Battery Charger Design
in [Design]
|
From: mike on 6 Jan 2010 21:46 thorin92 wrote: > On Jan 6, 3:15 pm, mike <spam...(a)go.com> wrote: >> thorin92 wrote: >>> In the application, the output of the battery is regulated by an LDO >>> Voltage Regulator before powering the AVR, so the AVR supply is of no >>> concern. >>> The DC supply for the charger will be heavily filtered and regulated, >>> powered from a mains transformer. >>> I do not wish to use an AVR to manage charging, although if this is a >>> necessity for implementation of a timer then I guess its compulsory. >>> In the battery specs it states fast charge 70mA for 3 hours. >> Yes it does. If you start with a completely discharged battery, you can >> charge it at 70ma for three hours...not four or five or six >> hours...THREE. You tell by measuring the time. If you overshoot >> by too much, the cell will vent and you'll get poor reliability. >> If it's not fully discharged, you can't charge it for three hours. >> If you know the discharge current, you could time it and put back >> what you took out times some efficiency ratio. Could all be done with >> internal timers >> >> Charge termination is one of the more difficult parts of using rechargeable >> batteries. There is no free lunch. If you fast charge it, you shorten >> the life. If you overcharge it, you shorten its life. >> I already suggested you look up "zero delta V" charge termination >> strategies. >> >> You can get clever, but not unless you (we) know all the details. >> >> Thus if I> build the LM317 regulator with current control to 70mA this should >>> suffice, but how would I know when the battery is charged? Is there >>> some indication from the terminal voltage with this type of battery? >>> Alternatively, a trickle charge of 4.3mA implemented using the LM317? >>> Best >> You're all over the map with your requirements. 4.3ma won't get you >> 8-hours charge time. >> >> I'm trying to help you, but you're not answering some critical questions. >> How long does this thing have to work? >> You say it's a research project. >> If it needs to function for six months, >> you have more options than if it is expected to run reliably for years. >> Expected life under various overcharge conditions are right >> there in the spec. >> >> Your choice of battery is causing you more grief than if you could >> tolerate a bigger one. >> >> You're trying to make this much simpler than it is. >> Wishing won't make it so. >> >> Stuff should be as simple as possible, but no simpler... >> >> Excuse me, but I feel a speech coming on...I can't stop it...here it >> comes... >> >> System design is an interactive process. You've asked for help >> on a specific part. That's ok, that's what you're getting. >> >> It is very common for an engineer to design a system that he thinks >> should work...but creates some difficult problems in one area or another. >> Often, eliminating the tunnel vision and re-examining the overall solution >> results in much easier overall implementation. >> >> But not knowing what you're doing prevents us from suggesting things >> like using three cells large enough to give you the run time >> you need AND accept overcharge that eliminates most of your >> recharge issues, eliminating the ldo and using one port on the AVR >> to implement a boost converter to drive the leds. >> But there may be other issues that prevent that. >> >> Don't mind me...I'm easily frustrated... > > Hi, no problem and thanks for the help. > > Unfortunately, the Battery has been decided and it cannot be changed. Wishing won't make it so. > I have tested the discharge rate/capacity for the spec and all is > fine. The device will operate for 4-5 hours until the AVR brown-out > kicks in and shuts down the system at 4.5v. As previously stated, 0.9V/cell ==> 4.5V is more than fully discharged. As I stated before, I > assume the battery is not fully dis-charged at this point. I think you should check that assumption against the specs. Never assume when you can use real info. At this level of discharge you should probably pay some attention to equalizing the cells. Small overcharge can do that. > > I would like the Battery to last as long as possible, and a trickle > charge at 4.3mA is acceptable. Not unless you change your charge time requirements AGAIN!!! Quit using the phrase, "as long as possible." There is no MATH do deal with that requirement. Everything must have a NUMBER. And pick the number according to REAL requirements AFTER you've thought about it. Arbitrary numbers are no better than no numbers. > > Despite the years of experience I am obviously missing, I do want a > simple system, Yeah, and I want Taylor Swift to show up at my door and call me "daddy", or in my case, "granddaddy". But we're both likely to be disappointed. Wishing DON'T make it so. regardless. There is no "regardless" in electronic design. REGARD is the essence of design. Since this is for a research project, the > charger design is a secondary priority, thus I'm not interested in its > sophistication provided that it works, safely, reliably, and charges > the battery. That's a nonsense sentence..."It doesn't have to be perfect as long as it works perfectly". A charger is unimportant until you find that the project fails because of the charger. > > My basic question at this point therefore, is; will the 317 reg > current limited to 4.3mA work? A 317 can be made to limit current. There are "headroom" requirements that you probably meet. A 317 is serious overkill for this application. With 9V input and 4.3ma charge, you can get by just fine with a resistor. even if it takes 24 hours? Or is there > something I should be aware of using this method? Yes, you should be aware of the "division" button on your calculator. That is the only > question I am asking. The short answer to the question you asked is, "the path you're suggesting will not take you where you said you wanted to go." It's not because of the 317. It's because of the simple math you didn't do. > > Best > This is gonna be my last post...hopefully...on the subject. If you won't listen, I can't help. I apologize, but you've completely depleted my supply of tact. THEMOSTIMPORTANTPARTOFAPROJECTISTHE SPECIFICATION/REQUIREMENTS DOCUMENT. If you don't know what you're building, you can't design it. You're all over the map. First it was 3 hours...now 4-5. You want it to charge "as fast as possible" then 8-hours then 24 hours. It's very simple math. 150 mah / 4.3ma ==> 34.883721 hours plus some based on the charge efficiency. If you can't/won't do that level of analysis, there's not much I can do to help you. I gotta go to the store now and buy more tact. I'm sending YOU the bill.
From: Phil Allison on 6 Jan 2010 22:01 "mike" > You're all over the map. First it was 3 hours...now 4-5. > You want it to charge "as fast as possible" then 8-hours then 24 hours. > It's very simple math. 150 mah / 4.3ma ==> 34.883721 hours plus some > based on the charge efficiency. If you can't/won't > do that level of analysis, there's not much I can do to help you. ** Experience with NiMH cells tells me that at the C/50 rate little if any charging occurs and at the C/100 rate - no charging occurs at all. Only charging at current levels above these rates does the business. So, in the OP's example, the first 1.5 to 3mA does nothing but tread water. A second issue is being able to tell when the dish is cooked - ie, if one cell in a pack requires 3mA to tread water and another only 1.5mA, then the dish will only ever be cooked in parts. Far better to charge moderately fast - like 5 hours at the C/4 rate ( 38mA in the OP's case ) and KNOW when the dish is fit to serve. > I gotta go to the store now and buy more tact. I'm sending YOU > the bill. ** ROTFL !! .... Phil
From: ehsjr on 6 Jan 2010 23:44 thorin92 wrote: > On Jan 6, 3:15 pm, mike <spam...(a)go.com> wrote: > >>thorin92 wrote: >> >>>In the application, the output of the battery is regulated by an LDO >>>Voltage Regulator before powering the AVR, so the AVR supply is of no >>>concern. >> >>>The DC supply for the charger will be heavily filtered and regulated, >>>powered from a mains transformer. >> >>>I do not wish to use an AVR to manage charging, although if this is a >>>necessity for implementation of a timer then I guess its compulsory. >> >>>In the battery specs it states fast charge 70mA for 3 hours. >> >>Yes it does. If you start with a completely discharged battery, you can >>charge it at 70ma for three hours...not four or five or six >>hours...THREE. You tell by measuring the time. If you overshoot >>by too much, the cell will vent and you'll get poor reliability. >>If it's not fully discharged, you can't charge it for three hours. >>If you know the discharge current, you could time it and put back >>what you took out times some efficiency ratio. Could all be done with >>internal timers >> >>Charge termination is one of the more difficult parts of using rechargeable >>batteries. There is no free lunch. If you fast charge it, you shorten >>the life. If you overcharge it, you shorten its life. >>I already suggested you look up "zero delta V" charge termination >>strategies. >> >>You can get clever, but not unless you (we) know all the details. >> >>Thus if I> build the LM317 regulator with current control to 70mA this should >> >>>suffice, but how would I know when the battery is charged? Is there >>>some indication from the terminal voltage with this type of battery? >> >>>Alternatively, a trickle charge of 4.3mA implemented using the LM317? >> >>>Best >> >>You're all over the map with your requirements. 4.3ma won't get you >>8-hours charge time. >> >>I'm trying to help you, but you're not answering some critical questions. >>How long does this thing have to work? >>You say it's a research project. >>If it needs to function for six months, >>you have more options than if it is expected to run reliably for years. >> Expected life under various overcharge conditions are right >>there in the spec. >> >>Your choice of battery is causing you more grief than if you could >>tolerate a bigger one. >> >>You're trying to make this much simpler than it is. >>Wishing won't make it so. >> >>Stuff should be as simple as possible, but no simpler... >> >>Excuse me, but I feel a speech coming on...I can't stop it...here it >>comes... >> >>System design is an interactive process. You've asked for help >>on a specific part. That's ok, that's what you're getting. >> >>It is very common for an engineer to design a system that he thinks >>should work...but creates some difficult problems in one area or another. >>Often, eliminating the tunnel vision and re-examining the overall solution >>results in much easier overall implementation. >> >>But not knowing what you're doing prevents us from suggesting things >>like using three cells large enough to give you the run time >>you need AND accept overcharge that eliminates most of your >>recharge issues, eliminating the ldo and using one port on the AVR >>to implement a boost converter to drive the leds. >>But there may be other issues that prevent that. >> >>Don't mind me...I'm easily frustrated... > > > Hi, no problem and thanks for the help. > > Unfortunately, the Battery has been decided and it cannot be changed. > I have tested the discharge rate/capacity for the spec and all is > fine. The device will operate for 4-5 hours until the AVR brown-out > kicks in and shuts down the system at 4.5v. As I stated before, I > assume the battery is not fully dis-charged at this point. > > I would like the Battery to last as long as possible, and a trickle > charge at 4.3mA is acceptable. > > Despite the years of experience I am obviously missing, I do want a > simple system, regardless. Since this is for a research project, the > charger design is a secondary priority, thus I'm not interested in its > sophistication provided that it works, safely, reliably, and charges > the battery. > > My basic question at this point therefore, is; will the 317 reg > current limited to 4.3mA work? even if it takes 24 hours? Or is there > something I should be aware of using this method? That is the only > question I am asking. > > Best > > > > > Yes, the LM317 set for 4.3 mA will work. It is not the best solution. Here's a simple circuit with a 317 and a TL431, which is better than just the LM317 constant current circuit. The 317 is set to charge the cells at C/10, which for your cells is ~14 mA. The TL431 is set to steal current when the battery voltage rises approaching full charge. ----- + ----in|LM317|out-+ ----- | Adj [85R] | | +-------+------+------+-->|---+ | | | [680R] | | | | | __|_/ | | / / \ P | + /431\--->0 1M Battery ----- T | | | | Gnd ----------------------+------+-------+ The 85 ohm resistance sets the LM317 to provide about 14 mA. The pot feeding the 431 is set to make it conduct when the battery reaches ~6.5 volts. Prior to it conducting, the full ~14mA current, minus the ~ 7uA the 1meg pot draws, goes to the battery. When the 431 conducts, the 680 ohm resistor limits the current through the 431 to a max of ~ 9.5 mA, leaving about 4.5 mA to go into the battery. Note that you could change the 85 ohms to 18 ohms to get a charge current of about 70 mA, and the 680 to 100 ohms to allow about 65 mA through the 431. But the sheet at the url you posted mentions only the general** 14ma and 4.3 mA rates so depart from that at your own risk. **The 70 ma rate is specific for a completely discharged battery for 3 hours at 20 C Ed
From: robb on 7 Jan 2010 15:15 "thorin92" <thorin92(a)gmail.com> wrote in message news:36597a80-dd0f-4e96-a16b-ceda719358de(a)u41g2000yqe.googlegroups.com... > Dear group, > > I am trying to design a battery charger for this battery: > http://uk.farnell.com/varta/55615605940/battery-pcb-mount-6v-5-v150h/dp/863981 > I'm familiar with voltage regulators and current limiting etc. but am > unsure what parameters to use? > > If I simply provide 6v limited to 70mA will this work? > It seems (from the other posts) that this battery charger bit is not part of the research, just a project hurdle to get the colored cubes working ? the battery spec. says it is a drop in replace for other similar ni-cad & nimh batteries and so i presume there are existing ready made battery chargers that can charge this battery ? couldn't one just buy a ready made charger match for this battery and use that (**or the guts**) to solve the charger problem ? I have a box full of various rechargeable battery chargers for nimh/nicad from 4.5/6v/9v and for a variety of devices like camera batteries, RC batteries , rechargeable cells, etc. with many output ranges. maybe you just want to build one from scratch but that sounds like a project all it's own. my $0.02 robb
From: Phil Allison on 7 Jan 2010 19:17
"robb" > the battery spec. says it is a drop in replace for other similar > ni-cad & nimh batteries and so i presume there are existing ready > made battery chargers that can charge this battery ? ** Wot a silly presumption. The battery in question is a *solder in * replacement for MEMORY BACKUP purposes . So it fits on a large PCB and is supplied with a trickle charge by the same board. > > couldn't one just buy a ready made charger match for this > battery and use that (**or the guts**) to solve the charger > problem ? ** No. ..... Phil |