Electronics > Beginners

19V PSU charging 12V lead acid battery

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Zero999:

--- Quote from: nuclearcat on September 16, 2019, 08:52:32 pm ---
--- Quote from: KL27x on September 16, 2019, 08:50:26 pm ---
--- Quote ---Certainly not a buck. As far as a 19V PSU is concerned, the battery across its output is a dead short. It will dump its maximum output current into it. The output capacitors will rapidly discharge when the switched is turned on, possibly shortening their life after many such cycles.
--- End quote ---
Ok, geniuses.

You've gone this far. Add the frigging inductor. In this case you don't even need an inductor. Just a power resistor in series. Done.

--- End quote ---
Probably this circuit will generate less heat:

--- End quote ---
If you don't mind a linear regulator, then how about the LM317?

nuclearcat:

--- Quote from: Zero999 on September 17, 2019, 07:59:32 am ---
--- Quote from: nuclearcat on September 16, 2019, 08:52:32 pm ---
--- Quote from: KL27x on September 16, 2019, 08:50:26 pm ---
--- Quote ---Certainly not a buck. As far as a 19V PSU is concerned, the battery across its output is a dead short. It will dump its maximum output current into it. The output capacitors will rapidly discharge when the switched is turned on, possibly shortening their life after many such cycles.
--- End quote ---
Ok, geniuses.

You've gone this far. Add the frigging inductor. In this case you don't even need an inductor. Just a power resistor in series. Done.

--- End quote ---
Probably this circuit will generate less heat:

--- End quote ---
If you don't mind a linear regulator, then how about the LM317?


--- End quote ---
It will work too, but probably i will check if feasible to use regulator to charge in CV mode, a bit different circuit, and i have to check if i can offset Vadj from MCU, as its better to charge a bit faster empty battery (maybe PWM + RC + opamp?).

Zero999:

--- Quote from: nuclearcat on September 17, 2019, 08:58:25 am ---
--- Quote from: Zero999 on September 17, 2019, 07:59:32 am ---
--- Quote from: nuclearcat on September 16, 2019, 08:52:32 pm ---
--- Quote from: KL27x on September 16, 2019, 08:50:26 pm ---
--- Quote ---Certainly not a buck. As far as a 19V PSU is concerned, the battery across its output is a dead short. It will dump its maximum output current into it. The output capacitors will rapidly discharge when the switched is turned on, possibly shortening their life after many such cycles.
--- End quote ---
Ok, geniuses.

You've gone this far. Add the frigging inductor. In this case you don't even need an inductor. Just a power resistor in series. Done.

--- End quote ---
Probably this circuit will generate less heat:

--- End quote ---
If you don't mind a linear regulator, then how about the LM317?


--- End quote ---
It will work too, but probably i will check if feasible to use regulator to charge in CV mode, a bit different circuit, and i have to check if i can offset Vadj from MCU, as its better to charge a bit faster empty battery (maybe PWM + RC + opamp?).

--- End quote ---
The idea is to keep the current constant and turn it off, when the battery voltage rises too high.

nuclearcat:

--- Quote from: Zero999 on September 17, 2019, 09:10:10 am ---
--- Quote from: nuclearcat on September 17, 2019, 08:58:25 am ---
--- Quote from: Zero999 on September 17, 2019, 07:59:32 am ---
--- Quote from: nuclearcat on September 16, 2019, 08:52:32 pm ---
--- Quote from: KL27x on September 16, 2019, 08:50:26 pm ---
--- Quote ---Certainly not a buck. As far as a 19V PSU is concerned, the battery across its output is a dead short. It will dump its maximum output current into it. The output capacitors will rapidly discharge when the switched is turned on, possibly shortening their life after many such cycles.
--- End quote ---
Ok, geniuses.

You've gone this far. Add the frigging inductor. In this case you don't even need an inductor. Just a power resistor in series. Done.

--- End quote ---
Probably this circuit will generate less heat:

--- End quote ---
If you don't mind a linear regulator, then how about the LM317?


--- End quote ---
It will work too, but probably i will check if feasible to use regulator to charge in CV mode, a bit different circuit, and i have to check if i can offset Vadj from MCU, as its better to charge a bit faster empty battery (maybe PWM + RC + opamp?).

--- End quote ---
The idea is to keep the current constant and turn it off, when the battery voltage rises too high.

--- End quote ---
I am planning to measure battery idle voltage and probably apply higher current if state of charge is low. Also it will be nice if i can adjust offset, according to temperature and battery capacity (including approximating capacity loss with age). Its cheap to do, make charging faster (outages too frequency in my country to charge slowest way), and will increase lifespan of battery.

AVGresponding:

--- Quote from: nuclearcat on September 16, 2019, 06:26:30 pm ---
--- Quote from: ThickPhilM on September 16, 2019, 06:13:02 pm ---Perhaps a small automotive lamp in series with the PSU output?

More information would be helpful, like the output current of the PSU, the type of lead acid battery (wet, gel etc) and its capacity

--- End quote ---
Lamp is worse than adjustable linear regulator :)

Let's assume PSU current is sufficient in any case, i dropped idea relying on PSU current limit.
Most likely not smaller than 12V 2.6Ah gel, this means with max 0.1C it should be 0.26A. As i charge by PWM, probably in pulse i can use 0.2C (if i wont exceed 50% duty cycle). I plan to implement it by using typical 2 NPN transistor circuit.
But thats the question, if battery chemistry is ok with fact, that on 10-20khz PWM pulse current is 0.2C, but average current is 0.1C.

--- End quote ---

Well, I was making a couple of guesses based on your previous posts, ie that you were looking for lowest complexity due to 'third world issues', and that you were using small car or motorbike wet lead acid batteries.
Now that I know you're using very small gel types, that changes things significantly.

Gel aka AGM type lead acid batteries really don't like over-voltage situations, especially very small ones, and doubly especially the likely low quality ones you'll be using, which will have a lot of very impure recycled lead in the plates. Do not use a pwm regulated 19v supply, you will kill the battery in a year or so, depending on a couple of other things (ambient temperature, depth of cycling).
Cyclic charging of these batteries requires a constant current for the bulk charge, and a constant voltage for equalisation. PWM charging from a 19v source is not a good idea, a simple linear regulator and current limiting resistor are the best bet for such small batteries. A laptop charger is massive overkill.
C/2 is more than adequate for the bulk charge in this case, exceed C and you will cook it. Float current shouldn't be more than a few tens of milliamps at most, if it draws more, it's on its way out.

Do not believe the manufacturers datasheets on charge voltages; they want to sell you more batteries.

If you want a good lifespan, a float charge voltage 13.6v really is the highest you should go, cyclic use, 13.8v, but don't let it sit at that voltage for more than a couple of hours.

The quality of these batteries varies considerably. Cheap Chinese ones are not very different from cheap Chinese NiMH in their capacity claims in my experience; adjust your bulk charge rates accordingly.

With reasonable ambient temperatures, and sympathetic charge/discharge profiles, I've had good quality batteries last 20 years.

I've also seen good quality batteries cooked in months, by crappy quality/badly adjusted chargers. Some of the bigger ones can be quite exciting when they fail  :-DD

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