hi guys just got a quick question about ajustable regulators for battery charging

[steptoe1111]

hi guys just got a quick question about regulators for battery charging,
im curently using a lm317t 12v 1.5 amp reg, conected to a 16vdc 1000ma plug transformer, just to keep some scooter batterys maintained on a float charge here is the diagram http://i647.photobucket.com/albums/u...Picture201.png
very simple and has been keeping a 7ah 12v seald lead acid at 13.75v for the last week,
instead of the 2.4k resistor i added a 2.2k with a 500k 25 turn vairable resistor in series to ajust the voltage regulation it is adequately heat sinked,
now it does get warm with the 7ah battery, and gets very warm when being used on a 18ah battery, id say that would be the limit for it, wich is why im here to ask
i have a 100ah hour agm battery that gets stored for 7months of the year , and was hopeing some thing like this little circuit could do the same but i dont think the lm317t would cope with that due to heat mentioned above, i noticed the lm338k 3-35v ajustable 5amp reg does the same job,,, its rated at 5amps instead off 1.5 would this be suitable for the bigger battery??? increasing the transformer isent a prob either i have some old junk lying around i was thinking 4-5amps thats roughly 1/20th the rateing on the battery.
like i said it has been working great on these little batterys but id rather check with some pro's before puting on a $300 battery and potentaily damageing it
thanks guys any imfo is greatly apreciated
oh yeh and happy new year to all

[multisync]

the more current drawn from the charger and the lower the battery voltage will effect how hot the lm317 gets not how large is the battery. If a 12volt 2 amp hour batter is completly discharged you will have to supply 2 amps of current or more which the chip cannot do. It is limited to 1.5 amps. If a 100 amphour battery is partly or completly discharged it will charge at a maximun of 1.5 amps again because that is all the chip can supply. You can put a small resistor in series with the battery to limit the charge current ie 10-100 ohms. The power rating of the resistor esp if the value is small will be a few watts. You may also want to look up pulse chargers on this web site to help remove the sulfur from the batery plates.

[wwenze]

Sounds like you're using the LM317 in constant voltage mode, try a constant current circuit.

[steptoe1111]

thanks for that mate,
so the smaller reg should do then..just to maintain the batterys float ,
i use a seperate charger to charge them up, and already have a desulphator
i was just a bit unsure on the heat isue...but what you mentioned above does make more sense... i dident want to come home after work to charcoal and sinders but if any thing the reg will fail and cutout the circuit if it overheats wont it????

[steptoe1111]

yes the design is from that data sheet page 18 listed as.... 1.2V–20V Regulator with Minimum Program Current
whats the diference between constant voltage mode, and constant current circuit regarding this aplication
thanks guys

[wwenze]

Pardon me, i missed reading the purpose of your float charger. Ignore wat I said about constant current charging. Instead, check the amount of current being drawn from the LM317. If it is below 1.5A, you probably just need bigger heatsinking.

To get the full 5A from LM338 you will also need an appropriately bigger heatsink.

[steptoe1111]

i was about to ask what the diference is between .. constant voltage mode, and constant current circuit. the reg should swicth off when it hits the requierd voltage its curently holding a 1.2 ah seald lead acid battery at 13.75v and only drawing 20ma
so id say its full charged
all ill im trying to do is build a simple float charger to leave conected to batterys indefinatly to store batterys, and keep em top'ed up... mainly just to stop em from sulphateing in the cuboard hopefully extending there lives and if i can whip up something from some old junk lying around i will always try that first
like i said i have an expencive 100ah agm battery i bought recently used for 12v 500watt trolling motor that is going to be shelved for 9 months of the year
im kinda hopenig this will be siutable for the job without having to lash out on a smart charger of some sort
heatsinking is no probs ive some old computer heatinks lying around

[Chris Daly]

Hi
As a precision current limiter mode the LM338 delivers exactly the current required. So it can be generally left on, To connect a LM338 this way a resistor equal to Vref /R . In practice as your battery discharges with power drawn from it by a load, the current limiter should then top up the current required to charge the battery. Note it won't supply any more current than what is required, you choose the value of R to maintain the battery, which is in most cases what is needed, but you could have a switch for 2 or three values of R - however use a relay or a automotive type switch to minimize contact loss.

ie Vref 1.25v / 1.2 ohms = 1 amp, or lower value of resistance for more current. The LM338 has the aforementioned setting resistor from its output, The adjustment Pin then connects directly to the load ie at the outer joining point of that resistor.

See page 12 of the LM338 Nat Semi data:
www.jaycar.com.au/images_uploaded/LM138.PDF

Try it out, hope this helps. Cheers / Chris

[wwenze]

Originally Posted by steptoe1111

i was about to ask what the diference is between .. constant voltage mode, and constant current circuit. the reg should swicth off when it hits the requierd voltage its curently holding a 1.2 ah seald lead acid battery at 13.75v and only drawing 20ma
so id say its full charged
all ill im trying to do is build a simple float charger to leave conected to batterys indefinatly to store batterys, and keep em top'ed up... mainly just to stop em from sulphateing in the cuboard hopefully extending there lives and if i can whip up something from some old junk lying around i will always try that first
like i said i have an expencive 100ah agm battery i bought recently used for 12v 500watt trolling motor that is going to be shelved for 9 months of the year
im kinda hopenig this will be siutable for the job without having to lash out on a smart charger of some sort
heatsinking is no probs ive some old computer heatinks lying around

A constant voltage regulator keeps the Vout fixed regardless of load or amount of current drawn, a constant current source keeps output current fixed regardless of load and achieves this by varying the voltage across the load.
Constant current is good for charging battery where voltage of battery keeps changing and you need a consistant charge rate (to prevent too fast charging @ low charge and too slow charging @ high charge, both bad for the battery), constant voltage is good for keeping the battery at a fixed float charge voltage regardless of the self-discharge rate at the voltage.

And unless you have an external trigger circuit to sense the battery voltage and disconnect/reconnect the battery when necessary, the regulator will always be trying to push current into the battery.

Since it is for float charging a LM317 should be enough; even if assuming the battery has an impossibly-high self-discharge rate of C/100, for a 100Ah battery that's still 1A.

[tatus]

Using pure current limiting will eventually destroy a lead acid battery. The LM338 circuit on page 12 does not provide a voltage limit. This type of battery requires constant voltage mode to 13.4V ~ 13.8V (depending on cell type) during float charging. A current limit may only necessary for the regulator to avoid overheating. Luckily the LM317 has a temperature monitor that shuts down the device to avoid overheating. So I'd recommend keeping the circuit as it is, maybe increasing the size of the heat sink (at which temperature does it run - as a rule of thumb: when it starts feeling painful touching the heat sink permanently you are at 65 to 70°C, which is completeley safe for the LM317).