lead acid battery charger design

If you really want to do what you asked, you've got a problem...several problems. The voltage current relationship for a charging lead-acid battery is dependent on current conditions and history. Plot the voltage/current curve that you want then compare that to what the battery wants at different parts of the charge cycle. You'll probably find an unstable condition that oscillates.

Not what you asked, but I'd use constant current charge and sense the voltage. When it hits the determined level, switch to constant voltage at whatever voltage you think you want. You're gonna need some kind of digital latch to remember.

I'd probably use a PIC processor. Use the A/D to sense the voltages and the PWM to set the currents. Fewer parts than most anything else you could do. Trivial to customize. mike

No. It doesn't work that way.

Whaddyamean it doesn't work that way??? If you design it that way, it works that way.

Maybe you meant something other than what you said?? If you intend stuffing a voltage source with unlimited current into a battery, you're asking for trouble. If you limit the current, you've got what I described. You can make it more complicated from there if you feel the need.

Measure the dynamic imepdance of the battery, voltage vs. current for various states of charge. Plot the family of curves. If you can draw a straight line that solves your problem for all conditions, you can make a simple linear circuit to do it. I'm betting you can't unless you choose a very limited subset of the set of conditions. I'm betting that even the straight line represents negative resistance. And the battery is nonlinear and time variant. You've got an oscillator.

The way out is to define a sequence of states triggered by the charge conditions. Simplest way to do that is with a processor.

Or you could simplify the requirement.

mike mike

You can sense the current through a resistor, use a current mirror to develop a voltage, use an op-amp comparator to compare the voltage to a reference, and use the output of the comparator to electronically control the constant voltage source for a stepped voltage reduction. There are some nifty power management/battery charger IC's available that will avoid all the design work for you.

Ed

A circuit with feedback.

Actually this can be done with a comparator.

Lead Acid has three main charge phases:

True. That's one of the reasons why voltage/current control for SLA batteries typically has only minimal feedback.

That's bulk charge.

The 14.4V mark for 12V batteries.

Bingo.

As would I.

Exactly. Such a charger is on my project list.

agreed.

BAJ

It does. Here's a web page on the subject.

Bulk charging is current limited. That limit essentially forms a constant current charge. You do switch at a predetermined level. You do switch to a constant voltage absorption charge.

I believe it meets all of the criteria.

BAJ

I won't use unlimited current. In the original post I asked how to implement the transition from the absorption stage to float stage. I read your response in that context. It didn't occur to me you'd be talking about bulk charging! Regarding the original question, I know I saw a fairly simple circuit somewhere that used an op-amp.

found it:

I think we're in heated agreement. Take a close look at your circuit. Ignoring the issues when you hook a dead battery up to it. Let's assume a subset of all possible initial conditions that don't go up in smoke.

U2 is a comparator providing digital latching function. It's a two state state-machine controlled by S1 and the battery voltage.

During the first part of the charge cycle, U2 does absolutely nothing. Vout gets pulled down with prejudice to almost the battery voltage and tracks up as the battery charges and it's voltage increases. So, you have initial charge current limited only by the 0.2 ohm resistor and the current limit in the LM350. Go look up the GUARANTEED specs on the current limit for this part. We have some random current limit that's dependent on the LM350 and the ambient temperature and the capabilities of the heat sink it's mounted on. Once the system comes out of current limit at some battery voltage, the current starts to taper as a function of the vout-vbattery across the resistor R7. When the current drops to the threshold of u2, it's output goes low and reduces Vout. But guess what...R7 is a resistor. Vout is known. So measuring the battery voltage is IDENTICAL in function (mathematically equivalent) to measuring the current (voltage across the resistor). You have three variables, knowing any two determines the third.

So, you get a constant current (of indeterminate value) charge that terminates when the battery voltage reaches the threshold. At that point you have a state transition to lower Vout and remember that state with a digital storage element, u2 in this design.

We're in heated agreement about the process.

IF only the circuit shown had predictable characteristics. Just put a splash cover over the battery so when the LM350 finally gives up and melts itself shorted and the 18V causes the battery to emit hydrogen that's ignited when R7 catches fire....

But heck, it probably works most of the time.

Are we having fun yet??? mike

That circuit INCREASES voltage into the battery as current subsides, until the set point is reached. That does not satisfy what you posted: "What kind of circuit will reduce its output voltage as current into the load subsides? "

Prior to the 150 mA set point: You have a fixed voltage in through a fixed resistance to produce Vout to the battery. Vsource-----R7------Vout Vout = Vsource - Vdrop Vdrop = I*R7

As current goes down, Vdrop goes down. For example:

Vin I Vdrop Vout

So, until you reach the set point, voltage will not decrease as current decreases.

To say nothing of the disasterous design. It is actually worse that what I'll post below, but I'll use the numbers posted at the site.

He says it will start charging at 2 amps with

It gets really ugly if you analyze the circuit.

Ed

Ignoring the issue of why don't you just go out and buy a charger. I got a really nice one from Walmart for $40. The proper way to charge a lead acid battery is:

Please don't take those numbers as gospel, do your own research cause thats from the top of my head from looking into this a few years ago. This also ignores the issue of desulphation which involves periodically injecting high frequency AC current pulses into the battery in order to knock off sulphur crystals from the battery plates. You are trying to resonate the plate so the exact frequency may be difficult to determine a square wave might be more useful but you still probably want to sweep pulse width.

Ta heck with all these pics and op amps. Real men do it with discrete components!

pos power in_____________c e______________________ B+ | \\ / | | \\___/ | |_/\\/\\/\\____| \\ | / | \\ R1 \\c / \\ | | | |_______| | | / / /e \\ R2 | / | \\ | | R3 | |__/\\/\\/\\___ | | | | | | | neg power in | | | |______________________|__________B-

The pass transistor (top line) should really be a darlington... forgive me for not wanting to spend all afternoon drawing ASCII. The R1 and R2 voltage divider defines float voltage. R3 is a power resistor on the order of 0.1 ohms for a small VRLA, or a much smaller value for a car battery. Current into the battery causes a voltage across R3 that has the effect of raising the voltage setpoint controlling the the shunt resistor.

Excuse me, I meant "shunt TRANSISTOR"