# LM317 Sealed lead acid charger

I have a couple of Yuasa NPW36-12 12V 7.2Ah batteries from a phased out UPS unit. The batteries are barely one year old and have less than 100 charge cycles on them. I don't have a lead acid battery charger and thought I could whip something quick on perfboard using the LM317s I do have lying around. This is what I came up with:

Basically a constant current/voltage charger which works for other battery types. Input is 19V from a laptop brick charger (3.5A) and R1 is made up of 2 resistors in series, each with a rating of 10W. I aimed for something close to 14.7V on the output so that I can compensate for the drop on U2 and D1.

Now to my question:

1. Is this a viable solution for an overnight charge since I'm charging at about 0.1C?
2. Will it harm the batteries if left attached for longer periods of time?
3. Should I worry about gassing at V>14.2V?
4. Lastly, I saw one where only one LM317 is used in conjunction with a TL431 at the adjust pin. Would that be a better option? I can attach the image if relevant.
• (1) Shouldn't your U1 pin 1 go to the other side of the current sense resistor R1? (2) Have you added up all the voltage drops across U1, R1, U2 and D1? Is there enough left to reach your charging voltage. (I'm only asking. I haven't done the sums.) Commented Jun 18, 2017 at 9:05
• oops, an oversight in joining using the eda at hand. On breadboard, output of U1 is connected after R1. Pulling around 800mA, I would be dropping almost 2V across U1 and U2 each. D1 drops around 0.6V at 25°C, corrected to 0.8V for my application. I guess I didn't consider R1 since I thought it is part of the U1 voltage drop. Either way, I have almost 4V margin from input to output. I can measure the voltage drop all over and post again if desired. Commented Jun 18, 2017 at 9:42

You don't need U1. Both U1 and U2 have the same nominal current limit. Just readjust R1.

You don't need D1, with its variable voltage drop, but in its place, you would need a diode from U2 out to U2 in, to power the input in the event that you connect the battery without input power.

Once you have removed D1, set U2 to deliver 13.8v. This voltage will safely float sealed lead acid indefinitely.

Heatsink U2 appropriately for its expected dissipation. While the current limit of the TO220 version is specified to be >1.5A, it can often be higher, I've seen 2.2A. The 317 is thermally protected, which means if it gets too hot, it will shut down for a while while it cools down, so an inadequate heatsink means not a fried 317, but a slow charger. You can reduce the expected dissipation by increasing R1, which moves the heat from U2 to R1.

With those voltages, you might want to consider making R1 an incandescent bulb. The strong positive tempco of the filament resistance means they are sort of constant current components, much more so than a resistor. For example, a 12v 12 watt halogen will draw the thick end of 1A at a large range of voltages across it. A bulb has this neat trick of being able to lose a lot of power in a small space safely.

Some sealed lead acid cells state that you can use a higher charge voltage if you time the over-voltage carefully. Doing this will result in a quicker full charge. Check the book of words for your specific cells carefully to avoid destroying them like this.

• Won't that let the LM317 quickly fry as it's going to deliver its max rated ~2A at the output when charging? That is why I have U1 to limit the current. I thought SLA batteries in general liked being charged at about 0.1-0.5C.. Commented Jun 18, 2017 at 9:46
• @jm11011 updated answer. Commented Jun 18, 2017 at 10:11

Just a warning: If you care about a long service life from the battery, you can't toss it together. There's a precise science to charging lead-acid batteries. It requires enough sophistication that I consider it a waste of time to hand-build one.** A "12V" (6 cell) charger is the ultimate commodity. Building one of those is like building a USB charging block.

A circuit as simple as the above, if charging at 0.1C, will then overcharge the battery. If you turn down the voltage to float level, then it won't charge very quickly. A 2-stage charge (high rate on a rundown timer switch) is a better choice but really for best performance, go 3-stage.

Familiarize yourself with the other oddities and annoyances of the lead-acid chemistry, e.g. if you discharge more than 30% of the nameplate capacity, you are prematurely aging them. I generally recommend looking for any other chemistry.

** unless you are working in unusual voltages, e.g. older boats and train cars use 32V (16 cells). Then you often don't have a choice but to fabricate, as the few vendors who support odd/arbitrary voltages want a king's ransom for their units.)

• 3 stage charging is not necessary, it just permits a faster charge cycle. If the battery is kept at room temperature then float charging at 2.2-2.3V per cell is fine. On my alarm panel I used an LM317 set to 13.6V and the original battery was still working after 20 years on float. industrial.panasonic.com/cdbs/www-data/pdf/ACD4000/… Commented Jun 18, 2017 at 21:16