Best approach to power 4.6V motor from li-ion - Page 1
Forum section: electronics / projects

author: fagiano
author_rank: Regular Contributor
timestamp: February 28, 2016, 11:55:00 am
content: Hi,I'm planning to convert a device from using NiMH batteries to using Li-ion. The device is pretty simple, it just has a 4.7V DC motor that runs about ~3.5A. I'm trying to use batteries that are rated at 2.5A max discharge (I can fit 4 in the device). Is it better to step-up ~3.7V to 4.7V(3 or 4 in parallel) or create a battery pack and step down ~7.4V to 4.7V?(Stepping up is simpler because I don't need a balancing charger circuit).Any thoughts?ciaoAlberto

author: FreddyVictor
author_rank: Regular Contributor
timestamp: February 28, 2016, 04:34:14 pm
content: a fully charge LiPo will have voltage of 4.2v, so really not that short of your 4.7v, so my point would be why bother stepping up ?I'm no expert, but 3.5amps for a 4.7v motor seems a little excessive....

author: Siwastaja
author_rank: Super Contributor
timestamp: February 28, 2016, 05:10:48 pm
content: 2s li-ion doesn't need balancing circuit or even center tap monitoring. Just use quality cells and use conservative cutoff voltage for discharge, such as 2*3.0V = 6.0V. It's often done like this in commercial products.It's a lot easier to step down because you don't need a full DC/DC converter - the motor works as the inductor by just driving it with PWM. With a bit oversized FET, you are probably fine with unregulated converter without current feedback, i.e., one mosfet, freewheeling diode and fixed PWM generation.

author: fagiano
author_rank: Regular Contributor
timestamp: February 28, 2016, 05:34:37 pm
content: Quote from: FreddyVictor on February 28, 2016, 04:34:14 pmLooking at the discharge curve of the battery at 1A it looks like it will deliver 90% of the capacity from 3.8V to 3.5V that seems to make the motor spin somewhat noticeably slower than 4.7.Quote from: FreddyVictor on February 28, 2016, 04:34:14 pmThat's what I measured from running the motor from a bench PSU. I was thinking of limiting current at 3A anyway to be safe as I'm no expert.Quote from: Siwastaja on February 28, 2016, 05:10:48 pmMmh this is interesting but I think because of the current I'd need 2s2p for it not to draw to much from each cell, can I still go without balancing?. 2A is the maximum recommended per cell and I rather go 1.5A (Lion scares me a bit).I'll do some test with just cells is series without step-up and see if is fast enough.thank you for you timeciaoAlberto

author: Zero999
author_rank: Super Contributor
timestamp: February 28, 2016, 10:28:16 pm
content: Quote from: FreddyVictor on February 28, 2016, 04:34:14 pmIt depends on the size of the motor and what it's doing.Power = V*I so for a lower voltage motor of the same size, the current would be higher.

author: Audioguru
author_rank: Super Contributor
timestamp: February 28, 2016, 11:01:06 pm
content: Quote from: Siwastaja on February 28, 2016, 05:10:48 pmWhy would you charge a lithium cell to only 3V when it is dead when it is 3.2V and then its load should be disconnected? Normally a 2-cell lithium battery is balance-charged to 4.2V for each cell.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 29, 2016, 11:15:52 am
content: Quote from: Audioguru on February 28, 2016, 11:01:06 pmYeah, why would you? You said that, so please tell us.

author: Audioguru
author_rank: Super Contributor
timestamp: February 29, 2016, 03:05:15 pm
content: I think some people know nothing about Lithium rechargeable batteries because they never learned about them atwww.batteryuniversity.com.

author: station240
author_rank: Unknown
timestamp: February 29, 2016, 05:26:16 pm
content: BatteryUniversity is a discredited site, it's hard to tell what is true and what is republished rumours and nonsense.Seriously why do people recommend a site that is little more than one person's options ?If you want accurate information, read the datasheets from the company that makes the batteries.