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How much sag to expect from 36v 20ah Q30 cells and 25 a controller

Posted: Feb 21 2019 6:01pm
by 1boris
I have a Bafang bbs 36volt 500 watt motor with 25a controller and a ebay battery with samsung 32 e cells 19,2 Ah
It sags from fully charged to 38volt at full throttle.Is this to be expected or will a high quality lion battery with 30Q cells sag less?

Re: How much sag to expect from 36v 20ah Q30 cells and 25 a controller

Posted: Feb 22 2019 8:04am
by dogman dan
4v sag not bad really. Fairly typical. Cheaper cells would definitely sag 6-8v under the same load.

Re: How much sag to expect from 36v 20ah Q30 cells and 25 a controller

Posted: Feb 22 2019 8:11am
by izeman
Not to forget that you have the biggest sag at fully charged batteries. They lose the top voltage very quickly. Building a 20Ah battery with 30Q cells could be a little challenge though 8)
Let's assume a 21Ah one, so 7p battery. They should handle 25A quite fine, as it's only 3,5A per cell which is way below their max discharge limit.

Re: How much sag to expect from 36v 20ah Q30 cells and 25 a controller

Posted: Feb 22 2019 9:17am
by 1boris
Thanks,but would it be less than 4 volt sagging with 30q cells?

Re: How much sag to expect from 36v 20ah Q30 cells and 25 a controller

Posted: Feb 22 2019 10:30am
by izeman
1boris wrote:
Feb 22 2019 9:17am
Thanks,but would it be less than 4 volt sagging with 30q cells?
Depends on many factors. Not cell related only. ALL you wiring (and this includes ALL serial connections inside the battery) go into the equation. How the nickel strips are soldered/welded (hopefully) and how thick the nickel is. What connectors you use and how good they are, how long the wires are, especially if they are relatively thin ...
Take the quote 26mO per cell, 7 in parallel and this gives you 3,7mO. Then you have a 10s pack, so this gives you 37mO for all the cells. Another 5mO/m 12AWG wire. Then add nickel strips and spot welds and connectors and you should be somewhere near 60mO if all done right.

U = I x R. We're talking 25A and 36V. So it's 36 - 25 * 0,06 = 34.5V (1.5V sag)

Does this answer your question? ;) I hope i calculated correctly ....

BTW: The 32E has 35mO internal resistance. So sag with the same configuration with 30Q should be 3/4 of the 32E pack. Or 1/3 more for the 32E if you see it like that = 1,5V x 1,3 = 2V

Re: How much sag to expect from 36v 20ah Q30 cells and 25 a controller

Posted: Feb 22 2019 11:08am
by 1boris
Nice,Thank you very much for taking the time :D