Hi, I am thinking of building a battery charging pack to use with iSun Solar Charger. It will charge at least 3 AA NiMH batteries (the more the better, what ever makes sense). I will use the Energizer AA NiMH 2100mAh rechargeable batteries. Since I am an electronics/rechargeable batteries newbie, I would like to ask for some pointers on how to construct an appropriate circuit to safely charge the batteries, without damaging them or the iSun. ISuns peak output is: [email protected] or 145 [email protected] . I suspect in imperfect world I would never get that anyway. I am thinking about putting 4 AA 1.25v batteries in series and maybe putting a diode to protect against revers current flow. What diode would you recommend? How do I ensure that I have appropriate voltage across the batteries? I guess I need about 4*1.25 + (about 0.2V to maintain voltage difference when they are fully charged). That would mean I would need to ensure that the voltage never gets above 5.2V. The current: if we follow C/10 that would mean I need about 210mA. Again, I would need to ensure it does not get any higher then 210mA. Or is it too high for NiMH batteries already? How can I limit the current and voltage? What would you recommend? Is there a way of simulating the solar charging of AA NiMH batteries using software like CircuitMakter, Multisim or others? Thank you for your help, Andrew
1n4001 is cheap and will work fine. 290mA forever is not good for 2100mAh cells. However, it's not too bad, and unless you leave the cells in for weeks in full desert sun pointing the panel directly at the sun there will not be a problem. Set the panel up to 7.6V, connect a diode in series, and you can safely charge from 1-4 cells. In theory, you might just charge it in one day in the desert if you keep the panel pointed at the sun. In practice, 2 sunny days in most places, or more cloudy. Put the cells in the cool away from the cells. Don't leave the cells in for more than a few days at a time.
Use any diode capable of handleing the current, with max curent output of 290mA and 4 cells NiMh 2,1 Ah. no ekstra circutry are necessary. Steinar
Thank you for the replies. I thought I'll take this opportunity to learn something about electronic circuits. So I have read a little and found a wonderful component called LM317. I thought – it would be great if I can limit the voltage and ensure a limited constant current. Here is what I have come up with: Fig 1. Constant Current (~210mA) and regulated Voltage (~6V) ___________ | LM317 | 6 *---------*-------|IN OUT|---/\/\/\--*------*---------* | | | | | | | | | | COM | \ | | | | ----------- / 265 | | | Solar | | \ | | +| Panel | 1uF | / | 1uF |+ --- --- | | --- --- 4AA - 7.6V --- *-----------------* --- - NiMH --- | | | --- - | \ | - | | / 1k | | | | \ | | | | / | | | | | | | *---------*-------------*------------------------*---------* Fig 2. Constant Current (~210mA) only ___________ | LM317 | 6 *---------*-------|IN OUT|---/\/\/\--*------*---------* | | | | | | | | | | COM | | | | | | ----------- | | | | Solar | | | | | +| Panel | 1uF | | | 1uF |+ --- --- | | --- --- 4AA - 7.6V --- *-----------------* --- - NiMH --- | | --- - | | - | | | | | | | | | | | | | | | | *---------*--------------------------------------*---------* I also realised that putting extra circuitry, especially diode wastes the Solar panel power. Therefore I am inclined to connect the batteries in series directly to iSun. Apparently iSun already has a reverse current protection so I do not need a diode. Don't have to worry about voltage drop caused by adding a diode. It probably does not make sense to limit the current to 210mA. There is not much difference between 290mA and 210mA. As long as I limit the time the batteries are being charged it should be OK. So the remaining questions are: 1) What is the minimum and maximum voltage I can charge the 4 AA batteries with? Can I charge them with iSun set to [email protected]? Maybe 15V iSun setting and then reducing the voltage to 7V or 8V is more efficient.? If there is not enough sunlight and the solar panel drops to 10V or less the batteries will still get charged. Do I need extra circuitry to regulate/limit the voltage or can 4 AA batteries take it? 2) Can I charge 1 or 2 AA batteries connected to 7.6V/290mA ? 3) What if the batteries have some residual charge and they just need a top-up? How will I know when they are fully charged and I should take them out? 4) What if some batteries in the pack have different residual charge? Some are fully discharged and some need just a top-up, is this a problem? Thank you for your help, Andrew
I've failed at guessing what you're trying to do. Sounds like you want to plug in random cells with varying state of charge and charge them in the sun?? You don't have much choice but to use constant current if you want to keep it simple. First thing to do is to plot the current vs voltage for the solar panel at varying light levels. You may discover that under normal circumstances the current output is way less than you expect. When I tried that experiment, I just gave up cause there wasn't enough output from my panel to charge anything unless it was noon in June. The slightest angle to incident made a dramatic difference in output. There's no substitute for nor better learning tool than real measured data. A light bulb is a PTC resistor that can be used to advantage to limit charging current to something the cells can stand. Another low-tech solution is to use a lead-acid battery charged from the sun. Now you have a pretty constant source of juice to charge the NiMH any way you want. Clamp the lead battery at 14V (or 7V) so it can't be overcharged. Then you can use a commercial fast charger that will optimally charge and protect your NiMH. Or you can build one similar. A charge pump is another interesting method to transfer charge from one battery to another. If you want efficiency, your problems multiply. mike -- Bunch of stuff For Sale and Wanted at the link below. Toshiba & Compaq LiIon Batteries, Test Equipment Honda CB-125S TEK Sampling Sweep Plugin and RM564 Tek 2465 $800, ham radio, 30pS pulser Tektronix Concept Books, spot welding head... http://www.geocities.com/SiliconValley/Monitor/4710/
15v or 7.6v is your peak output V. Much of the time you wont get even close to that out. So if you charge a 6v pack on a peak 7.6v solar panel you'll get no charge at all for much of the time. Using the 15v output will get things charging, and also provide current limiting. The batteries and solar cell combination will limit the voltage already. Solar cells have significant source resistance, and batteries have a zener like characteristic. So that already covered. And the solar cells will also limit the current, theres only so much they can put out. right ok. Diode already built in. It would be wise to look at the charge current requirement for your cells, make sure youre not going to nuke them or blow them up, or yourself. minimum is going to be a bit above their nominal voltage. Theres no maximum since your source is i limited. right, no reg needed tho. yes if they can take the charge current, if not no. Its better to charge battery packs as a complete pack though, to avoid mixing chagred and half charged cells. Yes: Dont divide the pack. If your appliance uses 4 AA cells, charge that 4 cells as one unit. Regards, NT
Hi, Just two weeks back I made a NiCad charger (AA 1.25v 650mAh x 2) with a LM317, like what Andrew has drawn. But there was a small difference. I had a diode in series in the LM317 o/p (otherwise battery dischages). Then I though diode is no good for a charger, cause its I/V characteristics. So I put 50 Ohm resistor also in sereis. I had two batteries in series. These are my observations: 1. Batteries had starting voltage of .7V each. 2. I was pumping around 65mA (I was monitoring the current and varing the LM317 o/p voltage so I get this current all the time) 3. If I don't increase the o/p voltage of LM317, charging current drops below 10mA. 4. Within minutes battery votage increased upto 1.4v each (2.8 both) and it stayed there (reason for 3). So I figured this is some sort of stable stage for about 10hour charge. But with this cct configuration, when starting up from very low battery voltages, it would take lot of current which might damage the batteries. That's why I thought I should put a little R in series so it gives desired I/V characteristics (for larger V change small change in I) this is actually the opposite of a diode I/V and can be easily obtain with high R. but then U run into the problem high R means U desipate lot of W in the R itself and Ur i/p should be kept high. Harshana
I goggled "Charge pump" and found many varation. I am interesting in fast recharging my lead acid lawn mower battery from an external battery safty. My current plan is to use small wire size to limit the current. I also have a stand-by AC power system, using an inverter, for my home and plans to connect the deep cycle battery to my car battery for recharging, using the same small wire. http://www.billcotton.com/standby_ac_home_power.htm Where can I get more information about the charge pump you mention?
You just need a couple of switches and diodes. Charge a cap from the source battery and dump it into the load battery. Probably need an inductor to limit peak current. C, delta-V and rep rate define the average current. It's not a lot different from a standard switching supply. mike -- Bunch of stuff For Sale and Wanted at the link below. Toshiba & Compaq LiIon Batteries, Test Equipment Honda CB-125S TEK Sampling Sweep Plugin and RM564 Tek 2465 $800, ham radio, 30pS pulser Tektronix Concept Books, spot welding head... http://www.geocities.com/SiliconValley/Monitor/4710/
Not to seem too impertinent, Andrew, but is there a problem with using the product as designed by the manufacturer? http://www.icpglobal.com/html/battpak.asp
for $20 I bought a NiCd/NiMH charger from Walmart, that included 4 AA NiMH batteries. It has a 12vdc as well as a 120vac interface, so I just plugged the 12vdc interface into my 15 watt solar panel with a ups battery as a buffer. -- Steve Spence Renewable energy and sustainable living http://www.green-trust.org Donate $30 or more to Green Trust, and receive a copy of Joshua Tickell's "From the Fryer to the Fuel Tank", the premier documentary of biodiesel and vegetable oil powered diesels.
