BM3451 BMS - Page 1
Forum section: electronics / projects

author: Mjolinor
author_rank: Frequent Contributor
timestamp: April 16, 2021, 09:59:16 am
content: I bought some BMS from fleabay with the intention of replacing the battery type in my drill with Liion.The chip is a BM3451 which does seem to be quite capable in terms of balancing three, four or five series batteries.I cannot decide from the datasheet whether or not you can charge it from a supply that is over 21 volts but lower than 30. Is there anyone familiar enough with this chip to answer the question?Obviously the higher the voltage the higher the charge current so the faster the charge. If I charge from 21 volts then it takes maybe 10 hours, if I charge from 24 volts it will take only a couple of hours but is it safe? I cannot decide whether the series FETs take care of the overvoltage supply or not. They certainly disconnect when the thing is charged.Datasheet:http://www.si-semic.com/upload/1460800283.pdf<<< A link from another thread on the forum, no point attaching it if it is already here.For info these are the ones I bought:https://www.ebay.co.uk/itm/3S-4S-5S-25A-18650-Lithium-Battery-BMS-Protection-Board-Charging-Module-Charger/363085628922?hash=item54899705fa:g:N0UAAOSwIulfQ5PWTypical ebay seller sent me the 3s ones so I had to mess with them. Well designed really. It is quite hard to spot but on the right side in their photos you can see the difference between the different cell numbers of the BMS. Just missing links so if you are planning on getting some the 3s is usually cheaper but the only difference is that it has three, zero ohm resistors that the 5s does not have.

author: horvat.kamca
author_rank: Contributor
timestamp: April 16, 2021, 11:42:47 am
content: Lithium batteries are charged using CC-CV scheme. That means, you need power supply- with voltage 5x4.2 = 21V- with current limit based on type of used batteries, for 18650 it's usually about 1ATo improve charging speed, you need higher current, not higher voltage.What cells are you planning to use?

author: Mjolinor
author_rank: Frequent Contributor
timestamp: April 16, 2021, 12:55:35 pm
content: Quote from: horvat.kamca on April 16, 2021, 11:42:47 amYes. I know that.That is why I am asking the question.I am using (in this specific case) cells from fuji453562(21CP9/36/64 in a 2P 5S configuration to give me 2 AH @ 21 volts.At 21 volts (supplied) it takes whatever current it pleases, from flat maybe 200 ma or so dropping to zero when charged. It does not clip the voltage obviously. Running it from a 2 amp or a 200 amp supply will make no difference.What I need to know is if the BMS will regulate correctly for voltage up to the requirements of the battery from any source provided that source is greater than the charged battery voltage and less than the chip maximum.Sorry if I did not explain the question correctly, I thought it was clear enough.

author: horvat.kamca
author_rank: Contributor
timestamp: April 16, 2021, 01:41:09 pm
content: BMS doesn't regulate anything, it only disconnects battery if it's voltage is over or below limit. If you charge battery with higher voltage than 21V, overcharge protection will trigger before the battery is fully charged.Charging current should be regulated by power supply.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: April 16, 2021, 03:06:31 pm
content: Quote from: horvat.kamca on April 16, 2021, 01:41:09 pmThe BMS regulates the voltage and therefore current on a per cell basis by applying a variable impedance in parallel with each cell.It does not simply disconnect if voltage is too high or too low.Specifically what I find with this BMS chip is that if I charge at 21 volts it takes half a day from flat and when the BMS disconnects from the supply the 5 voltages are all either 4.19 or 4.20 giving 21. If I charge from 24 volts then when it disconnects I have the same 5 voltages but the charge takes maybe 2 or 3 hours, again the voltage is 21. If I charge at 24 until cut off then reduce to 21 and turn on again then it charges for maybe 2 seconds before disconnecting again.I conclude that the 24v is fully charging the cells and that this is contrary to what I thought should happen which is why I am specifically asking if someone is familiar with this specific chip.

author: M0HZH
author_rank: Regular Contributor
timestamp: April 16, 2021, 03:41:05 pm
content: The BMS balances at the top end, bypassing cells that have reached max charge voltage; it will not have a significant impact on charge times.Applying 21V to a flat 5S 2Ah battery (12.5V is considered completely flat, but it might be close to 13V) would draw as much current as the PSU can supply; 200mA sounds very low. You do have to use a slightly higher voltage PSU to get the fastest charge as there is wire drop, but normally 0.5-1V or so would be enough. Laptop chargers use 19V for 4S (16.8V max) batteries.