LM350K as a charger

"Jason S"

** Then find a schematic intended for exactly that purpose.

The usual method is to use a *current limited* 13.8 volt DC regulator.

A regulated 13.8 volt supply with a suitable bulb will do at a pinch.

........ Phil

If you google for - electronic circuits for RC models - you will find a number of circuits for SLA chargers, and some good info about them

David

Jas> Hi, I need someone who knows a little about LM350 voltage regulators...

(snip)

If you really want to charge a SLA battery, I'd suggest using a smarter circuit/device - have a look at the Unitrode (now TI) UC3906 at:

That adaptor would be quite suitable for a charger based on the UC3906. There was a ?SC kit using them - may still be around ay Jaycar, Altronics or TrickieDickie's.

On Sat, 22 Oct 2005 15:46:22 +1000, "Jason S" put finger to keyboard and composed:

If we ignore the contribution of the LM301, then the regulator's output voltage is given by ...

Vreg = 1.25 [1 + R1/(R2 + R3)] = 16.6V

If the output of the LM301 is 0V, then ...

Vreg = 1.25 [1 + (R1||R4) /(R2 + R3)] = 14.0V

The LM301 compares the voltage drop across R2 (76.5 mV) against the voltage drop across R6 (0.2 x Icharge).

The charger is kickstarted by momentarily disturbing the offset of the opamp via the Start switch.

The circuit attempts to maintain a constant charging current of 383mA. It does this by sensing the charging current in R6 and adjusting the charging voltage via the LM301. The regulator voltage is controlled by varying the resistance at the ADJ terminal of the LM350.

Any general purpose PNP transistor (eg BC557) should work OK. A 1N4148 or 1N914 signal diode should be fine in place of the 1N457.

The LED lights whenever the charging current is low and goes out when the current exceeds ~400mA.

I don't see an R7.

In any case I would avoid this circuit for several reasons, the main one being that it provides no current limiting (other than that provided inherently by the LM350) in the case of a dead flat battery, or in the case of a collapsed cell. A better approach is to use a

13.8V constant voltage, current limited charger. WES Components have one for about $20 to the trade. If you'd like to build your own, then I can scan the circuit of a commercial SLA charger for you.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

"Franc Zabkar"

** WES have several SLA chargers, the smallest is rated at 0.5 amps and others are 1 amp and 2.7 amps.

The 0.5 amp (Arlec PS664 ) model is suitable for SLAs rated at 2.5 Ah or greater.

The OP says he has a 1.3 Ah SLA.

The Jaycar MB-3517 SLA charger ( rated at 350mA) is more suitable.

BTW

Small SLA chargers are cheaper than the cost of the parts to build one.

........ Phil

Yes I know that, but that doesn't apply to me, as I want to incorporate it into a circuit. I want to be able to control how the charger works. I dont want to have an external transformer for the charging. I just want a transformer that can be used for 2 purposes: A charger, and a power supply for the rest of the circuitry.

"Jason S

** No one gives a flying f*ck about YOU - Jason Shithead.

Points of fact are made for the sake of accuracy and to benefit others reading now or possibly years later.

........ Phil

*** Hi Franc!

Thanks so much for your effort to respond the way you have, question by question. I admit I made an error with the R7. I actually meant the R6 (0.2 ohm), sorry. I thought the 10W 'R6' had something to do with current limiting, no? Can't current limiting just be added to it, or is the circuit in general still 'not-so-good' in your opinion? What were your other reasons, just out of curiosity?

Anyway, when the LED lights up, does the charging actually stop (to a certain degree) as well?

Yes, I want to build my own because I want to be able to incorporate it with other circuits at a later time (that require battery backup), and I don't want to have an external battery charger plugged into the wall... i know they exist, and are handy, but it doesn't suit my requirement.

### SCENARIO ### Incase you don't fully understand the scenario (I dont want the wrong charger type!), maybe this will give you a better understanding............. Basically, I intend to have the battery isolated from both the External Load, and the Battery Charger (separated by relay contacts). The battery is only used when it is needed (for backup). Instead of having the battery connected to the charger at all times (keeping it on trickle charge), I will probably just have a battery voltage checking circuit that monitors the voltage of the "isolated" battery at regular intervals (baring in mind batteries self discharge), and if the voltage is getting low (around 11.8V), a relay will connect the battery to the charger. The charger will do its job, but when the battery gets full, it either stops charging, or somehow signals out to the relay driver that the battery is now full and to disconnect the battery from the charger again.

- Battery Voltage Monitor : "Battery, are you getting low dear?"

- Battery : "Yes! Charge me!"

- [Relay connects battery to the charger]

- [Charging commences]

- ...Time...

- [Charge is complete / Relay disconnects battery from the charger / Fully charged battery "isolated" again, ready for use].

P.S. Your idea of scanning a commercial SLA charger for me sounds great > If you have a scan for me, let me know and I'll give you my email address. But I would need you to briefly explain what the charger does, especially at the end of the charge cycle. Sorry for the long email... might as well say as much as I can, instead of emailing back and forth hundreds if times!

Thought I might as well make use of that 18V 1.2A transformer that I can no longer return now =(

Thanks again, and best regards,

Jason.

yeh, I believe it - but in my case, I have no option - I need an "onboard charger".

Thanks for the reply (better than your 'first' one anyway!). But what happens if I dont want to use an External "plug-in" charger? Is there a good schematic somewhere I can use (if so, what do you like about it)? You see, the battery is going to be used as a "backup" battery, and I don't want to monitor it myself - so the charger should be fully automatic.

"Jason S"

** Why do you so rudely IGNORE what is said to you ???

Are you a disciple of that Mark Harriss imbecile ???

ALL you need is a voltage regulator with **13.8 volts DC ** output that is CURRENT limited to around 300mA. This will charge the 1.3 Ah SLA at a safe rate and then hold the battery with a tiny trickle of current at 13.8 volts. A 12 volt, 6 watt bulb will do in series with the 12 volt battery and an LM317 set to 13.8 volts.

This is the standard and preferred method of SLA charging !!!

Get it ???????

......... Phil

I don't recommend that you try to determine when the SLAB is in need of charging by measuring the voltage, since the battery voltage is pretty constant over a wide range of the possible state of charge. You might find that when you need it, the battery is only 10% full.

If you want the charger to be off most of the time (e.g. to reduce the standby power consumption of your installation) then I would suggest just turn on the charger for 1 hour every week, plus whatever time is necessary after the battery has been on-load.

I would suggest you buy (or if you insist, build) a normal, voltage regulated, current-limited SLA charger. Check with an ammeter to make sure that it does not discharge the battery at all if it is left connected with the mains switched off. Then put a timer and a relay in the mains to switch it on for one hour a week as I suggested above. Perhaps one of those plug-in timers would do. It would even have electrical approval too, not that this says much about electrical safety...but more about legality etc.

If you find that your charger =does= draw some leakage current away from the battery when the mains is off, then you might be able to arrange a relay in between the battery and charge that is switched on when the charger is on.

Chris

Jason, you'll lose so little by expanding your kill-file by one, you'll wonder why you didn't do it earlier.

Cheers.

Ken

On Sun, 23 Oct 2005 18:16:24 +1000, "Jason S" put finger to keyboard and composed:

R6 sets the charging current according to this formula:

Icharge (mA) = 76.5 / R6

It doesn't actually *limit* the current.

You could add current limiting to the circuit, but you could achieve the same end more cheaply using an alternative approach.

One other reason I don't like this circuit is that it charges the battery as a current source rather than as a voltage source. I think you'll find that the latter is the more widely accepted approach for SLA batteries.

It seems to me that the LED's brightness can vary throughout the charging process. I'm not quite sure what its exact function is.

Here is the circuit out of a camcorder battery pack, WES code VBV860.

It's basically just an LM723 regulator with a TIP41C pass transistor. Some component values are missing but the principal info is all there. Note that although the unit is sold as a 6V battery pack for 6V camcorders, it actually contains four series connected 2.0V SLA cells. I've had to modify mine for 6V using a 6V SLA brick and some minor circuit changes. You will need to change some resistor values to convert it for 12V (13.8V) use.

R4 determines the output current limit, as follows:

Icharge(max) = 0.6V / R4

A value of 0.56 or 0.68 ohm should give you a 1A max charging current.

The potential dividers formed by R16/R17 and R6/R18 determine the charging voltage.

Transistor Q2 needs to be heatsinked.

FYI, there is a discussion going on at sci.electronics.repair regarding a homebuilt SLA charger. It uses a Unitrode UC3906 sealed lead-acid battery-charger IC:

It may be just what you need.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

really? I wasn't aware of that!

Ok thanks Chris, I'll take that into consideration. Thanks for your reply.

Jason.

Thanks for your reply. I've had a look and the UC3906-based circuits look interesting. I'll consider this in my design.

Jason.