Lead Acid Battery Charger

[aaron]

Hey guys 

A mate of mine was going on about how he built a Sealed Lead Acid Battery charger. I think he just said he was using a LM317 to feed the battery it's stand-by charge voltage of 13.5v (Minimum). I'm having trouble finding this out exactly, can it be left charging at its minimum stand-by charge voltage for.... forever?

I'm curious to this, I would love to build one as a side project, it wouldn't take long at all and I would use it a lot! But, I don't yet have the know-how to build chargers that follow a specific charge algorithm or anything, and of course I want it to be safe. So yeah, can I do this??

From what I have gathered, each cell has a maximum floating charge voltage, so it would be (13.5/6 = 2.25v/cell for a 6 cell?), which it can sit at, so if the above is correct and safe, it would be awesome, my charger would be a little slow, but work. 

Thanks in advance guys! 

[Balaur]

The recommended minimum approach to charge lead batteries is a constant current followed by a float charge.

While frugal, a LM317-based charger is quite able to fill both phases by (hopefully) an internal power/current limiting when the battery is fully discharged and a voltage regulation for float charge.

The only issue is the possible 3A when initially charging an empty battery. Make sure that the AC transformer / primary power source can safely deliver that.

A float charge voltage of 13.5V-13.8V is the recommended level for a lot of applications (such as HR 1224W batteries used in UPSs as an example).
 And yes, it can be leaved on forever. HR1224W batteries have service life in excess of 5 years when charged at 2.275V/cell (i.e. 13.65V/battery).
You may not like the relatively slow charging, though.

Obviously, "intelligent" chargers may use algorithms involving several steps, which aims at dealing with batteries living a hard life.

Best regards,
Dan

[aaron]

Thanks Dan. 

I'm not sure if this is correct, but my little 12V Sealed Lead Acid battery states an initial charge current of 1.26A, and if I'm reading the LM317's datasheet correctly, would I be right that it's maximum rating is 2.2A?

It also stated that it's internally limited, I could be wrong so I like to check, would it technically be safe to charge with the LM317, as long as it has a heatsink? (Assuming as you said, the supply can do the 2.2A) =)

Thanks heaps! 

[tld]

A mate of mine was going on about how he built a Sealed Lead Acid Battery charger. I think he just said he was using a LM317 to feed the battery it's stand-by charge voltage of 13.5v (Minimum). I'm having trouble finding this out exactly, can it be left charging at its minimum stand-by charge voltage for.... forever?

In a word: Yes.

When the battery-voltage reaches the minimum voltage you'd charge to (say 13.5V), your regulator wouldn't supply anything to the battery, so it'd be quite happy.  This is often called a float charger.  It's not uncommon for people to simply connect wall-warts to batteries, so let them float up to the float voltage.

Not that if you do this, I'd at least try to make sure you don't supply too high a voltage, or too much current.

Easiest way would be to use two LM317s, one as a constant-current source, the other as a constant voltage regulator.  That way, you'll supply the current set by the first, until it reaches the voltage set by the second.

Probably a good idea to heatsink them as well.

I generally do this when I need to charge anything I don't have a charger for, such as LiFePo4-cells, and small Li-Ion coincells etc.  For safety-reasons I don't fully charge the latters, but it does work quite well.

tld

[notsob]

OR you can get one of these, not only does it charge Pb batteries, it also does LiPo, NICad etc

http://hobbyking.com.au/hobbyking/store/__5548__IMAX_B6_Charger_Discharger_1_6_Cells_GENUINE_.html

[aaron]

Thanks tld 

So, am I correct to say that, to limit the current to 1.26A (Rating of the battery I have) the LM317's Output goes through about a 10Ohm resistor, back to the Adjust pin, then that output (Which I hope would be limited to 1.26A) is run through a second LM317, which is voltage limited to 13.5v?

Sorry, still learning. 

Oh and notsob, thanks, all though I would rather build one myself! I don't even have the $20 for that thing. XD

Edit: This is just my mucking around: http://puu.sh/2knhw

[tld]

So, am I correct to say that, to limit the current to 1.26A (Rating of the battery I have) the LM317's Output goes through about a 10Ohm resistor, back to the Adjust pin, then that output (Which I hope would be limited to 1.26A) is run through a second LM317, which is voltage limited to 13.5v?

Yes, exactly.

The first will make sure current isn't above 1.26A, and the second will make sure voltage isn't above 13.5V.  At first, you should see the voltage rise slowly (as 1.26A is delivered), but when it's approaching 13.5V, this slows down, as the second regulator is starting to limit.

Note that it should be a 1 Ohm resistor, not a 10 Ohm.  There are calculators you can use to check stuff:

http://www.reuk.co.uk/LM317-Current-Calculator.htm

Also, note that the resistor needs to be able to handle the current that flows through it, the calculator will show you which rating you need.

And finally, for needing a 1 Ohm resistor capable of 1.57 Watt, you can instead use 2 x 2 Ohm resistors, capable of 1 Watt each.

I'd probably go for some safety-margins, and either get a bigger resistor, or multiple smaller ones.  If you need to be able to handle 1.57 Watts, might as well aim for 4-5 Watt rating, such as 4 x 4 Ohm resistors.

There's also huge things, which should keep you well covered: http://www.ebay.com/itm/2x-1-OHM-50W-Wirewound-Aluminum-Housed-Resistor-50-Watts-/400310239042?pt=Vintage_Electronics_R2&hash=item5d34597f42

Oh, and finally, with this, you'd have a voltage-drop across both regulators, and the resistor, so your input-voltage needs to be high enough to allow for that.

Sorry, still learning. 

Learning is good.

tld


CAUTION: Keep in mind that lead acid batteries can supply insane amounts of current, so be safe at all times.  I recommend permanently attaching a fuse, and only accessing the battery through the fuse, for either charging or discharging.

[tld]

Edit: This is just my mucking around: http://puu.sh/2knhw

The resistor (1 Ohm) needs to be between the regulators, in series with them, so the current flows through it.

Also, you probably need to add some caps.  The calculator I linked has an example of how to wire for constant current.

tld

[aaron]

Awesome, was thinking I would have gotten everything backwards, thanks heaps.

I plan on using a 12VDC Source, I have at least a box full of AC/DC Adapter things, hope shorting them won't cut the houses circuit breaker though... 

Thanks for your help, and further input is welcome

I'll get back to you on how it all goes very soon 

[tld]

I plan on using a 12VDC Source, I have at least a box full of AC/DC Adapter things, hope shorting them won't cut the houses circuit breaker though... 

You can't do that.

What the LM317s (and all linear regulators) do, is that they convert excess voltage to heat.  If you'd like to drop a voltage from 20V to 15V, then they burn off 5V as heat, and give you 15V out.

So you can't deliver 13.5V from a 12V supply, through a linear regulator.

LM317 isn't very efficient, so might need the input to be 2-3V above the output.  For two regulators, that's 4-6V.

Going by worst-case, that means that for 13.5V output, you'd need 19.5 or above input.

Better to look for a used laptop powersupply, those can often be up to 20V.

tld

[aaron]

Oops, yeah I missed that, my bad.
Guess I would be looking for something 20v to 24v, I'm sure I have tons though.

[aaron]

Well I went and got some missing parts (.1uF Caps and Fuses)

Here's an updated schematic: http://puu.sh/2kIbx

Apart from the resistor issue (Still gotta get a higher wattage 1Ohm one) does this look good?

And you stated I would likely need more caps, I did a little research and according to other schematics I need to have a .1uF cap on the input, as in my schematic, but none of the websites seem to explain why, would anyone know? It's such a low value, just doesn't make sense how it could have an effect, that's all.

Thanks 

[aaron]

Bump 

[stryker]

Hi,

And you stated I would likely need more caps, I did a little research and according to other schematics I need to have a .1uF cap on the input, as in my schematic, but none of the websites seem to explain why, would anyone know? It's such a low value, just doesn't make sense how it could have an effect, that's all.

Section 6 of the datasheet, starting page 9, explains that it's one of the external components which can be used to improve the performance of the regulator - link here.