charging lithium battery packs (18650 cells)

[Steve Dally]

Okay, I have a number of questions concerning the safety of charging my DIY 18650 battery packs.
1. for a 2s battery pack, 8.4 volts, 3000 mah (total of 2 cells), is it okay to charge it with a 'compact power adapter' that I found while cleaning out the basement. I think it went to a handheld camcorder that I threw out. On the back it says, "output 8.4 volts, 0.6 amps" so the voltage seems spot on, the amperage seems low at C/5. But it doesn't say, "Battery charger" anywhere on it. Is there something special in a charger, specifically, that's more than just providing 8.4 volts? (just for some background, the cells themselves are in good shape and I am using a BMS on the pack)

2. For a 3s pack, 12.6 volts, 3000 mah (total of 3 cells), how do I determine the proper amperage for a charger? does too big a charger 'push' too much juice into the batteries? Or does the battery 'pull' whatever is needed? In short, should the charger be on the big side or the small side, relative to the battery? I have an official, legit Lithium battery charger with an output of "12.6 volts, 1.5 amps). That seems appropriate for 3000 mah, but what would happen if I were to use my big 20 amp charger to charge this little pack? Does the battery blow up or does it simply 'pull' what it needs. Opposite scenario: What would happen if I were to try charging my 560 ah LiFePO4 batteries in my RV with the little 1.5 amp charger. Methinks the charger would melt down.... help?

Sorry for the newbie questions. Obviously, I'm quite new to this, but I'm learning a lot and really loving the process!
Thanks again for any and all help and suggestions,
Steve

 

[sunshine_eggo]

Okay, I have a number of questions concerning the safety of charging my DIY 18650 battery packs.
1. for a 2s battery pack, 8.4 volts, 3000 mah (total of 2 cells), is it okay to charge it with a 'compact power adapter' that I found while cleaning out the basement. I think it went to a handheld camcorder that I threw out. On the back it says, "output 8.4 volts, 0.6 amps" so the voltage seems spot on, the amperage seems low at C/5. But it doesn't say, "Battery charger" anywhere on it. Is there something special in a charger, specifically, that's more than just providing 8.4 volts? (just for some background, the cells themselves are in good shape and I am using a BMS on the pack)

If 2S BMS installed, fine.

2. For a 3s pack, 12.6 volts, 3000 mah (total of 3 cells), how do I determine the proper amperage for a charger?

Cell charge rating. A 2Ah cell may only be able to handle 1A.

does too big a charger 'push' too much juice into the batteries?

No, but it allows them to pull as much as they can, which is likely more than is recommended, possibly much more. Best to select a charging source that can't deliver more current than the cells can handle.

Or does the battery 'pull' whatever is needed? In short, should the charger be on the big side or the small side, relative to the battery? I have an official, legit Lithium battery charger with an output of "12.6 volts, 1.5 amps). That seems appropriate for 3000 mah, but what would happen if I were to use my big 20 amp charger to charge this little pack?

Nothing good. Reduced cell life at a minimum.

Does the battery blow up or does it simply 'pull' what it needs.

It "pulls" based on its internal resistance, which may allow it to take the full 20A.

Opposite scenario: What would happen if I were to try charging my 560 ah LiFePO4 batteries in my RV with the little 1.5 amp charger. Methinks the charger would melt down.... help?

It would take 560Ah/1.5A = 373 hours. Charger might not be rated for that kind of duty cycle.

 

[cyberfed]

If there is a BMS on those 2S or 3S packs, then if you were to hook them up to a charger pushing 20A if the BMS isn't some cheap Chinese junk off amazon and actually works it should cut off the charging saying 'nope too much'. If there is no BMS then those batteries are going to take the 20A and depending on the size of pack that could be bad news bears. In terms of chargers, if you are going to be messing with Li-ion or even LifePo4 chemistries get yourself a good charger that is customizable for all types of voltages and chemistries. I use one from Grin Technologies out of Canada. Not cheap about $300 but I can charge batteries of many chemistries not to mention enter my own custom profile so that I can charge batteries to their sweet spot to maximize their life. There are many chargers out there to choose from. Most cheap BMS's will allow Li-ion batteries to charge to 4.25V +-.5v and let them drown to 2.75v +- .5 v. I'm not a big fan of pushing the battery to 100% capacity. The return in lifespan by just dropping it a tad pays off 3x to 4x. Once in awhile I do fully charge them to their limits to keep the chemistry happy. In terms of what amperage you should charge at just lookup and download the datasheet for the batteries you are using. They will have all the information there. Recommended charge rates, max charge rates, discharge rates ect.. I do not treat all say 18650's as if they are equal I always get the spec sheet for them to charge them within spec. Now .6 amps is not alot but in fact many 18650's like this level of charge, yes its slow (for a big pack impossibly slow and obvisouly you wouldn't use that amount of current), but its easier on the battery (say a single battery). Li-ions like slow charging. If you are in a hurry yes there are some that can be charged much faster at the cost of wear and tear of the battery. Most Li-ions will be fine with a 1A charge but CHECK.

You mentioned your charger is a from a camcorder, does that match the battery chemistry? Its not as simple as voltage and amps. Lithium batteries charge in stages starting with a CC to bulk charge and then slowly decreasing that amperage down as the battery gets full and the charger knows, toward the end you'll be down to .100ma of current going to a battery (speaking of just 1 battery cell here). So not all chargers are created equal. Something to think about.

3S in Li-ion is not ideal for 12V because if you are going to draw a lot of power quickly the voltage sag may drop too low for your device. A 4S configuration works fine for 12V *IF* you charge it to the right limits and dont push it. Again here's where an adjustable charger pays off in spades.

I'm sure you have heard of the book "DIY Lithium Batteries - How to build your own battery packs" by Micah Toll. If not you should pick it up, its a short easy read and I learned some things in there that were solid advice.

For me when I build packs, of whatever chemistry I tend to follow these rules, and I know maybe not everyone can afford to do so.

1. I always use brand new batteries from a reputible source. Liion wholesale is legit, there are others too such as IMR batteries.
2. I test every single battery I get with an expensive IR tester to ensure the pack has cells that have almost idential IR's.
3. I do a full capacity test charge/discharge to make sure its delivering what its supposed to AND keeping its temp at what the manufacture says it should be, I use a thermal imaging device for this.
4. I only use quality BMS's (Batrium, JK ect...) they MUST have a wifi or bluetooth interface so I can see what each cell is up to. Plus the active balancing keeps all the batteries at almost perfectly even voltages.
5. Once the pack is fully assembled it goes through stress tests pushing out the max amps the BMS allows (with the prroper inverters). I again measure with the thermal imager to ensure nothing is heating up more than is expected. After about 3 runs like this with no issues. I consider the pack ready for use. It is then placed inside essentially a fireproof bomb proof military surplus container lined with welding insulation rated for over 6000F. Everything is locked down so nothing moves around. I then bolt the inverter to the outside of the pack with a switch so I can keep the inverter from being a paracitic voltage hog. Everything is fused of course. I also install an ESP32 microcontroller that monitors the environmentals of the inside and if anything reaches a level I dont like I have it email me and text me immediately as well as set off alarms. I have an MQTT dashboard using grafana to monitor things while away from home. Though I NEVER run my packs unless I'm home. Various other fire block materials are placed around the case too. Just in case.

I'm sure many will think this is super overkill, and maybe it is, but 1 its fun for me and 2 I like safety!
Best of luck with your builds. If you take away anything from this post is, get a proper charger.

Stay safe and keep DIYing!!
Cheers