Am I right to doubt the capability of the LM317L in Bill's design above as it is temperature compensated and therefore the voltage requirements can be low or high, requiring a voltage source approaching 20V. With 20V in and possibly as little as 12V out I would think there might be some heat issues.
Wouldn't want the built-in heat safety mechanism of the tiny T0-92 pkg to work like an oscillator!...although I was considering a "Pulsed" maintenance charging routine.

Uhh, the only LM317L was mentioned in your post. I stated this circuit...

would deliver 100ma if it were shorted, which is true. I suspect the part number is a LM317T, in a TO220 package. Total max current 1.5A, but I brought it down a lot to keep things cool.

The other two schematics are interesting, and I would probably use Wookie's if I redesigned this circuit.

 

I have been following this thread with great interest. Does anyone have any thoughts about charging a bank of four 12V 12Ah SLA batteries connected in series? I have already had 8 batteries destroyed because the chargers failed to switch to float charge. I have now resorted to charging each battery individually which means having to open the battery casings.

 

I have been following this thread with great interest. Does anyone have any thoughts about charging a bank of four 12V 12Ah SLA batteries connected in series? I have already had 8 batteries destroyed because the chargers failed to switch to float charge. I have now resorted to charging each battery individually which means having to open the battery casings.

What casing would you have to open if you ave four 12V 12Ah batteries. You would just need to disconnect the wires that create the 48V.
A "smarter" smart charger is what you need, one that will time out after a preset charge time. If the charger only switches to float when a certain current is sensed, then you will have this problem time and again, especially with multiple series connections.

 

I think MrChips means that he has to open up the container that the batteries are housed in.

One way to get around the issue of having to disconnect the batteries to charge them would be to have four isolated supplies driving regulators; but you really wouldn't want to buy something like that - too expensive.

Charging four batteries in series would be OK when they're new and all the same, but they can age at different rates. When one battery develops a shorted cell, the remaining batteries will be overcharged, causing them to fail pretty rapidly.

 

I found my bag of adjustable regulators and have a few that might offer a possible alternative to the 317 regulator. Most importantly, I have the parts already.

1) LM2941CT
Io -1A, Vo-5V - 20V, Drop=Out - .5V

2) LM2675N-ADJ
Io - 1A, Vo - 1.2V - 37V, Simple Switcher(DIP-8)

3) LM2575T-ADJ
Io - 1A, Vo 1.23V - 37V Simple Switcher

 

Well, those regulators would require a complete redesign of the LM317-based regulator schematic, as their reference voltage is between the GND and ADJ or FB pins, rather than between OUT and ADJ like the LM317. You couldn't plug any of those into the existing schematic and have them work.

 

I suspect the 2941 would be the closest pop-in solution. I do have the LM317T and should probably just use it for now.

Does anybody know if National's Webbench online sim is working? I was testing a few designs out yesterday and it did not allow me to build or purchase the parts. Is this because TI bought them out...maybe?

For a 60AH SLA the float current should be about C/100, correct? 600mA. Is that not more than the self discharge rate?

 

I think MrChips means that he has to open up the container that the batteries are housed in.

One way to get around the issue of having to disconnect the batteries to charge them would be to have four isolated supplies driving regulators; but you really wouldn't want to buy something like that - too expensive.

Charging four batteries in series would be OK when they're new and all the same, but they can age at different rates. When one battery develops a shorted cell, the remaining batteries will be overcharged, causing them to fail pretty rapidly.

SgtWookie is dead on! I think one bad battery is killing the rest.
I am now planning on bringing out connections to each battery to a charging connector so that each battery can be monitored and charged individually.

 

From Wikipedia: http://en.wikipedia.org/wiki/Trickle_charging

A float or trickle charge might be as low as C/300 (a 300-hour discharge rate) to overcome the self-discharge. Allowable trickle charging rates must conform to the battery manufacturer's recommendations.
For a 12 V 60 Ah battery a C/300 rate would mean 60 A / 300 = 0,20 A = 200 mA

 

I just remembered - A year or two ago, I started to design a "round-robin" type charge controller (I think iONic was involved), but there were just too many things going on at the time, and I forgot about it. Never really got beyond the conceptual stage.

I'd started out thinking I could get away with using N-ch MOSFETs on the low sides of the batteries, but then quickly realized that wouldn't work due to the body diodes.

You'd really need something like DPDT relays for each battery that was being monitored/charged, switched in one at a time using a microcontroller. A 4017 5-stage Johnson counter was briefly considered to do the switching, but that was a rather "dumb" solution; really not much of a way to figure out if one or more of the batteries were bad.
[eta]
If individual battery temps were going to be individually monitored, a 3pdt relay would be needed per battery, or the uC would need a lot of I/O pins that can connect to an internal ADC.

 

From Wikipedia: http://en.wikipedia.org/wiki/Trickle_charging

Wikapedia to the rescue! Maybe the C/100 was a trickle charge rate whereas C/300 is the float charger that can be left on indefinitely.

I'll have to see if I can find out what chemistry the battery is, LA or GEL. It will be important to tweak the circuit.
And perhaps I need a two stage charger... Trickle - Float. So much for decisive thinking. This battery is already half in the bag and barely has the CCA to start the car in. I truly suspect sulfation to be the mail culprit. I should just replace it and use the desulfator on it or limp it through to the end. I think I will stop with the over-thinking and use the 317 circuit pretty much as is and buy a new battery in the spring. My riding lawnmower batter is in the same boat. The desulfator circuit is done except connecting the power and batt terminal wires, and scoping it out......ha that means it's only half done!

 

I just remembered - A year or two ago, I started to design a "round-robin" type charge controller (I think iONic was involved), but there were just too many things going on at the time, and I forgot about it. Never really got beyond the conceptual stage.

I'd started out thinking I could get away with using N-ch MOSFETs on the low sides of the batteries, but then quickly realized that wouldn't work due to the body diodes.

You'd really need something like DPDT relays for each battery that was being monitored/charged, switched in one at a time using a microcontroller. A 4017 5-stage Johnson counter was briefly considered to do the switching, but that was a rather "dumb" solution; really not much of a way to figure out if one or more of the batteries were bad.
[eta]
If individual battery temps were going to be individually monitored, a 3pdt relay would be needed per battery, or the uC would need a lot of I/O pins that can connect to an internal ADC.

I believe this was the discussion:
Intermittent Battery Float Charger

It didn't deal with series connected batteries but rather using a single charger
to maintain series of out-of-use batteries. It would essentially change which battery was connected to the charger apply a top off charge and switch to the next battery. Trying to charge a set of series connected batteries would be a hole other ballgame, lots of wiring and DPDT relays I guess.

 

Yes, that was the thread; and now I remember that I was pretty distracted with illness at that time.

 

Yes, that was the thread; and now I remember that I was pretty distracted with illness at that time.

...and I had started to build it and was distracted by what usually distracts me....just about anything.

 

Thus with the circuit bellow:

From a 50AH battery(changed from 6AH)
Float charge of C/300 = 167mA
R4 Float(R4Fl) = .62V/.167A = ~3.7 Ohms

and

Trickle Charge of C/100 = 500mA
R4 Trickle(R4Tr) = .62V/.5A = ~ 1.2 Ohms

R2(pot) = 500 Ohm??

R3 in the prior version is replaced by Q2, R3, and R5. Q2 must be thermally coupled to the battery positive terminal. This is a very simple and cheap (1 resistor, 1 transistor) change to get a greatly improved charger.

Values for R3 & R5 were selected to give a reasonably close match for the temp compensation required. As the battery temp goes up, the float voltage is decreased.

Decreasing the resistance of R3 and R5 while keeping the ratio of resistances proportional will decrease the temp compensation effect. Total resistance of R3+R5 should not be reduced below about 2k Ohms.

Changing the ratio of R3 to R5 will change the voltage drop across Q2, thus affecting the output voltage.

 

Look at the later schematic. I inserted a Schottky diode to keep the charger resistors from draining the battery if the power went off.

One problem with either of these circuits is that it won't recognize if a cell is shorted. Shorted cells will cause the charger to output maximum current; it'll run your power bill up. An intelligent charger would indicate a failed cell after trying to charge it for a bit.

 

Look at the later schematic. I inserted a Schottky diode to keep the charger resistors from draining the battery if the power went off.

One problem with either of these circuits is that it won't recognize if a cell is shorted. Shorted cells will cause the charger to output maximum current; it'll run your power bill up. An intelligent charger would indicate a failed cell after trying to charge it for a bit.

Good to know. I may even have one of them in my collection. I will have to get cracking on that BQxxxx battery charger then.
Can I assume that I can adjust R2 for use with a 6V SLA Battery?

 

R2 is 500 Ohms, but as it is, it's set to about mid-range.

I never tried it for 6v. However, I don't think you'd be able to use the existing temperature compensation circuit aka "rubber zener" with 6v.

A rough calculation tells me that you'd only be able to vary the output voltage by about ±2.6v at room temp with the temp compensation circuit in there, so no.

 

One problem with either of these circuits is that it won't recognize if a cell is shorted. Shorted cells will cause the charger to output maximum current; it'll run your power bill up. An intelligent charger would indicate a failed cell after trying to charge it for a bit.

A crude monitoring device one might be able t use is those WATT Meters devices that record Current, Voltage, KWHrs, Power...