Testing 18650 lithium cells with BMS - Page 1
Forum section: electronics / projects

author: praveen_khm
author_rank: Contributor
timestamp: May 29, 2018, 03:03:04 am
content: Hi,I have started making battery packs using standard 18650 lithium ion cells. Once the battery management system (BMS) is soldered, I charge it to check if it stops charging after a limit. For example, a 1S setup stops charging if voltage is above 4.2v-4.3v. Later I connect a load (motor) to discharge it completely. At around 3.4-3.5v, BMS kicks in and stops discharging.Now, I do similar charge and discharge tests for 2S battery packs (6.8v lower limit - 8.4v higher limit), 3S and 4S battery packs.Everything is good for single units. Now the question is how do I do mass testing these packs? Each charge and discharge setup takes approximately 1 hour. If I were to make multiple packs, how do I test multiple packs at once without taking all day.I understand there may be commercial setups to test this. But I need something I can do by myself - Cheap and easy. (Unless commercial solutions on eBay is available at few dollars). Any suggestions are welcome.Skills: Programming, Soldering, Hardware design and development, PCB designing. Adding these skills so that someone might suggest a setup even if it takes some or all of these skills.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 29, 2018, 06:53:10 am
content: You normally won't. You buy reputable cells which are guaranteed to be good; and you buy reputable BMS which is guaranteed to work. Both parts are production tested already and guaranteed to work in combination. (Finding a proper BMS, however, can be extremely difficult, especially for small players.)If you need to test the combination in a "real world" test case, I can't see any means to speed it up reliably while still getting some extra benefit apart from the separate production testing which already happened.What you are expecting to find through this testing? Your test is going to be limited in finding one single cell hitting HVC, and another single cell hitting LVC, so possible faults in other cell modules are going undetected, so you rely on the BMS manufacturer's production testing anyway.If you want to verify how the power switching (MOSFET?) of the pack, or your system integration works, you can test it separately by injecting a high or low voltage event in any of the cell voltage connections, instead of connecting it to the actual cell.Most BMS's have a series resistor after the cell connection, forming an RC filter for cell voltage measurement. You could inject a small current, positive or negative, after this resistor, to cause HVC/LVC event even when the BMS is already connected to the pack.If you really need to do full pack charges and discharges on production level, you'd need to buy a battery tester (around $1M investment if you want a brand name), or design your own, which is fairly simple given you don't need microvolt accuracy or fancy features. For production scale, bidirectional switch mode converter makes sense by redistributing energy instead of burning it as heat. I have designed a few such systems. It's still quite a big task to do just for this purpose.

author: praveen_khm
author_rank: Contributor
timestamp: May 29, 2018, 11:44:42 am
content: Thank you for the detailed explanation. Can you please post dune links of making a simple tester as you mentioned?

author: nctnico
author_rank: Super Contributor
timestamp: May 29, 2018, 12:44:50 pm
content: If you want to test charge/ discharge limits then you are only testing the BMS and for this you should test the BMS without cells. A remotely controlled PSU with a few relays to reverse the current should work fine. If you are serious about the batteries should test & match the cells. A charge/discharge test is only useful to determine whether the battery meets its capacity. This can be done using a remotely controlled PSU and a DC load.

author: praveen_khm
author_rank: Contributor
timestamp: May 29, 2018, 12:56:17 pm
content: ok. I will do a setup to test only the BMS and that makes sense. Battery are Samsung ones and they hold their reputation and all I check if they have nominal voltage before soldering a BMS.