Solar powered battery charger for Nickel batteries
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From: krw on 27 Jun 2010 09:51 On Sat, 26 Jun 2010 22:41:19 -0700 (PDT), dagmargoodboat(a)yahoo.com wrote: <snip> >They say they can make an NiMH pack last the life of a car. No problem. They'll be expensive enough so when they're dead, so is the car. Kinda like a Chrysler transmission.
From: Payala on 28 Jun 2010 12:18 On Jun 26, 1:11 am, Raveninghorde <raveninghorde(a)invalid> wrote: > I am used to charging NiCd or NiMH cells however I haven't done it > from a solar panel before. I'm happy I can hold the solar panel near > the point of peak efficiency but I am concerned about charge > termination due to the variable power available from solar panels. > > Normally for NiCd I would use negative delta V for termination with a > back up timer. For NiMH I would use a thermistor for delta T > termination. However I can see potential problems with solar as the > power source. > > In principle I could lay out a 60W panel in Arizona and charge the > batteries without problems - until someone parks a truck and shades > the panel dropping the charge current. > > So what is the best way to terminate charge on nickel based batteries > with a variable power source? Hi, I have designed a solar battery charger for an autonomous system much like the one you describe. It uses NiCad batteries. The batteries used aren't much like typical AA Nicd's, they are ranged from 45A·h to several hundred A·h's. As you have already guessed, in a photovoltaic power system, the charging is made on a "opportunity charging" scheme. You don't have control on how much power you can get at a given time, the panels output power will change drastically on a cloudy day. You can't expect to have anything like constant-current charging, so dV/dt charge termination will give you problems, since you can have negative dV/dt due to the panels decreasing its output current. AFAIK, there are three types of charge controllers, and all of them rely on final battery voltage for charge termination. - ON/OFF regulators: These have two threshold voltages, a higher one which disconnects power to the battery and a lower threshold which reconnects power. Much like a thermostat does. These are the oldest and less sofisticated ones, but you can see that power disconnection relies on battery voltage compared against these thresholds. - PWM regulators: These have a final charge voltage setting, and do a PWM on the transistors that control power from the panels to the battery. They can gradually control how much power goes to the battery. The way they work is, if the battery voltage is much lower than the final voltage setting, they let 100% of the power to the battery, as the battery voltage rises and it gets closer to the final voltage setting, the regulator lowers the power delivered to the battery. Normally, battery voltage will equal the final battery voltage value. - MPPT (Maximum Power Point Tracking) regulators: These regulators have a DC/DC converter and can adjust the voltage seen by the panel when connected in order to obtain maximum power from the panels. I can't give you much detail about these types of regulators, since I don't know them very well. But I can tell you that they are supposed to give you the full rated power from your panels, which PWM or ON/OFF regulators wont do. Normally, the batteries used in these systems are Pb based ones, mostly because they are cheaper. But if high temperature behaviour and in some high reliability applications, NiCd's are used. If you take a look at Saft's Sunica + range of batteries, you will see how these batteries are oriented for photovoltaic applications. And if you dig a little bit in the technical manual, you will see that they recommend a final voltage-based charge termination. They even recommend different voltage settings based on daily depth-of-discharge. So the short answer is; If your application looks anything like this one, use final voltage charge termination. Cheers, Pedro
From: Raveninghorde on 28 Jun 2010 12:49
On Mon, 28 Jun 2010 09:18:32 -0700 (PDT), Payala <ppayala(a)gmail.com> wrote: >On Jun 26, 1:11�am, Raveninghorde <raveninghorde(a)invalid> wrote: >> I am used to charging NiCd or NiMH cells however I haven't done it >> from a solar panel before. I'm happy I can hold the solar panel near >> the point of peak efficiency but I am concerned about charge >> termination due to the variable power available from solar panels. >> >> Normally �for NiCd I would use negative delta V for termination with a >> back up timer. For NiMH I would use a thermistor for delta T >> termination. However I can see potential problems with solar as the >> power source. >> >> In principle I could lay out a 60W panel in Arizona and charge the >> batteries without problems - until someone parks a truck and shades >> the panel dropping the charge current. >> >> So what is the best way to terminate charge on nickel based batteries >> with a variable power source? > >Hi, I have designed a solar battery charger for an autonomous system >much like the one you describe. It uses NiCad batteries. > >The batteries used aren't much like typical AA Nicd's, they are ranged >from 45A�h to several hundred A�h's. As you have already guessed, in a >photovoltaic power system, the charging is made on a "opportunity >charging" scheme. You don't have control on how much power you can get >at a given time, the panels output power will change drastically on a >cloudy day. You can't expect to have anything like constant-current >charging, so dV/dt charge termination will give you problems, since >you can have negative dV/dt due to the panels decreasing its output >current. > >AFAIK, there are three types of charge controllers, and all of them >rely on final battery voltage for charge termination. > - ON/OFF regulators: These have two threshold voltages, a higher one >which disconnects power to the battery and a lower threshold which >reconnects power. Much like a thermostat does. These are the oldest >and less sofisticated ones, but you can see that power disconnection >relies on battery voltage compared against these thresholds. > - PWM regulators: These have a final charge voltage setting, and do a >PWM on the transistors that control power from the panels to the >battery. They can gradually control how much power goes to the >battery. The way they work is, if the battery voltage is much lower >than the final voltage setting, they let 100% of the power to the >battery, as the battery voltage rises and it gets closer to the final >voltage setting, the regulator lowers the power delivered to the >battery. Normally, battery voltage will equal the final battery >voltage value. > - MPPT (Maximum Power Point Tracking) regulators: These regulators >have a DC/DC converter and can adjust the voltage seen by the panel >when connected in order to obtain maximum power from the panels. I >can't give you much detail about these types of regulators, since I >don't know them very well. But I can tell you that they are supposed >to give you the full rated power from your panels, which PWM or ON/OFF >regulators wont do. > >Normally, the batteries used in these systems are Pb based ones, >mostly because they are cheaper. But if high temperature behaviour and >in some high reliability applications, NiCd's are used. If you take a >look at Saft's Sunica + range of batteries, you will see how these >batteries are oriented for photovoltaic applications. And if you dig a >little bit in the technical manual, you will see that they recommend a >final voltage-based charge termination. They even recommend different >voltage settings based on daily depth-of-discharge. > >So the short answer is; If your application looks anything like this >one, use final voltage charge termination. > >Cheers, >Pedro Thanks. I am trying a method based on Maxim App note 484. http://www.maxim-ic.com/app-notes/index.mvp/id/484 I am going to use a comparator to give an interrupt to the PIC which will disable the buck convertor. This should keep the solar panel sitting close to it maximum power point. I like the pwm termination method of lowering the current as you get close to the maximum battery voltage. It should be possible to get over 90% charge without risking overcharging. |