Lithium Battery Charger (help?) - Page 1
Forum section: electronics / projects

author: joblessalex
author_rank: Contributor
timestamp: July 12, 2016, 04:30:07 am
content: I have recently built an electric bike and the battery for it is quite massive. The charger I would need would have to charge 14 cells in series. I decided to build a charger for it out of buck converter modules and have run into a snag. I bought two sets of modules, one set of 110 to 24v converters and one set of adjustable buck converters. I wired the 110 to 24v modules to the input of the converters, and the output of the converters in series to a balance lead. I then set all the voltages on each converter to 4.2v. Here's the huge problem... When I plug them all in, they fire up just fine, but when the battery is connected, some of the modules begin charging and some just flash on and off like they are drawing too much current for some reason. No matter what I tweak, they all seem to shut down. It's unpredictable. Sometimes module 1 and 4 flash, sometimes it's another set. The modules I am using are these two.http://www.ebay.com/itm/222164125653?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIThttp://www.ebay.com/itm/261963340848?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITHere is an extremely fancy drawing of my setup:

author: joblessalex
author_rank: Contributor
timestamp: July 12, 2016, 04:59:15 am
content: The buck converters are for sure not isolated. The other ones clearly say isolated and show no path to ground on my meter. There are only two wires for input, so no way to reference them to ground. I've tried swapping live and neutral with no luck there either. They are all current limited to 5a max.

author: joblessalex
author_rank: Contributor
timestamp: July 12, 2016, 06:42:13 am
content: So new information. If I set the current limit to 3A, the charger works flawlessly. It gets fairly hot, but it works. Are all the amperages adding up and being forced into one module somehow?

author: MosherIV
author_rank: Super Contributor
timestamp: July 12, 2016, 12:44:13 pm
content: HiIt is NOT a good idea to use a power supply to charge any Lithium type battery.Please re-read evb149 post, his 'edit' section describes the proper charge cycle for LiIon, LiPo is almost the same.If charged incorrectly Lithium can catch light. Once Lithium starts to burn it CANNOT be stopped.There are special chips for doing charge control for both LiIon and LiPo.Use one of these in conjunction with the PSU for safe charging of Lithium.

author: Ravenghost
author_rank: Contributor
timestamp: July 12, 2016, 03:07:08 pm
content: What battery cells are you using 18650? A battery pack?If you don't want to build custom charger use something like thishttp://www.hobbyking.com/hobbyking/store/__64345__Turnigy_Accucel_6_80W_10A_Balancer_Charger_LiHV_Capable.htmlbut it will take 3 turns to charge your 14 batteries.

author: nctnico
author_rank: Super Contributor
timestamp: July 12, 2016, 07:03:02 pm
content: Quote from: MosherIV on July 12, 2016, 12:44:13 pmBurning Lithium batteries can be extinguished with sand. But still a Lithium battery pack needs a proper BMS. I strongly advise against building this yourself. Lithium batteries can do a lot of damage and your insurance company may not cover damage caused by a DIY battery pack.

author: MosherIV
author_rank: Super Contributor
timestamp: July 12, 2016, 07:17:23 pm
content: QuoteI do not beleive sand can put out a Lithium fire. Sand is used to contain the fire because it is inert and resistent enough to heat that it helps to contain the fire. The safety procedure for a Lithium battery fire is to somehow put the battery into a metal container full of sand and move the container outside (somewhere where it will not set alight somethung else) and let the fire burn itself out.

author: KL27x
author_rank: Super Contributor
timestamp: July 12, 2016, 08:39:38 pm
content: QuoteYour 120AC to 24VDC converter says "Output current: 500mA"At theoretical 96% maximum efficiency of your adjustable buck converter output stage, that would give you a maximum of 2.74A at 4.2V out. I would further reduce the maximum current to no more than 2A, probably, pending thorough reliability testing.As long as max current protection is working and you are testing/verifying the function and stability, there is no reason you can't charge the batteries with your setup as far as you can personally trust these eBay circuits to not fail. Safety is paramount, though. You probably want to verify mode of failure for several examples to see how this circuit is going to eventually fail.

author: joblessalex
author_rank: Contributor
timestamp: July 13, 2016, 04:27:27 am
content: Thanks everyone for the input. The safety concerns are all valid. The reason I didn't want to use a turnigy is because of the 6 cell limit. That's currently what I am doing is two 6s packs and a 2s and then throwing them on a standard hobby charger. This method takes forever, especially to balance because it's charging the whole pack, then discharging the high cell. This leads to cell damage over time and takes waaay longer. The batteries I am using are recycled laptop cells, so they are already handled very carefully and never charged unwatched. To those who mentioned not to charge to 4.2, you are absolutely right. This was just a test setup to see if it could fully charge the pack. My goal is to only charge to 4.15 max. I have a meter on my bike which shows voltage to avoid deep discharge. The cells life isn't really that big of a concern either as a full pack for me is about $5. As for the 5% accuracy, after a short test, they seem to be staying all very close to where I set them(+-0.07). Each of these modules contains current limiting, so I am charging them correctly (CC, CV). The pack is 20AH, so running it at such a low rate of 5A is nothing. Thanks to kl27x for the calculation. I overlooked that as I was getting the current figure from my bench supply set at 24v. It was reading 470ma so I sized it for that. I completely overlooked the cable losses and the fact that it was short circuit current, not current at 4.2... The charger seems to be working now at the 3A rating with no issues. I will look into back feeding and a startup procedure. For now just not connecting the battery before plugging it in works for me.

author: Ravenghost
author_rank: Contributor
timestamp: July 13, 2016, 09:04:42 am
content: If you already have li-ion battery charger like accucell-6 you could look into parallel charginghttp://www.rcgroups.com/forums/showthread.php?t=932319Connect your 14 cells in 2-3 parallel cell packs and charge packs in series. This covershttps://www.eevblog.com/forum/projects/what-do-i-need-to-know-about-paralleling-li-ion-li-poly-batteries/what you need to know about paralleling your battery cells.

author: David Hess
author_rank: Super Contributor
timestamp: July 13, 2016, 03:51:50 pm
content: Coach! Coach, put me in! Put me in coach!Almost all switching regulators have a negative input resistance (power factor correction switching regulators and fixed duty cycle switching regulators do not) so when you apply power, some of the 24 volt off-line switching regulators go into current limit lowering their output voltage causing the 5 volt switching regulators to draw more current causing the 24 volt off-line switching regulators to lower their output voltage even more eventually reaching a stable 0 volt output.Solutions include using higher current 24 volt off-line switching regulators, lowering the current limit on the 5 volt switching regulators, and maybe applying power to the 5 volt switching regulators *after* the 24 volt off-line switching regulators have started.

author: Kilrah
author_rank: Unknown
timestamp: July 13, 2016, 07:50:58 pm
content: Quote from: joblessalex on July 13, 2016, 04:27:27 amBut why?You'd want to connect only the first 6 cells to the charger/balancer, charge them completely, then connect the next 6, charge them, then only the last 2.That's 3 "charge cycles" but no time/energy lost in balancing.