how many batteries in series for a 12V system? - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 24, 2017, 11:49:24 am
content: I am in a dilemma regarding the number or series Li-ion cells for a 12V system.My loads are BLDC motors running on the 12V power and other circuits that would run on 5V / 3.3V.Should I use 3 cells or 4 cells in series for Li-ion.The charging voltage of the cells is 4.2V.so for3 Cells = 4.2 x 3 = 12.6V.4 Cells = 4.2 x 4 = 16.8V.If I take the nominal voltage of the cell as 3.7V3 Cells = 3.7 x 3 = 11.1V.4 Cells = 3.7 x 4 = 14.8V.So my first question is should I consider the voltage as 4.2V or 3.7V?Currently I'm planning to use a 19V charger because I had thought of using 4 cells.The other questions are can I directly provide this voltage to the motors which would probably result in overvoltage in a 4 cell system and under voltage in a 3 cell system.Or should I put in a 12V SMPS regulator after the batteries?TIA

author: retrolefty
author_rank: Super Contributor
timestamp: May 24, 2017, 12:03:35 pm
content: QuoteA proper balanced Li charger will adjust it's voltage and current automatically to suit the number of cells in use. One does not just provide a constant voltage to charge these kinds of battery.QuoteI would go with a 3 cell Li to power a nominal 12 volt load or system.QuoteOne normally doesn't use regulators for powering higher current motor loads.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 24, 2017, 01:09:43 pm
content: Quote from: retrolefty on May 24, 2017, 12:03:35 pmI meant to say that I am currently planning to use a 19V adapter.For the charging system I was planning to use the BQ24600 from TI it can charge upto 6 cells in series however I don't see that has any cell balancing features.So you say 3 series cells that would give a voltage range from 9 to 12.6V if I consider 3V as the min voltage and 4.2V as the max voltage of the cell. Is this OK?Ok! agreed on the regulator part. You see that I have to power the bldc motor controller which is 3rd party and I don't know if it will operate from 9 to 12.6V but I can check on that

author: tszaboo
author_rank: Super Contributor
timestamp: May 24, 2017, 01:35:31 pm
content: I usually shortcut these problems by using LiFePO4 cells, with a nominal voltage of 3.2V. Unless you have a specific reqirement, or price is an issue they are better anyway.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 24, 2017, 02:35:49 pm
content: LiFePO4, that chemistry is highly safe right? So 3.2 nominal you say? but charge voltage would probably be 3.6so3.2 x 4 = 12.8V. (For nominal) and3.6 x 4 = 14.4V. (for max)but the min discharge voltage would probably be 2.8V2.8 x 4 = 11.2V. (for min)So this gives you a range of 11.2 to 14.4, which is quite good, it seems to be centered around that 12V mark.How do you manage to charge these do you use a charge controller and what about cell balancing?Know any reputed source for these?

author: Audioguru
author_rank: Super Contributor
timestamp: May 24, 2017, 02:41:39 pm
content: Why don't you use brushless motors and balanced chargers designed for Lithium batteries? My electric radio controlled model airplanes do.

author: tszaboo
author_rank: Super Contributor
timestamp: May 24, 2017, 02:49:38 pm
content: You buy a cell balance charger, or build in a board dedicated for that. I dont know the application, since you havent told us, nor capacity.

author: grifftech
author_rank: Frequent Contributor
timestamp: May 24, 2017, 03:06:53 pm
content: My "12V" lithium cordless drill battery is just over 11 volts open circuit and has 3 cells

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 24, 2017, 03:57:20 pm
content: Quote from: ZeroResistance on May 24, 2017, 11:49:24 amuse 3 cells, its considered as 12V battery.Quote from: ZeroResistance on May 24, 2017, 11:49:24 am(4.2 + 3.7) / 2 = 3.95Code:3 Cells = 4.2 x 3 = 12.6V. ----> 0.6V overvoltage3 Cells = 3.7 x 3 = 11.1V. ----> 0.8V undervoltage4 Cells = 4.2 x 4 = 16.8V. ----> 4.8V overvoltage4 Cells = 3.7 x 4 = 14.8V. ----> 2.8V overvoltageQuote from: ZeroResistance on May 24, 2017, 11:49:24 amgeneral answer is yes. undervoltage, motor runs a little bit slower and cooler. overvoltage, motor runs a little bit faster and hotter. thats all.Quote from: ZeroResistance on May 24, 2017, 11:49:24 amfrom my experience... no.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 24, 2017, 07:33:35 pm
content: Excellent answer Mechatrommer!!Thanks also to Audioguru, Griggtech and Nandblog!!So the general consensus is to use 3 cells of 3.7V (nominal) and 4.2V (max) voltage. This approximates a 12V system. Although Nandblog also suggested to use LiFePO4 batteries.One point that I missed out is that the system is normally being powered by the mains and when the mains fails it goes to battery backup mode.So if we take 3 cells of 4.2 volts that gives us 12.6V charging voltage so out input voltage from the would have to be higher than this so lets say it would need to be at least 2 volt higher so 14.6V or approximate it to 15V.So in mains operation the motors will be operating at 15V from the mains adapter and when on battery it would move to 12.6V. Would this higher volatge be ok for the motors especially considering the fact that the system will be in mains mode for a larger percentage of time.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 25, 2017, 05:30:25 am
content: Quote from: Mechatrommer15V to the motors should'nt be a problem right? it will only run hotter and faster?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 25, 2017, 07:20:56 am
content: Quote from: ZeroResistance on May 25, 2017, 05:30:25 am12V motor supplied with 15V should'nt be a problem. it will only run hotter and faster... and shorter life. how short is anyone's guess. i run a 12V motor on 24V supply for few months and then went dead.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: May 25, 2017, 10:15:22 am
content: Thanks mechtrommer , I would want the motors to have as long life as they could possibly have without overstressing them.How about going for a 12V mains adapter and then boosting it to 15V and feeding that to the battery charging circuit.While the loads are fed directly through the adapter.

author: Nusa
author_rank: Super Contributor
timestamp: May 25, 2017, 11:27:42 am
content: You're asking for validation without giving information about the motor or application. We don't know, so we can only cite generalities.I'd start by finding the spec sheet for the particular motor you have in mind. It'll likely specify an operating voltage RANGE as well as a nominal voltage. It'll also likely cite a noload rpm and current, an nominal rpm and current, and a stall current. I'd also expect torque info and maybe thermal characateristics.And then there's the nature of the application. For instance, a power screwdriver is a potentially high current heavy load application that may even stall, but it typically only runs for a few seconds and then rests for a while. Contrast that with a cooling fan that runs continously at a constant load.