I needed a good battery charger that I could observe how well it was working with a battery. I have been doing a lot of work lately on batteries. I will be working on a section of remote land come spring and will need to use large batteries as my main source of power. So far I have built some good indicators so I can see at a glance if the battery is ready for work. I have built some testers to " Stir Up" a battery and get it ready for work. With this project I want to be able to charge a battery and watch it and see the juice it is taking on. Then I would know that if I left it on overnight and it should have say 20 amps to give me. If it doesn't give it back to me I know something is wrong. I didn't want to get fancy with the charging part as I am more interested at the moment in getting a set of batteries that do their job of storing current and giving it back to me when I want it.

With my Chinese connection on Ebay I got these great 7 segment displays to show Voltage and Current for about $10 apiece. As you can see it looks pretty good and shows me in glowing red and blue the amount of Voltage and current I am providing to the battery.

To start with I had this old battery charge sitting around and it had a large transformer supposedly able to supply 10A. It don't look pretty but it works fine.

Princess Auto our Toy Store here in PG had some pretty good battery chargers on for about $50. But their clumsy little analogue display just doesn't give me the confidence that the above displays do.

The Voltage display works real well, just watch the polarity and it will indicate the correct voltage up to +30 Volts with an accuracy of 5%.

The current display is another story which I will talk about later. I finally got it working after many smokey try's but its accuracy is only about 10%

I had to do quite a rewire job on that old charger and only kept the transformer, the two diodes for the full wave bridge and the thermal fuse. I put in new terminal blocks and rewired it all so it should be good for 10 years or so. The transformer is good because the windings are set up for 6 Volts and 12 Volt charging. I put in a filter capacitor with the bleeder resistor just to give it a smooth DC as possible to the battery. It is set up as a full wave bridge rectifier so with the capacitor in there it is pretty smooth DC. The DC turned out to be about 16 Volts when all was said and done. I didn't want such a high voltage across my 12 Volt batteries so I put in a couple of diodes which dropped it down a volt. This helped out quite a bit and actually dropped the current down by about 30%. Which is cool because I like to charge a battery slow and steady.

The Voltage display goes in easy. I have made a mistake on the schematic. The + lead for the Voltage display goes on the other side of the diodes. This is nice because when the charger is off: hooking the clamps across the battery will show the voltage on the battery. Good for a quick check on the condition of the battery.

As said before, getting the current meter to work was quite a problem.and I had to put in a separate battery to run the display, thus the little 9 V battery shown on the schematic. I also put in a switch so I could conserve the power of that little 9 V battery. So I only turn it on when I want to see what is happening.

In the picture of the current meter you can see the SHUNT is quite large. It is 0.0001 ohms and is meant for a 100 Amp display of current. So it is way overrated for the application I will be using at only 10 Amps tops. The inner workings of the little A/D converter for this display has to have a totally isolated supply voltage. The docs said it needed +12V to operate. The back has four wires.,(2) for the +12 supply and (2) to go across the shunt resistor. but if you tries and I did to pick up 12V from the power you were measuring you get a smokey wire on the power lead of the display. So I had to assume that the box needed it own isolated supply. With that taken care of it powered up fine. The next problem was that it indicated a total wrong current. And to add more weirdness to it if I put my current meter in the positive lead the display smartened right up and displayed the correct current. This had me scratching my head for awhile.

It had to be because the unit is measuring such a small voltage across the shunt and I am working at a max 10A when it is good to measure 100Amps. So there was an adjustment rheostat inside that I tuned in and it finally indicated very close to what it should, But I could only get it too about 10% of what it should be. This is good enough for me as this unit is just to fill a battery at a decent rate of charge and no bells on whistles on the operation. This unit is for charging larger lead acid heavy duty batteries and my deep cycle batteries for the ranch.

So when all is said and done I have another unit that I can use with my up and coming battery arsenal. This battery business is a lot of work and I have enough stuff to do with these batteries without working too heavy on the charging business which I see can be quite the study in itself. This unit will load my batteries just fine and I will worry about the science of charging batteries later.


You can consult the PDF for some of the details of this project.

Revised 2013 by Larry Gentleman