Switching Lead-acid battery charger IC - Page 1
Forum section: electronics / projects

author: Srbel
author_rank: Frequent Contributor
timestamp: November 23, 2015, 08:37:22 am
content: I was thinking of making this battery charger (for 12V car battery), that would use laptop adapter (19V, 4A) for power, and would charge at about 3A or so.Has anyone used an IC that is suitable for that? I can't seem to find any.

author: xReM1x
author_rank: Regular Contributor
timestamp: November 23, 2015, 01:40:30 pm
content: Quote from: Srbel on November 23, 2015, 08:37:22 amcar battery have high charging rate and high dischrage rate (not sure about the exact C rating).if you have 60Ah car battery it will take alot of time to fully charge it with 3A CC. (about a day and a night ).why do you want to charge it with 3A? if you are using 4A 19V supply you can easily step down the voltage and get a higher current, but generaly speaking lead acid battery is the easiest to chrage,read here:http://www.batterystuff.com/kb/articles/battery-articles/battery-basics.html

author: eneuro
author_rank: Super Contributor
timestamp: November 23, 2015, 03:35:34 pm
content: Quote from: Srbel on November 23, 2015, 08:37:22 amMine is classic step down buck converter with ATTiny85 MPU for logic control, Hall effect current sensor to limit current, optoisolated gate driver, two IRFZ44N in parallel, inductor, freewheel diode.No display-I use three position switch to turn off, tndaby mode where floating voltage 13.5V is provided, and charge mode with 14.2Vmax with temperature compensation, so at lower ambient temperature charge voltage is increased, at higher decreasedTL431I is used as voltage reference for ADC sensing current, voltage.Probably there are plenty of ready ICs to charge lead-acid, but sometimes it require messing with its datsheet, while when well known AVR MPU is used it is easier find any bugs for me and adjust quickly MPU code when needed...Are you looking for IC to control whole charge process of lead-acid?

author: Srbel
author_rank: Frequent Contributor
timestamp: November 24, 2015, 07:41:04 am
content: Quote from: xReM1x on November 23, 2015, 01:40:30 pmI know it will take 24 hours or so to charge, that is not the problem.I want about 3A because high current inductors are not really cheap (if I want 5A rms then I would need an inductor that handles 10A, because that would be the peak current?). And because laptop adapter is some "Terabyte" brand, so I don't want to load it too much. And it is actually rated for 4,74AQuote from: eneuro on November 23, 2015, 03:35:34 pmIt would be ideal if IC does everything (and even has some advanced features, like desulfurization, if possible).If I don't find a suitable chip, I will make a linear charger. I already started designing it for 1A (don't want to go over that because of heat dissipation). It is basically a modified dummy load that Dave designed. OP-AMP forces a voltage on a power resistor via FET, and instead of load there is a battery. Then other OP-AMP subtracts 2 voltages from battery terminals and gives a battery voltage on the output. That output goes to the input of comparator, and when it reaches 13,2V (that is set on the other comparator input) it will pull the OP-AMP input low via transistor, which will force 0V on a power resistor, and the charging will stop.Can you also tell me at what voltage should it stop charging? I was thinking like 13,2V. I don't want to fully charge the battery to 100%, and stress it. I just want to be able to "top-up" the battery a bit, so it becomes usable, and to revive some flat batteries if possible.And, will the battery voltage while charging be "clean"? There is voltage ripple from the adapter and current ripple from the OP-AMP regulating voltage on the power resistor. So, I don't even know if I could correctly detect those 13,2V on the battery with comparator. Maybe it will just keep turning the charging on and off because of ripple.P.S. Microcontrollers? I don't know programing. Not a single command. In fact, I hate it.

author: macboy
author_rank: Super Contributor
timestamp: November 24, 2015, 03:09:20 pm
content: Quote from: Srbel on November 24, 2015, 07:41:04 amDon't stop at 13.2 V. With lead batteries, you MUST charge them fully, rather than letting them sit around partially discharged. If they sit partially charged (or discharged), then lead sulfates begin to form on the plates. The sulfates are an excellent electrical insulator, and will destroy the battery. If the sulfation is not too bad, you can use a desulfator to break it down (returning the sulfur to the acid) but a very heavily sulfated battery is beyond repair. So, if you want to give your batteries a long and stress-free life, keep them FULLY charged, and charge them immediately after using them.The standard "float" voltage is 13.8 V, but that is for 22 deg C ("room temperature"), and the voltage varies with temperature. At higher temperature, the voltage needs to be reduced slightly, and at a lower temperature, it needs to be increased slightly. You canpermanentlyconnect a charger at the float voltage without damaging the battery, and keep it fully charged at all times. To keep your batteries in good condition, they should either be floated continuously or be periodically charged up to keep them fully charged.The "fast" charge voltage is about 14.4 V (again varying with temperature a little). You can apply 14.4 V until the current drops to 0.05 C (so for a 20 AH battery, that is 1 A). Then youmustreduce the voltage to the float voltage for last part of the charge. Otherwise you will damage the battery, albeit slowly.I'd recommend just float charging. It takes only a little longer and is easier on the batteries. You will need to limit the initial charge current to about 0.1 C (so 2 A for a 20 AH battery). The easiest way to do this is a constant current/ constant voltage power supply. Set the current limit to the desired charge current, and the voltage to the float voltage (13.8 V adjusted for temperature if needed). The voltage on the battery will slowly increase over time until it reaches ~13.8 V, then the current will slowly drop over time until it reaches nearly zero. At that point, the battery is fully charged and you can either remove the charger or just leave it on.You can buy inexpensive adjustable constant current/voltage buck converter (step down switching) power supplies on ebay or wherever. They will work fine. Some have a special LED to indicate when the charge is complete. This usually lights up at 1/20 of the initial charge current, and is adjustable on some. (pay more attention to the output power, not output current when looking at these modules. At 14 V and 3 A, you need over 40 W of output power... look for 50 W or better modules). I quickly searched and found this:http://www.ebay.com/itm/DC-7-40V-12v-to-1-2-35V-5v-24v-Buck-Converter-LED-Drive-Constant-Current-Voltage-/171923708476which may or may not be suitable for your needs. It's just an example.You can find float voltage vs. temperature charts with google.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 24, 2015, 03:18:46 pm
content: Quote from: Srbel on November 24, 2015, 07:41:04 amIt depends somewhat on the switching frequency, but I think most inductors will be able to take the peak currents, they have enough thermal mass to "filter" the pulses - so I think you should be able to select based on RMS current without any issues.Also, Gigabyte is an OEM that is quite popular when it comes to high-end "gaming" computer hardware. While I'm not going to say its built like a medical supply pack, it's far from some noname chinese brand. It's probably some re-branded FSP or similar powersuply. This is their website:http://www.gigabyte.com/When I first came into contact with them I also thought it was some gimicky company but from what I've seen they actually deliver decent stuff.

author: Srbel
author_rank: Frequent Contributor
timestamp: November 25, 2015, 06:51:14 am
content: LOL, it actualy "Terabyte" brand. I know about Gigabyte, I have their motherboard.So, 13,8V it is. I don't want to buy a finished charger, I want to design it myself.Can I charge the battery with constant current until voltage climbs to 13,8V? Seems better than "forcing" a voltage on the battery.

author: macboy
author_rank: Super Contributor
timestamp: November 25, 2015, 01:38:56 pm
content: Quote from: Srbel on November 25, 2015, 06:51:14 amThat is exactly what you do; constant current (or current limited) until the voltage climbs to 13.8 V, but you don't stop when it reaches that voltage. The battery is half to two-thirds full at this point. Then you need to keep 13.8 V on the battery until the current drops to less than 1/10 of the initial charging current.

author: eneuro
author_rank: Super Contributor
timestamp: November 25, 2015, 02:05:33 pm
content: Quote from: macboy on November 25, 2015, 01:38:56 pmWhen we already have current limit, there is no problem keep battery at its floating voltage, so we do not need stop charger, but I'd also use temperature sensor on battery to stop charging (disconnect power source and turn on beep sound, blinking red LED, etc) when battery temperature goes above given limits and use the same sensor to make temperature compensation within defined min/max floating voltage values for given battery based on manufacturer datasheet-sometimes they specify those floating voltages and temperature correction coeficients

author: Srbel
author_rank: Frequent Contributor
timestamp: November 25, 2015, 04:47:46 pm
content: Ah. :/ Well, that is why I want to use a dedicated IC.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 25, 2015, 06:33:22 pm
content: I remember using the LT1512 in a project last year. Was essentially just a switching regulator that sensed the battery voltage. Went up to 1.5A of current but afaik you could use an external mosfet to increase the current but don't quote me on that.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 26, 2015, 02:10:26 am
content: If you're looking to leave it charging for a long time, what works better is to charge up to 14.4V or so and hold it there for something like an hour or two, then taper the voltage down to 13V or so (just enough to prevent the battery from discharging) and leave it like that for a few days (or until the battery has discharged some) before repeating the cycle. From what I have seen, internal corrosion is a failure mechanism that is particularly common with batteries left on float charge for long times. The switching between the 14.4V and 13V seems to be the best compromise between corrosion and sulfation, even more so than a constant voltage of 13.8V that seems to be the norm.It is also noted that as far as service life goes, lack of filtering is actually a plus! The high frequency pulses tend to reverse any sulfation that takes place. So what you really need is more or less a buck converter without the output capacitor. Then you need to filter the voltage sensing (a RC filter works nicely) so the controller doesn't get confused by the ripple and then slow down the loop so it remains stable despite the delay from the filter.

author: Srbel
author_rank: Frequent Contributor
timestamp: November 26, 2015, 08:17:43 am
content: Quote from: TheUnnamedNewbie on November 25, 2015, 06:33:22 pmYes, I have seen that one. But for 19V input it can only charge the battery at 700mA max. Datasheet does not give an example of using an external switch, and this IC senses current internally, so if I add an external FET, it will not measure output current.And it does not give an example circuit for lead-acid nor buck configuration. Why are LT datasheets so basic...

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 26, 2015, 11:25:14 am
content: I must have been mistaken then, I thought I rememberd the datasheet doing a great job of explaining how the thing works. Well, good luck on your hunt!

author: Srbel
author_rank: Frequent Contributor
timestamp: November 26, 2015, 12:11:49 pm
content: Well, I haven't read the datasheet, just skimmed through it. Maybe it is this circuit on he page 8:http://cds.linear.com/docs/en/datasheet/1512fc.pdfBut it uses 2 inductors, and I want just buck configuration, not SEPIC.

author: DanielS
author_rank: Frequent Contributor
timestamp: November 26, 2015, 09:00:24 pm
content: Quote from: Srbel on November 26, 2015, 12:11:49 pmThe "two inductors" in a SEPIC converter are on the same core, which makes it a transformer.I like SEPIC: while the design might be a little more complex, it provides intrinsically current-limited buck-boost capability along with capacitive isolation between input and output to stop current flow when switching stops for whatever reason including catastrophic failure of anything other than the coupling capacitor.

author: langwadt
author_rank: Super Contributor
timestamp: November 26, 2015, 09:27:23 pm
content: Quote from: DanielS on November 26, 2015, 09:00:24 pmThe two inductors in a sepic does not have to be on the same core

author: Srbel
author_rank: Frequent Contributor
timestamp: November 27, 2015, 12:22:24 pm
content: Anything better than LT1512?

author: DanielS
author_rank: Frequent Contributor
timestamp: November 27, 2015, 01:27:24 pm
content: Quote from: Srbel on November 27, 2015, 12:22:24 pmThere are plenty of options but you have to do some of your own homework. That was only one example to help you get started.http://www.linear.com/parametric/Battery_Charger_IC#!cols_1806,1112,1067,1068,1367,1069,1033,1032!s_1067,0!gtd_!1033_%3E=12!1067_%3E=1!1806_Lead%20Acid

author: Srbel
author_rank: Frequent Contributor
timestamp: November 27, 2015, 01:43:05 pm
content: I have already looked at various datasheets, and did not find any dedicated lead-acid charger IC (they are all multi chemistry). I have only found linear dedicated lead-acid chargers, but not switch mode.I am not asking you to do that for me, I am asking if someone already used such a chip, to tell me which one is it.

author: DanielS
author_rank: Frequent Contributor
timestamp: November 27, 2015, 02:16:37 pm
content: Quote from: Srbel on November 27, 2015, 01:43:05 pmWhy insist on lead-only charging ICs? IC manufacturers design chips that support multiple chemistries for a very simple reason: the fundamental charge cycle is the same for all of them, all you need is a handful of extra resistors to set application/battery-specific limits.