Building AA NiMH solar charger to use with iSun

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
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