Characterizing 12v SLA batteries - Page 1
Forum section: electronics / beginners

author: waoj
author_rank: Contributor
timestamp: October 12, 2021, 08:11:17 am
content: I just inherited a literal tonne of 12v 7A, 7AH SLA batteries in a UPS and I think they're aout 75% through their useful life. I'm preparing to replace them, but don't really have the money to replace all at once, nor do I want to transport so much at once or deal with that much waste at once.Instead, I want to run a test on each battery to identify the things that make it still have life. I assume that's a low internal resistance, and a high watt hour capacity.I have a simple, resistive car battery load tester, but I suspect that's too 'strong' for these batteries. I don't want to damage them putting a load on them comparable to cranking over an internal combustion engine. I also want to gather data electronically, which a dumb auto store tester probably won't do.What kind of equipment do I need if I want to safely charge and discharge a 12v battery like this a few times and record the performance of it? I want to halt the testing if it gets too hot, or draws too much current. I was thinking of rigging up a dumb load, and a dumb car battery charger with relays on an RPI, but I want to know if there's a better solution out there with ethernet or serial already built in.

author: CaptDon
author_rank: Super Contributor
timestamp: October 12, 2021, 04:12:46 pm
content: There is an expensive answer, programmable power supply + programmable loadbank set for constant current + data logger for voltage probably talk to them throughGPIB and write a simple control program in Visual Basic or one of the many controloriented computer languages. Set a charge voltage limit of about 14.5 volts max andfor those 7ah gel cells a charge rate limit of 250ma to 400ma max. Charge for maybe20 hours and then put a constant current drain of perhaps 2 amps on the battery andplot the discharge curve or simply the time it takes to discharge to 10.5vdc. As the batterycharges the current will fall off as the battery reaches 14.5 volts. A tired battery will drawlittle charge current unless it has a shorted cell or two. A cooked dry battery (Happens whengel cells are on constant trickle charge) won't draw hardly any charge current and willsag severely under load immediately. You could do all of the charge/discharge/timingmanually if you can't do the GPIB/SERIAL thing. Gel cells are weird in one way, some seemto last for a decade even with abuse and others seem to rapidly die after about oneyear of even moderate or light duty. High charge current seems to kill them faster thanhigh discharge current.

author: james_s
author_rank: Super Contributor
timestamp: October 12, 2021, 05:30:36 pm
content: Keep in mind you can get money for the dead ones from a scrap dealer. A few years ago I took a load of I think 330 lbs of old car batteries and SLA batteries that had accumulated at my friend's machine shop over to Schnitzer (recycler) and got almost $80 in beer money for the guys. I haven't checked the prices for scrap lately but it's worth looking into, certainly don't just throw them out.

author: Jwillis
author_rank: Super Contributor
timestamp: October 14, 2021, 12:45:05 am
content: Quote from: waoj on October 12, 2021, 08:11:17 amAn off the shelf battery tester wouldn't damage the battery but it would give you false negatives . A typical load tester will draw around 20Amps and your batteries simply don't have the charge to supply that so the voltage on the test would drop off sharply leaving you thinking the battery is no good. Remember when using those testers , your only suppose to load the battery for a few seconds .If your interested in a data logging load tester , you can get a 150W to 180W electronic load fairly cheap . Now they have a Bluetooth module built in and a downloadable application for monitoring the load under test . Look for a DL24 150W or DL24 180W .

author: BrokenYugo
author_rank: Super Contributor
timestamp: October 14, 2021, 01:04:36 am
content: A toaster tester will indeed be too big of a load, they're usually a nominal 100 amp load, sometimes 130.

author: waoj
author_rank: Contributor
timestamp: November 04, 2021, 03:58:08 pm
content: Sorry all for not seeing your replies. my notifications were broken and I'm a slacker! Thank you for the time you took to contemplate my situation and offer advice.@jwillis "off the shelf battery tester" feels a little ambiguous to me with all the shelves that are out there.What someone gave me was a "car battery tester" which is a resistive heating element about 1.5mm thick. If I had to guess, I'd say it could pull 500-800 amps for a few seconds. It's meant to test automotive batteries whose cranking amps often are advertised above a thousand.@BrokenYugo is my tester what you refer to as a "toaster tester"? I do believe if I were to run it continuously for 30 seconds on a big enough battery it could very well toast a slice of bread. It gets hot in a hurry. I've only intentionally used it on cells I was pretty sure was dead. But I have a dc clamp meter, and it briefly went up to 50A on one of these poor SLA1079 batteries!Ideally, I think I'd prefer something more accurate than a clamp meter. But I don't have a meter that can pass more than 20A through it and honestly I don't trust mine not to melt much last 5A continuous. I always take my equipment apart before using it and those traces look a little paltry to me.This is the general type of battery I havehttps://duckduckgo.com/?t=ffab&q=SLA1079&iax=images&ia=imagesSpecifically the Apex ones pictured are the old ones I am weeding through, and I'm replacing them with interstate batteries sla1079's.Check out this carnage from some of the bad ones that failed... spectacularly!Enjoy some photos, including FLIR videohttps://photos.google.com/album/AF1QipOb7JaFXMkNp6TlyEsu55ai_uGKj98sPSTJRTIThe smoke doth left the building. I think that was caused by more than a couple amps...I do think I'd like to use a constant current load. I suspect the current of the toaster varies with it's element temperature, and I would like something more repeatable.My UPS is rated for 80 KVA. And it's at about 60KVA load right now. Shared across 256 of these batteries. They're arranged in series packs of 8. The packs themselves are series'd in groups of two. And these groups of two are placed on either side of a DC neutral bus, to create a DC high side and low side bus. the inverters supposedly use the low side bus, through igbt's to create the low side of the sine waves, and the high side of the bus to create the high side of the sine waves. It sounded a bit eclectic to me, but I think it has something to do with this UPS having no single large transformer. It's entirely solid state/silicon driven.I'm not entirely sure how many amps are pulled through these cells when it's running on battery. I suppose I could Jerry rig a wire through the front face of a battery module that I could clamp meter under actual production load. But I suspect it's pretty high. It only runs for 3-5 minutes. Long enough for the generator to start, have a hiccup, start again, and stabilize, and still have enough capacity to handle the switch back to mains. (Transfer switch breaks for a minimum of 2 seconds each time.)I've used support from Schneider for UPS system specific stuff, but they don't help much with understanding battery chemistry.I was wondering how/why these batteries seem to fail in a 'chain'. i.e. I almost never just find one bad one, but they're grouped together. My theory is that as cells in a battery go bad, they short closed, raising the apparent voltage in everything else in the same series circuit. So all remaining cells in all remaining batteries in that string are over-charged, leading more cells to break down and short, causing yet higher overcharging in a runaway scenario.I fee like I'm chasing after a Chernobyl reactor core sometimes with this thing!

author: SeanB
author_rank: Super Contributor
timestamp: November 04, 2021, 04:50:44 pm
content: Simplest test is to take each one, and measure open circuit voltage on them. Any below 10V are scrap, and those between 10V and 12V are worth charging for 3 hours at 500mA, and see if they go up in voltage over 12V. Then take all of them, and put a car headlamp on the battery for 3 minutes, and see the voltage under load. Any under 11V are toast, and the rest are still usable. Another test is to look for signs of corrosion on the terminals, again any is a fail. Then take them and shake, any that rattle again are failed. Quick, easy and minimal equipment needed.Just after charge them all up to full, either in a large series string, with a current and voltage limited supply to the float voltage of 13.8V, or in parallel, with a higher current supply limiting to 1A per battery, and a open circuit voltage of 13.8V at ambient of 20C.

author: SmallCog
author_rank: Frequent Contributor
timestamp: November 05, 2021, 02:32:00 am
content: Quote from: SeanB on November 04, 2021, 04:50:44 pmI agree with the above, and it will quickly sort the wheat from the chaffOtherwise I use one of these to test similar batteries at work. It seems to do a good job on 7Ahr batteries however the clamps can be a little unwieldy on the terminals.https://www.batteriesdirect.com.au/shop/product/22830/mabtj41.html

author: BradC
author_rank: Super Contributor
timestamp: November 05, 2021, 07:00:22 am
content: I use one of these :https://www.skyrc.com/Discontinued_Products/BD200It's discontinued, but has been replaced by the BD250. Same same but slightly bigger.I'm in the middle of running through a set of 55ah and 17ah SLAs right now for a UPS upgrade, so timely.I charge them thoroughly, then select a sane load from the battery data sheet. Usually something like a 3 or 5 hour load and let 'er rip.