Cheap hack to keeping dozens of 12vSLA batteries topped off? - Page 1
Forum section: electronics / beginners

author: waoj
author_rank: Contributor
timestamp: July 27, 2023, 04:31:08 am
content: I have 60-80 SLA batteries I got for free. I had to weed out a few dead ones, and none of them are new. But I'd like to keep them charged and ready to use in a UPS next time we have a multi-hour outage. I also probably have a near indefinite supply of these 4 or so a month so I feel like I want to make use of them and then when the individual batteries no longer charge, pile them up and take them to a local scrapper. When I get them its either because "they're old and the machine told me to change them" or "the pack is failing self-test" (because one cell is at 6v, and all the others charge up to 13 just fine).I have 3 or 4 12vSLA chargers, and I don't want to manually swap them to different cells every day.I considered paralleling them all together. But if one cell "fails short", the current from the others will likely melt the hell out of it.Putting an individual fuse on every single SLA sounds excessive but perhaps the only way to be 100% safe, if wasteful of fuses. DC Breakers cost so much more though I'll shut up about fuse costs.But what if I put an incandescent lamp in series with each battery? If they were all functioning normally the bulbs wouldn't have enough energy to light, but if one failed, its partner bulb would light up to signal a problem and give that energy somewhere to go that doesn't burn my house down. I think I've seen this before in products. maybe with a neon. Is that a primitive "ballast"?Maybe if I put a diode on each battery, so current could go into it but not out of it? But diodes lose a volt and a half or something right? so that's no good for charging unless I can find a charger that's the same voltage too high. (unlikely)And 'chargers' base the voltage and current they put out on how much energy the battery is absorbing right? so how could that possibly work for a bank of paralleled or seriesed cells?To make things more complicated, my UPSes use 48v "packs". And it has no ability to balance within a pack. So maybe I should group my SLAs into 48v strings as well, after charging them all up one by one and matching them as close as I can by voltage.What's the best way to go about this?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: July 27, 2023, 06:48:48 am
content: I wouldn't worry about it. They have a decent shelf life. They should be able to hold a charge for 6-9 months.

author: MrAl
author_rank: Super Contributor
timestamp: July 27, 2023, 08:58:55 am
content: Quote from: waoj on July 27, 2023, 04:31:08 amHello there,Wow, 60 or more batteries is a LOT of batteries to maintain.You can't connect them all in parallel, you know that's out.You also can not use a diode, well you can use a diode, but it will not help anything.Charging batteries in series really isn't a good idea. People do it but there are risks that are very hard to control with certainty.Your light bulb idea sounds very good and i actually did that myself a long, long time ago to charge a car battery from 120vac line voltage. I used a big bulb like 60 watt or 100 watt or something. It limited the current to a safe level.For your purpose though, you probably want to go with a smaller 12v bulb. You want maybe 1 amp current. To get that you have to look up some bulbs and think about how much resistance the bulb will have when "cold" as that will tell you what current you will get to each battery. The only fear I have is that we won't be able to find a small enough bulb to fit the bill so the cost will be high. There is a lot of information on the web for bulbs though. We really need to know the 'cold' resistance.Some problems come up though.If you have 60 batteries connected with 60 bulbs that means that whatever the charger puts out will be divided among 60 batteries. If the charger puts out 60 amps, many that would be enough because 60/60=1 amp. If the charger puts out only 10 amp however, that only 167ma per battery, which may not be enough. There is such a thing as a minimum charge level where if there isn't enough current the battery will not charge effectively. There is a slight chance it would work, but I would think 500ma should be a target level or something close to that. You probably don't even need fuses then unless you feel that you want to be more sure nothing can go wrong.The other problem is how to get each battery to stop charging. There is a chance that the bulb will limit the current enough such that when it gets close to the final voltage the current will be small enough so that the battery no longer charges much.This would depend partly on what charger you are using, and what the output voltage is.There should be no harm in testing. We can look up some bulbs and you can get some and try it with maybe 2 batteries to start with. If the charge regimen turns out to work well, then you could invest in more bulbs. Your charger will have to be able to support the minimum current though.It would be good if you could mention what kind of charger you intend to use, and what the maximum charge current it will put out, and what the charge voltage it puts out is. I don't think you should use one of those 4 stage chargers for this because each battery will be charging at a different place in the charging profile.It would also be good if you could mention the ampere hour capacity of these batteries, or how big they are physically.

author: waoj
author_rank: Contributor
timestamp: August 02, 2023, 11:58:55 pm
content: 3.7" tall 2.56 wide 5.94 longor in metric, 9.4cm: 9.4 tall 6.5cm wide 15.1cm longa photo of the shape/form:https://download.schneider-electric.com/files?p_Doc_Ref=SPD_STOS-7RSR8V_FL_H&p_File_Type=rendition_369_jpg&default_image=DefaultProductImage.pngThese are the "most common computer ups batteries" with F2 male spade tabs, at least in North America but I would suspect globally. They're 4.85 or 6 lbs ( 2.19 or 2.72kg ) for 8ah and 9ah versions. I think 7s and 10ah versions "exist" but I don't see many of those and suspect lies. I suspect the higher AH ones just have thinner lead components and more of them (for more surface area.I believe their max amp draw is 2.7 amps, and 2.1 max charging currentThis data comes from a spreadsheet I have that journals a dozen or so different models of the same interchangeable battery type.Interstate SLA1079 is one model number I have a tonne of. (perhaps literally...)the two 12v chargers I have are Schumacher DSR118'shttps://manualzz.com/doc/en/54208487/schumacher-dsr118-battery-charger-and-engine-starter-ul-104...My other thought, is that i have two APC SMX1500 UPSes that run at 48VDC. Maybe I'd be smarter to keep all but three of these batteries actually wired into that UPSes DC bus at all times???They will do me more good on the UPS than on a shelf right? the UPS doesn't really have the facility to charge balance, or monitor the state of individual batteries even with the expensive addon battery frames that I can't afford.I opened it up to get the part number of the Anderson connectors it uses. And i found out that the external battery connector is just paralleled to the internal one with 10 AWG wire. And those damn anderson connectors are made of hens teeth apparently, so I decided to steal the connector from the internal battery packs that I already have in those SMXes and them to bridge the connection from Anderson to external spade connectors, and from there use WAGOs to split off into external battery strings.I mounted the UPSes to plywood on my basement walls. Maybe photos will be coming when I get this all nice and pretty....I bought some 30A 50v circuit breakers for cheap on amazon, and some 10AWG/30A-rated WAGOs as well for less cheap but I don't want to burn down the house.I discovered that APC uses a 60 a fuse in their internal 48v "packs"! yikes! I plan to use 10or 15A fuses on each of my 48v packs with spade connector cables and keep the fuses out and visible, and 30A breakers on each end of the long cable that will join the ups to the battery rack since there will be power at both ends.As far as monitoring what packs have a dud in them, I am thinking I will use my thermal cam once every couple months to look at all the fuses when idle, and during a UPS calibration (under load) and any that are super hot, or cold will get attention. And hopefully if a battery dies shorted, the current it causes will pop that 10a fuse.For a battery cabinet, I had started to think about how to safely store all these connected batteries. I bought some 50-cal ammo boxes to wall-mount. And then I remembered that I had a Symmtra LX I wasn't using. It has 5 battery slots, with cartridges meant for this same type of battery, and I'll just drill a hole in each of those flame-retardent cartridges and run my wires out the front. They're meant to withstand meltdowns in that metal frame. One cartridge already did, years ago when I was willing to spend the money for a double-conversion ups at home. These days I prefer a standby-style ups, for power savings...Good plan? terrible plan? better plan?

author: MrAl
author_rank: Super Contributor
timestamp: August 03, 2023, 09:54:19 am
content: Quote from: waoj on August 02, 2023, 11:58:55 pmHello again,Since you only have a few chargers then you would have to connect the batteries in parallel for one bank. I'm not sure how good of an idea that is because chargers are made to charge one battery alone, unless they are very cheap chargers that just use a transformer and rectifier or rectifier bridge. Modern chargers have more than one stage for charging and that means they must be monitoring the battery voltage, and it is known that different batteries have different internal series resistance. That means one battery gets charged more than the others, and since the charger monitors the voltage, that means some will get barely charged at all. This is going to be especially true if the batteries are old. This means we would have to know something about the charging circuit also, and what method it uses to charge a single battery. The older chargers just pumped current into the battery, and you took it off charge after some hours and assumed it was charged. The current may decrease as it gets charged, but that's about it. No four-stage charging or anything like that.See if you can find out what method your chargers use to charge a single battery.If you do get to test two batteries in parallel, you will probably have to run them down to see if they got charged, but then again it is going to be hard to tell unless you charge each one separately and test them after that first, in order to establish what the capacity is for each one first. Then when you charge the two in parallel, you can test again and see if they both got charged properly.I myself do not like charging batteries in parallel or in series, I prefer to do one at a time because that's the safest way. I know people do them in parallel and in series and in both parallel and series though.