance between B+ and B- were 0.
I immediately disconnected the batteries and decided to check with you about what is happening.
My setup seems simple enough, and while i think it can be improved with a charge-discharge protection i still don't understand what happened.
I would appreciate any help about this, what happened, was it dangerous and how to save against it next time.
Also: the batteries were all around 4.1V . The setup is exactly as on the picture, cables are good quality.
Are you really sure that is the 'whole setup' as in all the connections ?
You certainly do need discharge protection that cuts of the batteries when they get to circa 3.0v, letting that number of batteries go down to 0V and then attempting a recharge is asking for trouble.
And yes, letting the batteries charge up abov the normal limit is very dangerous, they can start to swell an then viloently explode, that pack could produce a nice fireball. If you dont understand how to use Lithium batteries safely, its best not to experiment.
srnet:
Are you really sure that is the 'whole setup' as in all the connections ?
You certainly do need discharge protection that cuts of the batteries when they get to circa 3.0v, letting that number of batteries go down to 0V and then attempting a recharge is asking for trouble.
Sorry, I thought I answered this one. I'll try to be more on point next time.
Yes, this was the whole setup, however it was intended to be improved later with the charge-discharge protection module.
My plan was never to complete discharge the batteries, I don't know much about them, but I do know that.
Think of this more as a one-time setup to check if the arduino is powering up.
But even if I had the charge-discharge protector on batteries I don't see how it would solve this behaviour as the batteries are connected in parallel.
Or would I need the module on every single battery?
You can't check the resistance of a battery with a meter. A meter on resistance range puts a voltage across what it assumes is a passive resistance. But a battery is creating its own voltage so you get nonsense results and may damage either battery or meter.
What exactly is the 5th component, your so-called "power-bank-ish controller"? Is it connected to anything other than just the battery?
slipstick:
You can't check the resistance of a battery with a meter. A meter on resistance range puts a voltage across what it assumes is a passive resistance. But a battery is creating its own voltage so you get nonsense results and may damage either battery or meter.
Thank you Steve, this is exactly the kind of information I'm looking for. So this would mean that when the resistance is showed as 0 on the meter the batteries are in the state of rebalancing?
But how does the voltage increase fit then?
Also what about the charge discharge module? Would it work if I just connected one on all batteries at the same time?
Will it work if I connect two of them at the same time? (the powerbank module and the module for sim800+arduino)
VielenDank:
Thank you Steve, this is exactly the kind of information I'm looking for. So this would mean that when the resistance is showed as 0 on the meter the batteries are in the state of rebalancing?
No it means that you can't tell anything from that reading. It is meaningless.
And from the minimal description of your powerbank thingy I can't tell anything about what it is doing. It looks like its main purpose is to provide a 5V 2.1A supply from a 3.7V battery. If it does anything else like charging and how good its charge algorithm is there's no information on.
Although it's normal for a lithium battery to recover some voltage when it has been on load and the load is removed it shouldn't ever go as high as 4.3V unless you have been overcharging it.
BTW for all practical purposes you can treat any number of Li-ion batteries connected in parallel as though they were one large battery.
slipstick:
And from the minimal description of your powerbank thingy I can't tell anything about what it is doing. It looks like its main purpose is to provide a 5V 2.1A supply from a 3.7V battery. If it does anything else like charging and how good its charge algorithm is there's no information on.
Sorry about this but that's what you get when you buy the cheapest module online.
Although it's normal for a lithium battery to recover some voltage when it has been on load and the load is removed it shouldn't ever go as high as 4.3V unless you have been overcharging it.
Hmmm, the batteries were not being charged at that moment but they were fully loaded couple of days before. And once I used the sim800 to connect to the network the voltage started to increase slowly at the rate of about 0.1V/30sec.
I've read that the sim800 requires a short burst of 2amps, which might have triggered something like this.
BTW for all practical purposes you can treat any number of Li-ion batteries connected in parallel as though they were one large battery.
So there are no issues like balancing when they are connected in parallel ?
e.g. one has 4.1V while the other have 3.7 so the 4.1 one transfers energy to 3.7 ones
VielenDank:
Hmmm, the batteries were not being charged at that moment but they were fully loaded couple of days before. And once I used the sim800 to connect to the network the voltage started to increase slowly at the rate of about 0.1V/30sec.
So if the circuit in the first post is how you had it all wired up how was the Arduino and the SIM800 working, there are no connections shown ?
I actually was reffering to the whole circuit not being charged by the 957-kc module at the time.
The connection is the same as mentioned in the picture. Batteries connected to the sim800 and the stepup module from which the arduino is taking its 5v supply.
Basically what i need is a setup which i can connect the sim800 and the buck directly to. Is there some solutions which can handle this
VielenDank:
So there are no issues like balancing when they are connected in parallel ?
e.g. one has 4.1V while the other have 3.7 so the 4.1 one transfers energy to 3.7 ones
Think about it. If they're connected in parallel they are all by definition at the same voltage. The only way anything could happen like you describe is when you first connect them together in parallel and you'd have be daft to try paralleling batteries with widely different charge states.
The nominal voltage for a lipo cell is 4.2V 3.7V (that was a typo)
Charged - the voltage of a fully-charged LiPo cell is 4.20V, and charging above this will damage the cell. Nominal - this can be considered a sort of "half-charged" voltage, as it is 3.70V, in between charged and discharged. Nominal voltage is what manufacturers use when describing the voltage of their batteries.
You are using a 5V USB charger to charge LIPO batteries which should NEVER exceed 4.2V.
What's wrong with this picture ?
You CANNOT USE A USB 5V CHARGER TO CHARGE LIPO BATTERIES THAT SHOULD NEVER EXCEED 4.2V !
Is that clear enough for you ?
If you get these and plug wire all the black wires together and all the red wires together and use that
parallel red/blk pair to charge your lipo batteries it would be ok.
You may have already damaged your batteries but if you switch to a lipo charger the next batteries will
be ok.
Don't ask any more questions about these batteries. You can use them as long as they work but they have been compromised by charging from a NON-Lipo charger.
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.
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.)
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.
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.
