BMS boards overcharge protection - Page 1
Forum section: electronics / projects

author: santoshgurral
author_rank: Contributor
timestamp: March 27, 2020, 09:35:48 pm
content: HiI am using a 3s BMS board for my battery pack to make it use as a DIY UPS from my modem. So I need to plug in the charger adapter to the battery pack and battery pack output is connected to modem always. Whenever there is a power cut the modem gets its power from the battery pack otherwise battery charger adapter will be charging the battery as well powering the modem.My concern is since I always have to plug in the charger adapter, will this not overcharge the batteries? I know BMS boards will have overcharge protection which will cut off the charging when the battery pack is fully charged. But i want to know how does a 3S BMS board does this overcharge protection? Because I have a TP4056 charger module as well with me, which does not cut off the charging when the load current is greater than100mA. Which is what in my case the load current will always present since the modem is connected. Please help me understand how does a 3s BMS board manages to cut off the charging even when there is always a load present while charging?Please see attached images for my setupThanksSantosh

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 27, 2020, 10:35:55 pm
content: Your diagram looks wrong. Notice the two unused connections B2 and B+?B+ and B- should connect to the ends of the battery pack. B1 and B2 should connect to the cells in the middle for balancing.Don't connect the modem to the charger directly. How do you expect the modem to work when the charger is unplugged? It would be better to connect it to the battery.The BMS board should cap the voltage on each cell at 4.2V (approx.). The current will drop to zero as the batteries become near fully-charged.

author: santoshgurral
author_rank: Contributor
timestamp: March 27, 2020, 10:56:25 pm
content: My diagram was just a pictorial representation to understand the setup. hence I did not show all the wiring there. Anyways I have updated the diagram and attached hear to avoid confusion.Since the charger point and battery points are the same P+ and P- terminals on BMS board. so when there is no charger battery will supply the power to modem.My doubt is when there is always a load(modem) present on the P+ and P- terminals how does the li-ion charger terminates the charging. I am assuming a li-ion charger will look for a change in charge current which should be less than 1/10th of programmed charge current. How does the charger terminate charging if there is a load >1/10th always present?Also, i have attached a screenshot form Tp4056 charger IC datasheet for your referenceThanksSantosh

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 28, 2020, 12:43:25 am
content: Quote from: santoshgurral on March 27, 2020, 10:56:25 pmI don't think so. Look closer at the board. There is no direct connection from P- to B-.If there was a direct connection, what would this board even do? It would have no way to disconnect the charger, drop the voltage, regulate the current. It would be useless!I had a look at some ebay listings for this board, which had information ranging from okay to spectacularly useless. None clearly said what the maximum voltage input on this BMS is. It could be 13.6V.If you have 13.6V on the P terminals and the batteries were almost fully charged, the BMS would be dropping 1 volt between its input and output.How do you know this BMS uses TP4056s? It doesn't say anywhere that I could find.In conclusion:You should attach a DC power supply to P+ and P-. It doesn't control the charging cycle.Modem should be connected to the batteries.

author: santoshgurral
author_rank: Contributor
timestamp: March 28, 2020, 02:06:47 pm
content: P- to B- is a short normally(by default) this BMS opens the switch/MOSFETs in case of an overcharge or over-discharge or short circuit event.No, this BMS board does not have a TP4056 charger IC on it. Because the BMS board does not charge. we need a CC/CV li-ion charging adapter for that. This BMS board does protecting and balancing the battery pack but not charge.I referred to TP4056 here just to make my point understand that in case of TP4056 module chargers the charging is never terminated if there is a load on the output which greater than 100mA.To simplify my question how does a 3S BMS board overcharging protection work? or is it part of the Li-ion charging adapter to terminate the charging? If so how does it terminate charging when there is always a load present on the output

author: roman.isaikin
author_rank: Contributor
timestamp: March 28, 2020, 07:38:57 pm
content: This types of BMSes OV/UV/SC protection has only one way to disconnect the charger - disable the mosfets between P- and B-. That should happen only if charger will try to charge above BMS threshold, in which case the load will be powered from charger only. After that, even if you disconnect the charger, BMS will probably not turn itself on without removing the load.So if the charger is good, it's ok-ish to connect load in parallel with the charger.