Cell Phone Charging and Power Usage.

One of the comments on this blog made me start wondering about the little parasitic devices we have all over the house.  I've always assumed that they drew so little power they wouldn't matter when compared to to the kilowatt guzzling motors and heating elements we have in the larger appliances.  One little device that annoys me is the cell phone charger.  Every time the phone gets to a full charge it tells me to unplug the charger to save energy.  Sheesh, leave me alone, let me worry about how much power I'm using.

However, the commenter said his measuring device recorded 15 watts for several of his devices.  If I can confirm that, I may have to think about doing something since I have a bunch of them around the house.  So, I'll take on an annoyance and see what the power usage really is for my cell phone charger.

This little phone sucks 12 watts when it first starts out, so I need a really good wall wart that can supply over two amps to get the quickest charge.  Since I used the 'genuine' Samsung charger for this test, I let the phone drain down to (approximately) the same point and tried one of those 'Samsung' chargers that are available on Ebay.

Don't let the scale confuse you, this charger never gets over 1 watt.  Since my granularity (using this monitor device) is 1 watt, it could have gone a little higher or lower and still read this way, so I plugged in an amp meter in series and watched a while and it didn't ever go over 500 mA.  It doesn't have the stair steps of the real Samsung charger and took much longer ( less than an hour compared to over 2.5).  It looks like buying a real brand name may result in much, much better performance.  However, how the heck do you tell if it's really from the manufacturer?  Both of them are labeled 'Samsung' and they are the same form factor.  The ratings on the side are the same, so how do us folk out here in the world tell the difference?

I don't have an answer to this, and I'm certainly not going to buy a few hundred different ones and try them out.  I'm seriously thinking about making a load to test these things before I try to depend on them for anything.  If they fail the test, I'll do some serious complaining to the supplier.  The other thing I'm thinking about is putting together a power supply that will give me 5V at three amps reliably.  This would be useful to see how the charge characteristics of the phone look when it has enough current available.

Just for fun, I weighed each of them.  The real Samsung charger weighed 37 grams and the other one came in at 25 grams.  The real one has 12 grams more stuff in it, or thicker plastic.  Since I plan on using the fake for other things, I'm not going to dismantle it ... yet.  But here's a picture of the two of them:

The fake is the black one.  

Just to let you folk know though, there are two differences between them. 1. The real one has a UL certification on it, the fake doesn't.  That's really easy to overcome, they simply add another certification stamp to it.  2. The fake says 5.0 volt at 2 amp, and the real one says 5.3 volt at 2 amp.  Once again, that's easy to overcome.  I've already mentioned the weight, but no supplier tells you the weight of the device.

Anyway, I started this as an investigation into parasitic power and wound up researching chargers and their capabilities.  Sigh.  At any rate, let the buyer beware.

Battery Charging - Part 6

My float charger that is based on ideas I stole from the Harbor City 42292 device has gone through another iteration.  I decided to actually have a circuit board made that would hold the components and allow me to mount the board properly.  Using Eagle, I drew up a board which included the charger parts and everything necessary to support the XBee.  It was fun and came back looking like this:

It was so cool getting my first printed circuit board design back in the mail, so I rushed right in and assembled it.  Naturally, there were a lot of problems.  I hooked the voltage divider for the analog measurement to the wrong pin on the XBee and had trouble telling it was transmitting because I didn't include  troubleshooting lights to look at.  I got Eagle back up and changed the design, and ordered another board.

On this one I also increased the size of some of the traces and moved the voltage regulators around so I could use the heatsink I kept from the original charger.  So, I populated the board and hooked it to the lid of the box.  The entire device looks like this:

I happened to catch the leds on in the picture, but they are disconnected in a running device because they draw unnecessary power when it is actually in use.  It works real nice, and keeps the battery voltage at a constant 13.4 volts.  I've had several hand wired ones in place for months now and they are doing the job.

I'm still not done though.  I discovered a way to sense current being put out by the device.  I can put a hall sensor over a trace on the board that carries current and it will change output relative to the current flowing through the trace.  I doubt that I can tell the exact amount of current passing through the trace, but I can certainly tell if it is actually charging the battery.  This idea will take some development though and I don't know what components will be necessary for the final result, so I'll just assemble the rest of them and put them to work. I hope to get back to the hall effect device idea in a couple of months and work out the details.  The previous post on these devices is here <link>

I learned a lot creating the board, and I think I can develop boards for other ideas I have.  Things like a timer that will catch the time broadcast on my XBee network to operate lights and other devices would be great. It gets to be a real pain tediously connecting wires on a protoboard, especially when you want several identical devices.  Having an XBee network and something like this means I can finally get rid of the X10 devices that are so unreliable.  It would be so cool to completely switch over to XBee throughout the house.

Battery Charging - Part 5 (Harbor Freight item 42292)

Yes, I finally assembled and tested the various ideas into a single box for my float charger for lead acid batteries.  This project has been going on for over a year now and may be getting close to having a nice working prototype.  I incorporated a voltage display and an XBee set up to transmit analog readings into the mix.  So, what happens is you plug the charger into the wall and the XBee comes on, samples the voltage before the output diode and sends it to whatever device you set it up to talk to.  This way I can monitor the battery state of charge remotely without even having to go into the garage.  Here's an interior view of the charger:

On the lower left is the charging circuit that I came up with based (roughly) on the Harbor Freight 42292; at the top, stuck to the side is a series 2 XBee plugged into a breakout board; and on the right is the voltmeter display.  This is all enclosed in a plastic rectangular enclosure.  When assembled it looks like this:

Of course, since the XBee is a 3.3V device, I had to add a power supply for it.  I took the power directly from the float voltage and used an AP1117T33 regulator.  When I first started testing the combination, it got pretty hot pretty quickly, even when it wasn't hooked to a battery.  A little checking (I unplugged the XBee) told me that the XBee and its two LEDs were sucking enough power to heat it up, so I enabled sleep on the XBee.  This way, it would be off most of the time and only turn on to take a couple of measurements and transmit them.  This totally solved the problem.Here's the schematic I eventually settled on.  

If you're following this, you'll see that it's a simple variable supply that I set to 13.4V with resistor R2 which feeds a 3.3V supply to power the XBee.  The voltage divider R4, R5 sets the level that is in the 1.2V analog input range of the XBee.  Since these regulators sometimes tend to oscillate, I added capacitors C1 and C2 just to make sure I didn't drive the XBee nuts.  That's also why I added the resistor R6, to keep it away from reset.

To set it up, I put a 1K resistor across the output and set the voltage using R2 to 13.4 volts.  Then I centered the pot R5 and checked what was being transmitted.  By adjusting the pot, I was able to get a nice reading at the receiving end which I translated back into voltage and can display with an XBee connected Arduino.

There's a little jitter in the XBee readings caused by transients.  I can solve that two ways, add a capacitor on pin 20 of the XBee or do a rolling average on the receiving device.  I haven't decided which to do yet. I'm leaning toward a rolling average, but experience will tell me if it even needs to be fixed.  Here's a short sample of the Arduino output I set up.  Obviously, this isn't what I'll eventually use, but it's nice to be able to see the charger out in the garage:

A fun item I noticed is that the circuitry for monitoring the battery and transmitting the value can be separated from the charger.  That way I could hook a device to the battery and it would transmit the voltage to something that could alert me to go attend to the darn thing.  I may pursue that for something in the future.

No, I'm still not done.  This thing was a real pain to build.  It came up from several prototypes and kept getting more complex and looks like a mess because of this.  I think I may journey into getting a custom PC board made to put the thing together with since I want at least five of them.  Also, since I now have a box that will hold it, I'm going to mount the components on the lid with a wire to the display on the box part.  That way I can actually screw stuff down so it doesn't bounce around over time.

Apple Device: "Charging is not supported with this accessory"

I don't have a single Apple device in my house, no iPad, iPhone, iPod, iAnything.  I don't even have any apples.  However, the little wall warts I bought <link> have a bad reputation because they don't work on some iDevices.  While it's true that they don't, it really isn't their fault, it's Apple's.  This is the message:

Notice that it doesn't say that the device is the wrong voltage, wrong polarity, or wrong anything else?  That's because the device just doesn't meet the Apple standards of being made by Apple specifically for a particular Apple device.  They really, really want to lock you into buying your stuff from them and them alone.

My little chargers are fine and will work with Apple stuff with a simple modification that I won't do since I don't need to charge Apple devices.  What Apple did was use the D- and D+ (the two middle pins on a standard type A usb plug) to supply specific voltages that Apple could look at to tell if the charger was one of theirs.

Well, this little trick didn't last long before the engineers that make battery extenders and chargers figured it out.  Obviously, they grabbed a real apple charger and ripped it open to see what kind of shenanigans Apple was up to.  One manufacturer Lady Ada wrote up a nice description of what was happening and how she worked around it for one of her products, the MintyBoost.  She has a nice description of how she overcame the problem and made her stuff work <link>.

This also explains over 90% of the complaints on the web about various iDevices not working with a charger.  Basically, these two voltages (D1 and D2) are tiny and can vary a lot based on contact resistance, length of cable, net draw of the device, etc.  Basically any darn thing at all.  For example some of the solutions are to turn off the iPhone, plug the cable and charger into the phone, plug the charger into the wall, turn on the phone, repeat a number of times to allow the gremlins to align correctly.  And, this occasionally works.  Another solution is to lower the screen intensity, put the phone in airplane mode, then hook up the charger. There's always someone that says you need a new cable, better charger, or hold your mouth differently.  In the case of the iPhone, there was an OS update that caused a significant number people to have charging problems.  The fix for many of these non-technical folk was to go to the Apple store and get a new device or have a new part installed that will work.  Of course there are the Apple cultist that insist that only a new Apple charger will solve the problem.

But people, electrons are electrons and it isn't you or your device when a problem like this turns up, it's Apple exercising their control over their cult.  Annoying.

Don't despair yet.  There are ways to get around this problem that actually make sense.  No, you don't need to wet a Qtip with perfume and clean the contacts, letting it air dry for an hour(this is a real suggestion), there are much more mundane solutions.  For example, this site <link> talks about a little connector that has Apple plugs on both ends and works around the problem.  However, the darn thing is too expensive (I'm not a fan of this solution, but some of you might be).  There are several other similar devices out there that solve the problem.  You can also solve the problem by building something yourself (gasp !); I know this goes against life philosophy of the myriad teenagers that talked their parents into getting this cool new gadget, but life isn't fair.

Just cut a USB charging cable that mates with the device and add a little voltage divider.  The schematic of the changes is:

If this picture doesn't look familiar, take another look at Lady Ada's description, it was taken directly from there.  See, Apple uses the voltage divider to set specific voltages on D+ and D- to identify their devices.  Other manufacturers don't do this kind of thing unless they're trying to meet the USB charging standards, and for a low price device it isn't worth the effort or cost. Just another effort of Apple to wrangle you into their pen.

So, you ask, "What do you care, you don't own an Apple device?"  I don't, but my less fortunate friends, neighbors, and relatives do.  They get really annoyed when their devices won't work on one of my chargers when they come to visit, and they hate it when a charger stops working and gives them this silly, non descriptive message.  Also, I found zero, nada, explanations of what was going on to help people with this problem.  That annoyed me more than anything else, so I decided to post it here and a couple of other places, hoping that poor souls that are eating up Google cpu cycles could, at least, understand the problem better.

Soak a Qtip in perfume and clean the contacts?  Really?

Tiny Power Supplies for Micro Controller Projects

When you build micro controller projects, eventually you want to do something real that will have to work long term.  For this you need a power supply that is small, maybe small enough to put in a light switch, ceiling fan, or dryer control panel.  These things are hard to find.  I've spent hours on Google trying to find really small 5V supplies with some success, but never fitting my needs.  Nice little supplies that you have to buy in 100+ quantities, nice little supplies that cost US $50, etc are out there, but who wants to invest that kind of money in a technology that changes almost daily?

In a few project postings I describe how I cut the guts out of a wall wart like this:

These little devices work great as microcontroller power supplies.  The one above will put out up to 2A and doesn't seem to get hot doing it.  I've had several in service, out in the weather for two years.  At a cost less than US $5 each (usually with free shipping) they are a fine solution.  I've taken the supply out of the plastic case and installed it into devices like this <link>:

The little board in the upper left is the guts of the wall wart.  I've even used the entire wall wart as a mounting platform for another device:

This device <link> has an XBee mounted on the side of the wall wart transmitting temperature every minute or so.  All-in-all, a pretty useful device repurposed from the iPad craze.  However, technology moves on and I want to try a couple of ideas that would work better if I had an even smaller power supply.  I asked on a couple of forums and was pointed to one of these:

I already had two of them charging mobile devices in the house so I decided to gut one of them and see what was inside:

This little supply is TINY. Yes, I had to cut the case off with a Dremel to get to the guts; the top was seriously glued to the bottom and refused to come off. The two board with connecting cable configuration could create a problem mounting, but nothing I can't overcome, and the wires to the AC pins might give me a problem, but it couldn't be too hard to solve; I like this.  So, looking around the web to see what kind of deal I could get on them I found them for US $1 on Amazon.  Yes, you read that right, a single dollar, with free shipping.  So:

Here's my pile of stock devices.  

Since supply and demand act on everything, the price at Amazon has gone up to US $1.45 as of this writing and could go either up or down based on activity, but they are still a bargain.  

Even with this solution available, I'm still searching for possibilities.  The rough requirements for the supply are: minuscule size, 5V at a least an amp (relays, you know), easy to mount, things like that.  

To you suppliers out there, there's a market here that you aren't exploiting.  SparkFun, Lady Ada, Itead, etc. are you listening?  You folks in China, what are you waiting for?

Battery Charging - Part 4 (Harbor Freight item 42292)

Part 1 of this project is here, part 2 is here, and part 3 is here.

I probably will never actually finish this project to my satisfaction.  If you have been following this for the last many months, you might remember that I started off trying to get a float charger for lead acid batteries that actually worked and didn't dry the cells out in a couple of weeks ruining the batteries...without paying U$50 or more for each of them.  Rural living requires a lot of batteries for various machines that we use a few times a year and it is really annoying to have the battery fail between uses because of inattention.  Anyway, the latest charger variation (see part 3) works really well to keep the batteries charged with minimal loss of water, but checking on it is a pain in the butt.

I have to hunt down a multimeter and traipse out to the barn and check the voltage on each device.  It would be nice to have a voltage indicator on them to tell me if it is working correctly.  So, off to ebay I went and ordered a few of these:

These little guys just hook to the wires and get power from the source being measured.  Not the perfect situation since you have to power the LEDs from the supply that you are using, but it's a battery charger, not a lab instrument, so this will be fine.  To test them, I just hooked it to the charger output and stuck it on the wires leading to the battery on one of the cars.  This thing is cool.

The actual measured voltage is 13.4 on my multimeter, so it is relatively accurate and it gives me a highly visible indication of the state of charge from across the room.  Now, I have to figure out how I want it hooked into the circuitry and what form factor I'll use for mounting and using it.  The little black box I have in-line won't do the job, so I'll have to look around for something to house the display.

This brought up another idea: suppose I use one of my XBees and transmit the battery voltage over my network?  I could then check the state of charge from anywhere (yes, anywhere in the world) any time I wanted to.  I could even set up an alarm to send me email or a text message when something went wrong like a rat eating through the wiring.

I know what you're going to say, "What happened to the U$50 price goal?"  I don't have a good answer to that question; the parts and pieces so far are around 15 bucks, the little voltage display cost me a little under 3 bucks, and now I'm looking at an enclosure and an XBee.  This will push the total cost up into the 40+ range.  

I guess I'll just have to compromise my principles.....

Battery Charging - Part 3 (Harbor Freight item 42292)

Part 1 of this project is here, and part 2 is here.

Yes, I'm still working on a good float charger for maintaining my batteries.  See my other posts on this here and here for a recap of the project.  I finally gave up on the LM7805 circuitry that comes with these devices.  The Harbor Freight 42292 charger used to be controlled by a LM317 variable voltage regulator a long time ago.  They probably changed the regulator at some point to lower cost or overcome a supply problem.  However, it originally was a much better device; it even had a variable resistor in it to allow the end user to tweak it a bit to fit his purposes.  I got a clue on this during my searches to get ideas on what to do.  Here is the link to a schematic of the original device (link).  Notice how different it is from the current circuitry?  Just so I have a picture to talk about, here is the schematic from that site:

Back in those days, the wall wart was just a transformer so the diodes to rectify the supply were in the little black box.  IC1 is a LM317 and notice how they set up the resistor array to limit the range of available voltage and have a transistor to limit current?  Nice design that you can find all over the web.

I just duplicated part of this on a small piece of protoboard that I cut to match the size of the existing board.  It came out like this:

I still have a diode in series with the output to prevent problems with reversing the connection to the lead acid battery and used different values for all the resistors than the original had.  The reason I changed the values was purely practical; it's what I had on hand.  Notice that the parts count is a grand total of five?  So, why doesn't the manufacturer do something like this?  I don't have a clue.  So, my schematic looks more like this:

The input on the left is the wall wart's 19 VDC (apx) output; VR1 is a 1K, 25 turn potentiometer I picked up on eBay; R2 is 1.2K; and R3 is 180 ohms.  This gives the charger a range of about 9.6 to 16 or so volts.  To calibrate it, I put a 1K resistor across the output, set it for 13.4 volts and then took the resistor off.  When I hooked it to the battery, it floated the battery up to 13.37 volts over an hour or so and just held it there.

I rely on the LM317 to take care of itself.  It has over current and over temperature protection built in and regulates the voltage quite nicely.  If I discharge the lead acid battery by pulling a lot of current such that the voltage drops down to 12.4 or so, the IC will get hot, but not dangerously so.  Then over time, it charges the battery back up to the 13.4 volt level and just holds it there.

So, since I'm only using the wall wart, plastic enclosure, and heat sink of the original device, did I save any money?  Yes.  The wall wart alone is worth the price I paid for the original charger and the rest of it serves as a platform for the charger I ended up with.  

So far, I've converted three of these devices to the new design and they are chugging away keeping batteries charged.  I chose 13.4 V as the float point based on experience and a suggestion from a helpful person on the Arduino forum; this seems to be the best point to hold the battery without losing water to the charge current.  I haven't had to add any water to any of the batteries yet.  Over time, I'll know if I need to make adjustments to this, but at least now I can.  

Thinking about this a bit, it brings up an interesting point.  This wall wart can supply an amp easily and the output seems to be very good, that means I have just made a variable voltage 1 A supply that can serve various general purpose uses around the house.  The next time they have these on sale with a 20% off coupon I may pick up a bunch of them and stockpile them for later use.  Heck, since the LM317 can work as long as the difference from voltage in to voltage out isn't more than 40 V, I could use it to control the voltage from a 24 VAC transformer like I needed way back in the thermostat project.  I needed 5 VDC from a rectified 37 VDC taken from an air conditioner power supply; this would have saved me about $20 in power supply cost.

Battery Charging Part 2 (Harbor Freight item 42292)

Part 1 of this project is here:

I've worked out my modifications to the little charger. Here is the schematic I ended up with:

I used two 100 ohm resistors to keep from hitting the supply rails on each end.  These aren't strictly necessary, but I had them handy and it seemed a little safer.  The diode on the output is to prevent the battery from discharging if the power is down for a long time; like I kicked the plug out of the wall.  It really isn't a 1N4001, it's some generic 1A 400V power supply diode that I had around.  The pot is 1000 ohm 25 turn and cost about a buck or so off ebay.  It allows me to set the voltage precisely.

Here is a picture of the way I hooked it up:

This will give me the float voltage that I want and won't discharge a battery if the power dies.  That makes it reasonable to put on a solar cell for things that don't have power near them.  A tractor parked out in the back somewhere, a quad setting next to a tent on a hunting trip; things like that.  Notice I got rid of the wires with the clamps on them.  I put a plug on the end and can now just plug it into the vehicle.  Improvements could be made like adding a volt meter to it to tell how it's doing and another light that says everything's OK, but those are for the future.  Now all I have to do is put it back in the case and label it with the voltage I set it to and let it do its job.

Battery Charging (Harbor Freight item 42292)

Part 2 of this project is here and part 3 is here.

I have a tractor and two older quads in my barn.  Yes, I have a barn.  The problem is that about half the time I go out to use one of them the battery is dead.  I drag out a charger and extension cord from the house, charge it up, and then get to use the vehicle; usually a day or so later than I wanted.  All too often, I find out the battery won't recharge and needs to be replaced.  This is not only annoying, it's darned expensive.  And eventually I want to get a nice big portable generator with electric start; I really don't want the battery dead on that when I need it.

Obviously, I need a float charger on the vehicles to keep the batteries charged.  One that doesn't boil the water out and can run unattended for months if necessary.  I can't find such a thing at a reasonable price.  Yes, there are devices like the BatteryMINDer, Battery Tender, and hundreds of float chargers.  These things can cost a heck of a lot if you need several of them for different machines that have a battery in them.  Sure, it's cheaper than replacing the battery, but you have to leave them plugged in all the time, and some of them just don't work as advertised; if at all.  Wouldn't it be nice if there was a cheap little float charger that could be adapted to a solar cell?  One that I could control the float voltage it puts out so I could use it on the AGM battery in one of the cars as well as the standard small lead acid battery in a quad?  One that wouldn't drain the battery if I kicked the cord out of the wall and didn't notice until a month later?  Such a device would run me at least $40 and I need several of them.  Crap.

Looking around, I found a coupon for the Harbor Freight float Charger, but it has some draw backs.  It will drain a battery if it is unplugged from the wall, it puts out too much voltage to be left alone essentially forever, and the stupid wires on it suck.  But, it also has some compelling positives, it's cheap, it has a DC wall wart for power, it's cheap, it has simple circuitry that can be modified to do what I want, and it's cheap.  The DC wall wart is expecially attractive since it can be cut off and a solar cell substituted to run it where there is no power.  So, credit card in hand, I went to Harbor Freight and bought several of them (I did mention they were cheap didn't I?).

First inspection showed me that the little devices are set to 14.1 volts constant voltage with a current limit of around .9 Amps, it will discharge a battery through a status LED, and the combination of thick clumsy wires with smaller power wires is somewhat hard to deal with.  Time to take one apart and see what's in it.  After prying on the cover for a while unsuccessfully, I gave up and cut it open with a Dremel.  Reverse engineering it gave me the schematic below:

Harbor Freight Charger 42292

So, it has a good old 7805 power regulator set up as a variable supply and a TIP41 transistor to regulate the current.  The S9013 transistor is set up to turn the device off around 9V, because a 9V lead acid cell is probably beyond recovery for a float charger.  The resistors in parallel are to get a specific value out of the various dividers in this thing.  My first question was, "Why do it this way instead of a variable resistor?"  The answer was obvious when I checked the prices of the various components; fixed resistors cost a fraction of a cent in quantity, variable resistors are much more expensive.  But, the question I don't have an answer to is why they didn't put in a diode to stop from discharging the battery if it's unplugged from the wall?  Oh well, I have a few (hundred) diodes laying around that can do this job for me.  

When I cut open a second one I found a different set of components.  The second one appeared to be an older version and did not have the parallel resistors.  It had one resistor in each position and was laid out slightly differently.  The circuitry was essentially the same, it just appeared to have been changed over time.  Same drawbacks and advantages as the first one.

So, I hooked one of them up to an AGM battery I have in my car and let it work for a few days.  Sure enough, it float charged the battery up to 14.1 volts and kept it there.  This is not a good thing.  The specs on the Optima AGM automobile cranking batteries warns against this and tells me to keep it between 13.2 and 13.8 volts.  A little rework should take care of this problem; especially if I add a diode to the output which will cut .7 volts off the top and get me right in range.  It also appears that I can set the voltage by adding a simple little potentiometer and setting it to be what's needed for a normal lead acid battery like my tractor.  This would be really useful since the instructions for float charging a lead acid battery vary by each author I look at.

I've seen instructions that insist I shouldn't charge a flooded lead acid battery to more than 2.1V per cell for a 12.6 volt float.  Then when I look at another site it says the float voltage should be 2.2V per cell which gets me up to 13.2 volts float.  Then, they start talking about surface charge, waiting 24 hours to check it, parasitic current draws from onboard devices, and temperature corrections.  Then some of them talk about how as little as one tenth of a volt will shorten the life of the battery considerably.  Obviously, many of these people are spouting off stuff they don't understand and others are just repeating things they read on some forum somewhere.  What to do?  I'm just going to start with the info from Wikipedia and see how the water in the battery holds up.  I'll set the AGM voltage at 13.5 and the flooded cell batteries at 13.9 for a month or so and see how much water goes away.  I'll adjust down if the flooded cells lose water.  Note that using a constant voltage supply will take care of parasitic devices like the little LED that blinks for the alarm system on my car.  That's why I prefer constant voltage with current limiting techniques.  

One of the interesting items is that various articles say that the float voltage has to be correct to 0.02 volts.  First, I've never seen anything that kept that close a tolerance, and second, how the heck is someone supposed to measure that?  Sure, there are meters that give that resolution, but how accurate are they?  It would be relatively easy to set up a little computer to measure the voltage and adjust the bias on a voltage regulator to keep it right on the money, but how expensive is a meter that is calibrated to be that accurate?  And, how can you trust it to be that accurate over time?  My method is going to be much more pragmatic.  I'm going to measure the voltage I set the devices to on several meters and just average them.  Then I'll set the voltage slightly high and adjust downward to minimize water loss.  I know for a fact (long years of experience) that it's better to overcharge a tiny amount than to undercharge.  Undercharging causes sulfation and one of those sulfate crystals can pierce the lead causing a short that cannot be corrected.  Been there, done that.

Here are the two devices I've messed with so far:

Notice that the top one has a smaller component count than the lower one, but essentially the same circuitry?  This is a result of the parallel resistors to get a particular value that I described above.  The schematic I show applies to both of them, just adjust for the difference in methods.  Also, the various values of the resistors involved are different for the two devices.  A friend of mine suggested that they change the various values based on a particular run of 7805 and TIP41 components involved.  Also, variable resistors can be bumped and change value; an important consideration in something that will be tossed around a garage by people like me.  I also want to point out what, to me, is another shortcoming: the wire sizes and types used.  Notice the wall wart has a nice long wire that is quite small and the part that clamps onto a battery has a heavy coiled wire.

Someone out there want to tell me the logic in this?  The coiled wire is strong, relatively inflexible, and drags the device around in the car.  This thing puts out less than one amp and just doesn't need that kind of cable.  They did use good strain relief techniques though so the cables won't just pull the thing apart when you stretch it out.  The heat sink gets warm, not hot, as does the wall wart, and after reaching the float point, cools right down.  On my car with the parasitic draw from various devices, the device stays a little warm because it is supplying current to overcome this instead of sucking the starting battery down to nothing.

Obviously, this project is going to take weeks, if not months, to complete.  I have to test for days at a time and monitor the voltage and current feed to be sure I have something that will maintain the batteries not destroy them.  Then I want to get a solar cell to power several of the devices at once (in my barn where there is no power) and install a nice permanent solution.  You'll be seeing more on this over time.