Battery Charging and Ohms Law

[EasyGoing1]

Ok, as some of you know, I've been building a LiIon battery charger with an Arduino, and I noticed something today that I thought was ... odd ...

When I put a depleated 18650 in the Arduino, and hit it with 4.1 volts, best current draw it will do is .8 amps. It's got a C rating of 3000. However when I was at my friends place and I charged the same battery on one of his wiz bang $100 battery chargers, it said that the battery was charging at 1.5 amps.

So I thought I'd bring something up that I noticed when I was experimenting with the arduino project. A few weeks ago when I was trying to get one of my 18650's to charge at 1 amp, I cranked up the supply voltage until I hit that current. But what I noticed was when I took a voltage reading off the battery while it was charging, it was only at 3.9 volts HOWEVER when I disconnected it from the charging source, it was showing a voltage of like 4.8 FROM THE CHARGE SOURCE... so connected to the battery, it reads 3.9 but disconnected, it reads 4.8 and the only variable that's changing is the connecting and disconnecting from the battery being charged.

Now people say never apply more than 4.1 volts o a standard LiIon cell, but if the voltage across the battery never exceeds 4.1 even though the charging source may be cranked higher than that, is it possible that this might be the only way to charge a battery at half its C rating?

Thoughts?

[redwire]

Re: Battery Charging and Ohms Law

It's the same problem as your original door lock solenoid circuit- the switching transistor is only partially on and thus has a large voltage drop across it and limits your charging current. 4.8-3.9V=0.9V at 1A losses as heat.

LiIon battery charging are 4.2V +/-0.03V to +/-0.05V which is better than 1% precision.

Having much more charging voltage than spec'd is playing Russian roulette, as far as

[sam_sam_sam]

Re: Battery Charging and Ohms Law

Ok, as some of you know, I've been building a LiIon battery charger with an Arduino, and I noticed something today that I thought was ... odd ...

When I put a depleated 18650 in the Arduino, and hit it with 4.1 volts, best current draw it will do is .8 amps. It's got a C rating of 3000. However when I was at my friends place and I charged the same battery on one of his wiz bang $100 battery chargers, it said that the battery was charging at 1.5 amps.

So I thought I'd bring something up that I noticed when I was experimenting with the arduino project. A few weeks ago when I was trying to get one of my 18650's to charge at 1 amp, I cranked up the supply voltage until I hit that current. But what I noticed was when I took a voltage reading off the battery while it was charging, it was only at 3.9 volts HOWEVER when I disconnected it from the charging source, it was showing a voltage of like 4.8 FROM THE CHARGE SOURCE... so connected to the battery, it reads 3.9 but disconnected, it reads 4.8 and the only variable that's changing is the connecting and disconnecting from the battery being charged.

Now people say never apply more than 4.1 volts o a standard LiIon cell, but if the voltage across the battery never exceeds 4.1 even though the charging source may be cranked higher than that, is it possible that this might be the only way to charge a battery at half its C rating?

Thoughts?

************You need to be careful charging this types of batteries...........

I really HOPE that you are using batteries with built in protection board or are using an external protection board because if you are ARE NOT you are asking for trouble I should know I have charge these batteries with OUT protection board BUT I limit the voltage to 4.2 volts and the current to no more than 500 milliamperes and keep a very close eye on them to make sure they are NOT becoming warm

Also one note

When buy these protection board test them to make sure they work properly
I had bought some that did change them fully but would not hold a 1 amp load that were suppose be

These measurements were taken at the battery terminals not after the protection board

I have had some protection board that would slightly over charge and or would let batteries drain a little low on voltage or would not let batteries drain down to 3.00 on each cell

Do not take it for granted that the protection board that you are buying work the way they should

Now I will use them if and only if they

first

That they do not over charge meaning that the changing voltage does not go over 4.3 volts and

Second

when you discharges them they do not go below 2.8 volts

Just a word of caution

[sam_sam_sam]

Re: Battery Charging and Ohms Law

Now people say never apply more than 4.1 volts o a standard LiIon cell, but if the voltage across the battery never exceeds 4.1 even though the charging source may be cranked higher than that, is it possible that this might be the only way to charge a battery at half its C rating?

Thoughts?

You can charge them a higher voltage than the 4.2 volts BUT only if you make sure that the protection board works the way it should you have to monitor the battery voltage at the battery not the voltage going to the protection board these are two different things do not get confused by the two readings

Do not let the battery voltage go beyond 4.3 volts

[eccerr0r]

Re: Battery Charging and Ohms Law

If you charge beyond 4.2 volts, make sure it really is 4.2 volts and your voltmeter isn't inaccurate... uh... ahem... yeah...

If the impedance of your battery is very low, that 0.1 volt difference between 4.1 and 4.2 can mean a lot of amps. You can see this with a good lead acid car battery (which have similar charging requirements to lithium ion except voltage and is MUCH more forgiving during overcharging though can also have the same outcome, i.e. exploding) and a variable CC/CV power supply. As you get near the voltage of the battery the current will rise and then skyrocket because of the internal impedance of the battery. Current limiting is a must.

[EasyGoing1]

Re: Battery Charging and Ohms Law

It's the same problem as your original door lock solenoid circuit- the switching transistor is only partially on

There was no switching transistor in my experiments... just an adjustable power supply, a volt Meter and a battery.

[EasyGoing1]

Re: Battery Charging and Ohms Law

If you charge beyond 4.2 volts, make sure it really is 4.2 volts and your voltmeter isn't inaccurate... uh... ahem... yeah...

I have a Fluke 115 thats about two years old ... I'll assume I can trust it.

As you get near the voltage of the battery the current will rise and then skyrocket because of the internal impedance of the battery. Current limiting is a must.

This is a curious statement .. everything ive seen and read shows that the more charge the battery has, the greater its internal resistance therefore it consumes less current...

[eccerr0r]

Re: Battery Charging and Ohms Law

This is a curious statement .. everything ive seen and read shows that the more charge the battery has, the greater its internal resistance therefore it consumes less current...

Internal resistance of batteries do not go down unless the battery is worn out or slightly due to chemistry. Rather the bulk of the charge rate slowdown is due to the battery voltage going up. When that voltage goes up, the difference between it and the charger goes down (as long as the charger is now in CV mode), and I=Vdiff/Rint and current goes down. It's exponential because lower current means less charge to do the reaction, less reaction going on results in less voltage change, and hence the curve seen.

That is, unless the charger voltage is stuck as CC and doesn't honor the voltage limit of the cell. Then the difference remains the same when the charger voltage keeps going up to maintain current flow, and possibly cause battery contents to spew everywhere, uncontrollably.

[EasyGoing1]

Re: Battery Charging and Ohms Law

That is, unless the charger voltage is stuck as CC and doesn't honor the voltage limit of the cell. Then the difference remains the same when the charger voltage keeps going up to maintain current flow, and possibly cause battery contents to spew everywhere, uncontrollably.

Its interesting ... just makes me wonder ... if a battery has a certain C rating, and if we can charge them at half the C rating safely, why would the level of voltage applied to the battery actually matter if the current is maintained with spec?

[eccerr0r]

Re: Battery Charging and Ohms Law

As we've said before, Li-ion, MAX VOLTAGE TRUMPS charge current, period, specifically to prevent explosions. If you cannot satisfy voltage limit before hitting C/2 then you have to respect voltage or throw the battery away and get one that has lower internal resistance. Usually if you cannot hit C/2 and voltage is at termination voltage, this means that charge termination should occur soon - the battery is already at around 80%+ charged and the rest of the way is just absorbing the rest of the way to 100%. This portion of the charge is indeed the slowest because of the explosion prevention.

For lead acid batteries, you can play a little looser with voltage but ultimately it's not safe for it either as going over voltage will tend to produce gas and you lose electrolyte. Like Li-ion, it should also be voltage limited and the final bit of charge is slow.

ONLY NiCd you can increase voltage get to C/2. But even then you should honor temperature rise, if temperature goes up to high, this too should cancel C/2 and likely means you have to throw these cells away as well. (BTW, NiCd charge termination is quite a bit more tricky, and if you get charge termination correct, you can charge at C or even 2C. Temperature rise is actually used for charge termination.)

[EasyGoing1]

Re: Battery Charging and Ohms Law

As we've said before, Li-ion, MAX VOLTAGE TRUMPS charge current, period, specifically to prevent explosions. If you cannot satisfy voltage limit before hitting C/2 then you have to respect voltage or throw the battery away and get one that has lower internal resistance.

So you're saying that under 80% charge ... the battery should be able to hit C/2 without going over 4.1 or 4.2 volts?

[eccerr0r]

Re: Battery Charging and Ohms Law

Yes, if the battery is "good" and the company says it should be able to charge at that rate. And more likely than not, you'll have to use 4.2 volts to get that high charge rate. Using a lower charge limit voltage means it's going to hit its "80%" sooner, so it really is like 70% charged(and limit at 90% of max capacity compared to 4.2V charge).