making a battery gauge for a 12v 9Ah UPS battery.  - Page 1
Forum section: electronics / beginners

author: inifus
author_rank: Newbie
timestamp: October 11, 2011, 10:34:52 pm
content: Hi!I just replaced my RBC17 12v lead acid battery in my UPS because the UPS kept nagging me that the battery was no good anymore. I had watched the video blog of the lithium ion battery gauge with LM3914. I was thinking I'd make a similar device for this particular battery. I need to know, is there some difference in how lithium ion and lead acid batteries operate in terms of voltage drops? and what should I avoid to not have the battery explode in my face?Also. I have yet to find a datasheet for the battery. if anyone have seen it please post a link for me, or if anyone knows the operating voltages and other information needed

author: amspire
author_rank: Super Contributor
timestamp: October 11, 2011, 11:06:43 pm
content: All the lead acid batteries behave pretty much the same, so you don't need the exact data.This should help:http://www.efirstpower.com/PDF/FP1270T.pdfLook at the graph showing Open Circuit Volts vs Capacity.Now it is temperature dependent, so if you want to be really current, you could look at volts vs temp for a battery and compensate with a thermistor.But it is probably not that important.Richard

author: sonicj
author_rank: Frequent Contributor
timestamp: October 12, 2011, 12:22:08 am
content: im not sure that a LM3914 circuit would provide much relevant information in your specific application. reason being, a battery in poor health but with plates unshorted would still show 13.8V (or whatever) when floating. you would need look at the battery voltage under load to determine whether or not it is capable of sustaining a suitable voltage to keep the UPS alive (see:coup de fouet) and then look at how long it can maintain that usable voltage (capacity).there are complicated & expensive devices available for testing battery health without placing them on a load, but i think sort of technology is out of reach for most of us non-corporate ups users. not saying it can't be done... i just don't think it can done effectively with only a lm3914.-sj

author: fernandohdz
author_rank: Newbie
timestamp: June 25, 2012, 03:50:10 pm
content: Hi did any one can help me to make it for my motorcycle 12 v batteryThanks

author: 6502nop
author_rank: Regular Contributor
timestamp: June 26, 2012, 02:46:10 pm
content: Quote^ I pasted the key points here. Re-read them. Again.Now, if you still don't see it. allow me:As much a fan of the 3914/15/16 chips as I am, proceeding with your project is not only asking for disaster, butcompletely unnecessary. The "RBC" is what APC calls their "Replacement Battery Cartridges" (just a good ol' SLA), so I'm guessing that you have one of their models. Which means that youalsohave the CD that came with it, and the cable (USB) to hook up communications. The software is called "PowerChute", and depending on the model of UPS, will doexactlywhat you're trying to do. After all, it was the UPS thattoldyou the battery was flaky, right? Well, PowerChute does this in a GUI. Some models can even track power usage, brownouts, and load capacity over time.So, throwing in a bar graph indicator doesn't do squat as far as determining the real state of the battery, as my older APC (and Belkin) units run periodic tests on the batteries to see if they can stand up to the current draw of an outage without crapping their britches. They do this about twice a day, IIRC.(Test Mode: If you're close enough to it, the UPS will click {relay}, and you'll start hearing a 60Hz hum {transformer driven by the battery and internal driver MOSFETs}, then after a few seconds, another click, and either nothing {good!} or an alarm {bad battery!}. Essentially, it loads the battery by simulating an outage, and then measuring the voltage drop. If it gets too low, the alarm goes off.)As for the inside of those things? You're inveryclose proximity to mains voltage. One little slip or extra-long lead somewhere could end up with you in the obituaries. Not good.So, just replace the battery, and let the UPS do what it was designed to do. Install PowerChute if you want to look at all the blinkenlights as it's doing it.QuoteEverything you need is in the very lengthy datasheet from National:https://www.national.com/ds/LM/LM3914.pdfbut the thing to remember is that the Rlo and Rhi are just the lower and upper points of the resistor "ladder" to the comparators. So, by tying them through your own resistors to set the Lo/Hi points, you'll have the ten segments in between to display. There are also many circuits out there if you Google for "3914 automotive".However, I'd can this idea, as LEDs are bad at displaying when you're riding on a sunny day (best for motorcycles!). So, I'd splurge and grab a gauge set with back lighting for night use.nop

author: 6502nop
author_rank: Regular Contributor
timestamp: June 27, 2012, 08:04:14 pm
content: And, just after I logged off, I couldn't the @#$%ing problem out of my head, so here goes:Grab an LM339 Quad Comparator (or two) and an LM78L09 Low-current Vreg (you won't need much juice, so the 500mA 7809 will work great).Now, the problem stems from having to both power the tester and measuring the voltage of the thing we're testing. Which won't work, as the darn thing will always show as being the high-voltage level (unlike a simple analog meter).Hmmm.So, the brilliant idea wasNOTto measure the voltage of the battery, but to measureHALFthe voltage of the battery! We then use a Vreg to provide for our reference voltage, about 60%+ of expected battery output. This would be about 9V for a 12V battery . As it happens, "there's a Vreg for that".A quick run through some modular-type math, some clicky-clicky with gEDA, a few notes, and...

author: 6502nop
author_rank: Regular Contributor
timestamp: June 27, 2012, 08:17:57 pm
content: And the first one, without the automotive factor involved...nop