Battery Charger Design
in [Design]
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From: JosephKK on 8 Jan 2010 18:24 On Wed, 06 Jan 2010 07:15:22 -0800, mike <spamme0(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... I kind of have to agree. The very poor assumption that you can get a full 50 mA for 3 hours out of a 150 mAh cell scares me. If you want more than a few cycles of battery life 200 mAh or 250 mAh is a better choice.
From: thorin92 on 12 Jan 2010 01:45 Hi Ed, thanks for the schematic. This looks appears to be a good solution and I'll be prototyping this. I contacted VARTA directly and their recommendation is exactly the same scheme as originally proposed, using a regulator with constant current, but with a timer. Thanks for the help here.
From: thorin92 on 12 Jan 2010 01:50 Sorry, when I mentioned 50mA for 3 Hours this is the worst case. The actual circuit nominally draws around 20mA. When I tested the Battery lasted for around 5 hours.
From: ehsjr on 12 Jan 2010 21:37
thorin92 wrote: > Hi Ed, thanks for the schematic. This looks appears to be a good > solution and I'll be prototyping this. I contacted VARTA directly and > their recommendation is exactly the same scheme as originally > proposed, using a regulator with constant current, but with a timer. > Thanks for the help here. You're welcome. You might want to consider adding an LED that glows when the TL431 steals current, to let you know that the charge is complete. Ed |