efficient way to disconnect battery when voltage drops? - Page 1
Forum section: electronics / general

author: ja421
author_rank: Contributor
timestamp: August 08, 2016, 06:30:45 pm
content: We have an off grid water pump that runs off a deep cycle battery and is charged by solar.It receives a wireless signal from the thank when the tank is 1/2 way empty and to start pumping. (on 3/4 low, an alert inside the house is given so we know there's a problem)This is all working well, except... When the battery is drained too much, the solar charger locks it out as a bad battery.So what I want to do is just have something that monitors the battery voltage, and if it should drop (say below 11 or 10.4v) it will not engage the pump. I think most of the time, we can go a day or two w/o it pumping, and that should give the solar panel (and sun) enough time to recharge the battery.I have toyed with a zener diode & transistor to energize a relay to cut the battery in/out, but this seems to waste a lot of energy (I have not tried to use a mosfet instead of a relay). I am thinking about using an attiny85 & sleeping as much as possible...It seems there might be a better / lower power way to do this. Any thoughts?thank you

author: Seekonk
author_rank: Super Contributor
timestamp: August 08, 2016, 07:04:17 pm
content: 11V is way too low. I don't go below 12.6V. You could just switch the control circuit on and off, that would be low current. Even an opto isolator may be sufficient. I use the little arduinos for everything at only $2 each in my solar applications. A TL431 is an alternative for discrete. Do you really need to pump at night?

author: wraper
author_rank: Unknown
timestamp: August 08, 2016, 07:12:07 pm
content: Quote from: Seekonk on August 08, 2016, 07:04:17 pmReally??? What's the point in the battery which will get cut off even before discharged by any sensible amount?

author: ja421
author_rank: Contributor
timestamp: August 08, 2016, 07:21:56 pm
content: Quote from: Seekonk on August 08, 2016, 07:04:17 pmI'm not sure I agree with 12.6 -- deep cycle batteries can provide a lot of current at lower volts, and 12.6 seems like a battery just sitting idle, so it might never get used... Either way, the cutoff/in voltage shouldn't be the primary focus; it's the method for monitoring the voltage I'm after a fairly effecient way."could just switch it on off" -- yes, I could; but that requires walking a fair bit up/down a steep hill to the pump house.The pump does run at night if it needs to (drops to 3/4 low); no one likes waking up and finding out the hard way that there is only enough water for 1/2 a shower.I guess I'll experiment with arduino more and see how power compares to the zener method.

author: Ian.M
author_rank: Super Contributor
timestamp: August 08, 2016, 07:35:11 pm
content: If its a Lead Acid battery, don't worry too much about getting ultra-low power consumption. The typical self-discharge is 5%/month, which for a 100AH battery is equivalent to about 7mA continuous load. It wont even notice a 1 or 2 mA load.You do however need to add some hysteresis to make the turnon threshold sufficiently higher than the cutoff threshold so that the rise in terminal voltage when the load is removed doesn't turn the pump back on causing it to short-cycle.Probably the best option would be a TL431 driving a high side big P-channel MOSFET. Stick a NPN emitter follower on the TL431 cathode resistor to boost the +ve going gate drive, with a reverse biassed Schottky diode B-E so the TL431 can pull the gate -ve quickly. Put a high value resistor from the MOSFET drain to the tap on the potential divider feeding the TL431 ADJ terminal to add hysterisis. You should be able to get the quiescent current down to 1.1mA without any problems.

author: edavid
author_rank: Super Contributor
timestamp: August 08, 2016, 08:09:09 pm
content: A relay is fine, just don't energize it unless the level switch is calling for water. The relay power will be negligible compared to the pump power.But if I were you, I would adjust or disable the lockout on the charger, or get a different charger.

author: LabSpokane
author_rank: Super Contributor
timestamp: August 08, 2016, 10:12:26 pm
content: Quote from: Seekonk on August 08, 2016, 07:04:17 pm10.5V = 0% charge with lead acid.12.6 = fully charged.11V-ish is a reasonable cutoff that will protect the battery while providing a reserve charge.

author: Seekonk
author_rank: Super Contributor
timestamp: August 08, 2016, 10:22:22 pm
content: Quote from: ja421 on August 08, 2016, 07:21:56 pmThat's the way I run my system. What is different is my system only runs in the day and uses car batteries (ones from vehicles not in use). I don't allow anything to start unless I have 13.7V, quite easy to obtain with only a couple amps charge, even at 8am. 13.5V is the low battery warning and 12.5 is the desperate warning light. These voltages are easy to obtain with just a couple amp charge. I can always run the system manually. Draining battery down is just running behind the 8 ball. Easy to get in situations that can't get caught up. This is what works for me living many years off the grid. You are better off storing in tanks than a battery. Letting it go to 11V is only something someone would say who hasn't worked with batteries.

author: LabSpokane
author_rank: Super Contributor
timestamp: August 08, 2016, 10:43:47 pm
content: Quote from: Seekonk on August 08, 2016, 10:22:22 pmLead acid batteries only sit above 12.6V either while being charged or immediately being disconnected from a trickle charger. Disconnect your batteries and let them sit idle for an hour and you'll watch the voltage drift down to 12.6V.I've spent a fair bit of time with lead acid technology and while very discharged, 11V is not a damaging discharge level for a battery in good condition - particularly if it is discharged under a substantial load. Discharging to 11V at low loads is a very different thing and will cause the battery to be at a greater state of discharge. Discharging past 10.5V causes damage unequivocally.If your lead acid batteries are weak at 12.6V after sitting idle for an hour after a full charge, they are likely damaged or weak.

author: Seekonk
author_rank: Super Contributor
timestamp: August 09, 2016, 01:51:48 am
content: My system doesn't even wake up till the battery is charging. I make my decisions always under charge conditions. If he uses a UNO he can eventually have a smart system that can make decisions based on what is happening. If diligent he will able to correct for all the bad advice he has been given. If the system experiences occasional moderate discharges, it is a bad design. It is pretty easy to get into a situation of heavy discharge and stay there. That is not what batteries like. My batteries have to go back into vehicles and I expect normal life expectancy from them. My system is rather small but it supplies all my needs for 5 months, including hot water and it has to work every day without fail. People love to have big dumb systems because that is all they can understand.

author: Monkeh
author_rank: Super Contributor
timestamp: August 09, 2016, 02:02:30 am
content: In other words you have a system of inadequate storage capacity using inappropriate cells.

author: LabSpokane
author_rank: Super Contributor
timestamp: August 09, 2016, 02:34:55 am
content: Quote from: Seekonk on August 09, 2016, 01:51:48 amYou *cannot* decide the health of a battery under charge conditions only.QuotePrepare for disappointment. Vehicle batteries are not designed for this application.QuoteThat's a pretty bold assertion considering you're ignoring the very fundamentals of lead-acid storage design. Yes, you're getting away with it ... for now, but the numbers you cite indicate that your batteries are already failing.

author: LabSpokane
author_rank: Super Contributor
timestamp: August 09, 2016, 02:46:09 am
content: Quote from: ja421 on August 08, 2016, 07:21:56 pmAn Arduino with a relay output would a simple way to handle this. You could simply use a voltage divider to drop down the 12V side and use the internal 5V reference to measure the battery state. Crude, but quick and simple and would get you up and running. Then you can play with the parameters and not have to do any more plumbing. The voltage regulator on the standard Arduino is pretty inefficient, but you can buy a DC-DC converter easily enough and power the system through the 5V pin if necessary.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: August 09, 2016, 11:12:29 am
content: I wouldn't bother re-inventing the wheel. There are dozens of manufacturers who make 'battery protectors' which are designed for this exact application.Here is an example:http://www.windandsun.co.uk/media/303907/Battery-Protect-Data-sheet.pdfAlso I'm going to throw my opinion in too: 12.6v is fully charged for a lead-acid battery so it's dumb to use that as your cutoff voltage, you are only using up the tiny amount of surface charge from the charging process that way.Also in my opinion, 11v is too low. There's not a great deal of useful energy to be had once the discharge curve starts to turn downwards and it will affect the long term health of the battery going down that far.I'd regard 11.5 to be 'fully discharged' but if you want long life out of your batteries then I'd stay above 50% discharge.

author: LabSpokane
author_rank: Super Contributor
timestamp: August 09, 2016, 04:34:04 pm
content: The battery disconnect Denzil posted is nice since the voltages are programmable and it has hysteresis designed in. The default disconnect voltage is 10.5V, which is fully discharged. The choice of a disconnect voltage above 10.5V is really down to what the user wants, be it fully depleting the batteries or leaving enough to start a car after leaving a load on too long. There is no "right" answer.An arduino would be a fun, simple project where the user can control his/her destiny for better or worse. Otherwise, you can buy a solution. Be wary of prefab vehicle disconnects because many have the 10.5V cutoff hard coded as that may not be what you want.

author: ja421
author_rank: Contributor
timestamp: August 10, 2016, 04:04:18 pm
content: Thanks for all your thoughts & reinforcing that my cut off values are reasonable.I had looked at some of the pre-made units (ebay, reuk.co.uk, etc) but most missed a quiescent drain spec; as did almost every schematic I found. I guess it's a matter of make a few & test them to find one that makes me happy. Finding this onehttp://arduinomaster.myfreesites.net/battery-saver-cut-out-switchmade me think; maybe just using a attiny & sleep it, test, sleep & use a mosfet might be a really easy way to go; and allow me to easily adjust the low/high values.I really do appreciate your ideas & input. THANK YOU

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: August 11, 2016, 01:18:01 pm
content: I think you're over-thinking the quiescent current thing. Unless your system is very very small or only operating very infrequently, I can't see it being a significant factor. A Victron BP-50 will probably suit you. (unless your pump is bigger than 50A continuous/200A peak) It has a quiescent current draw of 1.5mA when on and 0.6mA when off.I don't know which country you're in but you can buy one here for £39.90.http://www.kurandamarine.co.uk/battery-protect-bp-50?language=en&currency=GBPIf you can build a similar spec system with a lower Iq for that price just in parts (even if you work on it for 20 hours for free) I'll buy you a beer!

author: ja421
author_rank: Contributor
timestamp: August 11, 2016, 03:00:11 pm
content: Quote from: DenzilPenberthy on August 11, 2016, 01:18:01 pmYou may be right... I do get hung up on thinking about the little stuff sometimes, for fear that the little stuff will add up somewhere else when I'm not looking... but you make a valid point.Quote from: DenzilPenberthyWell, now I have to work on doing it.

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 11, 2016, 10:44:14 pm
content: I built a small portable power system for camping/emergency use and wanted the same thing. I ended up going with a latching relay as it only requires power to turn it on or off, then it latches in that position. You connect the positive, then there are two grounds that can be completed by a transistor to turn it on or off depending on what ground you apply power to. This is a "dual coil" relay, there are some single coil ones but you have to reverse the polarity, which would be a bit more tricky to do with a MCU. I think a H bridge is what you need. Dual coil is just simpler to setup.In my case the microcontroller has other tasks such as controlling LCD display (voltage and other messages). I can then connect a laptop to set different parameters such as cut off voltage and calibrate voltage/amps readout. I can't seem to get the amp readout to be accurate as it jumps around too much, but that's another story. If I was to do it over again I would have used a high side current sense IC instead of shunt/op amp like I did.11 volts is a decent voltage set point to go with. You could do 11.5 to play it safer and extend life of battery. 12.6 is way too high, you'll barely get any use out of it. That's practically a full charge sitting idle with no load. It will go lower with a load. I have seen some 48v systems that stick around 50 volts for hours though with a load on them (that would be like 12.5v on a 12v system) but batteries vary.Idealy whatever system you design you should not hard code stuff like shut off value, so you can field adjust it as needed. That's what I did for my camp solar system, I can plug a laptop and there is a text based interface to setup different parameters such as voltage calibration, shut off, shut off timer, etc... Was fun to code.

author: Seekonk
author_rank: Super Contributor
timestamp: August 12, 2016, 05:49:35 pm
content: I think that is the totally wrong concept. A simple philosophy, Don't start what you can not finish. Whatever switch you have to turn on the pump, supply that to a TL431 and a relay. That relay is used to trigger the pump circuit. This only powers the relay when there is pump demand. No hysteresis is needed because it always checks at rest condition. You don't get short cycling. This assumes the pump generally operates for a fixed period. Otherwise use a timer. This 10.5 & 11V comes from inverter manufacturers trying to protect inverters from excessive currents from under size wire power leads. No one in their right mind would drag a battery down to that.As Sheldon would say. If I were wrong, don't you think I would know it.

author: arlipscomb
author_rank: Contributor
timestamp: September 08, 2016, 06:08:13 pm
content: The lower the battery is discharged, and the longer it stays in that state, the shorter its life. If you take a lead acid battery charge down to the point where it is at 12 volts at rest (no load) and stays there for a day or so and you will start to get sulfation. Of course if you need water, you may not have a lot of choices. A couple of things come to mind:Check the charge voltage against the recommendations of your specific battery's manufacturer. Many times the charge controllers are too conservative on charging voltages and you end up running out of daylight before your battery is fully charged. A half volt can make a lot more of a difference than you would think it would.If you are building a programmable solution maybe start backing off on the pump run cycle earlier in the discharge cycle? For example if the battery is 75% discharged only run the pump for half the normal time. I am not sure what the water is for so you would have to figure out if something makes sense based on the need.Add a small wind turbine to supplement the solar. Often when the sun is not out you get some wind.

author: Tibby
author_rank: Newbie
timestamp: September 23, 2016, 11:27:05 am
content: We build chargers for SLA's and use a relay to disconnect load when battery drops to minimum acceptable. Since the load current is quite high we energize the relay while load is active and release when battery needs to be disconnected. Then its only our little sleeping PIC keeping an eye on battery that draws small current when it wakes up momentarily.You can also use a latching relay, only uses short energy pulse when switching.

author: KL27x
author_rank: Super Contributor
timestamp: September 29, 2016, 09:19:25 am
content: QuoteWell, then. Perhaps this is why your numbers are so far off from the rest of the forum. Some of us may have assumed a low voltage cutoff would be more useful when the battery is at rest or under load. This is usually where the voltage is actually going down, in my experience.

author: Seekonk
author_rank: Super Contributor
timestamp: September 29, 2016, 06:45:49 pm
content: Not responsible for peoples reading comprehension levels. OP never said if he had L16's or a Wallmart deep cycle battery. Voltages for various deep cycle batteries are not the same. People just pop up a chart and think they know something. You should have a smart system that can determine if you can even start and then monitor end current. The OP may or may not figure out 2 years from now why his batteries have failed. There are solar pumping systems that have no batteries.

author: ja421
author_rank: Contributor
timestamp: September 29, 2016, 08:24:47 pm
content: The OP is on year 7 for these batteries, at least a majority of them; 1 is 10 year and a couple are less than 2 years... Most of that longevity is a manual monitoring process. My goal, before my health completely craps out, is to make the system smarter so that it is easier for my wife to maintain after I cannot.

author: tautech
author_rank: Super Contributor
timestamp: September 29, 2016, 08:43:13 pm
content: Quote from: ja421 on September 29, 2016, 08:24:47 pmShouldn't be too difficult to cobble something together around this:https://www.maximintegrated.com/en/products/power/supervisors-voltage-monitors-sequencers/ICL7665.html

author: ja421
author_rank: Contributor
timestamp: September 29, 2016, 10:23:27 pm
content: Interesting chip -- thanks. I ordered one and will experiment...

author: tautech
author_rank: Super Contributor
timestamp: September 29, 2016, 10:34:15 pm
content: Quote from: ja421 on September 29, 2016, 10:23:27 pmYeah, you've got Q andQoutputs but do try and find more than 1 datasheet for all the info on these.You may be able to have enough drive for a small FET but look at using a gate driver too.You'll need to give a regulated supply, something like an LM7805L should be OK.I found getting the divider values just right a little tricky, some tweaks may be required.Pretty easy with a DMM or two and an adjustable PSU and something to monitor the output when it switches.

author: ja421
author_rank: Contributor
timestamp: September 30, 2016, 04:12:24 pm
content: Great! Thank you for the tips.

author: Codebird
author_rank: Regular Contributor
timestamp: December 19, 2016, 07:19:27 am
content: I think you might be over-engineering this. You don't need high precision for this - you just need something that will disconnect a load once the supply voltage goes under 11V or so, and turn it back on when it reaches 13V? Exact limits adjustable. It happens that I faced this exact problem myself some time ago, and still have the design I came up with. You can built it easily on stripboard and through-hole too, so cheap to build a one-off.https://birds-are-nice.me/ipfs/QmeCqXxspQN8Fa1dZ7W94nucyXA28qUcGbzc3c712PRHDa/

author: ja421
author_rank: Contributor
timestamp: December 19, 2016, 02:54:31 pm
content: Thanks @codebird -- I ended up using an attiny85 (and most of the time it's in sleep mode); I did this as it was really easy to configure low & high voltages (via potentiometers) as well as build in hysteresis much easier than other solutions I came up with.It is definitely not the most efficient, or elegant, method, but it seems to work. Part of me keeps thinking "a processor will crash & hang" but so far it has proven to work well.

author: Jeroen3
author_rank: Super Contributor
timestamp: December 19, 2016, 03:40:28 pm
content: You can buy battery protectors for marine batteries. They have similar situation where batteries are charged with solar energy and must not deep discharge.Often the solar charger contains these features as well.Stop DIY work, you're putting it outside. Which requires solid and reliable solution. Which isn't achieved on some protoboard in tupperware.

author: Seekonk
author_rank: Super Contributor
timestamp: December 19, 2016, 03:56:15 pm
content: Go on ebay and search Low Voltage Disconnect. You can get a nice ready made board for $10 with switch selectable pickup and dropout.

author: Codebird
author_rank: Regular Contributor
timestamp: December 19, 2016, 04:36:24 pm
content: Jeroen is right. Just about anyone here could put together a low voltage disconnect - microcontroller, or chip designed for such a purpose, or comparitors and latch as I suggested. They aren't hard to build, but if it's going to be outside then you will have to worry about weatherproofing. This applies even if you buy a ready-made board for the purpose - I can tell you from experience that even when sheltered in an outbuilding, unenclosed circuit boards do corrode. Stripboard goes fast, PCBs last longer, but all die in the end. You could build a suitable enclosure as well, with watertight seals and gaskets on all cables, but by that point you're reaching the level of expense and time investment where you're better off buying a ready-made solution, board and enclosure together.The easiest way might be to just get a bog-standard PWM charge controller off Amazon. I wouldn't trust a low-cost charge controller for anything over perhaps 100W, a lot of them have very 'unconventional' ideas about proper battery management, but most of them do have a low-voltage disconnect and reset function.But, if you really do want to build your own just for the satisfaction... you could just pot the whole thing in epoxy or silicone.

author: ja421
author_rank: Contributor
timestamp: December 19, 2016, 05:12:56 pm
content: I had a $200 solar controller that was supposed to be "weather resistant"; it was inside a shed (insulated,dry, & heated); after 2 years, the components had corroded to the point that it shorted, literally blew up several components so that they were unrecognizable, and no support from vendor (schematics or repair) -- their solution was to "buy another". So I don't hold that a "pre made" unit is somehow better than what I can come up with. Having made a few things, and setting them in water proof project boxes & using liberal amounts of sealant where wires came thru, I am convinced that they last just as long. I now coat all bare metal with some nail polish as well for anything that will be near the weather.

author: Codebird
author_rank: Regular Contributor
timestamp: December 19, 2016, 06:14:51 pm
content: It is inconsistent. I've got two pieces of stripboard in my garage - the garden light timer, and the charge controller. Both made in exactly the same way, and housed in the same room. The bell board is only slightly older, but seriously corroded - while the charge controller is entirely untouched by corrosion. Microclimate matters. But it's certainly a concern. Even if you buy an off-the-shelf unit, we all have horror stories of the 'quality' of low cost electronics.Personally, I'd make it myself, because I know then that even if it does go wrong in a few months I can fix it and make it better. I'd also make sure to include a fuse though, just in case! This has actually been on my to-do list for a while: Making some test boards, applying different means of corrosion protection, and exposing them to the harshness of good British weather.

author: ja421
author_rank: Contributor
timestamp: December 19, 2016, 06:27:12 pm
content: Electrolysis, I am sure, also plays a part in corrosion. Different components may use different metals which could contribute. It's not so much the 'off the shelf' part, it's the part that claims like "Weather resistant" should be able to handle more than a 2 year lifespan inside. So making it myself gives me A) exactly what I want, B) helps me learn what is going on, C) gives me a lot more satisfaction, D) allows me to fix it if something goes wrong later.On one project, I used the nail polish method & some shoe-goo to protect (or pot) all the components from vibration and it's installed on a motorcycle and has held up very well; it is not well protected, gets moisture, water splashes, rain, etc. and has held up rather well. The wire at the far ends are the only thing that have given me a problem so far.... so I think for inside a shed or under the overhang where it never gets water on it, it will hold up, provided I don't run into an electrolysis issue.

author: Red Squirrel
author_rank: Super Contributor
timestamp: December 19, 2016, 11:21:46 pm
content: Anything you buy or make you'll probably want the whole setup to be in some kind of outdoor/wet rated Nema box anyway. The battery, charge controller, and any other controls. May also want a small heater in the cabinet to keep it at a half decent temperature, just need to figure out the best balance as if you use the heater too much you're just using up the battery, but if you don't heat it at all the battery could freeze when it's low. May even be worth insulating the box with some fire rated insulation.

author: ja421
author_rank: Contributor
timestamp: December 19, 2016, 11:42:51 pm
content: I use a small incandescent light bulb with a thermal switch to keep it from near freezing temps. This has worked well. The box it's in is a large ammo box that is sealed and insulated. During the winter, I close the lid, during the summer I remove the lid & use a small (thermally controlled) 12v fan to keep things from over heating. So on top of the individual bits (like the battery disconnect) being inside their own sealed project box, they are inside another, which is then inside a building under a roof protected from the raw elements...Every little bit adds up; which is why I try to save as much power from every thing.... .8 amps vs 1 amp isn't much different, but when you look at a dozen things, that's over 2 amp savings... same applies when I'm dealing with 100ma vs 80ma -- it all adds up.