Lithium Ion Battery Shut Off

Hi, my name is Jackson Pugliese, and I am currently building a battery pack that will be consisting of multiple lithium ion batteries. Getting the pack itself together, is not an issue at all, however, what is an issue is battery shut off when charging. 

I would like to be able to charge my battery pack with some form of laptop charger, that outputs 12v, for a 14.8 volt lithium battery pack. The main issue is overcharging.

Lithium ions explode, or catch fire if they are over charged, and thus is not an option for me, so to find a solution, I searched the internet, and two things came up. 

Option one, use some form of battery charger, that automatically shuts off when It is fully charged (which i do not want to do because I want to do it differently hence the instructable)

And Option two, which came up alot, but i do not fully understand is I use something such as this 
http://www.amazon.com/Yeeco-Current-Polymer-Lithium-Protection/dp/B010NM52LG/ref=sr_1_2?ie=UTF8&qid=1456701414&sr=8-2&keywords=protection+circuit+module+14.8+volt

I do not understand what it truly is, nor how to use it, and would like someone to explain to me both of those things.

Everything I am doing is safe, because I am using a en closer for lithium ions, in case if they explode or catch fire, so know one has to worry about anything not working.

Thank you very much everyone for your time, and have a wonderful day!

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dudes1 year ago

Well, you basically have two options here.

1 You could buy a charger or charging module. The charging module you could actually incorporate into the battery pack if you wanted to.

2 You could build a charger. This one seems to be your preferred choice, but comes with some serious risks. Li-ion batteries are delicate and sensitive and must be charged specially. You said 14.8v which means a 4 cell series pack. That gives a fully charged voltage of 16.8v, or 4.2v per cell, that means that anything over that will cause damage and potential failure of the batteries. Unfortunately, you can't simply charge them with a 16.8v power supply. While it would be technically possible to charge them to 16.8v, cut the power, and let them be, it is not ideal, and has potential for major problems. The biggest issue is that, unless your batteries are exactly identical, they will charge at different rates, meaning that the 16.8v will not be equally divided among the 4 cells, so one cell may get 4.1v, another 4.3v and so on. This is particularly dangerous if you have very different cells, such as two 2000mAh and two 2600mAh cells, or a bad cell. In the latter case, the bad cell will probably not hold much voltage, maybe 2-3v, so the other three will have to make up the voltage, resulting in overcharging all three. The solution is to monitor each cell individually and charge them each to 4.2v. A circuit to do this would look something like the image below (pretend the capacitors are batteries and the voltage sources are chargers) Using a setup like this will ensure that if a cell is bad, only that cell can be damaged. It also allows the mixing of different rated cells, although that is still not a good idea for other reasons. For information on charging individual cells, google is a great place to look ;)

If you really want to make the whole thing from scratch, I designed a circuit a while ago that charges a single li-ion battery. I've never tested it in real life, but if you'd like to look at it and modify it or whatever, heres the link

http://everycircuit.com/circuit/6485575270924288/l...

I hope this helps a little, if you have any questions, I will be happy to try to help.

~Dudes

Screenshot_2016-03-08-13-18-23.png
merlinj (author)  dudes1 year ago

The link that I gave seems to be something similar, it has multiple slots for multiple batteries. Each of my batteries are 3.7 volt, and they are in series, thus bring the voltage up to 14.8v. If I use the charger that was in the link I gave, would that work with a laptop charger?

Would I have to charge my battery pack with 3.7 volts instead of 14.8? would it be a problem if I charged it with 12 instead of 14.8?

The information you gave is super helpful, I am just trying to make sense of it all. Thank you for your Patience!

dudes merlinj1 year ago

The module you linked is not actually a charger, it's a protection module, which means it simply prevents dangerous over charging and over discharging. This would certainly be a good thing to include in the battery pack, but you would still need a charger. I would suggest a charger like this one, which will charge 2, 3, and 4 cell battery packs.

Li-ion batteries are 3.7v nominal, or normal, voltage, they are considered charged at 4.2v, dead at 3.2v, and dangerously low at 2.7v. So the four cell pack is 14.8v nominal and 16.8 fully charged. Using a 12v charger would give each cell 3v, which is below what is considered dead, so 12v won't work. At the bare minimum you could charge the cells to 3.7v each or 14.8v, but that is somewhere around a 50% charge, so the battery life would be about half of what it could be. However, undercharging a li-ion repeatedly can damage it, so it really isn't a good idea to use anything less than 16.8v.

~Dudes

merlinj (author)  dudes1 year ago

That you, that was one of the most helpful things I have read. So what is I switched it to use only three 3.7 volt batteries instead. It would charge up past the 3.7 volts, but not surpass the 4.2 volts per. Since it doesnt surpass the voltage for over charging, can it overcharge at all?

Logically if it can only reach as high as the input voltage, and that voltage is 12, and the max charge it can handle is 12.6, then would I even need some sort of protective circuit?

Also, how exactly would I wire that protective circuit that was in the link?

dudes merlinj1 year ago

Technically, if you used three in series you could use 12v. However, undercharging a lithium ion battery can cause permanent damage. So, it could not overcharge, meaning it should be safe, but it will undercharge, meaning you will be using a lot less capacity than you could.

As for how to wire the protection circuit, I drew up a rough (very rough, I'm clearly not an artist) schematic of how it should be connected. Again, you still need a charger if you use this, but this helps make the battery pack safe.

20160309_120436.jpg
merlinj (author)  dudes1 year ago

Your help as been absolutely amazing. I just need to know 2 things.

1. If I use 3 batteries, with a 12 volt charger, and no protection circuit, is their any way for it to possibly overcharge (my logic is it doesn't reach a high enough voltage to do so)

and

2. If I do only charge it to 12 volts each time, how much does that diminish the battery? like would it be similar to if I don't fully charge my laptop?

Thanks a ton!

dudes merlinj1 year ago

If you use three batteries and a 12v source, you should be ok. If you're using a balancing circuit (something to make sure the batteries charge equally, the circuit you linked should work fine) then you could connect 12v to it and charge them. It would be like charging your laptop to about 90% everytime, which, if done often, reduces the capacity of the battery. So, it can be done, and it would be safe, but I don't recommend it.

Two things if you do decide to charge it with 12v; One, every once in a while (every other month or 15-20 charge/discharge cycles, whichever is more often) charge it fully, with an actual charger. That will prevent or at least reduce the effects of undercharging it. Two, you need to limit the charging current. If you connect the batteries to a 12v 5A power supply, they will pull 5A, which can seriously damage them. A good rule of thumb for charging is .2c for normal charging and .5c if you're in a hurry. So if you have a 2000mAh pack, you'd want 12v at 400mA for normal charging and 1000mA for fast charging. If absolutely necessary, the batteries can take up to 2.5c (5000mA for a 2000mAh pack) but that's not a particularly good idea.

merlinj (author)  dudes1 year ago

Thanks a ton!

So what would the balancing circuit look like? also, could I buy one?

Would it change anything to have 6 batteries instead of 3, but to have it be 3 and 3 in parallel? ( it would still be the same voltage). would you need to change the balance circuit? if so, what would the new on look like?

dudes merlinj1 year ago

The circuit you linked is a balancing circuit and overvoltage and undervoltage protection circuit in one, designed for 4 cells. This is the same thing but designed for 3 cells;

http://www.ebay.com/itm/8A-Li-ion-18650-14450-Batt...

It would be connected in the same manner as the other.

If you wanted to have 6 cells with two parallel strings of three series you would simply treat the two parallel batteries as a single battery. The balancing circuit and the charger will see the two as one bigger battery and will charge them normally. The picture below should help with understanding the connections.

20160309_163330.jpg
merlinj (author)  dudes1 year ago

hey, I know its been a while, but I finally built everything, and it works accept for one thing, The amperage the board pulls is not that much. I bought a board that can handle 15 amps, and lipos that can handle that as well, and for some reason, my 150 watt 12 laptop charger is only suppling it with about 1.7 amps. The pack can defiantly supply more, the lipos can take more, and so can the board. When I tried measuring the resistance between the two power terminals with only the batteries connected to the board, the resistance came up basically undefined. It didnt say zero, and didnt say any number, it just had a decimal place, and absolutely no numbers.

My Question is this, what determines how much current it draws? and How do I make it draw more?

Your help as been so helpful for me man, thank you so much

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