Battery Charger Design
in [Design]
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From: mike on 6 Jan 2010 10:15 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...
From: Spehro Pefhany on 6 Jan 2010 10:48 On Tue, 5 Jan 2010 04:34:21 -0800 (PST), thorin92 <thorin92(a)gmail.com> wrote: >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? > >Thanks in advance Find some actual manufacturer's recommendations with algorithm for these NiMh cells. dV/dt charge termination does not work if the current is too low (typically trickle charging is not recommended) dT/dt charge termination doesn't work if the current is too low (typically trickle charging is not recommended) You really need to look at cell voltage *and* temperature and control current at least to one of several levels (pre-charge, fast charge, and top-up) to reliably handle these things. If the cell voltage is too high or too low, or the temperature is too high or too low, you do different things, and if the rate-of-rise of cell temperature is high enough you can terminate fast charge.
From: Spehro Pefhany on 6 Jan 2010 10:50 On Wed, 06 Jan 2010 05:09:20 -0800, mike <spamme0(a)go.com> wrote: >Putting a constant voltage across a NiMH battery is a good way >to make smoke. Perhaps not smoke, but a nice spray of caustic electrolyte. Past a certain point there is an internal "meltdown" and the reaction continues without externally applied current, at least with some cells.
From: thorin92 on 6 Jan 2010 10:44 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
From: Phil Allison on 6 Jan 2010 21:12
"thorin92" Unfortunately, the Battery has been decided and it cannot be changed. ** A choice made in ignorance of the consequences is NOT engineering. You have painted yourself into a very small corner. 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. ** And you have already been told that is a WRONG assumption. I would like the Battery to last as long as possible, and a trickle charge at 4.3mA is acceptable. ** You have no idea what you are crapping on about. Is a 60 to 80 hour charge time really OK ???? Despite the years of experience I am obviously missing, I do want a simple system, regardless. ** You are starting to sound like an arrogant fool stamping its foot. You must in software design - right ? 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. ** But have not the tiniest clue what the issues are. My basic question at this point therefore, is; will the 317 reg current limited to 4.3mA work? even if it takes 24 hours? ** It could take up to 60 hours at that rate and be impossible tell what state of charge the battery is in. If the pack has dropped to 4.5 volts under load - then it IS fully discharged. So you only need to TIME the recharge. Need help to design a 4 hour timer ?? ..... Phil |