How to charge 3.6VDC battery with DC Power Supply
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From: Robert Roland on 2 Mar 2010 16:40 On Tue, 02 Mar 2010 08:52:24 -0800, Tim Wescott <tim(a)seemywebsite.now> wrote: >Could be three Ni-Cd in series, though. It never even struck my mind. Thanks. -- RoRo
From: Robert Roland on 2 Mar 2010 17:05 On Tue, 2 Mar 2010 11:45:05 -0800 (PST), jg <juangarcia(a)sacbeemail.com> wrote: >They are NiMH. In that case, forget everything I said in my earlier post. >So would it be OK to push to 4.2 vdc and set max current to something >like 200mA? I would prefer formulas rather than a straight answer. >(Teach me how to fish.) NiMH (and also NiCd) cannot be charged based on absolute voltage. Their voltage is not predictable enough, and also changes very little as the charge level changes. There are several ways to charge NiMH batteries. The simplest one is to charge them with a constant current for a specific amount of time. You can only use this method if you know the charge level before you start charging, which, in practice, means you must fully discharge the battery before you start charging it. If you cannot find the data sheet, a safe bet is to charge at C/10 (a current corresponding to one tenth of the battery's capacity, 250mA in you case) for 14 hours. New batteries should be overcharged considerably the first time in order to "activate" all of the electrolyte. 24 hours is usually recommended. You can configure your lab power supply to be a constant current charger: Set the voltage higher than the battery voltage will ever reach, but not so high that you make big sparks when you connect the leads. I'd say 10-12V is OK. Then set the current limit to 250mA and charge for the specified amount of time. At this charge rate, you won't ruin the battery if you leave it connected for a couple of days, but overcharging will slowly kill the battery even at this rate. If you want to charge partially discharged batteries, you need a more intelligent charger. As Tim says, watch the voltage while you charge, and when the voltage stops rising, you're done. This method relies on the internal temperature of the cells to change, so it will not work reliably if you charge very slowly. -- RoRo
From: Tim Wescott on 2 Mar 2010 17:08 jg wrote: >> --------------------- >> The cell size is 4/5A, about 1500 to 2000 milliAmphour >> Standard charge is 1/10 capacity (150mA) for 14 hours. >> It's more than 10 hours because charging efficiency is not 100 percent. >> >> Do not let the cells get over 40 degrees C on charge. Heat is not good to >> NiMH >> You can charge faster as long as the cells don't overheat. >> Faster charge profiles require more careful design, with thermal safetys. > > Thanx, what about voltage? More than 3.6v right? More than enough to keep the battery charging at the intended current. I can't recall the exact number for a NiMH cell, but you want significantly more than 1.25V/cell -- 1.5V/cell is probably the bare minimum. Set the voltage to 12V and let the current limit do it's job, maybe. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: jg on 2 Mar 2010 17:17 On Mar 2, 2:05 pm, Robert Roland <f...(a)ddress.no> wrote: > On Tue, 2 Mar 2010 11:45:05 -0800 (PST), jg > > <juangar...(a)sacbeemail.com> wrote: > >They are NiMH. > > In that case, forget everything I said in my earlier post. > > >So would it be OK to push to 4.2 vdc and set max current to something > >like 200mA? I would prefer formulas rather than a straight answer. > >(Teach me how to fish.) > > NiMH (and also NiCd) cannot be charged based on absolute voltage. > Their voltage is not predictable enough, and also changes very little > as the charge level changes. > > There are several ways to charge NiMH batteries. The simplest one is > to charge them with a constant current for a specific amount of time. > You can only use this method if you know the charge level before you > start charging, which, in practice, means you must fully discharge the > battery before you start charging it. If you cannot find the data > sheet, a safe bet is to charge at C/10 (a current corresponding to one > tenth of the battery's capacity, 250mA in you case) for 14 hours. New > batteries should be overcharged considerably the first time in order > to "activate" all of the electrolyte. 24 hours is usually recommended. > > You can configure your lab power supply to be a constant current > charger: Set the voltage higher than the battery voltage will ever > reach, but not so high that you make big sparks when you connect the > leads. I'd say 10-12V is OK. Then set the current limit to 250mA and > charge for the specified amount of time. At this charge rate, you > won't ruin the battery if you leave it connected for a couple of days, > but overcharging will slowly kill the battery even at this rate. > > If you want to charge partially discharged batteries, you need a more > intelligent charger. As Tim says, watch the voltage while you charge, > and when the voltage stops rising, you're done. This method relies on > the internal temperature of the cells to change, so it will not work > reliably if you charge very slowly. > -- > RoRo Excellent. Thanx Robert, this is exactly the info I was looking for. I have about 10 of these. Eventually they will find their way into RC boats. Kids are going to love having the extra batteries. Thanx again.
From: Tim Wescott on 2 Mar 2010 18:42
jg wrote: > On Mar 2, 2:05 pm, Robert Roland <f...(a)ddress.no> wrote: >> On Tue, 2 Mar 2010 11:45:05 -0800 (PST), jg >> >> <juangar...(a)sacbeemail.com> wrote: >>> They are NiMH. >> In that case, forget everything I said in my earlier post. >> >>> So would it be OK to push to 4.2 vdc and set max current to something >>> like 200mA? I would prefer formulas rather than a straight answer. >>> (Teach me how to fish.) >> NiMH (and also NiCd) cannot be charged based on absolute voltage. >> Their voltage is not predictable enough, and also changes very little >> as the charge level changes. >> >> There are several ways to charge NiMH batteries. The simplest one is >> to charge them with a constant current for a specific amount of time. >> You can only use this method if you know the charge level before you >> start charging, which, in practice, means you must fully discharge the >> battery before you start charging it. If you cannot find the data >> sheet, a safe bet is to charge at C/10 (a current corresponding to one >> tenth of the battery's capacity, 250mA in you case) for 14 hours. New >> batteries should be overcharged considerably the first time in order >> to "activate" all of the electrolyte. 24 hours is usually recommended. >> >> You can configure your lab power supply to be a constant current >> charger: Set the voltage higher than the battery voltage will ever >> reach, but not so high that you make big sparks when you connect the >> leads. I'd say 10-12V is OK. Then set the current limit to 250mA and >> charge for the specified amount of time. At this charge rate, you >> won't ruin the battery if you leave it connected for a couple of days, >> but overcharging will slowly kill the battery even at this rate. >> >> If you want to charge partially discharged batteries, you need a more >> intelligent charger. As Tim says, watch the voltage while you charge, >> and when the voltage stops rising, you're done. This method relies on >> the internal temperature of the cells to change, so it will not work >> reliably if you charge very slowly. >> -- >> RoRo > > Excellent. Thanx Robert, this is exactly the info I was looking for. > I have about 10 of these. Eventually they will find their way into RC > boats. Kids are going to love having the extra batteries. > If you don't mind spending the $$ get a peak-detect charger at your local hobby shop. Or look around and see if there are any used ones on Craigs List -- there are probably still some electric fliers that are converting to LiPo and have an old NiCd only charger around, or who are upgrading. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com |