12V DC UPS - Page 1
Forum section: electronics / general

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 22, 2022, 05:09:11 pm
content: So the battery died on my very small home UPS that I used for the modem and router. I used it to give me maybe 5-10 minutes of connectivity to save stuff in case of power failure.The thought occurred that it's quite silly to have this DC->chop to AC->wall wart chain just to have 12V for the modem.So there are 12V mini-UPS things on Alix... Any good? Experiences? Thoughts?It's the same price as trying to track down a new 12V gel cell for my UPS, so...https://www.aliexpress.com/item/1005004107179526.html

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 23, 2022, 12:33:46 am
content: Just get a 3S pack with BMS and connect it to the 12V bus using a Schottky diode and resistor, or directly if the DC supply is current limited to within the charge current limit of the battery. It turns out that 12V yields 4V/cell for 3S, which is OK to leave connected indefinitely.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: July 23, 2022, 01:17:17 am
content: Under the mountain of paperwork on my desk there should be an envelope on which I've drafted a plan to build usb powerbank with power pass through and a 5v->12v boost converter module for the router/firewall in the garage in case of power failure.I don't doubt it would work, I just haven't got around to doing it.Cos I'm so lazy, I ended up just buying a 7Ah gel cell and put it in this:https://www.altronics.com.au/p/m8561-12v-dc-ups-battery-backup-power-supply/That was about 7 years ago. Still works OK as of two nights ago when our power got knocked out by some trees.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 23, 2022, 05:06:30 am
content: For the price of the Alix thing I'm not motivated in the slightest to kludge something together. I just had this feeling that the clunky, chunky, and heavy lead-acid inverter UPSs, with their tiny battery life and at the same time six or eight AC outlets for 150$ CDN, just make no sense. I still can't really believe they're actively sold.So I ordered the Alix thing.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: July 23, 2022, 05:22:53 am
content: Quote from: Alex Eisenhut on July 23, 2022, 05:06:30 amWRT the lead-acid consumer UPS, not sure about your locale but here there was a push some years ago among the office supply big stores to shove these things down everyone's throats. And yeah, you're right. They're shit. I have a fine collection of dead soldiers.QuoteI like the wide input voltage range. Again, locale. I'd need to be watching the package heat during summer time.

author: Emo
author_rank: Regular Contributor
timestamp: July 23, 2022, 07:53:44 am
content: Hi,Basically this is an existing product. I have used this one for several years nowhttps://www.amazon.de/-/en/Mini-UPS-Uninterrupted-Power-Supply-Black/dp/B07PHQNLFY/ref=sr_1_4?crid=K3JBXYVSLM3J&keywords=12v+ups&qid=1658562540&sprefix=12v+ups%2Caps%2C81&sr=8-4Eric

author: Miyuki
author_rank: Frequent Contributor
timestamp: July 25, 2022, 05:00:25 am
content: I use this DIY solution and it works great, offers days of backup when powering just a network infrastructureIt is a Mean Well power supply with a backup feature, so it manages the battery24V system with two 12Ah lead acids, they had the best price/capacity ratioAdded 12V step-down converter and 48V boostThe main part of the network are Miktorik based routers/AP and they use 24V PoE, which is why choose the 24V systemI need the 48V for the DSL modem as it uses a TP-Link PoE extractor and this need 802.3af voltage levelsAnd have 12V output for things like NASIt is cheap and robustI wanted to add some more features, like measuring, more protection and so, but maybe sometimes in the futureIt just works

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: July 25, 2022, 05:34:00 am
content: Quote from: Alex Eisenhut on July 23, 2022, 05:06:30 amThe downside being, that kind of gear is designed without caring about creepage, clearance, or any safety measures really. If it doesn't kill people on the day of sale, it's good enough. Never mind if it catches fire next week.Quote from: Miyuki on July 25, 2022, 05:00:25 amHm. "Strain relief" (knotted mains cord) that leads to a gaping hole larger than the knot, exposed high-current battery wires, masking tape on the battery terminals.. is that Meanwell even secured or is it just sitting on the shelf? Good God, man, I think that's even more dangerous than the Aliexpress product. Have you no shame?

author: Miyuki
author_rank: Frequent Contributor
timestamp: July 25, 2022, 05:49:12 am
content: Quote from: Geoff-AU on July 25, 2022, 05:34:00 amA little shameIt is considered a "test" and "not finished"Normally it is closed and sits in the cornerThe power supply is secured in placeExposed wires are not directly from batteries, they go from the Meanwell and it is fused, so not as dangerous as it looksBut I agree it is not a example how it shall be used

author: beanflying
author_rank: Super Contributor
timestamp: July 25, 2022, 05:58:44 am
content: Quote from: NiHaoMike on July 23, 2022, 12:33:46 amNumber of times you have FALSELY posted about BMS circuitry and charging Lithium cells is now +1. Please go learn once and for all how Lithium batteries are charged and a tip that is NOT USING A BMS.STOP IT !Lithium Batteries are a multi step charge process so that is neither just constant current or using a Diode on a random power supply as some sort oflimiter.

author: tom66
author_rank: Super Contributor
timestamp: July 25, 2022, 07:04:12 am
content: I did something similar for our CCTV DVR. Modded the power supply to output 13.2V instead of 12V (within the +10% range for the ATX hard drive) which kept a small lead acid battery semi-charged. It's sufficient to keep the system going for about half an hour in a power cut, and it backpowers the cameras too through the 12V socket.The only problem I've found is the hard disk is the first thing to cut off, but if the power comes back before the DVR dies, it takes a while before the DVR detects the HDD again. I need to add a 'UVLO' circuit to properly reset things.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 25, 2022, 12:46:31 pm
content: Quote from: beanflying on July 25, 2022, 05:58:44 amhttps://www.electronicsweekly.com/market-sectors/power/float-charging-lithium-ion-cells-2006-02/Quote

author: beanflying
author_rank: Super Contributor
timestamp: July 25, 2022, 01:13:44 pm
content: Quote from: NiHaoMike on July 25, 2022, 12:46:31 pm4V is 'safe' for most Lithium options BUT you do not get there by ANY METHOD in your initial post! Don't randomly Google for answers YOU WANT to suit your argument instead do some actual reading on how done safely to suit the Lithium Technology in question.Start with understanding what a BMS board is and is not.

author: tom66
author_rank: Super Contributor
timestamp: July 25, 2022, 01:50:38 pm
content: Float charging Li-ion batteries is simply not possible: you cannot apply a fixed 4V if the battery is at say 3.5V, the current drawn during charging will be excessive which will overheat the cell. You need both current limiting and voltage limiting. Now, you can modify a standard li-ion charger IC with an external comparator so that it shuts off once the cell goes over e.g. 4V, which will create the 'effect' of float charging, but the result will be that the charger cycles on and off around that point, so not really float charging. It will certainly extend the cell's lifespan, the rule I have heard is doubling calendar life for every 0.1V less than 4.25V.It is NEVER permissible to charge a li-ion cell with only a resistor or simple current limiter, cell voltage will increase until the cell is destroyed, if the current is high enough this may well result in thermal runaway.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 25, 2022, 03:23:05 pm
content: Quote from: beanflying on July 25, 2022, 05:58:44 amHow aboutyoudropping your attitude and go learn how lithium ion batteries are charged.But if you can't even get the name (lithium ion; lithium batteries are a different thing) right, I guess there is very little hope for you.I see no big issues in NiHaoMike's suggestion. Of course, he leaves out the detail what heexactlymeans by "BMS". But he often leaves out details to be worked on.Clearly, you don't have any idea about the "multi-step process" you are talking about.Also the battery does not know or care if current is limited using a resistor or something more fancy, and I think it's quite revealing you can't understand even this.In the future, you might want to consider this: if you think someone is giving poor or dangerous advice, learn the thing first, and then properly explain what is wrong. And remember, to be able to teach others, you need to have quite solid understanding.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 25, 2022, 03:32:58 pm
content: Quote from: tom66 on July 25, 2022, 01:50:38 pmOf course it is, I regularly do it.The suggestion was using a constant-voltage supply plus a current limiter resistor. This provides both current and voltage limiting.But you need to understand Ohm's law to be able to dimension the resistor. If you need help with that, you can ask about it in the Beginners section. Thank you.Mike's suggestion of having a BMS - practically this would be most easily satisfied by using "protected cells" with built-in single-cell protection PCB - would take care of adding another layer of security in case the upstream 12V supply fails in a way making it output a higher voltage - unlikely but possible. Resistors basically never fail short, so current limiting will be very robust. This basically just leaves the low-voltage cutoff functionality on the shoulders of the protection boards. BMS boards would also open the circuit if gross imbalance develops, but this is again unlikely. Or one can use a balancing 3S BMS.If you use decent quality non-bullshit cells, this is one of the least risky mods one can do. But as always, there is some risk to anything. I just find it extremely funny people jump on this particular li-ion usage tip, because we seeorders of magnitudeworse every day on this forum and no one gives a shit.Probably because they don't understand what they are talking about, and get fixated on some single words like "float charging", after reading from Battery University or some other BS site that "float charging a lithium ion cell is impossible".

author: tom66
author_rank: Super Contributor
timestamp: July 25, 2022, 03:54:24 pm
content: Siwastaja, what happens to your float charging circuit and the battery, if the battery runs to <2.8V and you connect it to the 4V supply with the current limiting resistor?This is not as simple as Ohm's law: below about 2.5-2.8V, Li-ion cell current must be around C/25 or lower. Otherwise, you risk dendritic build up in the cell in the recovery phase which can lead to future runaway.Now what happens if the cell is totally dead (0V terminal) and you power the charger circuit up? A good Li-ion charger circuit will not charge the cell at all in that condition. But, your circuit will attempt to charge the cell -- and it will charge it with considerable current given the volt-drop across the resistor -- which could result in the cell releasing its magic smoke. (Note that relying on the battery protection board to do this is not necessarily OK. The protection boards often allow charging in this condition, just do not allow discharging.)I don't believe you can ever build asafeLi-ion charging circuit with just a CV source and a resistor, and there is no manufacturer of Li-ion cells that will recommend it. For other lithum rechargeable chemistries, for instance lithium manganese silicon (example Seiko ML414) then this is tolerable, but this is only because the parasitic reactions in the cell are so high as to keep the cell in a safe condition at all times. This definitely does not apply to Li-ion cells.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 25, 2022, 05:17:35 pm
content: Quote from: tom66 on July 25, 2022, 03:54:24 pmThat was already covered. I won't repeat it because it is futile to do so with people who lack the skill to concentrate on the text. Instead, please read, re-read, and re-re-read until understood.It was already present in NiHaoMike's original post, which was extremely short and to the point. I just elaborated on it.I'll give you a hint, though: a three-letter acronym was discussed.

author: tom66
author_rank: Super Contributor
timestamp: July 25, 2022, 06:23:10 pm
content: Read and re-readyet I'm still baffled; if you think a BMS, which I'll assume is the acronym in question, is going to prevent the runaway condition, you are sorely mistaken. But thank you for questioning my reading comprehension; I'll question your battery charging comprehension instead, since this is a technical discussion.Thevast vast majorityof 3S packs (e.g. hobby RC packs) have no in built BMS and so the advice suggested (4V constant voltage with resistor per cell) is very dangerous for inexperienced users. If they DO have a proper balancing BMS thismaywork, however it will need to be close to the protection circuitry that for example a laptop battery offers.You will still need to provide protection in the case of 2.8V/cell or less. Many battery protection circuitsDO NOTisolate the battery from charging currents when the battery is under-voltage; the reason for this is to allow the battery to be recovered by an appropriate charger, which is usually possible down to about 1.5-1.8V/cell with some deterioration in the capacity if the charge is replaced very slowly. The exceptions are things like power tool batteries, because users are idiots and will try anything to recover their 100 euro battery that they left discharged, these tend to prohibit almost all zero volt charging as the terminals are too tempting to hook up to some external source. But I can confirm that the majority of BMS circuits will not block charging below 2.8V; for instance, the BQ20z70 device used in a great deal of laptop batteries allows zero-volt charging, and it applies no special current limit in this case; neither do common single-cell Li-Ion isolators like AP9221 or the venerable DW01 which is seen in pretty much every cheap single cell protected Li-Po. There are a few laptop batteries with pyrofuse devices like 12AH3 which can semi-permanently isolate the pack if it's gone below a certain voltage. Again, quite rare, and usually only seen in very high volume consumer kit.Therefore, I continue to insist that you should not recommend charging any Li-Ion battery pack with only a voltage source and resistor. It really is very little extra cost to use a proper Li-Ion charger chip and protection circuit, and offers a significant improvement to the safety of the system. The danger, as ever, is that advice is copied by those who have no appreciation for the danger that Li-Ion batteries pose, and then implement chargers like this thinking it's perfectly sufficient.

author: beanflying
author_rank: Super Contributor
timestamp: July 26, 2022, 02:05:23 am
content: Quote from: Siwastaja on July 25, 2022, 05:17:35 pmSpeaking of 'attitude' over something you chose to step intoMike's suggestion was WRONG on multiple levels.Your own LACK of knowledge on charging multicell Lithium packs is showing. I have been charging LiPo's for well over a decade so I have more than a little idea about multistage charging of them. Resistors, Diodes and other simplistic current limiters are also NOT the appropriate way to 'limit current' lower voltage or 'float' charge a Pack of Lithium batteries at X volts. Cells are not matched for impedance, capacity and in most cases not even from the same batch run.A BMS is a safety cutout no more and no less is is not now and never will be an appropriate method to terminate a multicell Lithium Charge process. This LIE and a potentially dangerous one is spread all over the web and as this is an Engineering forum this needs to be stamped out here at least. Stack Exchange, Reddit and the countless YouTube videos are completeQuoteThis is called a Balancing CHARGER congratulations you finally mentioned the appropriate thing to use to charge Lithium multicell packs. The only thing you added INCORRECTLY (in 95% of cases) that is not part of the charging circuit is the BMS which is a battery protection device

author: PlainName
author_rank: Super Contributor
timestamp: July 26, 2022, 06:54:26 pm
content: HelloJust to shortcut even more angriness, perhaps I can butt in and point out:QuoteMy understanding (of the suggestion, not batteries) is that the BMS is intended to stop the cell being depleted below 2.8V (or whatever the safe minimum is). It's not intended to be part of the charging system, which is what the constant voltage and resistor is. The CV/R is thus 'safe' because the battery will be disconnected by the BMS on over-discharge, so the <2.8V and 0V situations won't arise. Maybe a small fuse just in case would round it off.You're welcome

author: tom66
author_rank: Super Contributor
timestamp: July 26, 2022, 07:58:20 pm
content: The point is many BMS devices only provide balancing and overvoltage/overcurrent protection. They are not smart enough to provide full protection for the cell, like adjusting the current limit at low state of charge. That is the responsibility of the charging IC / controller.

author: PlainName
author_rank: Super Contributor
timestamp: July 26, 2022, 11:23:32 pm
content: Maybe those would be the wrong BMS to use, then. I mean, if your design is based around a 5V zener it's a bit pointless to note that all those snazzy diodes and diacs and what have you would really suck at the job.But I reckon I am best out of it now...

author: beanflying
author_rank: Super Contributor
timestamp: July 27, 2022, 02:03:16 am
content: Even the basic BMS boards typically provide over and undervolt battery protection including evilbay/AliX rubbish.Because of the low impedance of most Lithium cells you can't charge as Constant Voltage from near flat (circa 3V) toward a 4V or better a 4.2V charged state. You need to use a controlled current source for the initial bulk charge a resistor and diode is not such a source. Toward the top for the last 10% of capacity (generally over 4V) this is where a Constant Voltage type of charge is done.And before anyone bleats again about my use of the generic 'Lithium' rather than a specific type be that ion, polymer, Fe ..... it is for a reason not all Lithium Rechargeable Cells are the same but most share similar charge and use conditions.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 27, 2022, 02:19:47 am
content: Quote from: beanflying on July 27, 2022, 02:03:16 amThe resistor value is chosen to keep the current below the max charge rating of the battery. The current will go down as the battery charges, that's perfectly fine if fast charging is not the goal.The 4.2V figure is to get the full rated capacity which is not desirable in this case. A lower voltage reduces the usable capacity but also reduces the degradation over time so in the long run it ends up cheaper. It's a completely different use case than for a drone where weight reduction is everything.

author: beanflying
author_rank: Super Contributor
timestamp: July 27, 2022, 02:53:21 am
content: And the termination of charge here is what exactly?Your BMS NON solution will see the Pack going from open to closed circuit at the top of charge EVERY CYCLE which is anything but desirable for a UPS. Your Schottky diode option is a dud as the forward Voltage drop across it varies with current so how again are you picking your end point and exactly what is it? Add to this you are already requiring a stable supply as part of your charge circuit. Balance charge solutions exist for a reason and are CHEAP.Long term 'storage' of LiPos is generally recommended well below 4V for better life but below the 4.2 is 'better' but not a lot. Want to talk about weight not being a factor here and cost being one then clearly there is a good reason most UPS supplies use AGM Lead batteries

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 27, 2022, 03:18:31 am
content: Quote from: beanflying on July 27, 2022, 02:53:21 amThe diode is for discharge. The charging stops by means of the supply voltage being limited. I suppose one could use a current limited LDO instead of a resistor if a fancier solution is desired.QuoteMaybe you have been living under a rock but lithium is cheaper than sealed lead acid nowadays, especially in small sizes. And unless you go for an even more expensive premium brand, lead acid nowadays generally doesn't last past 3-5 years even when kept under optimum conditions. Contrast that to (name brand) 18650s which easily last past 10 years when treated well.

author: beanflying
author_rank: Super Contributor
timestamp: July 27, 2022, 03:43:17 am
content: Quote from: NiHaoMike on July 27, 2022, 03:18:31 amSize and non specified brand specificStop making stuff up to suit YOUR narrative. Lithiums are across the board offer less capacity for more $.I have AGM's doing well after your apparent time frame. I still have a few NiCads that are well over 20 years old that doesn't make them a good option because of this. Some of my Lithiums are close to or might be over 10 years old in spite of regular high current use and frequent charged state storage. I have also had LiPo's in particular (good branded ones) puff well under max current after a few usesDon't throw insults you can't back with actual fact!Quote from: NiHaoMike on July 27, 2022, 03:18:31 amSo you are now suggesting a voltage fully regulated power supply and LDO's and ..... instead of the $3 Balancing Charger board (good for 500mA-1A)? Stop trying to weasel this out as a good solution it is NOT. Use a proper charger that exists and works instead of an imagined make believe bad solution.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 27, 2022, 04:23:41 am
content: Quote from: beanflying on July 27, 2022, 03:43:17 amHere's a source for pretty cheap name brand lithium batteries:https://batteryhookup.com/As an example, here's 15x 2.5Ah cells for $8.50:https://batteryhookup.com/products/genuine-lithium-polymer-4v-2520mah-10-08-whIn 3S configuration, you get 12.5Ah. I have never seen a 12Ah 12V lead acid battery for that cheap.Lead acid is really in the "they don't make them like they used to" category, especially for the smaller sizes which are mostly used in low cost applications.QuoteFind me a low cost 3S charger that can be configured to stop at 4V/cell or 4.1V/cell instead of the usual 4.2V/cell. The only ones I have seen with that capability are R/C chargers which aside from being relatively expensive even for the budget ones, are not at all desirable for this application in that they need to be manually started.

author: beanflying
author_rank: Super Contributor
timestamp: July 27, 2022, 05:35:49 am
content: Wow for one specific instance and size of YOUR choosing you 'allegedly' find and option to prove what exactly? Someone selling bulk single cell leftovers sourced from the factory floor in China is cheaper than someone selling a packaged 12V AGM Battery? This is a bogus argument Lithium is across the board more expensive than AGM this is just a simple fact.As to the second part of your attempted weasel it is not rocket science or hard to design a comparator to switch or reset the trigger of a micro to start and stop a charge process at certain over or under voltages. There you go it even has built in BMSeBay auction: #384879258895Something like this from Lewis is likely an ok start pointhttps://www.bristolwatch.com/ele2/battery_charger.htm

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 27, 2022, 01:28:11 pm
content: Gentlemen, this is EEVblog, not Twitter.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 28, 2022, 01:59:18 pm
content: Quote from: beanflying on July 27, 2022, 05:35:49 amFor another example, 108 new Japanese 18650s for $100:https://batteryhookup.com/products/new-case-of-108-panasonic-ncr18650a-2900mah-cellsIn 3S configuration, you get 104Ah. There are cheaper 100Ah conventional lead acid but sealed lead acid is more expensive, even in that fairly large size.QuoteThat doesn't handle current limiting, so we go back to the idea of using a resistor for limiting current.