Built me a battery charger...

S

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
...for nothing. From parts I had lying around -- and one of my mother's pencils.

Yep, a pencil.

I had a spare SLA battery from a (very much) broken outdoor motion-sensitive lamp. Date on the battery is 9/2004, so that explains why the PCB in the lamp was starting to come apart. We've probably had that lamp since '05.

Battery is made by a Sanshui Samtech Battery Co., Ltd., model # JL3-XM-4.5 and the datasheet for it is a little hard to find. I've put it up here, if anyone cares: http://www.datafilehost.com/download-12b9893e.html

Source for the datasheet, in case that gets deleted, is here: http://www.ajcbattery.com/products/sanshui-jl3-xm-4-6v-4-5ah-emergency-light-battery.html

OK, so it's a 6V, 4.5AH battery. Nice. According to the datasheet, I can pump up to 7.5v into it at once to charge it. Trouble is, I don't have an SLA battery charger...

...but I do have a 7.5v 1a wall wart, courtesy of a friend of mine who likes to give me all sorts of stuff that he can't really use. A check with the multimeter revealed a working 7.5v output when that wall wart was plugged in.

So I google around, and I find this page: http://sparksandflames.com/p6.html

sparksandflames.com said:
Do not connect the wall wart directly to the battery.
<snip>
...if there is no current limiting circuitry in the wall wart, it will provide much, much more current for a short period of time, and burn itself out, start the wires on fire, or cause the battery to explode.
Click to expand...

On this page of that site is a very simple adaptation. All one needs is a resistor and Ohm's Law.

R = E / I
R = resistance desired
E = energy in volts (more precisely, difference in voltage between wall wart and battery output)
I = desired current in amperes (amps)

A note before the (rest of the) math: on this other page here, it's stated that the safest way to charge a battery is to charge it at 1/10 of its rated capacity, for 14 hours. That's the method I'm using.

R = (7.5-6) / (0.1*4.5)
R = 1.5 / 0.45
R = 3.333...

Actually, 3-1/3 ohms is the exact amount needed. Tricky bit here is that I don't have a 3.3ohm resistor, particularly one that can take 3+W (7.5*0.45 = 3.375). So I made one.

Googling around for a way to make a resistor gets you a lot of stuff about smearing pencil lead on paper and measuring it with a multimeter. I also got lots of stuff about resistors needed for airbags in cars. None of it was useful. Even HackaDay didn't help much.

Then I had an idea. My mother insists on using #1/B pencils because they somehow are easier to read (never mind that they must be sharpened insanely frequently -- which is why I use mechanical). She has what amounts to a lifetime supply (and then some) of them from Ticonderoga.

So I asked Mom if she would let me have one. She was quite willing to share. This page right here gives all sorts of helpful info. For a 125mm length of #1/B pencil, the resistance is 7ohms.

3/7 = 0.428571...
(3/7)*125=53.571428...
53.6mm = 2.1"

So I cut the pencil up into a 2-1/4" length. That's the teeniest bit over 3.2ohms --

2.25" = 57.15mm
57.15 / 125 = 0.4572
0.4572*7 = 3.2004.

So there we go. I cut the barrel jack off the wall wart, and separated the two wires. One got a wire nut to the black wire off the old lamp -- ending in a spade connector. The other wire (the positive lead off the wall wart) got an alligator clip for interfacing with the makeshift resistor. The red wire from the old lamp (with a different size spade connector) got an alligator clip as well.

So the circuit is:
positive lead of wall wart -> resistor -> battery -> negative (ground) lead of wall wart
Simple.

Here, have some pictures. These are all clickable thumbnails.




BTW, it's been plugged in for the ~30min it took me to write this. That pencil ain't even warm. I'd call this one a success.

Comments, anyone? :D
 
Nice hack. You gotta be careful, though--as the battery charges up, it's voltage will increase, and you don't currently (heh) have a way to keep your 7.5V power supply from overcharging that battery. The datasheet for the battery likely has a float charge voltage which is safe for the battery. Otherwise, you'll need to periodically check the voltage going into the battery, to make sure you don't overcharge it.
 
Well, it was at 1/3 capacity when I poked it with my multimeter... so I'd say that after 9 hours 20min it should be all charged up ;)

Not going to leave it plugged in overnight, though, for sure!
 
No thanks -- I can remember exactly two letters of morse code, and they are S and O ;)
 
Haha that is awesome! I never realized a battery charger could be that simple, I thought there was more to it than that, but now that I think about it, it does make sense. Ohms law. More resistance = less current, so resistor is used to limit the current. Brilliant!

Just need a way to monitor the voltage and perhaps a couple values of resistors so you can slowly step down the current as it's fully charged. Could be as simple as a couple relays that open/close to bypass resistors.
 
Heh. That's not a bad idea...

However, I do have some bad news. After two hours of charging, the battery is at about 5.5v -- or ~91.6% of a full charge.

That's nearly 7-1/3 hours early!

...which means that this battery's capacity is [2/(9.333...)]*4.5 = 0.964AHr.

Well, shit :( At least I didn't spend any money on it!

Proof? You don't need no steenkin proof! Oh well, here, have a pic of the reading on my amazingly shitty Radio Shark multimeter. Can't believe I paid $20 for this whomping pile of doo!

 
An 8-year-old SLA is likely to have lost a fair bit of its capacity, but it's not clear if the battery was fully discharged when you started. It's likely that if the circuit went bad, the battery got over-discharged and the lead plates got heavily sulfated, in which case, yeah, the battery's toast.
 
When I started, the battery was at ~2v. I figure it's toast.

Next time I have $11, I'll buy a new one. These things are handy!
 
"No code for 6M and up" not sure what that means.

I have no way of building a large antenna, nor could I cover the fees for membership/callsigns/license/etc. and/or permits to build the antenna, nor do I have particularly all that much interest in using a radio. Although building one from tubes sounds fun, that's a little different ;)

That said, my next project is a joule thief. Shouldn't take that long once I can get to a radio shark and grab two parts I need ;)
 
I mean no Morse code test is required for the lowest tier of license which gives you access to all the bands from the 50 MHz 6 Meter band on up, including the workhorse 2M and 70CM VHF and UHF bands. As for large antennas, a quarter wave on 2M is...uhhhh...half a meter. You have already built the cantenna, you can literally hack up VHF antennas out of odd bits of salvage metal.

I don't want to belabor the point, but you are technically inclined, and given that you do not live in a metro area with a hackerspace, amateur radio would be a really good opportunity to find a community of geeks around you.
 
starhawk said:
When I started, the battery was at ~2v. I figure it's toast.

Next time I have $11, I'll buy a new one. These things are handy!
Click to expand...
Oh yeah, that's a dead battery. Normally you don't want to let a 6V battery to fall below 5.5V or so.
 
Those gelled electrolyte units do not recover well

There are circuits for desulfating pulsers out there, I have had pretty good results with those.
 
If you could email me some circuit diagrams for battery revivers, I'll see what I can do. As long as it doesn't involve programming a chip (or SMD/SMT parts), I can put it together. My hunch is that (as the saying goes in this family) "even Jesus couldn't bring that back from the dead" but I've been wrong before ;)

As for the ham radio thing... also shoot me info on what kind of equipment we're talking and how much it costs to either buy or make, as well as license fees and all that stuff. I can say that I've got what I want to say is a 3' antenna off a dead CB radio, but I can't guarantee either length or origin there without digging the damn thing out of a box, and I'm not doing that right now.
 
Sorry to bring a thread back from the dead here, but I may have some good news. I was mucking around with my Dell fan and my spare C batteries (from this thread) and, having found that all my C's are dead from old age, still in the package (they're all ~1v batts now :( ) I remembered this thing.

Dug it outta storage and amazingly enough it had 4v to it... I'm going to charge it till midnight (3hrs) and see what happens. When I last was charging it, earlier in this thread, and posted that it was at 5.5v... if my memory serves me that was while it was charging -- I didn't disconnect the charger to get that reading so it probably was bogus. The battery may indeed be good after all!

I was able to rig up a series circuit with two of the ~1v C cells and the SLA @ ~4v -- and I got a whine out of the fan. Push-started it with my finger and it slowly spun up to a pretty serious clip... if the battery can be charged to a full 6v, it'll run the fan for prolly an hour or two...

I used google calculator to tell me 14/3 = 4.66, since I can't math for shit... if all goes well with the three hour charge tonight (that is, it isn't like 8v across the terminals or some shit, and nothing burns or explodes), I'll charge it an hour and forty minutes tomorrow and see what voltage it has across the terminals then...
 
Don't worry ;) I don't intend to... if the battery feels even kinda hot at any point it's getting disconnected.

I'm checking it every few minutes cause I've a nervous disposition by nature.

EDIT: also the wall wart involved is plugged into a power strip with a circuit-breaker switch. Anything bad happens I hit that switch fast.
 
Put the battery into a 4 cup glass measuring cup. The cup is at least a half inch taller than the battery, so unless I wind up embedding battery lid parts in the ceiling (incredibly unlikely, to say the least!) I'll be OK for sure.
 
Multimeter reads 5v. That's good :D I'll see how it is in the morning tho...

@jojo69: no disasters yet :p
 
Plugged it in this morning without running a voltage test, but I think it can hold some volts so I'm not worried 'bout that. Charging now, at quarter to one EST I'll unplug the charger and see what I've got.
 
Quarter o' one is here. Unplugged the charger, hooked up the multimeter.

Five volts.

Should I try charging it longer?
 
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