Using a larger battery - Page 1
Forum section: electronics / projects

author: Mike Warren
author_rank: Unknown
timestamp: October 24, 2013, 03:38:34 am
content: I want to run a 1 cell Li-ion powered video camera from a 2 cell Li-ion battery. The idea I have is to use a LT3995 to drop the voltage . I would like the existing battery meter system in the camera to remain operational with this larger battery, so I would need the output to track the input. Basically, the output voltage should always be half the input voltage.My first thought was to use a transistor driven from the battery to offset the adjust pin on the regulator, but that seems like it might be a bit difficult to get to track properly.I have a small microcontroller (ATTiny24A) which I'm using to do a couple of minor things and I could use the PWM output to control the regulator and an ADC to read the battery voltage. That way I could fine tune it in software to get it to track.Anyway, the reason for this post is in case I've missed an easier way to do this.Anyone have any thoughts on the subject?

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 24, 2013, 03:47:04 am
content: Put the cells in parallel.

author: Mike Warren
author_rank: Unknown
timestamp: October 24, 2013, 03:49:25 am
content: Quote from: NiHaoMike on October 24, 2013, 03:47:04 amI can't get access to the cells as they are in a sealed pack.

author: staxquad
author_rank: Frequent Contributor
timestamp: October 24, 2013, 03:50:03 am
content: Quote from: Mike Warren on October 24, 2013, 03:38:34 aminternal or external battery? (picture)why not use a larger capacity 1S Li-Po? (assuming you're trying to add capacity and will be running the battery externally)

author: Mike Warren
author_rank: Unknown
timestamp: October 24, 2013, 03:52:51 am
content: Because of physical restrictions and using batteries that I use on other equipment.

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 24, 2013, 09:52:39 am
content: just use a high value resistive divider with 2 small caps on it to the battery gauge; i assume you're using the batteries at a reasonably low discharge rate so they should discharge fairly evenly.

author: Mike Warren
author_rank: Unknown
timestamp: October 24, 2013, 10:48:29 am
content: Unfortunately, I can't do that either. I have no access to the battery gauge. It's built into the camera and also includes graceful shutdown when the battery is exhausted, which is the main reason Id' like to simulate the original battery. The camera draws between 700mA and 1.1A depending on what it's doing.

author: dr_p
author_rank: Frequent Contributor
timestamp: October 24, 2013, 11:01:39 am
content: From the 7.4V battery, two 100k resistors to ground with a 1K trimpot in the middle, adjust to precisely half.Feed this in the Positive input of an op-amp, feed the regulator's output in the negative input, and op-amp output goes to feedback pin of the regulator.So the op-amp does whatever's neccessary to keep the output of the regulator at 1/2 of the input voltageedit:I'm not sure if you should disconnect the feedback path before or after the RC stuff connected to the feedback pin of the LT3995.

author: Mike Warren
author_rank: Unknown
timestamp: October 24, 2013, 11:07:04 am
content: Hmm, that's something to consider. Anyone know of a readily available op-amp with extremely low quiescent current?

author: Mike Warren
author_rank: Unknown
timestamp: October 24, 2013, 11:29:32 am
content: Quote from: dr_p on October 24, 2013, 11:01:39 amThe feedback resistor and capacitor will need to remain connected because this regulator uses extremely small pulses during low current mode and I expect it would either be unstable or the ultra low quiescent feature wouldn't work properly. So using your op-amp idea would need to be an offset situation much like my transistor idea, but it should be easier to get it to track.Unfortunately, I don't yet have a LT3995 to experiment with.