Mimicking the behaviour of an almost flat set of alkaline AA batteries - Page 1
Forum section: electronics / projects

author: jokkydee
author_rank: Newbie
timestamp: June 10, 2016, 04:43:23 am
content: I want to test out the power supply characteristics of my battery powered design and I want to know how it behaves when the internal resistance of the batteries increases, i.e. when a set of alkaline batteries are nearly reaching the end of their life and surge currents in the power supply will cause the battery voltage to droop. There will be eight AA batteries in series - meaning a fairly high total series resistance when they get flat. I know that putting some series resistance and limiting the current on a power supply can roughly simulate a set of almost dead alkalines, but I'm not sure on what the typical values would be. I'm thinking about 1 Ohm series resistance and ~1amp current limit on the supply could be a good start to simulate the effect of the dying batteries but not sure. Any advice on this?

author: Brutte
author_rank: Frequent Contributor
timestamp: June 10, 2016, 08:12:28 am
content: I think there are no standardized behaviors of alkaline you could use as reference. If this is a one-off project then I suggest that you should not mimic anything but simply buy a batch of reputable alkalines , wear out 8 of those for testing purposes and then use the rest in final application.The alkaline is a generic term and many chemistries and constructions are used. What you could use are the datasheets with performance curves some manufacturers publish. These are not guaranteed parameters but could be useful for such purpose. Most reach down the 0.9V level, IIRC.

author: Fungus
author_rank: Super Contributor
timestamp: June 10, 2016, 10:56:11 am
content: Quote from: jokkydee on June 10, 2016, 04:43:23 amYep.Quote from: jokkydee on June 10, 2016, 04:43:23 amIt's just Ohms law.eg. If a 1.5V battery is showing 1.0V under a 200mA load then you have a 0.5V drop and the resistance is 0.5V/200mA = 2.5 Ohms.

author: Gyro
author_rank: Super Contributor
timestamp: June 10, 2016, 11:39:31 am
content: Check the reputable manufacturers' websites. In their technical sections they have battery datasheets which include discharge curves into various loads. By comparing voltage curves at different loads you can approximate internal resistance as different levels of discharge.

author: tszaboo
author_rank: Super Contributor
timestamp: June 10, 2016, 12:40:00 pm
content: One thing to look out for: If you have an undervoltage lockout, that can cause interesting issues. When you discharge with higher current the voltage drops, so your UVLO engages. This is not purely ESR voltage drop. The current drops to zero, and the battery starts recovering the voltage. That can take few milliseconds to few seconds.And then your device starts working again, resulting a "hiccup".

author: dannyf
author_rank: Super Contributor
timestamp: June 10, 2016, 12:58:27 pm
content: "but I'm not sure on what the typical values would b"If you have an ear meter, you can measure it through a cap.Otherwise, put a resistor across it and measure the voltage across the battery vs. The open circuit voltage. Applied that percentage of decline to the value of the resistor used will give you the battery's internal resistance.

author: edavid
author_rank: Super Contributor
timestamp: June 10, 2016, 03:10:31 pm
content: Forum member Jay_Diddy_B put together an LTspice model that is perfect for this:https://www.eevblog.com/forum/projects/modeling-an-alkaline-battery-in-ltspice/