help with smart nimh solar charger

Skyko

Hi guys, newbie here.

My background is EE and I have quite a bit of experience designing circuit boards and working with microcontrollers. I also really enjoy hiking and like my gadgets.

So... I recently purchased a 26 watt Brunton CIGS foldable panel and a 14 watt Powerfilm rollable panel. I got both because I wanted to compare peformance and useability while out in the wild. I also have a 60 watt monocrystaline rigid panel for test comparisions. I live in an area with poor insolation (Seattle) but like to hike in the southwest too.

For hikes of about 2 weeks I am liking the Powerfilm rollable panel. This thing is pretty cool...marine grade and a very cool demonstration video of it getting shot 7 or 8 times with a 9mm handgun and still outputting 98% of capacity! Definately a rugged panel weighing about an ounce a watt. This seems a reasonable tradeoff vs taking a ton of extra batteries since the batteries themselves are about an ounce and produce maybe 2 watts. 2 hours of full sun and the panel has paid for itself in weight (in hiking everything is expensive in cost so that is not really a consideration).

Ok, so now I want a super light super compact super reliable way to charge several arangements of batteries in various insolation regions and conditions. If I use NimH, I want a charger that deals correctly with this type of battery while performing at maximum efficiency for the various condtions. Unfortunately, this product does not seem to exist. I am therefore considering building my own based around an Atmel microcontroller running in ultra lowpower mode with an internal RC oscillator. Everything small and surface mount. I am trying to consider the various options to put on the board to handle all of my present and future needs. I would like suggestions. This will probably just be a one-off product, but I may do a small run of boards if there is interest (not sure how many of you guys are backpackers).

Here are some things I want to consider:

1) I want the charger to use whatever available power from the panel to the best efficiency. If I am getting a full 15.5 volts at 900mA on the 14 watt panel, I may want to fast charge 2 AA batteries at 1C (say, 2600mA) so I will need a DC-DC converter (or several) to boost my current. I can see this situation best as hiking along in mostly shade but stopping for an extended lunch and spreading the panel out in the sun for an hour. I want that hour to be productive.

2) I want to sometimes wear the panel draped across my backpack (it fits perfectly and fastens on easily with grommets. In this situation I may get anywhere from 0 to 14 watts depending on groundcover and orientation to the sun. The charger needs to deal with this and keep on charging at maximum efficiency. It will need a small super capacitor onboard to keep the Atmel microcontroller alive during full shade, but this is not a huge issue as the internal RC mode consumes uAmps at 3 volts. I would not want the microcontroller to reset constantly because I want to perform timing and power data logging with it.

3) I want to detect when the cells are charged to peak capacity. I may use some combination of dV drop detection, temp rise, and possibly monitor the total charge over time supplied to the batteries. I may run out of A/D ports 8)

4) I would like to power any USB device from the charger, including higher current draw devices like Kindle and 3GS phones that sometimes will not charge or operate off of the AA battery units available in stores. Why they limit the output current to 500mA (which is the USB standard, but...) is beyond me. The Kindle can take up to an amp at 5V and I would like to provide this.

This is just a first step but you can see I have been thinking about things. Mostly I spent several frustrating hours searching the web for a store bought device because I never want to re-invent the wheel. If there is something that meets 90% of what I have outlined and will work with a variety of panels from small to large, please please let me know before I start building.

Thanks.

BB.

Re: help with smart nimh solar charger

Skyko wrote: »

...marine grade and a very cool demonstration video of it getting shot 7 or 8 times with a 9mm handgun and still outputting 98% of capacity! Definately a rugged panel weighing about an ounce a watt.

Note to self, need bigger gun.

Ok, so now I want a super light super compact super reliable way to charge several arangements of batteries in various insolation regions and conditions. If I use NimH, I want a charger that deals correctly with this type of battery while performing at maximum efficiency for the various condtions. Unfortunately, this product does not seem to exist.

From what I have seen (no expert)--I agree. Nothing exists that meets your criteria (very reasonable ones at that).

1) I want the charger to use whatever available power from the panel to the best efficiency. If I am getting a full 15.5 volts at 900mA on the 14 watt panel, I may want to fast charge 2 AA batteries at 1C (say, 2600mA) so I will need a DC-DC converter (or several) to boost my current. I can see this situation best as hiking along in mostly shade but stopping for an extended lunch and spreading the panel out in the sun for an hour. I want that hour to be productive.

The ideal method would be to create a true MPPT (maximum power point tracking) controller... However, that is probably not easy.

So, the second method would be to make some assumptions... The first assumption is that Vmp of the solar array is, more or less, based on temperature. You could either assumed a fixed minimum Vmp (hot panel)--or take temperature into account). Vmp is not very dependent on light levels. Imp is, however, pretty much directly porportional to sunlight levels. There is not much use in trying to collect power with low light and low "Vmp" as Imp will be near zero anyway.

There is the issue of shadows--but you would have to check if the panel have any by-pass diodes. A panel that small may not. Try blocking 1/4 of the panel and attempt to find Vmp/Imp (or check Voc and Isc) to see if there are bypass diodes. In either case, it would be better to keep the panel in full sun (no shade) and just assume a fixed Vmp.

2) I want to sometimes wear the panel draped across my backpack (it fits perfectly and fastens on easily with grommets. In this situation I may get anywhere from 0 to 14 watts depending on groundcover and orientation to the sun. The charger needs to deal with this and keep on charging at maximum efficiency. It will need a small super capacitor onboard to keep the Atmel microcontroller alive during full shade, but this is not a huge issue as the internal RC mode consumes uAmps at 3 volts. I would not want the microcontroller to reset constantly because I want to perform timing and power data logging with it.

I would try a super cap. and a down converter circuit that would turn on anytime the super cap is > Vmp (or whatever voltage you pick). The down converter would pulse on and off and efficiently down convert to your intermediate voltage (another super cap or even a rechargeable battery bank that could power your micro processor and supply power for recharging).

Reason is that solar power is variable and you want to capture it any time you can. When you need to recharge your batteries, you will probably want to do that quickly (sun or no sun) and you would drain the intermediate storage bank to quick charge the portable batteries.

This would also allow you to operate your portable devices at any time.

3) I want to detect when the cells are charged to peak capacity. I may use some combination of dV drop detection, temp rise, and possibly monitor the total charge over time supplied to the batteries. I may run out of A/D ports 8)

dV drop pretty much uses a constant current that causes the batteries to drop voltage (and heat up) when "full". Doing this from variable sun power is going to make it very difficult to accertain charging cut off when performing fast charge on NiMH (C/2, C/1 etc.). Another reason for an intermediate battery bank in the pack--provide consistent energy for fast charging of batteries.

4) I would like to power any USB device from the charger, including higher current draw devices like Kindle and 3GS phones that sometimes will not charge or operate off of the AA battery units available in stores. Why they limit the output current to 500mA (which is the USB standard, but...) is beyond me. The Kindle can take up to an amp at 5V and I would like to provide this.

See above.

This is just a first step but you can see I have been thinking about things. Mostly I spent several frustrating hours searching the web for a store bought device because I never want to re-invent the wheel. If there is something that meets 90% of what I have outlined and will work with a variety of panels from small to large, please please let me know before I start building.

You can see I have been thinking about this too...

My thoughts have been leaning towards a, 12 volt for example, solar panel that would charge 5-6 NiMH cells at a C/10 rate (or less, due to variable sun). At C/10, you don't need temp control or dV sensing for charge termination. Also, the panel Vmp*Imp power transfer is fairly efficient if you have the "right number" of batteries in series. Also, while it is not a great idea, you can mix charged and uncharged batteries in a string.

Or, because solar panels are current limited, you could come up with a battery holder/switching array that would charge 1-6 batteries at C/10 rate (no need for voltage protection)... Although, efficiency would be terrible. (you could charge 6 batteries in the same time as 1).

The intermediate bank could either be simply used to now charge other devices (including fast charge). Or, the bank could be your batteries themselves. Say Vmp = 14 volts, use 6x AA (~12 volts charging)... And when you need a set, you pull them out and put in the depleted ones from your other devices.

The problem with using an intermediate battery bank is that small NiMH/NiCad, etc. are not very efficient batteries--they tend to be something like 60% charge efficient or so... So, every conversion/intermediate step tends to waste a lot of power.

The ideal of creating a 14 watt MPPT type charge controller for portable battery charging is an interesting project...

There are a few folks here that design MPPT solar charge controllers and they may be able to direct you in the proper direction if that is your plan.

-Bill

mike95490

Re: help with smart nimh solar charger

My thoughts - have the PV charge a "super cap" and then have that 12V source power a small 1-4 cell charger, something like the MAHA MH-C401FS-DC, that has high or low charge rates, and 4 independent channels, so each battery gets a full charge. Ir just get a $5 battery holder, wire them up to the PV and hope for the best.

Kamala

Re: help with smart nimh solar charger

What is the weight of a "super cap?"

niel

Re: help with smart nimh solar charger

the option of going with the maha charger may work, but pv voltages may in fact go too high for the input of the charger so i'd recommend some means of regulation to keep it from going too high before powering the charger from pv. yes, it has an auto adapter, but automotive alternators are also regulated and are not over 15v output like pvs are. something like a sunsaver pwm controller of the proper current capacity may be used in such a feed-through regulatory capacity if one lacks the ability to design a small regulator circuit.
ps-just what will the super cap supposedly do for this?

mike95490

Re: help with smart nimh solar charger

niel wrote: »

....ps-just what will the super cap supposedly do for this?

I was thinking that if while hiking along, and a brief shadow occurs, you would not zero volt the charger, and reset the charge cycle.

Super caps can be had about the size and weight of a D battery
2 ea, in series DK-6R3D105T $6ea = 13V, .5F

A 15V zener diode could be added to clamp the panel voltage to no more than 15V.
1N5352 might be the ticket, at $0.40

niel

Re: help with smart nimh solar charger

the cap can be electrolytic, but in any case most of the energy may go into charging up the big caps. i can see a small amount (edited to insert the words 'of capacitance') to smooth it though.
as to the zener you'll need more than just a zener to make it work.

Kamala

Re: help with smart nimh solar charger

I assumed a super cap was just a big cap, probably electrolytic.

Wrong! http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

Didn't know these things existed. From what I can tell they store about one tenth as much energy as a lead acid battery per unit mass, but charge much more quickly, don't require different charge stages and have a much greater life span.

niel

Re: help with smart nimh solar charger

Kamala wrote: »

I assumed a super cap was just a big cap, probably electrolytic.

Wrong! http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

Didn't know these things existed. From what I can tell they store about one tenth as much energy as a lead acid battery per unit mass, but charge much more quickly, don't require different charge stages and have a much greater life span.

unfortunately, the idea was to charge nimh batteries and not a super cap as a replacement for them. in any charging circuit you would find a huge surge of current due to a large capacitance and in thwarting that surge it would prolong the time needed to charge the batteries because the caps had to charge first.
also note that they don't store power longer than the nimh batteries do as your comparison is to standard electrolytic caps which i believe a small value of the standard electrolytics would be fine to smooth it out some. nothing, not even super caps, will smooth it out from a prolonged lapse of sun.
one day they may make those caps well enough to act as batteries, but in the meantime he wishes to charge nimh batteries.

Skyko

Re: help with smart nimh solar charger

Yesterday I scored a brand new Brunton I6 foldable CIGS solar panel (6 watt) at REI for $24! They apparently have clearanced the I6 version of the 6 watt foldable Solaris panel but the only difference between it and the normal $100 6 watt panel they sell is the I6 also includes a cable to charge an Ipod. Fine by me!

Anyway, today I took the little 6 watt panel on a test hike at Iron Goat trail near Stevens Pass, WA. There was actually a bit of hazy sun around 2pm and I carried the panel draped over the back of my daypack with a 100 ohm resistive load hooked directly to it. I monitored the voltage across this load with my Fluke dvm while hiking. With my back to the crappy Seattle sun, I actually measured 13 volts into the 100ohm load at around 2pm which is around 1.7 watts (P = (V^2)/R

The interesting thing is I measured almost 800mW with my back 90 degrees to the sun and amazingly 300mW when hiking *toward* the sun (so the panel was facing away from the sun!) The CIGS cells must do a decent job of gathering the reflected light off of granite and leaves.

On another note, I took apart a Belkin DC car adapter from an older cellphone and found a nice DC-DC converter inside (12-24V input, 5V 700mA output). The circuit is really simple and is based around a 8 pin dip chip MC34063. The data sheet is here http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF

It is actually capable of 12-40 volt input at up to 1.5 amps output with a output voltage selectable by the ratio of two resistors. In single quantity the chip is only $0.70 at digikey. I am going to modify the resistor ratio of this belkin converter first to give about 3.6V at 700mA from a 12 to 40 volt input and make a simple charger for 2 AAA. The data sheet claims a 86% efficiency operation so if I get enough wimpy seattle sun in the spring/summer to get 15V at 200mA (about half of the panel max output) I should be able to charge two 900mA AAA batteries in about 2 hours.

I think I am going to move ahead with a custom made microprocessor controlled charger but in stages. I want to do some experimentation with these chips and also build a simple data logging circuit to measure instantaneous and average power generated over a day long hike in various conditions.

Skyko

Re: help with smart nimh solar charger

I had already planned on using a small super capacitor (say a 1F 5.5V panasonic gold cap or similar) to keep the microcontroller alive during shadeouts. I don't believe a capacitor is really a viable storage medium for anything much more than instantaneous large current sourcing or powering micropower circuits (such as the atmel running on internal RC oscillator).

You can see this by the simple formula for voltage/current in a capacitor:

Current (I) = Capacitance (C) * Derivative of Voltage with respect to time (dV/dt)

I = C*dV/dt, or dV/dt = I/C

For a 1F cap being drained at 1 amp, the voltage will change at 1V per second. Thus your 1F cap will be totally flat after 5 seconds (if you could pull a constant 1Amp out of it down to zero volts). So not a lot of storage in a part that is 1/2 the size of a AAA NiMh battery (which has 900mAH capacity and could supply 1V for 1800 seconds or more at 1A.)

But at the 30uA draw of a microcontroller operating in the khz frequency:

dV/dt = 0.000030/1 = 30uV per second voltage drop for a 30uA draw. In this case the 1F cap would have lost 1V after about 9 HOURS.

There could be a case made for using the cap to build up a pulse current charge if this would be beneficial for the NiMh charging cycle. I am not totally up on the chemistry of the NiMh cells, but if they don't charge at all at say 20mA input current but would charge at 200mA, you could charge up a cap and then dump the charge at 200mA into the battery every 10 seconds. In this case a very low output system could still eventually charge a battery...maybe. Would a 200mA current at a 10% duty cycle charge a NiMh better than a constant 20mA? This would take some experimentation.

BB.

Re: help with smart nimh solar charger

In general, I am not a fan of "pulse" charging if efficiency is important.

You end up with high I^2 * R losses... (a 10% duty cycle plus charger would have 100x the I squared R heating losses). Wiring, electronics, etc. have to sized to handle these current pulses.

Of course, if R is very small (which it is in NiMH/NiCAD batteries)--a 100x a small value is still a smallish value.

-Bill

Skyko

Re: help with smart nimh solar charger

I take it you mean "pulse" charging.

I only mentioned this because I came across some reference that indicated that charging NiMh cells at very small currents (say 1/50 C) could be bad for the cells. I was just postulating that if you instead charged them at 1/5 C for a 10% duty cycle it might be better for cell lifetime. I have no idea if this is even true or if it even matters if you charge at very low currents.

BB.

Re: help with smart nimh solar charger

Errr, yea "pulse" charging.

When you you dig into the details of charging batteries--I am sometimes surprised that batteries work as well as they do.

-Bill

System2

Re: help with smart nimh solar charger

Interesting and timely discussion, I was just wondering about building something like this myself.

Skyko, would something like this (http://www.batteryspace.com/multi-currentuniversalsmartchargerfor96v-18vnimhbatterypackscigarettemaleplugullisted.aspx) meet your needs?
Not particularly light though (1lb).

You can enable the USB out put with a DC regulator from here: http://dimensionengineering.com/ They have fixed 5v/1A regulators or you can get the SWADJ3 that is adjustable and has a 3A capacity.

I'm looking into this for building a mini timelapse camera system for a research project. I'm looking for a charge controller that can be wired into a NiMH battery pack that will then run a canon powershot camera.

I'm planning to power the system with a 10W/12v panel that would slow-charging a small battery pack. Not sure the battery pack voltage yet. Probably either 3.4v for the camera or 12v that I'd then step down to 3.4v downstream of the batteries.

My first thought (other than using the commercial charger linked above) is to use the SWADJ3 from DE (http://dimensionengineering.com/DE-SWADJ3.htm) and set the output voltage to be around the max voltage of the batteries. My thinking is that once the batteries reach their peak voltage, the input voltage won't be high enough to charge them any further so they won't over charged. I assume I'm missing some steps since I'm a newbie to how charge controllers and battery chargers work. Does anyone out there understand the subtlties of battery charger function enough to explain whether or not this would work and if not, what factors need to be considered to make something that does work?

Thanks!

Tim


FYI, these guys also make DC DC converters and various battery chargers:
http://www.powerstream.com/

niel

Re: help with smart nimh solar charger

intersection,
thanks for the link as i see some converters that would fill some needs in the area of security cameras. they often need a stable 12v and 12v solar systems and their batteries tend to go higher and lower than 12v.
this particular brick may handle several cameras for those using a 12v solar system as it is a 20w 12v dc to dc converter,
PST-SU20-12-12S

Photowhit

Re: help with smart nimh solar charger

On time lapse photography;

You might consider a camera with built in timelapse feature, The Canon 'G' line through G9 do timelapse movies, so you just need to keep power to the camera.

I think the early Sony 'S' line does time lapse or schedualed photos, again so you just need to supply power to the camera. I think I had an S45? not sure now.

I've done some long distance cycling with solar, I didn't have the weight restrictions of hiking but I did a couple things, and considered others for future trips.

I charged 2 - SLA 6 volt batteries in series and discharge (mostly) in 6 volts, I did this with different connectors for load and charging. I used no charge controller and discharge some every night. it lasted through 8 months of cycling, running a radio, fan, light and even a small pocket TV.

I think this system might work for some of your uses between the kindle and what ever 2 AA uses. NiMH are somewhat tolerant of overcharging, This doesn't apply much to the quick charge plans you have.

I've used a 6 volt panel with a simple 5 volt, 1-2amp regulator to charge a nokia N800 which is pretty picky on the voltage range it excepts (I can't get a car phone regulator to work with it, though I haven't poped for the nokia version)

If I go out again, I'd use a 4 - 10 amp NiMH 'D; cells charged with a 6 volt 10 watt panel (or set of panels as I have a few 5 watt) if I went with a netbook I might use a 12 volt system. No regulator/controler. I might look into a lithium version if I choose to go with a regulator and CC as they require them. You might also see if the quick charge NiMH might be even more tolerant to over charging.

BB.

Re: help with smart nimh solar charger

If you are interested in digital photography on the cheap... Take a look at CHDK.

• Professional control - RAW files, bracketing, full manual control over exposure, Zebra-Mode, Live histogram, Grids, etc.
• Motion detection - Trigger exposure in response to motion, fast enough to catch lightning.
• USB remote - Simple DIY remote allows you to control your camera remotely.
• Scripting - Control CHDK and camera features using ubasic and Lua scripts. Enables time lapse, motion detection, advanced bracketing, and much more.

Basically, you can upload (open sourced) code to your Cannon Axxx, Sx, Gxx, SDxxx, IXUSxxx, SXx point and shoot cameras. Does not change the firmware--just loads from your memory card like an application. I have been using it on my Cannon A720 for a couple years now.

Very cool.

-Bill

Photowhit

Re: help with smart nimh solar charger

Looks very 'Kewl'!

To bad I'm still shooting some 30+ year old Nikon Lenses.

Have to check around for a Nikon group!

Thanks, very neat!

System2

Re: help with smart nimh solar charger

Photowhit wrote: »

On time lapse photography;

You might consider a camera with built in timelapse feature, The Canon 'G' line through G9 do timelapse movies, so you just need to keep power to the camera.

I think the early Sony 'S' line does time lapse or scheduled photos, again so you just need to supply power to the camera. I think I had an S45? not sure now.

Considering how many people do timelapse these days, it always surprises me how few cameras actually have onboard timelapse, its not like it would be hard to program into a camera's firmware. As Bill suggested, we are planning to use the great CHDK tools that have been developed to automate canon powershot cameras (http://chdk.wikia.com/wiki/CHDK).

Photowhit wrote: »

I charged 2 - SLA 6 volt batteries in series and discharge (mostly) in 6 volts, I did this with different connectors for load and charging. I used no charge controller and discharge some every night. it lasted through 8 months of cycling, running a radio, fan, light and even a small pocket TV.

So is it safe to assume that if the charging input voltage doesn't exceed the battery's max voltage than I should be ok?
--

Additional things to worry about:
(1) Do we need to make sure the input power doesn't exceed some max amperage when charging as well? (for example, if the batteries get very low and then there is a sunny day). I assume there is a limit to how fast it is safe to charge lead-acid or NiMH batteries?
(2) Over discharge - there needs to be a system in place to prevent the batteries from being too discharged. For the camera system I think this isn't an issue because the camera just stops working if the voltage gets too low but for Skyko's app this would be an issue wouldn't it?
(3) The folks at Dimension Engineering that if I'm using one of their dc-dc regulators in the place of a real charge controller I'll also need a Schottky Diode inline on the (+) side of the panel oriented in the direction of power flow (i.e. from panel to battery) to prevent backflow of power at night when it is dark. Anyone have any further insight into this?

Photowhit

Re: help with smart nimh solar charger

So is it safe to assume that if the charging input voltage doesn't exceed the battery's max voltage than I should be ok?

NO! I'm only speaking of nominal voltages, a 10watt panel designed to charge 12 volts in my original run. IN the case of a SLA the danger is of out gasing and battery damage, I guess some small chance of other problems.

Additional things to worry about:
(1) Do we need to make sure the input power doesn't exceed some max amperage when charging as well? (for example, if the batteries get very low and then there is a sunny day). I assume there is a limit to how fast it is safe to charge lead-acid or NiMH batteries?

Yes, NiMH batteries are made in several different types, some designed for quick charging, others for a "standard" charging. I've posted links at one point to a manufacturer suggesting an overcharge at a 10% of capacity (C10) rate would not destroy a battery designed for a standard charge, while it wouldn't be good for the battery it would be disapated as heat and leave the battery in reasonable good shape.

(2) Over discharge - there needs to be a system in place to prevent the batteries from being too discharged. For the camera system I think this isn't an issue because the camera just stops working if the voltage gets too low but for Skyko's app this would be an issue wouldn't it?

I don't know about the camera system, as for the batteries, in the short term I've found NiMH to take several total discharges. Biggest problem I see is unlike the SLA batteries, NiMH batteries are willing to release their energy over a very short period of time. Early after thier release I was photographing a wedding using them in vivatar 283 flash units, carrying spare batteries in my pocket (in holder so the pos and neg are on the same side.) they shoreted out against change and I was literally smoking!

(3) The folks at Dimension Engineering that if I'm using one of their dc-dc regulators in the place of a real charge controller I'll also need a Schottky Diode inline on the (+) side of the panel oriented in the direction of power flow (i.e. from panel to battery) to prevent backflow of power at night when it is dark. Anyone have any further insight into this?

Yep, a solar panel with out a charge controler will draw energy at night. w/o a charge controler, or disconnecting it at night (my guess is hiking you''ll disconnect to read or use light, whatever at night?)

Photowhit

Re: help with smart nimh solar charger

OK I thought I'd look around and couldn't find my links here, did a quick search and found a couple links;

http://www.powerstream.com/NiMH.htm

Includes language "...ld. Also, NiMH are sensitive to damage on overcharge when the charge rate is over C/10."

http://www.thomasdistributing.com/techfacts.htm

Speaking of Maha batteries states...

"Batteries can be continuously overcharged, at C/10 or lowercharge rate. This battery also has chemical built-in reverse polarity protection. "

System2

Re: help with smart nimh solar charger

I was looking around some more and found this site:
http://www.humanedgetech.com/shop/home.php
They have a lot of system aimed at solar charging AA batteries an powering low voltage devices:
Solar input enabled AA battery charger:
http://www.humanedgetech.com/shop/product.php?productid=115&cat=4&page=1

Multi-input/output battery and charge controller system:
http://www.humanedgetech.com/shop/product.php?productid=184&cat=4&page=1

System2

Re: help with smart nimh solar charger

Photowhit wrote: »

http://www.powerstream.com/NiMH.htm
http://www.thomasdistributing.com/techfacts.htm

Great links, thanks!

I just found this page that has a circuit diagram for a DIY smart charger for any type of battery. It is computer controlled but it might be a good starting point for developing a portable version.

And here's another one from the same person with a circuit diagram and build instructions for a 5v 450mA charger for a pair of NiMH batteries (designed for building a USB charger)

those links have been deleted by me and if you are interested in those stefenv links you can contact intersection for them.

niel

Re: help with smart nimh solar charger

intersection,
can you post that info without all of the side sales going on on the stefanv links? i'm giving notice this time so others can read it first, but the ads being linked to are technically against the rules and frowned upon by us.

they have been deleted seeing as how i gave plenty of time for some alternative solution and for some to read it.