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I need to build a lead acid battery charger

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electroyas

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I know there are many circuits available in the web about this, but as i understand they are not optimize for saftey of the battery or life time. What are the most importants point when making a battery charger for safety as well as quick charge without taking whole day.
 
1. Current limited by charger during initial charge-up (limit printed on battery)
2. Voltage limited by charger during charge accumulation phase. (2.400V/cell)
3. Lower voltage limit during indefinite float charging. (2.200V/cell)
 
Thanks for the quick reply, I was wondering how many opamps are needed to build the circuit without lot of circuit parts for long life time and easy replacement if ic get damaged. I like opamps because of their easy equations
 
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Two or three? Building a lead-acid battery charger is more like building a power supply; you need a transformer, rectifiers, heatsink, power semiconductor (Bipolar or FET), voltage reference, etc. For voltage limiting, you could use an opamp, or an IC voltage regulator IC.

It is much more than an opamp or two...
 
I forgot to mention what i meant by opamp was only for controlling part, other power section is needed for all circuits working from AC supply.
 
Hi,

Well it also depends on what size LA batteries you want to charge. For example, if you only need about 1 amp charge current you can build a charger with an LM317. If you need higher, you can use a higher current equivalent IC. These kinds of chargers are pretty simple to build using one IC and a few resistors and NO op amps required.

If you need higher current though you may be better off with a switch mode charger built using a buck converter topology. These can go up to any current level.

If you want to get more sophisticated, you can go to a micro controller chip and really make a very nice charger.

I often thought about modifying an older PC power supply which has high amperage 12v output. The voltage regulation circuit could be changed to put out more like 14.2 volts or something like that and that would boost the output enough to use as a charging source. A few more parts and that would be all that is needed. Never got around to it yet though :)
 
I would like to make the circuit simple so no switching no microcontrollers, LM317 is for fixed voltage , that means it takes long time to charge even small capacity battery. Nice idea about power supply thing, i guess if you want to boost it up just find where the feedback is coming to the control circuit , fix an external variable resistor to change it.
 
One of the first things you need to do is decide what capacity battery you want to charge. This is because to charge it fast you need to know the maximum current allowed. Lead acid batteries don't charge real fast in any case - several hours for a dead one.
So the question is what capacity??
 
I would like to make the circuit simple so no switching no microcontrollers, LM317 is for fixed voltage , that means it takes long time to charge even small capacity battery. Nice idea about power supply thing, i guess if you want to boost it up just find where the feedback is coming to the control circuit , fix an external variable resistor to change it.

You can use two LM317 devices in series, the first one has a single resistor that sets the max current. The second one is a voltage regulator that fixes the voltage at 14.5v, for the bulk charge phase.

Then if you want auto charging completion you need a way to switch the second LM317 from 14.5v to 13.6v to go to the final float charge phase. That is actually pretty easy, you sense when the charging current at 14.5v gets low enough, and when it gets <X amps you switch to 13.6v. That takes one opamp with some hysteresis.

Using the two LM317 chips gives you a bulletproof design as the voltage regulator and max current regulator are independent and reliable, and the LM317s also have fault current limiting and over temp limiting etc. If you just build from opamps and transistors you won't get that extra level of reliability.
 
I would like to make the circuit simple so no switching no microcontrollers, LM317 is for fixed voltage , that means it takes long time to charge even small capacity battery. Nice idea about power supply thing, i guess if you want to boost it up just find where the feedback is coming to the control circuit , fix an external variable resistor to change it.

Hello,


Well actually how long it takes to charge depends on how you set up the circuit. If you limit current to 1 amp you can charge just as fast as any other circuit as long as 1 amp charges fast enough. If 1 amp does not charge fast enough then you go to two or more LM317's or a higher powered similar device rated for higher current. You can get away with current limit as a single power resistor or you can use MrRB's idea to use another LM317 to limit current, or you can use a very small transistor with current sense resistor to limit current. There's a number of ways to do it. These circuits are about as simple as you can get and still get decent charging, and as MrRB also says they are pretty solid designs too. If you need more info, just ask :)
 
Also a LM317 will supply up to approx 1.8A where it's own current limiting kicks in, and is good for 1.2A or even 1.5A continuous in normal use.

I charge a 12v 7Ah SLA battery regularly, it takes about 1 to 1.5A for a couple of hours initial charge at 14.4v (and the current drops steadily during that process) then when the current has dropped to 200mA after a few hours at 14.4v I reduce the voltage to 13.8v and give it about 5 hours after that.

The point is that a LM317 will charge a large 12v SLA battery of 7Ah just fine. :)
 
Hi Mr RB,

Yes that was my point too :)

I do a lot of this myself and find that 2 amps works good for my 12v batteries around that same ampere hour rating. But 1.5 amps would be fine too. I think mine are rated for a max of 2 amps charge current or something like that too, so we cant go too high with these things anyway. The bigger ones like 50 ampere hour need a heavier duty charger though so im not sure what we would do there, we'd have to design a much higher current charger. But i think linear might be not such a good idea there unless the incoming voltage was not too high. For that through i would go to a switcher. When i charge mine most of the time i use a linear but with a switcher front end, so i get the best of both worlds...decent efficiency and smooth output, not that smooth output is mandatory here though, but what the heck if you already got it, might as well use it :)

On the other hand, i often have to charge Li-ion too and as you know it's the same algorithm just with tighter specs. And i wanted something that could work off of many different wall warts like 9v, 12v, 24v, etc., so i designed that circuit with a switcher so that it could take all those voltage inputs and not waste power. That's a good idea for charging batteries because they dont need a super smooth output anyway as i am sure you know. Amazingly, i even found that it works with a 6v DC wall wart and that was unplanned! It works because the peaks of the unregulated output are higher than 6v and that gets filtered by the units input capacitor a little more too. So i lucked out there. Max DC wall wart is about 24v unregulated but if it was regulated it could be higher.
 
How about the charger comes with the solar panels, they charge in a on off between 14 and 13V .is it safe for sealed type battery rated 35Ah?
 
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