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6V battery charger/power supply question.

JL Sargent

Diamond
Joined
Feb 7, 2005
Location
Birmingham, AL
I have a piece of automotive test equipment. It has an interal 6v lead acid battery. Im thinking a 6v hockey puck overnight occasionally would do the job of charging it. I pickup a used 6v puck and measure output the voltage at 9v ??? Thats not cool. Isnt the max voltage I need about 7volts? So I try another one that says 7volts out and it measures 11v??? What to do?

I checked a local automotive places machine like mine and theirs has the correct hockey puck on it I suspect. It reads 6.5v dc out. Any suggestions?
 
This is probably a Sealed lead-acid battery, You should first get the Ah Ampere-hour rating and the recharge rate, this is usually on the outside of the cell or on the manuf. site.
Especially with the sealed type, You don't want to use too high a charge current.
M.
 
Voltage regulator, bridge, and capacitor.

Get a little 3 amp variable voltage regulator, a bridge rctifier, and a cap. Build a little 6v charger, and set the voltage to whatever works best. I think the fully charged voltage of a 6v lead acid is like 6.4 or 6.5v. You could set the output to about 6.7v or whatever maintains a few miliamps of maintainence charge once the battery is fully charged. Then as the battery goes dead, it will draw more current, so it doesn't take forever to charge back up like a constant current charger. All electronics has a really cheap 3 amp voltage regulator. The datasheet gives you a schematic for a powersupply. Get a 10 turn pot so you have really fine voltage adjustment.

http://www.allelectronics.com/cgi-bin/item/LM350T/285/ADJ_POS._3_AMP_.html
 
I have made a couple "simple, stupid" chargers for small batteries. Not really for chargin, but for "maintaining".

Just a "wall wart" (what you called a hockey puck, I think), and a small box with full-wave (4 diode) rectifier and a resistor.

Voltage is somewhat irrelevant, you can pick a resistor value to give a current that is suitable, considering your AC voltage and current capability from the wall wart. Naturally, you need peak voltage to be more than battery charged voltage.

Charged voltage will be about 2.2 V per cell, since you won't be charging with a heavy current. So a 6V is around 6.6V.

Peak voltage is 1.41 x what your AC meter reads.

If you set up the charger to limit current to a maximum of 1/10 the ampere-hours, even if nearly dead, you should be OK.

Small 6V burglar alarm batteries are usually 3.5 AH or so, maybe 4.5. So 350 mA is the max you should charge with using a dumb charger (that is "C/10").

The nice thing is that if your charging voltage is close to battery voltage, say 9V or 10V peak for a 6V battery, the charge current will get less and less as you reach full charge. That way the charger may be able to be always connected, without gassing the battery.

If you charge at no more than "C/60 " when fully charged, you probably can leave it connected.
 
Thanks for the good ideas. The wall warts Im playing with are supposed to be 6v DC out (measured 9v). They are 500mah. My battery is 6v and 4ah. So I guess that output might be a little hot for my battery?
 
Thanks for the good ideas. The wall warts Im playing with are supposed to be 6v DC out (measured 9v). They are 500mah. My battery is 6v and 4ah. So I guess that output might be a little hot for my battery?

Not really, if you use a resistor.......

For a full-time charge, you would want maybe 40-60 mA at full voltage, which would be a resistor of maybe 50 ohms or so. Wouldn't charge fast, but should be OK even if you forget it. 40 mA is "C/100" and is usually safe long term.

If you want a more "real" charge, you would use a smaller resistor, of around 10 ohms, which would give you a higher charge rate in the 250 mA range. That is "C/16" which is an OK charge rate for bulk charging.

A switch would allow you to choose which rate you used. Low rate to bring up a dead battery, bulk to charge it, and low rate to maintain.
 
I'm assuming you are measuring your "prospective" power supplies with no load attached? Many, many of these are nothing more than a transformer, a rectifier, and a cap. Many don't have any regulation, or even a bleeder resistor. Therefore you are measuring the "peak" charge of the cap, not under load. Also, many times the stamped voltage output has really not much to do with "what it is."

It's a crap shoot with those supplies. Suggestions about adding a series resistor--and sometimes you can use a light bulb--are the easiest soloution.
 
A light bulb can be an excellent idea, since they tend to regulate current a bit. When voltage difference is high, bulb is hot and resistance is up, so current does not get out of hand.

As voltage rises, resistance goes down, allowing a larger bulk charge current than a straight resistor would. With clever selection, that can match a specific battery well. And it gives a charge indicator...... dimmer bulb means nearer full charge.
 
What's a hockey pack, is that a wall-wart?

Charging algorithm for lead acid is very similar to that of lithium ion. During initial charging stage, current should be limited to around 1/3C (for 10Ah cell, this means 3.3A). The voltage will continue to rise as it gains charge. You continue to charge until it reaches 2.42 to 2.5v/cell (x3 for 6v pack, x6 for 12v pack, cyclical application, such as portable devices) or you use 2.3v/cell and hold it there for a long time (standby application, such as UPS).

Simple dumb chargers don't shut off by voltage and holding it at 2.5+V/cell for a long time causes gassing. In open cells, you lose water. In sealed cell, you lose permanent electrolyte and reduce its life.


You can buy 12v smart-chargers at Wal-Mart, but if you look around online, you should be able to find one for 6v too.
 
What MetalWOT said.
Get yourself a smart charger for your 6 volt SLA battery. which will not overcharge it.
Excessive volts charging the battery will kill it quickly.
Regards.
Ray.
 
What MetalWOT said.

Excessive volts charging the battery will kill it quickly.

Which is why the resistor is important. It provides a way that various voltages can be limited to what the battery 'wants" and the current limited as well... (closely related things).

The high end-of charge charge voltage of 2.5V or so is for a high current charge...... For a slow charge or maintainer, no need.

The voltage under charge is dependent on the rate of charge (current).

If you want a fast, unattended charge, you should get a suitable charger.

For a gel-cell, make sure it is a gel-cell charger, as the finish rates are usually different for a wet vs gel type battery. You can gas a wet cell, but need to strictly limit gassing in a sealed gel cell.
 
Which is why the resistor is important. It provides a way that various voltages can be limited to what the battery 'wants" and the current limited as well... (closely related things).

The high end-of charge charge voltage of 2.5V or so is for a high current charge...... For a slow charge or maintainer, no need.

The voltage under charge is dependent on the rate of charge (current).

If you want a fast, unattended charge, you should get a suitable charger.

For a gel-cell, make sure it is a gel-cell charger, as the finish rates are usually different for a wet vs gel type battery. You can gas a wet cell, but need to strictly limit gassing in a sealed gel cell.


Given a relatively constant current, the voltage under charge will increase as the state of charge is increased. Series resistor limits current, but it does not regulate the voltage.

Active regulator is desirable for all SLAs, but if you don't have it, you should still do a top off charging occasionally. Nothing is as bad as letting it sit at less than fully charged state for lead batteries.
 
Given a relatively constant current, the voltage under charge will increase as the state of charge is increased. Series resistor limits current, but it does not regulate the voltage.

No, at a small enough charge current, the battery does the voltage regulation itself.

Very low rates are generally safe for continuous charging. Sealed batteries are capable of absorbing less than a wet cell, but still can do so.

obviously a true float service with a float voltage charger is ideal. However, that type of charger is not the issue here, at least as the OP seems to have set up the conditions.
 








 
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