Charging 18650s

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>>1168305
Pic related is the circuit I had in mind to connect the chargers to the cells. Basically, when 5VDC is applied to the relay coils, the relays flip and the batteries are each connected to a single charger circuit. When +5VDC is removed, the relays go back to their normally closed position which brings them all back to a series configuration. I’m planning on using one of the good 2A USB chargers to provide 5 volts, but after the current used to activate the coils, I’m down to less than 500mA for each charging circuit. This isn’t ideal, but it is enough for this project. I looked up the datasheet for the IC used in the charging circuits, and there is a program resistor on each board that I can change to tailor the output charging current to around 500mA. I understand that charging an 18650 a little slower tends to be better for the cell anyway.

Sorry for rambling. I guess what I’m looking for here is advice on the whole setup. Does it seem reasonable to use relays to switch the batteries in and out of the charging configuration? Is there an easy way to balance the voltage between cells? Should I even worry about balancing the voltage between cells, or is it not going to make much of a difference anyway? Since the batteries are in series, I can’t really fuse each one of them. Is there a need to fuse the series output, or do cells blow up in series configurations like this anyway? Should I use a thermistor and circuit interrupter instead of a fuse? I’m building a couple of these for a demo of another project, so they will be handled by laypeople who don’t know shit, and I just want to make sure that these aren’t going to blow up in their hands, no matter how they treat them. Expected life of one of these circuits is going to be about 1 year. Thanks guys!

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buy pic related
wire your batteries in series in your project
bring out a tap at each +/- junction and at the + and - of the pack.
connect wires to a plug that matches the white connector on the end of pic related
connect the + and - of the pack to the red and black jacks on the end of pic related
this will allow you to balance charge the four cells while they remain series connected

The iMax B6 is a great charger for many other types of batteries too.

>>1168410
Thanks. The price is definitely right, but I worry about sending one of these with each of the demos and then writing *very simplified* instructions on how to charge the batteries. Again, these are going to primarily buisness men who probably have trouble charging their own cell phones. I could probably replicate the logic in one of these chargers and put it on an arduino nano buried in there with the batteries, but I was hoping to not have to include a microcontroller.

What are the risks if I don't have a battery management system on these batteries? Are they actually going to blow up? They will have to dump a significant amount of current at one time (on the order of a couple of amps) but will only be on for 20 seconds at a time and will have a rest period in between on cycles. I'm not too worried about when they are plugged in and charging because they will only be charging with 500mA, and the charger will stop when the battery is full. I guess I'm really just worried about some idiot holding down a button for 5 minutes and a battery heats up enough to explode. If I can make a totally passive circuit with a few thermistors that will disable the pushbuttons when the batteries are too hot, that should be enough.

How do they do it with e cigarettes?

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>>1168428
OK, Look for a 4S battery management module.

There are ways of doing it much simpler than all those relays.

http://www.ebay.com/itm/4S-20A-Li-ion-Lithium-Battery-18650-Charger-BMS-Protection-PCB-Board-12-6V-Cell-/262735529819

>>1168305
Most tool batteries are five high current 18650 cells in series. Not to mention that laptops see five of these strapped in.

Is your goal to design a bms? If not think about the 18vs

>>1168512
Thanks again. So I'm sitting here looking at what you linked to, and it looks like that is a way to protect each cell while it is in the pack, which is definitely something I need. However, all of the diagrams that I see for these show how to connect each cell, and shows two wires that are used as + and - for both charge and discharge. So my next question is how to charge the cells with just two wires. I'm pretty sure that I shouldn't just connect 16 volts to the input and hope that it is split appropriately between cells. Some of these BMS' even explicitly say 'not a charging board'

Does anyone have experience designing something that uses both a battery management system and a separate li-ion charging circuit? It seems like the way they do it for drills and such is to have the BMS in the battery pack, and the charger and *custom* plug in a separate unit. What I'm looking for is more like a cell phone where the BMS and charger circuit are both in the device, and the plug is a standard micro usb.

I'm kind of thinking just include the BMS for the 4s cell configuration and also use the circuit that I already proposed for charging. I can just modify it so that the relays switch between the charging circuits and the bms.

I saw a guide on adafruit about charging multiple li-ion batteries, and they basically came to the same conclusions as me. The only difference was that they used a 3P2T switch instead of a relay configuration. For the relays I have in mind though, I can probably fit 4 of them in the same amount of space as 1 additional cell, so they are doable size wise.

The thing that I like about using a couple of relays is that if a charger is plugged in, the output of the batteries is automatically disabled and there are no additional switches needed. I've designed stuff for customers before and you might be shocked by how much they can break your circuit if they have access to things like buttons and switches.

>>1168748
Beleive me, I would love to have 18 volts. The thing with this project is that I need as much power as I can possibly get, but I also need to design it so that it doesn't look like a giant power brick. Ultimately, the application here is resistive heating, so this hand held battery pack is going to be dumping power in to a low resistance load; in some cases, significantly less than an ohm. I've noticed better heating results with a higher resistance though, say 4 or 5 ohms. Problem is, in order to get the same current through a larger resistance, I need a higher voltage.

The size and configuration of the demo that I have now works well with 18v (2 9-v batteries in series) so I would really like to get to 18 v with this charger. I'm custom modeling the bezel for this (think joystick minus the base) so if there is any reasonable way to fit 5 cells in, I will. On the other hand, if I have to, I will end up using 2 cells and a DC-DC converter to get somewhere between 12 and 18 volts, just to make it more appealing to the customer. For now, I'm thinking 3-4 cells and a DC-DC converter is the best compromise.

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>>1169007

I'm not trying to spoon-feed you a link to exactly what you need.

I posted that to encourage you to look at other options.

If you followed THAT link you would have found other boards with other specifications.

Like the one in pic related.

No need to reply to this post. I'm no longer following this thread.

>>1169024
Ok.

Wish you had stayed long enough to post actual experience instead of links.

>>1169073
not that guy.
cant be asked to read all this shit.
snoop around on the web, its pretty simple.
you can figure this out.

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>>1169024
>>1170705

If you have time to piss and moan, you have time to post useful information.

OP, I am in the same situation as you, building a DIY laptop battery. Here's what I've found out so far.

I am using battery holders instead of tabs, because I don't have a spot welder and want to be able to replace bad cells.

My pack is 2 groups of 4 cells in series (4s2p, 14.8v). I am adding a BMS with leads that go between each pair of cells wired in series, like the ones posted already. This is important because IIRC, unprotected cells will die if they are over- or undervolted during use.

My BMS has a "balance charge function" that cuts off individual cell pairs once they reach max voltage. So if you connect a power supply to the output leads, and more charge is going into the pack than coming out, the cells will charge up.

Constant voltage / current charging isn't as effective as a smart charger like an iMax. IIRC, I won't get the full rated capacity of the cells, and they won't last as long as if I had used a smart charger. For me, the convenience is worth it.

>>1172050
Thanks anon! I was starting to lose faith here when everyone kept posting links to BMS's that I had already looked at, then sent pictures of additional BMS's that were exactly the same as what they originally linked to, but acting like it was something different. Even more frustrating when an anon leaves the thread not even realizing that that he posted the same shit twice, like it was going to help somehow.

Anyway, I really appreciate the comment and the experience. Just to clarify, when you are pumping charge back in to the batteries, are you supplying 5v or 16v? I'm guessing 5v since that would be enough to provide to the cells (in parallel) but wanted to make sure.

Are you protecting your input voltage with a diode, so that you don't fry your (usb?) Charger?

I've noticed quite a bit of heat in my usb charging circuit. Leaving it plugged in for a while results in the chip heating up first, but eventually the entire board gets up to probably 60C or so. I was thinking of using some thermal epoxy to bond it to a small heatsink, just to add some thermal mass. Have you had to use any thermal management in your charger?

I'll keep updating as the project moves forward. Thanks again for your input.