BMS for small SLA set up - Page 1
Forum section: electronics / general

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2018, 09:22:21 am
content: I am building a very small room-level backup power system (UPS) which allows me to sit out a power outage. We get them here occasionally and it's bloody annoying. All I need to operate when the power is down is my transceiver and bench lighting. All I need to power is an FT-818, Signalink and an LED strip. Laptop is covered by its internal battery. Nothing fancy. So I figured that a simple 12Ah SLA will do the job for 3-6 hours which is plenty.However I need to look at top up/maintenance of this battery, preferably automated. I'd like to keep it on line so I don't have to change anything when the power drops. I need to pull 1.1A when off line average for 3-4 hours with occasional 2.5A peaks.Are there any off the shelf solutions or kits to keep this SLA maintained and in good condition which I can keep the above connected during the outage?Basically I want a 12V output UPS system with mains charge functionality.Edit: to note the input must be linear supply so it doesn't inject noise into everything! If nothing exists in kit form or off the shelf I will design something but would prefer to not bother.

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2018, 09:32:02 am
content: SLA's don't like being fast charged so just a LM317 reg will easy do the job and just be sure to set the float for standby use not cyclic. Just let the max current limit in a 317 do it's job.

author: nctnico
author_rank: Super Contributor
timestamp: August 23, 2018, 09:40:38 am
content: The key to keeping an SLA happy is not to overcharge it. Better set the float voltage too low than too high. An LM317 may not be precise enough for this job.

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2018, 09:44:54 am
content: That's one thing I was concerned with. I'm just drawing up what I have first... 1 min.

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2018, 09:50:40 am
content: Quote from: nctnico on August 23, 2018, 09:40:38 amOf course it is with a 10t pot and the right set/divider setup.

author: nctnico
author_rank: Super Contributor
timestamp: August 23, 2018, 09:54:20 am
content: Think about temperature variations.Putting a big electrolytic in parallel may also be a good idea to take the edge off peak currents.

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2018, 09:58:20 am
content: Ok this is where I started...Schottky drops is not a problem here as all the stuff works down to about 10V fine.Problem is I can easily substitute an LM317 or whatever charger in there but it has three drawbacks which I want to solve:1. No over-discharge protection of the battery.2. No intelligent cycle charge if the battery is suddenly discharged due to an outage.3. No over-current protection. They can deliver 100A or so. Woosh.This lead to this drawing which covers the above and over-voltage protection if the LT1083 fails. The main problem is what sits in the magic boxIf it handles the LT1083 regulator role and the diode OR gate function as well then I'm happy.I can design something which will do this with a UC3906 and MOSFET switch etc but I'd rather like to just buy something because I can't be arsed

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2018, 10:13:11 am
content: Just depends what you have on hand for the supervisory role.It doesn't need to be anything flash as long as it triggers within 0.5 of a volt or so.I keep a few ICL7665 on hand for jobs like this as they allow for two outputs and can easy be setup to the hysteresis and levels you want. Interfaces well with CMOS logic too.For the charge limit the range is not tight.........looking at a 12V 7.2AH SLA here Standby: 13.5-13.8 and Cyclic: 14.4-15v and up there, that's what kills them !

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2018, 10:25:14 am
content: Got plenty of LM311's and some LM385'sActually looking at it, this isn't rocket science.v < 10.5 .... failure condition. Do nothing10.5 < v < 13.65 ... constant current (cyclic)13.65 < v ... constant voltage (float)TBH after googling, this looks like it might do the job:https://quasarelectronics.co.uk/kit-files/electronic-kit/3193.pdf

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2018, 10:32:52 am
content: Yep, it's a piece of wee's really and all you need now is some LED indication of what's happening so to explain those WTF moments.

author: Red Squirrel
author_rank: Super Contributor
timestamp: September 04, 2018, 12:07:40 am
content: I think you can pretty much leave lead acid batteries at 2.25v per cell indefinitely (float voltage) and they'll be fine. They will charge and never overcharge. If you setup like that you don't need a BMS. This is how the telcos do it. Rectifiers are set to 54vdc, and loads and batteries are all connected to that same bus. There's monitoring such as mid point voltage to detect if a cell goes bad or what not, but no BMS or charging circuitry. Just constant voltage.

author: langwadt
author_rank: Super Contributor
timestamp: September 04, 2018, 12:47:21 am
content: Quote from: bd139 on August 23, 2018, 09:22:21 amhttp://www.ti.com/product/UC3906has most of what you need

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2018, 08:12:34 am
content: Thanks for suggestion. Was looking at that. But it's too complicated.For ref, I've actually designed something in the end. Outcome:1. LM723 / MJE3055 based CC/CV charger. Current limited to CC=1A / CV=settable between 12.8-14V (will dump at 13.65 as per battery datasheet)2. TL431 / SCR / fuse based over-voltage crowbar. Clamps if V > 15V3. TL431 / RFP70N06 based under-voltage cut off. Low side switch cuts battery off if falls below 11.9V.Whole system is immune to any part failing. At worst will pop a fuse.Will prototype it when I get some time. Main requirement is that it's cheap, reliable and I already have all the bits

author: splin
author_rank: Frequent Contributor
timestamp: September 07, 2018, 01:17:30 am
content: To save some effort just buy a $1.32 buck convertor module with CC mode for the charger:https://www.ebay.com/itm/New-CC-CV-Power-Supply-1PC-LM2596-LED-Driver-DC-DC-Step-down-Adjustable-ASS/263499546041?hash=item3d59cbfdb9%3Ag%3AAA4AAOSwlEdZtzrM&LH_BIN=1I think the third trimmer is for setting the led threshold to indicate charging - might be useful.For the undervoltage/overvoltage detection get one of these for $2.75:https://www.ebay.com/itm/12V-Storage-Battery-Charging-Controller-Module-Smart-Charger-Protection-Board/282206492472?epid=16018120565&hash=item41b4d15738:g:vigAAOSw8gVX82JAIt turns the relay on between the upper and lower voltage limits set by the trimpots, which isn't quite what you need, so disconnect the upper limit comparator output from the relay drive and use it to trigger your crowbar instead*.There are variants with two seperate opamps/comparators and digital versions for a little bit extra with digital displays, voltage settings and serial port output for status etc.Won't be quite as neat as a custom PCB job but what's your time worth?PS. I agree with Red Squirrel - don't bother with the LT1083 regulator and diodes - just keep the battery floated (with the buck module) and power everything from the battery via the undervoltage disconnect relay on the above module. Hopefully the battery will suppress any charger noise sufficiently. * If you eliminate the LT1083 and diodes you probably don't need the crowbar at all - use the battery control module relay as delivered to disconnect the battery if it gets too low or dangerously high (eg. because charger module fails). You might need to change a resistor value to get an appropriate range.[EDIT 2] That dual opamp/comparator is actually an NE555 - two comparators so should be ok however. The variant that I assumed had two seperate opamps/comparators:https://www.ebay.co.uk/itm/Battery-Charger-Protection-Automatic-Charging-Controller-Module-12-24-36-48V-ly/332472461174?hash=item4d68e72776:m:mB3r2OLEiwIo7iTK-9YLVdAThe chips are actually a 555 and an XL7005 buck convertor. Don't know what it's purpose is as the listing has nothing to say.

author: bd139
author_rank: Super Contributor
timestamp: September 07, 2018, 06:08:03 am
content: Can’t use a buck module. Switching noise gets into the RF side of things. Even the low impedance of the battery isn’t enough to kill it. Hell a ceramic cap won’t either. That incurs the cost of expensive magnetics in the end which ultimately compromises the original goal of cost.It’s worth designing a PCB for this one as I know a couple of people who would be interested in the outcome.Main thing here is safety. It’s connected to over £1500 worth of radio kit that better not go up in smoke. Explicit control gates for each failure condition are required there.