I know I have multiple threads going on on here right now, so I hope I am not asking too much but I have another question I am confused about.

When it comes to battery charging, I have found there are some very complicated (or at least, they look complicated to me) charging circuits out there.

My question is this: Why can't I just connect a 12v "wall-wart" AC-DC transformer to the 12v battery and charge it?

I know that this doesn't work out right and could damage the battery, but why do I need an overly complex circuit to charge it?

I have read about different types of charging, and if I don't mind a slow, maintenance style, "standby" charge for my battery, what do I do to "float" charge it?

I read somewhere that as long as you supply only about 25% of the current rating on the battery, you can't over charge or damage it while charging? Maybe I misunderstood this.

So, take this for example. I have a 12v 2.9aH battery. So if I connected a 12v 700mA "wall-wart" type transformer directly to it, would it safely charge and just remain fully charged indefinitely as long as its' connected?

I also read, though, that a higher voltage than the battery may be needed to charge it?

From the manufacturer of a specific battery, the Power-Sonic PS-1229 (a common battery I use for multiple applications) it states:

“Float” or “Stand-By” Service: Hold battery across constant voltage source of
13.5 to 13.8 volts continuously. When held at this voltage, the battery will seek its own current level and maintain itself in a fully charged condition.

How can I simply achieve this voltage? I have a mega-surplus of 15v 500mA AC-DC adapters on hand - is there something I could do with these?

 

You can use a LM317 IC (very simple, it only has 3 pins) and two resistors to give you a regulated voltage of 13.8v DC from a supply of >16v DC. Google for "LM317 voltage regulator" there will be a ton of info.

Remeber this won't give you perfectly charged battery to full capacity, but is functional enough for battery backups etc and is done all the time in alarm systems and UPS devices.

 

Was looking around and came across this:

If I understand correctly, I can put any voltage between 3-40v into it and get 13.75v out using R1 = 330, R2 = 3300 ?

Does this mean I could use the 15V adapters I already have with that simple circuit and safely charge any 12v SLA battery? Or am I over simplifying here? I noticed in your post you said it would need to be >16v input? Why is this?

EDIT: Was just reading around some more and finally found a site that noted the INPUT voltage must be at least 1.25v HIGHER than the OUTPUT voltage. So that answered my question on that. I am sure someone could explain why, but at this point I think I can just accept it as a spec and call it good. Still, I would like a response from someone who knows, using this circuit to get 13.75V, can I then just apply that to the SLA batteries and float charge them? As long as the batteries are 12v does it matter the amp-hours?

 

Hi

When charging SLA battery's one must take grate care because an sla battery has gel inside where a normal acid battery just has acid in it. If you charge the SLA batt. at too high amps the gel starts to cook and turn hard, or become a solid. The amp hour matters alot as far as it goes for the charging. Read the attached file to get a better understanding of the SLA battery

 

...I would like a response from someone who knows, using this circuit to get 13.75V, can I then just apply that to the SLA batteries and float charge them?

Yes, it'll work fine.

There are some issues. If the battery is discharged when first connected, the current will max out and ultimately be limited by either the LM317 or the battery itself, or maybe the transformer. This max current could be bad for the battery, and you need to find out. If the battery can tolerate charging at 1A, then it's probably not the weakest link. The LM317 will protect itself to some degree and prevent overheat and over-current. I guess you want the LM317 to be the weakest link, since it's the only piece that can protect itself.

The other issue is temperature and drift. The ideal voltage setting for the battery varies with temperature, and I think the LM317 tends to drift around a bit. Maybe it's due to the temperature of the resistor used to adjust it. Anyway, you could set it for 13.75 volts but it won't be there in 5 minutes. It doesn't move enough to be a real problem; I'm just saying you won't have control at the 0.01 volt level, barely at 0.1 volt.

 

the INPUT voltage must be at least 1.25v HIGHER than the OUTPUT voltage. I am sure someone could explain why,

Its a linear regulator that works by dropping voltage across it, thus is can only produce an output voltage less than the input. The 1.25V is the voltage drop across the transistor inside the LM317 device.

 

If the battery is discharged when first connected, the current will max out and ultimately be limited by either the LM317 or the battery itself, or maybe the transformer.

If the transformer was something a 500mA output, would that be enough to help the battery from overcharging/damaging itself? I assume the battery would only draw the max current of the transformer even if fully discharged?

 

I think the LM317 tends to drift around a bit. Maybe it's due to the temperature of the resistor used to adjust it. Anyway, you could set it for 13.75 volts but it won't be there in 5 minutes. It doesn't move enough to be a real problem; I'm just saying you won't have control at the 0.01 volt level, barely at 0.1 volt.

I've never seen an LM317 drift like this and have used them many times. In my experience they don't drift as long as you use quality resistors with a decent TempCo. [ Maybe you used a potentiometer to adjust the voltage? These drift a lot more than a fixed resistor]

As far as tolerance goes, the 100mV you quoted is pretty darn good for a linear regulator used at13.8V . (100mV/13.8V)X100 = 0.72%

 

If the transformer was something a 500mA output, would that be enough to help the battery from overcharging/damaging itself? I assume the battery would only draw the max current of the transformer even if fully discharged?

Right, you cannot get more out of the transformer than some limit. Whether that level is OK depends on the battery. It's a problem if the battery can't tolerate 500mA. Zero problem for a big car battery.

Using the transformer as the weak link could cause it to overheat, since (if it's really just a transformer, not a SMPS) it has no way to limit its own current to the rated max. An overload might cause it to draw 600mA for an hour, and that would be bad.

 

As far as tolerance goes, the 100mV you quoted is pretty darn good for a linear regulator used at13.8V . (100mV/13.8V)X100 = 0.72%

Right, I was just reacting to the OP quoting a voltage to two decimals. Not gonna happen. And yes, I probably used a pot and saw much larger drift because of it.

 

That should work fine as a slow charger. The worst case discharged ("flattest") 12v battery will be about 11v or so, if you put a power resistor before the LM317 you can limit the max current into a flat battery.

ie; 16v DC supply - (11v + 1.5v dropout) = about 3.5v across the resistor worst case.

If you want to limit the worst case current to 800mA to suit your 2.9Ah battery then a resistor of 3.5/0.800 = 4.35 ohms. A 4.7 ohm 5W resistor, or even a 3.9 ohm 5W resistor should be fine.

Otherwise if you have enough voltage overhead, you can make a proper current limiter with another LM317 and a single resistor to set the current, but that will cost you about 3v.

 

Why not use a charge controller chip designed for lead-acid batteries? There are plenty out there that control charge voltage and charge current and have temperature compensation built-in. The TI BQ24450 is an example of an easy one to use. Its a linear controller that uses an external PNP transistor and a sense resistor.

 

Why not use a charge controller chip designed for lead-acid batteries? There are plenty out there that control charge voltage and charge current and have temperature compensation built-in. The TI BQ24450 is an example of an easy one to use. Its a linear controller that uses an external PNP transistor and a sense resistor.

The reason I wasn't using such a thing is becuase I didn't know it existed. I was able to get a BQ24450, but looking at the data-sheets and such has my mind spinning again

How do I know what resistors and transistors and such to use? I simply don't understand all the math that I am sure is involved in the calculations somehow. I'm starting to think this whole electronics thing is just not for me...