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SteveF
01-28-2017, 04:01 PM
So I've got an old standard car battery. It's sufficiently worn out that my smart charger says it's bad and won't charge it. But it works just fine for running my chain saw sharpener.

I've got a nice DC variable power supply. Electronics is a weak area for me. Seems to me that I should be able to hook up the DC power supply to the battery, set it for 13.5 volts and just let it charge it up as best it can. Is there some non-obvious to me reason why this is bad for the power supply or the battery?

Thanks.
Steve

CarlByrns
01-28-2017, 04:09 PM
So I've got an old standard car battery. It's sufficiently worn out that my smart charger says it's bad and won't charge it. But it works just fine for running my chain saw sharpener.

I've got a nice DC variable power supply. Electronics is a weak area for me. Seems to me I should be able to hook up the DC power supply to the battery, set it for 13.5 volts and just let it charge it up as best it can. Is there some non-obvious to me reason why this is bad for the power supply or the battery?

Thanks.
Steve

Your sharpener doesn't require anywhere near the amperage the starter motor does. 2-3 amps versus 300-500 amps. The battery is probably mechanically worn out- over time the lead on the plates degrades and literally falls off (there is a dead space at the bottom of the battery case to collect the debris). Since the plates are now smaller (less surface area), they cannot provide that huge surge of current to the starter motor, no matter how charged up the battery is.
Replace the battery.

A.K. Boomer
01-28-2017, 04:14 PM
Yup plus all the stuff that falls off can be still somewhat conductive and collect at the bottom high enough to get to the plates internally shorting them out, it's toast and time for a new one...

lakeside53
01-28-2017, 04:27 PM
If your smart charger won't charge it then you may have low volts and bad cells. Measure the "charged" battery voltage after you take a few minutes of load on it. If it's not say 12.6 (2.1 volts per cell), don't leave it on a charger at 13.5

13.5 to 13.7 is ok IF the battery has all cells functioning. If one cell is shorted (one of many possibilities) it raises the possibility you will get to gassing voltage on the good cells. Also.. you don't say what capacity your power supply has - if large limit your current while charging, and then particularly when "floating".

Float charge is 2.35 per cell and gassing voltage is 2.35-2.41. (depends on cell type and temperature).

Joel
01-28-2017, 04:29 PM
I have an automatic charger that won't kick in if the voltage is too low. I had a battery that I charged a little bit with a non-auto charger then put it on the auto charger and it charged up OK. Can't speculate as to the condition of your particular battery, but I would check the electrolyte, charge it for awhile and see if it can be made to work acceptably. Just in case you didn't know, running a battery down really hurts its life.

J Tiers
01-28-2017, 04:33 PM
Nobody is answering the question.

The power supply as a charger CAN charge it. But the difference in voltage between the battery and the power supply will draw a lot of current.

If you can set a safe (for the charger) limit on current, sure, you can charge the battery.

I would do a few things.... I'd set a max voltage, I'd set a max current, and I think I would put a suitably rated diode between the battery and the supply, just as a safety measure. The output voltage would need to be higher by the diode drop.

Lacking a current limit, a high power resistor with a low resistance value would limit current, and allow you to charge without a lot of problems. There would be plenty of "losses", the resistor would get hot. Resistor value would depend on the power supply capability.

darryl
01-28-2017, 04:34 PM
Besides the usual issues with sulphation and battery plates shorting out with age, a battery needs sufficient charge to bring all the plates up to voltage so it can hold a charge and be able to deliver a high current when needed. To do this, a higher voltage of say 14 volts should be applied for long enough. After resting off charge for say 1/2 hour or so, the battery voltage should stabilize at around 13.6 volts or so, which is a full charge. Then you could apply a maintenance charge, which could be that level of voltage from your power supply.

Simply giving that voltage to the battery all the time doesn't mean it will come to full charge. You might find that after a rest period off charge the voltage will drop to perhaps 13 or so, maybe less.

Ideally you'd want an intermittent fast charge 'pump up', then the maintenance charge. You should also subject the battery to a high discharge current on a regular basis, just to help keep the plates clear of buildups. A lot depends on battery type.

PStechPaul
01-28-2017, 05:04 PM
A good limiting resistor would be an incandescent lamp, perhaps a headlight bulb. It will probably draw about 10-15 amps with 12V across it (about 1 ohm), but will be about 0.1 ohm when cold. So you could set the power supply to about 14 volts and the charge current will be well under 10 amps into a discharged battery reading about 11-12 volts. If your power supply is current limited you can just set the current to a safe value and set the voltage to 13.5 or 14 volts and it should charge at the maximum current and then taper off to a float charge of perhaps 100 mA. If your smart charger sees a low battery voltage you might be able to fool it by putting a diode in series to boost the reading by 0.7 volts, which might be enough. But it might expect a certain voltage from the battery before it will try.

J Tiers
01-28-2017, 06:19 PM
Bulb is a very decent idea. I like it.

You can select one that will not pull more than the supply can provide, even if the battery shorts completely. And you get a warning light if that happens!

lakeside53
01-28-2017, 08:38 PM
I used 2 x 120v 150 watt light bulbs in series as a "fuse replacement" when I was working on my washing machine 3 phase motor controller. Sure saved a bunch of fuses and fried diodes.

darryl
01-28-2017, 09:38 PM
I remember working on a speaker system in a local watering hole once. The speakers had inline bulbs for protection. It was interesting to power it up and watch the light come through the speaker cone when you cranked it. Never was a decent sounding system.

Need to know more about the power supply. Maybe it has everything you need on it already, but maybe not. If it doesn't have enough current capability then it could be working too hard constantly just to put a reasonable charge into the battery. If it has lots of current capability, then you might need to protect it in case of a shorted battery- the headlight bulb as resistor might be fine for this. If it has current limiting you can set that as a safety against a shorted battery, but you also have to be aware that it will probably overheat if left in that mode too long. It's possible that all you need is a voltmeter and some common sense. If the battery voltage comes up to say 11 volts or more when under charge, then it's not shorted. If the voltage comes up to whatever you can dial in and no current flows, then it's a dry battery. If lots of current flows but the voltage doesn't quickly rise above about 11, then you have at least one shorted cell.

Tell us more about the power supply-

danlb
01-29-2017, 02:16 AM
Generally, if the battery voltage after charging is some multiple of 2.1 you can infer a few things. For instance, 10.5 volts indicates that you have lost one cell, and it is shorted. 5 cells are still working. 10 volts is sometimes useful too.

Some of the smart chargers have an "anti sulfation" mode that is supposed to reduce or reverse the impact of overcharging. I've had success with that once.

JRouche
01-29-2017, 02:39 AM
So I've got an old standard car battery. It's sufficiently worn out that my smart charger says it's bad and won't charge it.

I've got a nice DC variable power supply. Electronics is a weak area for me. Seems to me that I should be able to hook up the DC power supply to the battery,
Thanks.
Steve

I also dont know much about electronics eiether Steve, worse at spelling :(

The "smart charger" is just a gimic. The best battery chargers are like old school welders. The bigger the transformer the better. Problem is, copper is expensive.


The electronic chargers see a short. So they fault out as they should. An unregulated power supply should not be used. JR

LKeithR
01-29-2017, 05:18 AM
...Nobody is answering the question...

Perhaps not specifically but they were providing the "correct" answer. The battery is toast, throw it away and get another one. Messing around with power supplies and resistors and capacitors is just a waste of time.


...The "smart charger" is just a gimic...

I dunno; we've had a Black & Decker "smart" charger at the shop for about a year now and it's the best charger we've ever had. It's load sensing; charges at 2, 10, 20 and 40 amps; has a "boost" mode as well as the ability to test and desulphate. It works like a charm--I'll never buy another "conventional" charger again. Oh, and it's very light for its size--easy to carry around...

SteveF
01-29-2017, 08:10 AM
Perhaps not specifically but they were providing the "correct" answer. The battery is toast, throw it away and get another one. Messing around with power supplies and resistors and capacitors is just a waste of time.

.......................

No, tossing it isn't the right answer. This battery became questionable for vehicle usage (voltage would drop down to about 12.5v after a few days) about two years ago. It sits in my shop, gets topped up with the smart charger periodically (desulfation didn't help) and does a perfect job of running my chain saw sharpener every couple of months. The last time I tried to charge it the smart charger didn't like it and I just got the DC power supply.

So thankfully Jerry is an engineer and knows how to stay focused on the question (as well as some of the others).

Thanks for the answers. Sounds like I'm good now. May just decide the run the sharpener off the power supply directly.

Steve

MikeL46
01-29-2017, 10:49 AM
My cheap brother always took 'dead' batteries and collected the acid, rinsed the battery to remove sediment, filtered and replaced the acid, charged the battery and adjusted the PH. He swears the battery was then good and would start his car.

I just recycle them and buy new.

Mike

TR3driver
01-29-2017, 11:16 AM
My cheap brother always took 'dead' batteries and collected the acid, rinsed the battery to remove sediment, filtered and replaced the acid, charged the battery and adjusted the PH. He swears the battery was then good and would start his car.

That does work, sometimes, to squeeze out a bit more life. But it's still an old, weak battery and won't last very long.

As far as the original question: Sure, go for it. A "nice electronic" supply will have it's own current limiting (usually adjustable), so no need to mess with resistors and such. Don't leave it on forever though, just charge the battery as far as it will go and then quit. And don't forget that any bubbling in the electrolyte (from overcharging) is a very explosive mixture of hydrogen and oxygen. You'll want a well ventilated area.

CarlByrns
01-29-2017, 11:53 AM
I also dont know much about electronics eiether Steve, worse at spelling :(

The "smart charger" is just a gimic. The best battery chargers are like old school welders. The bigger the transformer the better. Problem is, copper is expensive.

JR

Not true. The old-school uncontrolled chargers can cook even a new battery if not monitored carefully.

Ironwoodsmith
01-29-2017, 12:03 PM
Just power the sharpener from the regulated power supply and recycle the battery.

lakeside53
01-29-2017, 12:59 PM
"Smart chargers" work well on all batteries. If you are into battery life (like marine stuff...) 3 or 4 stage chargers are the only way to go. I get 9+ years out of a POS Lawn tractor battery on my generator by leaving a tiny "smart" charger connected. Summer 2015 I built 4 lake barges, powered by golf cart batteries. I expect 8-9 years out of those, but only because the charge is carefully controlled, as is the "maintenance charge" (not "trickle"). These chargers track the time since the last charge and do a saturation cycle ever 7 days or so.

There are many levels of "smart". Some smart chargers will even try to charge a dead battery by providing short high current pulses while watching the results. At about 5V (12v battery) they move to constant current mode (stage 1 of a typcial charge). They also monitor time... if you don't get to a certain voltage in a reasonable time, they assume bad cells and issue an error.

If you have sealed batteries, you better use a smart charger to stay below the "gassing voltage".

old mart
01-29-2017, 01:09 PM
I always used to charge up motorcycle batteries with a bulb in series to limit the amps output from a car charger. With your old battery, you could limit the time it is on charge with a cheap timer in the circuit, the type that plugs into the wall socket. An old battery will be lower voltage than a new one, so 13.4 should be fine. Car alternators will usually run to 14.2v.
I have a twelve year old car battery with the indicator that is still showing green. I keep it charged regularly to run a Dewalt 12v cordless drill and work light as the batteries are shot, I have removed the cells, connected up 9 feet of cable with crocodile clips and it works well.*

Willy
01-29-2017, 01:17 PM
Yeah I like the light bulb idea a well. I always use a 1057 bulb conveniently mounted in a socket with a lead on one end in series when charging, if I remember correctly this gives about a 6 or 7 ohm resistance. Nice thing about the bulb is that when first hooked up the bulb is bright and then slowly dims as the battery reaches a full charge. If the bulb stays bright for a half hour the battery is not accepting a charge and is toast.
Not only does this preclude baby sitting the battery charging process but it also serves as a nice quick visual indicator of what's happening.

Joel
01-29-2017, 02:03 PM
http://bbs.homeshopmachinist.net/images/misc/quote_icon.png Originally Posted by Joel http://bbs.homeshopmachinist.net/images/buttons/viewpost-right.png (http://bbs.homeshopmachinist.net/showthread.php?p=1095768#post1095768)
...The "smart charger" is just a gimic...

I dunno; we've had a Black & Decker "smart" charger at the shop for about a year now and it's the best charger we've ever had. It's load sensing; charges at 2, 10, 20 and 40 amps; has a "boost" mode as well as the ability to test and desulphate. It works like a charm--I'll never buy another "conventional" charger again. Oh, and it's very light for its size--easy to carry around...

I didn't make that statement, JR did.
I have a good automatic charger and like it. On occasion however, the battery voltage has been too low to kick it in and I have had to put a little charge in it by other methods prior to switching back to the auto charger. It is easier not to have to monitor the battery, so I almost always use that unit.