Best way to test battery discharge over time - Page 1
Forum section: electronics / beginners

author: blewisjr
author_rank: Frequent Contributor
timestamp: September 22, 2014, 06:43:10 pm
content: Hey guys,I am looking for a accurate way to measure the discharge of a certain battery over time under a specific resistance load. I would need to monitor both voltage dissipation as well as the constant current draw dissipation.The reason i need good accuracy is for safety and battery longevity as I am working with low resistance loads and high drain Li-Mn batteries and I am trying to figure out the best batteries for the particular application. Any ideas?

author: timb
author_rank: Super Contributor
timestamp: September 22, 2014, 06:54:20 pm
content: A Programable Load would do the job!Sent from my Smartphone

author: Christopher
author_rank: Frequent Contributor
timestamp: September 22, 2014, 07:59:32 pm
content: If it's a resistive load you can tie a few low-value high-power resistors together in parallel on a "heatsink" with some heavy wires.Nice & cheap. Couple of DVMs with some GPIB logging and you're sorted.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: September 22, 2014, 08:00:57 pm
content: When you mention "dissipation" I assume you mean power dissipation (as the power is spent as heat).If you're looking to just "record" and log data, I would suggest picking up a PC Programmable benchmeter like the Fluke 45. You'll get 4.5 digits on both readings, and it can do both current and voltage on the same unit, at (something like) the same time. I believeallof them include the RS232 port (but make sure you get one that has it!), so you can easily write a script or program to poll the meter for instantaneous voltage and current readings then stuff them away in a log file.There are a number of bench meters out there which can do this, and a few handhelds, so that would be the route I would suggest. You _could_ roll your own, but that would take you a very long way away from this project, and might be beyond your level right now.Cheers!

author: blewisjr
author_rank: Frequent Contributor
timestamp: September 22, 2014, 08:30:26 pm
content: When i said dissapation probably wrong word i meant the drop in voltage over time. For instance with a Li-Mn full charge on a 3.7V battery is 4.2V then it has a swift drop then tappers downward. It is bad to run a LiMn below a certain voltage because it kills the battery. The resistive load is actually a heat coil. The problem is as the voltage drops the output current of the battery changes due to the resistance load and if you hit a unsafe threshold it can leak or pop. So I want to monitor the trends over time to graph it to find a sweet spot for the load. I have a fluke 87V and one other not sure on name i do not think they log.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: September 22, 2014, 08:39:08 pm
content: If you're not able to buy a piece of gear to do datalogging, there is always the paper and pencil method of sitting and recording.

author: sacherjj
author_rank: Frequent Contributor
timestamp: September 22, 2014, 09:10:48 pm
content: If you are going to be there while testing: Paper and pencil, or Excel.If you are not going to be there, scripted data logging DMM or two (if you want current). Then a relay controlled by the computer to open at maximum discharge, if your Li-Mn cell doesn't have a discharge limiter.

author: Paul Price
author_rank: Super Contributor
timestamp: September 23, 2014, 03:36:39 pm
content: To check the charge acceptance, discharge and charge of a Li-ion cell I use an ordinary general purpose MCU analog to digital function along with an external current shunt whose voltage is amplified 100x to do the current measuring job. To create a constant or programmable load, I use one or more small two-prong 20W 12-V incandescent halogen lamps as a resistive load. I control the resistive load(either one lamp or lamp array)by use the the PWM function of the MCU to control a N-CHAN Power MOSFET to control the discharge current using feedback from the amplifier output of the current sense resistor. The MCU does all the calculations, which is simply to add up the amp-seconds to yield a total mAH result upon reaching a discharge voltage limit. I have used an off-the-shelf LCD(2 linesx16 Char) or else an RS232 serial link to HyperTerminal on my PC to monitor the discharge cycle and see the final results.For charging, I use the MCU PWM to create a voltage-limited step-down switching regulator to apply a constant current to the LI-ion battery while monitoring both the charge current and voltage across the battery. When the charge current diminishes to a cutoff value at slightly less than 4.2V, I can again use the same calculations to see the mAH charge acceptance of the DUT.Recently, using two 28-pin 16F886 MCUs I built a 4-cell LI-ion charger/discharger/battery analyzer using this design, all on a ordinary solderless breadboard.To power the whole setup, I use a laptop 18-20V 4A power supply to feed the 4 buck switching regulators and a 7805 regulator to create the 5V for the MCUs. One MCU does the displaying and charges two of the cells, while the slave charges/dischartges the other two Li-ion cells, all the while communicating its data to the master to be displayed using only one LCD.This is the best way to test battery discharge over time.

author: VK5RC
author_rank: Unknown
timestamp: September 24, 2014, 05:42:12 am
content: Pretty sure the 87 does voltage data logging, a low voltage cut off circuit, a few old halogen bulbs either in series or parallel and Bob's your uncle!

author: nowlan
author_rank: Frequent Contributor
timestamp: September 24, 2014, 03:06:15 pm
content: Out of interest, do the electronic loads, such as maynou let you set a end voltage?I was thinking you might be better off getting a protection circuit. I thought all the 18650 batteries came with them these days. Wondering if this board could be adapted to run off 1 cell. It says 10.8v. Else find a busted drill pack.www.ebay.com.au/itm/191242088646

author: sacherjj
author_rank: Frequent Contributor
timestamp: September 24, 2014, 08:19:47 pm
content: Quote from: nowlan on September 24, 2014, 03:06:15 pmMany loads allow you to set a disconnect voltage. I use the Array 3710A and it has this, unfortunately it is not accessible from the RS232 interface. But if you are logging while discharging, you can just send the load off commands. (Or you can set this value manually before you start the PC control.)

author: denelec
author_rank: Regular Contributor
timestamp: September 24, 2014, 08:47:10 pm
content: I once used this cheap 4 channels logger to record the discharge voltage of alkaline batteries.http://www.amazon.com/Velleman-K8047-Channel-Recorder-Logger/dp/B009N1TPRQThe voltage range is selectable (3V, 6V, 15V, 30V).The sample rate is selectable from 0.1 sample/s to 100 samples/sYou can record as long as you need. The results are stored in a text file on your computer.It came in a kit I had to assemble.Cheap but good enough for what I needed.If you discharge the battery in a resistor, you don't need to measure the current. I = V/R

author: denelec
author_rank: Regular Contributor
timestamp: September 24, 2014, 08:54:48 pm
content: This logger is not a precision device.The A/D is 8 bits.If you need millivolt precision, look elsewhere.

author: sacherjj
author_rank: Frequent Contributor
timestamp: September 24, 2014, 09:51:58 pm
content: Quote from: denelec on September 24, 2014, 08:47:10 pmYou might, depending on the level of current in the discharge and the temperature characteristics of the resistor.

author: mazurov
author_rank: Frequent Contributor
timestamp: September 25, 2014, 04:43:00 am
content: Quote from: XFDDesign on September 22, 2014, 08:39:08 pmOr you can shoot a video of two DMMs and later skip every minute, say, and record.