When to consider a battery discharged? - Page 1
Forum section: electronics / beginners

author: CBP
author_rank: Contributor
timestamp: July 18, 2020, 10:52:05 am
content: I have the task at work of checking whether 30-40 odd 12V SLA/Gel batteries are still holding a reasonable charge, we have no money for a proper charge/discharge tester so I have resorted to building my own.I've created a dummy load typical of what we would connect to the batteries, 2 x 12V bulbs that together pull about 1.4A. I measure the voltage across the battery with this load connected and then with a USB datalogger I get a text file of volts over time. I import that into a bit of Python script that works out how long until it reached a cut off point and then calculates how much current has been drawn and compares to what the capacity of the battery was to start with (some are different capacities) to determine whether it goes to the bin or the keep pile.I'm trying to work out when I should determine that the battery is discharged, we connect some digital stuff to them (monitoring equipment) that drops out out at about 10V so at that point the battery is no longer useful for our purposes but obviously it is still going to hold some additional capacity as the voltage drops. Would it still be reasonable to calculate the capacity based on what is useful for an application vs. what is there in total?Thanks.

author: Picuino
author_rank: Super Contributor
timestamp: July 18, 2020, 10:58:25 am
content: https://www.aussiebatteries.com.au/blog/how-to-troubleshoot-deep-cycle-battery-issues/

author: RoGeorge
author_rank: Super Contributor
timestamp: July 18, 2020, 11:04:52 am
content: It was never supposed do discharge them down to zero. The capacity is not measured down to zero volts, because discharging too much would not be good for the battery's life. Usually each manufacturer have datasheets with the allowed voltage range. Sometimes the voltage range is typed on the battery.I'll say 10V is already too low, and if you make a chart for how fast the voltage drop from 12V to 10V, you'll see there is not much energy in the 12V to 10V range anyway, when compared to the 14.4V to 12V.Depending on the exact type of battery, the minimum voltage might be different. The correct way would be to Google each battery model and follow the manufacturer recommendation for each battery model. If not possible, google "SLA minimum voltage" or alike.

author: CBP
author_rank: Contributor
timestamp: July 18, 2020, 04:18:11 pm
content: Thank you both...I did indeed notice the drop off between 12V to 10V and knew this was the death curve although I was expecting a more sudden drop than I logged but guess this depends on the battery type/characteristics. I think more may be destined for the bin than originally thought!

author: RoGeorge
author_rank: Super Contributor
timestamp: July 18, 2020, 05:21:50 pm
content: I hope by throwing them to the bin you mean recycle, or dispose them in a batteries recycle dumpster. Lead is not good as a spill, and is not so easy to mine either, so recycling helps two ways, by less lead poisoning and also less expensive batteries in the future.Here we have collection centers that give back a small amount of money for lead batteries (the ones for cars and motorcycles), or batteries recycling dumpsters in almost any shop or mall.

author: Ian.M
author_rank: Super Contributor
timestamp: July 18, 2020, 05:24:48 pm
content: Looking for the second differential of the voltage with respect to time to peak should locate the 'knee' of the discharge curve without unduly stressing the battery. You could normalise that to the rated capacity by doing the same test on a few new batteries.It may well be worth adding a switched 120R resistor or 100mA current sink to your load so you can step the current to detect excessive internal resistance early in the discharge test and reject the battery sooner.

author: Benta
author_rank: Super Contributor
timestamp: July 18, 2020, 06:55:36 pm
content: To be able to answer this question, you should post or link to the datasheet of the batteries in question. Lead-acid batteries don't really like deep discharge unless they're designed for it.

author: Rick Law
author_rank: Super Contributor
timestamp: July 19, 2020, 12:58:19 am
content: Quote from: CBP on July 18, 2020, 10:52:05 amIs 1.4A around how you typically use it?The difference between capacity vs current is huge. If you found it to hold X at 1.4A, it doesn't mean you would have X at 2.8A, and certainly a lot lot less if you have 14A. So you should size your test load close to what you expect the actual load to be.