I'm in search of a schematic (preferably not a finished product, as I am limited for space in my box), for a basic 12V SLA Battery Trickle Charger, that is capable of keeping a battery on trickle-charge almost indefinately. I need it to be as simple as possible. I am not too concerned about the speed of the charge, even the battery is slightly discharged.
Basically, my intention is to connect a fully charged battery to this charger for the first time (so the charger won't have to do much work I guess), and the charger is to keep maintaining the voltage for me until the battery power is "diverted" to a connected PCB via a relay or switch. It's for an emergency backup lighting system I am designing, so the charger will be built into the box, similar to that of an alarm system. I am aware you can purchase plug-in SLA battery chargers and the like, but that's not what I'm after, because I don't want to have to manually charge and monitor it.
Just use figure 10 and add the protection diodes. Choose 'Rs' (current limiting) to be fairly high, 10 ohms or so 5W wire wound. You don't really need a heatsink as the LM317 has thermal limiting, but it will get quite hot when the battery is being 'charged'!! But if you can mount it on some metal all the better.
The resistor values in the circuit will give 13.75 volts. If you can't get
It depends on how simple (aka pushed for time/money/space) Jason wants it to be.
Ideally, a charger based on the Unitrode/TI UC3906 which provided the proper care and feeding of SLA's. Kits available still AFAIK (Jaycar etc)
Next scheme I would use is that described by John.
The fig 10 approach has the charge current taper off as voltage rises, progressively to a poofteenth of what it starts at. This asymptotic charge curve is not real clever if he wants decent recovery time after his emergency lighting load has been connected. The two-in-series approach gives you constant current up to the point where the second reg chokes to death.
I have test gear costing >$20K that came with the "Fig 10" arrangement, and it roots batteries very effectively. In the end I reconfigured it to save my $$.
I agree, depends on how 'simple' simple needs to be. Typically emergency lighting is only used 'once in a blue moon', but that depends on your local electricity supply.
Figure 10 has limitations, but it is 'simple'. Battery life can be extended by dropping the voltage though. There is also no temperature compensation which the UC3906 does have - something else to consider.
The simplest scheme I have seen comes in a rechargeable torch made in China. A series resistor! Contains a 6V SLA battery charged from 12V. Instructions warn 'do not charge the torch for more than one day'....! I can see these being 'buggered' by the 1000's in very short notice. So they sell more torches - more turnover.
Yes I know the LM317... handy device! I see what you're getting at, but let me confirm with you, incase I misunderstood =)....
We use the 'Voltage Regulator with protection diodes' circuit (figure 5) with its output connected to the Input of the 'Current Regulator' (figure
7)? Also the R1 value for figure 7, what value would you recommend (including appropriate wattage), for the battery assuming it is already "fully charged" (let's say)? I don't get the formula for it =(
I don't like the sounds of the fugure 10 approach! What did you mean by the two-in-series approach? Were you referring to the Figure 5 and 7 approach recommended by John? If the second reg would choke to death, is there a way to somehow overcome this hurdle?
R1 determines the maximum current. The maximum current will be a constant current. Build the circuit and have a play with it. Connect different value load resistors and even a dead short and check the current for yourself.
To make your charger a bit nicer I was suggesting this
trfr/rct/cap ----> in (figure 7) out ----> in ( figure 5) out
If you just used figure 7 on its own. you have a constant current continuously passing 0.39 amps into your battery. This is fine if you disconnect it after 12 hours or so.
By adding the voltage regulator stage after the current regulator stage your battery charger will still deliver 0.39 amps max, when the battery is discharged but when the battery voltage rises to equal the voltage regulator output voltage, very little current will flow. The current "throttles" back. Budgie made a joke "where the second reg chokes to death" Get it? throttling... choking ...squeezing the life out of ... The current reduces to near zero current at the specified output voltage, set by you..
Lets say you put a heavy load on the battery of a couple of amps for a long time. Your charger would deliver 0.39 amps to the load, helping the battery. This is OK. This is good. When the 2 amp load current was removed the battery will receive a maximum current of 0.39 A from the charger. This is also good. As the battery voltage rises and nears the output voltage of the voltage regulator stage the charging current will reduce. The charger changes from a constant current mode, 0.39A maximum, to constant voltage mode at less than 0.39A, the current eventually reducing to nearly zero amps. While in this float charging mode, any little load on the battery, below 0.39A, the charger will take care of it. A big load on the battery and the charger will help up to its 0.39 A limit.
I hope that is a little clearer for you Jason. Make up the two circuits and play with them individualy and combined.
Ok, makes better sense now. So you recommend the following connectivity for such application: Transformer --> Rectifier/Smoothing ---> Current Regulator (fig7) --> Voltage Regulator (fig5) --> Battery.
I will build the test circuit on a breadboard soon, but I dont have a couple of the parts right now. I really appreciate your time to assist me with this in detail. =)
Figure of speech. Maybe I should have said "drops out of regulation".
The current source will limit the current through the voltage reg. A constant current source is really a variable impedance that drops excess voltage so that the load will see a constant current. If you place a dead short on a current source, you see ... the constant current into that short.
So the voltage regulator will determine the maximum voltage that the load (battery) can get from the duo, while the current limiter will do the same for the current under recovery after discharge.
There's also a circuit there in the data sheet for a current-limited voltage reg, but it's not really any getter or simpler to configure than the tandem arrangement John suggested.
I personally use the UC3906 on 6V SLA's and wouldn't change.
hmmm, everybody has different opinions about chargers. I really don't know now =( It says it can be left overnight safely. What about months, or even a few years later when it needs replacing? See, that's what I need. The battery is to be kept "topped-up" until it is required.