Battery Charger Design
in [Design]
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From: thorin92 on 5 Jan 2010 09:27 On Jan 5, 1:49 pm, mike <spam...(a)go.com> wrote: > thorin92 wrote: > > On Jan 5, 1:12 pm, mike <spam...(a)go.com> wrote: > >> thorin92 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/.... > >>> 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? > >> NO unless you comply EXACTLY with the conditions of that number in the > >> spec. Not likely. > > >>> Thanks in advance > >> To design a battery charger, you have to disclose a LOT about > >> how the battery is used. Simplest thing is to charge it at 4.2ma. > >> Spec at the link says it will last 6-years being abused like that. > >> But that may not meet your cycle requirements, that you haven't stated.. > > > Hi Mike, thanks for the reply. > > > The battery will be discharged at around 50mA over 3 hours, then re- > > charged as fast as possible. Its for a portable device running a load > > of LEDs (like a bike light) > > > A long life needs to be maintained because the battery will be > > permanently attached to the pcb. > > > Does that inspire any more help? Cheers :) > > Often, the most difficult part of a project is writing the spec. > Words like "as fast as possible" will get you loads of input that > may be VERY bad tradeoffs. > > Still insufficient information, but... > Unless you have some SERIOUS size/weight constraints, your battery > is probably too small. > > What's your definition of "discharged"? > If you mean voltage cutoff at a level that guarantees none of the 5 > internal cells gets reversed, you have a chance of success. > If you mean run it 'till the light goes dim, your battery will NOT have > long life. > > Charging is problematic. Depending on the NUMBER associated with > "as fast as possible", you may have no option but to use a smart battery > charger chip. > > I'm assuming this is a production device that requires agency approvals > and you care about customer retention. If it's an advertising giveaway > device, put lithium coin cells in it and ship it. > > Replaceable AAA-cells that the customer has to recharge externally > will save you a LOT of grief. Customers cuss the internal battery > when the light goes dim at midnight when they're half-way home. The battery is for a research project that involves small plastic glowing cubes, so the size/weight constraint is beyond AAA batteries. By "fast", I'm only referring to a sensible charge time that would be convenient. e.g. 8 hours is acceptable. The cubes only need to operate for around 3 hours between charges, and this is an AVR uC application so the system will switch off when the voltage drops below 4.5v. Therefore the battery would not be fully discharged (I assume). The charger chip approach sounds the most plausible, what would you recommend? Thanks
From: John Fields on 5 Jan 2010 11:58 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 --- That site's don for maintenance. What's the Varta part number? JF
From: thorin92 on 5 Jan 2010 12:24 On Jan 5, 4:58 pm, John Fields <jfie...(a)austininstruments.com> wrote: > On Tue, 5 Jan 2010 04:34:21 -0800 (PST), thorin92 <thori...(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/... > > --- > That site's don for maintenance. > > What's the Varta part number? > > JF The datasheet is here: http://www.farnell.com/datasheets/20247.pdf cheers
From: ehsjr on 5 Jan 2010 15:24 John Fields wrote: > 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 > > > --- > That site's don for maintenance. > > What's the Varta part number? > > JF Manufacturer Part No: 55615605940
From: mike on 5 Jan 2010 20:59
thorin92 wrote: > On Jan 5, 1:49 pm, mike <spam...(a)go.com> wrote: >> thorin92 wrote: >>> On Jan 5, 1:12 pm, mike <spam...(a)go.com> wrote: >>>> thorin92 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/... >>>>> 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? >>>> NO unless you comply EXACTLY with the conditions of that number in the >>>> spec. Not likely. >>>>> Thanks in advance >>>> To design a battery charger, you have to disclose a LOT about >>>> how the battery is used. Simplest thing is to charge it at 4.2ma. >>>> Spec at the link says it will last 6-years being abused like that. >>>> But that may not meet your cycle requirements, that you haven't stated. >>> Hi Mike, thanks for the reply. >>> The battery will be discharged at around 50mA over 3 hours, then re- >>> charged as fast as possible. Its for a portable device running a load >>> of LEDs (like a bike light) >>> A long life needs to be maintained because the battery will be >>> permanently attached to the pcb. >>> Does that inspire any more help? Cheers :) >> Often, the most difficult part of a project is writing the spec. >> Words like "as fast as possible" will get you loads of input that >> may be VERY bad tradeoffs. >> >> Still insufficient information, but... >> Unless you have some SERIOUS size/weight constraints, your battery >> is probably too small. >> >> What's your definition of "discharged"? >> If you mean voltage cutoff at a level that guarantees none of the 5 >> internal cells gets reversed, you have a chance of success. >> If you mean run it 'till the light goes dim, your battery will NOT have >> long life. >> >> Charging is problematic. Depending on the NUMBER associated with >> "as fast as possible", you may have no option but to use a smart battery >> charger chip. >> >> I'm assuming this is a production device that requires agency approvals >> and you care about customer retention. If it's an advertising giveaway >> device, put lithium coin cells in it and ship it. >> >> Replaceable AAA-cells that the customer has to recharge externally >> will save you a LOT of grief. Customers cuss the internal battery >> when the light goes dim at midnight when they're half-way home. > > The battery is for a research project that involves small plastic > glowing cubes, so the size/weight constraint is beyond AAA batteries. > > By "fast", I'm only referring to a sensible charge time that would be > convenient. e.g. 8 hours is acceptable. > > The cubes only need to operate for around 3 hours between charges, and > this is an AVR uC application so the system will switch off when the > voltage drops below 4.5v. Therefore the battery would not be fully > discharged (I assume). > > The charger chip approach sounds the most plausible, what would you > recommend? > > Thanks One thing to ask is, "how long does the research project last?" Doesn't make sense to go to a lot of trouble if it only has to last for a few months and a small number of recharges. Just charge it at C/10 and use a timer so it doesn't cook too badly. You can use the AVR to implement the timer internally. There are some considerations you may not have included in your analysis. I'm a PIC guy, so I can't speak for AVR, but assume similar issues apply. Have you considered that the maximum voltage on the battery during charge may exceed the supply voltage your AVR can tolerate? Don't forget to include the increased internal resistance of an aging cell. 4.5V is 0.9V/cell. That's below what's normally done, but not extreme. BUT, for such tiny cells at high discharge rates, it doesn't take a lot of imbalance to create problems. 4 cells at 1.2V can reverse the fifth one at 4.5V. It's not a matter if IF the cells become imbalanced, just WHEN. Depends on your actual discharge profiles, but for a timed charge, you might set a flag if the 4.5V shutdown was triggered and apply a longer equalizing charge for the next charge cycle. If you open up one of the NiMH 9V batteries, you find this type of cell construction inside. I've never had a good experience with rechargeable 9V batteries. They always leak or short. I don't have any experience fast charging such tiny cells, but you might look into 0deltaV charging. You have a processor, use that to monitor the charge. There are A/D converter resolution issues. You're probably gonna have to turn off the charge current to measure the volts. |