SLA battery charging with PWM but which frequency

[showtime]

I would like to make a smart SLA battery charger and controller with PWM. Which frequency should i use ?

thank you any comment for now.

[mackrackit]

If you are thinking of pulse charging, do not bother (my opinion ).

Constant Voltage is the best way to go. You can start off with a Constant Current but a CV charger sized correctly will do this automatically.

A maximum voltage of 2.4 volts per cell to bring the battery up to charge, then if you want to "float" if for a back up system or something like that, bring the voltage down to around 2.25 volts per cell.

For the "sizing" of the charger, 20 to 25% of the rated capacity is good. A 5 Amp Hour battery would have a current limiter of 1.25 amps. Even if you have a larger say a 12 AH I do not recommend mare than 2 amps. ( If you have ever replaced batteries from UPS and see the batteries swollen you will know what I am getting at ).

12 Volt example drained to 11 volts.
A charger built like this will when first placed on a "dead" battery will show a voltage close to the battery voltage and the maximum current allowed. As the voltage increases the current will decrease. When 14.2 volts is reached the charger will drop to the float voltage to prevent over charging.

In my experience this will get the most life from this type of battery.

For the past four years my lawn mower and trimmer have ran off of SLAs. This is pretty hard on the batteries but they will still last around two years. By the way, where I live the grass grows year round if I water it

I do use a "boost type" voltage regulator that use PWM to convert the nominal 12 volt from my main battery bank to the 14.2 volts.

[showtime]

Hi again,

Thank you for important informations about SLA charging rules. My SLA is 12V-7.5Ah.
-Yes I will use Constant Voltage.
-As many professional charging device producer say that PWM is the best way to charge SLA batteries. (I don't why it is). So I will do PWM charging system.

I have just uploaded an image. Which is better or any comment will be very very important for me.

[mackrackit]

You can not do PWM and have constant voltage. PWM will go from ZERO to HIGH voltage.

If you have a voltage source of 14.2 for a 12 volt battery and the voltage source is switched via PWM at say a 50% duty cycle, the typical meter will read 7.1 volts. This will take twice as long to charge as the battery is only receiving the needed voltage for half the time. The argument for this is a slow charge is better than a fast charge.

I have seen some of those professional chargers that will read close to the 14 volts with a meter implying a peak voltage well above the maximum limit. (remember the UPS example). The amperage will also be on the high side too. The argument for this is that the average voltage going to the battery is correct.

The image you uploaded.

The LM350 is good for 3 amps (if memory serves ) with a good heat sink. The Could switch between two or change the adjust resistance for float voltage.

Is this for a back up system?

[showtime]

No it is for a solar system. My mini panel supplu 18V without load. And shortcircuit current is 1.5A.

Is PWM usefull for this supply voltage?

[mackrackit]

In that case all you will need would be a way to cut the power from the solar panel when the battery is full. The panel may put out 18 volt open but will pull down to battery voltage.

If you want to get the maximum out of the solar cell, early morning , late afternoon, cloudy day, you can use a boost circuit to bring it up to charging voltage. Then when the cell is producing charging voltage on its own the boost circuit drops out and the solar cell is back to straight into the battery.

[showtime]

The end of your comment is most important thing to use the maximum solar energy. But would like turn back to PWM. Generally, what frequency is used for battery charging ?

Best regards

[mackrackit]

The frequency that is used (at least from the so called PWM chargers I know of ) is either 60 or 50 HZ, depending on where you live. They are simply using a half wave rectifier and regulating things from there.

This is from a battery manufacture of VRLA batteries. Same thing as SLA. You can not have a true SLA as it would blow up .

http://www.panasonic.com/industrial/...ingMethods.pdf
The referring page is here. http://www.panasonic.com/industrial/...eal/index.html

As you will see, a recommended frequency is not given as PWM is not one of the recommended ways of charging these batteries.

If you are looking for a boost circuit for those cloudy days, I am using one built around a LM2588T. http://www3.national.com/pf/LM/LM2588.html

[showtime]

mackrackit,
Thank you again and again.
Best regards

[showtime]

Hello again mackrackit,
When we use boost circuit and in a cloudy moment the boost circuit give us what we need. But what about the current? At this moment will the current be enough to continue the charge?

Best regards

[Acetronics2]

Hi, Showtime

Will you have a look to the CCS 9620 SL ( from BTI ) Datasheet ??? ...

YES ! It's a Pic !!!

Alain

PS: Cost is 16.9 Euros ... @ CONRAD.fr or CONRAD.de ... worth it !!!

[mackrackit]

Hello again mackrackit,
When we use boost circuit and in a cloudy moment the boost circuit give us what we need. But what about the current? At this moment will the current be enough to continue the charge?

Best regards

The current will be lower, but will also charge the battery, just slower.

The current from the solar cell will be lower in a cloudy condition so the over all current going to the battery will be lower.

Current is a measurement that involves Time.
I = Current
Q = electric charge in coulombs (ampere seconds)
T = time in seconds

I = Q/T

When the "boost" circuit is working it will take "TIME" to increase the voltage potential so the current over time will be lower than having a constant source. Simple circuits. http://www.intersil.com/engineeringt...ostcircuit.pdf

Basically in battery charging the only way to increase the current is to increase the voltage. If the source was the same voltage as the battery, no current will flow.

So even if the current over time is lower, it is still something. Something that would be going to waste other wise.

Feel free to say that my explanation is unclear and I will try to explain better.

[showtime]

It sounds like it is very smart technic for a charge system for a changeable source like solar system. I have started research boost technics. Do you use only an MOSFET/TRANSISTOR, inductor and capacitor in output of pic or only a boost IC ?

Maybe every professional charger producer use boost technic to get maximum efficiency in bad situations.

Best regards.

[mackrackit]

The solar charge controllers are built with this idea.

So far I use the LM2588 for most things like this. You could build your own though. Study how the 2588 works.

If you build one an IGBT would work better than a MOSFET as the switching time is normally faster. Controlled by a Gate voltage like a MOSFET.

Things to consider no matter how you do it.

The diode will need to be heat-sinked so on in a TO-220 package works well.

The inductor is critical, you will spend more time sizing this than anything else. One thing about national.com is the web-bench. it will help size things.

The switching frequency is critical also.

This would be a good project for a PIC. Boost, control, and Display all in one.

[showtime]

Ok.

I have a 12V 7.5Ah SLA battery and L200 IC constant voltage charge circuit. My voltmeter shows me 14.5V witout the load.I connect the circuit to the emtpy battery i see 11V and when the battery is full 13.9V. I think everything is normal for this charge technic. Because i use constant voltage charge.

But I don't know what should i do if i use boost type charge. Please correct me if i'm wrong:
-Pic will control the battery voltage for charge status. I think this is easy.
-How can we udjust the i.e LM3588 with a pic ?

Thanks...

[showtime]

hi again.
I see in the lm3588 datasheet. I understand that i can send pic pwm signal to the freq/adj leg (pin 1).

But sould i use lm3588 ADJ version. Because we don't need 12V regulated.

[mackrackit]

I have a 12V 7.5Ah SLA battery and L200 IC constant voltage charge circuit. My voltmeter shows me 14.5V witout the load.I connect the circuit to the emtpy battery i see 11V and when the battery is full 13.9V. I think everything is normal for this charge technic. Because i use constant voltage charge.

Sounds good, but if you can get the no-load voltage down to 14.2 would be perfect.

But I don't know what should i do if i use boost type charge. Please correct me if i'm wrong:
-Pic will control the battery voltage for charge status. I think this is easy.
-How can we udjust the i.e LM3588 with a pic ?

The LM2588 has its own adjustment with resistors, something like the LM317s. Set it as close as you can to 14.2 and that is all you need. For the straight charging, a PIC is not needed if you use LM2588. My main battery bank will go from 11.5 to 14 volts, (400 amp/hr). My chargers built around LM2588 to charge the small batteries will bring them up to 14.2 and hold them there. You could use a PIC to monitor this and when 14.2 is reached the PIC would shut the LM2588 off.

[showtime]

Thank you my friend,
I will set to 14.2V. When i connect the battery the voltage will drop. So will IC set 14.2V itself. Namely will we see 14.2V during charge?

By the way, I think you forget my ask about IC type. Normal LM2588 or LM2588 ADJ ?

[mackrackit]

When the battery is full and connected to the charger you will see 14.2 volts if the battery is healthy. After the battery is off of the charger the voltage will drop to 12.6 to 13 volts. This is the voltage of a fully charged 12 volt battery at rest.

I would go with the adjustable LM2588.

With the adjustable the voltage can be set pretty much where you want it.

[showtime]

Please give me more detail for first sentence.

[mackrackit]

To fully charge a battery it needs to be over charged . ( but not to much ) That is where the 14.2 volts comes from. (2.36 volts per cell ) The battery will only show this voltage when it is on a charger and fully charged. If something is wrong with the battery it may never reach this voltage. ( bad cell ) Or reach this voltage very quickly. ( shorted cell )

[chuck]

Floating voltage should not go higher than 13.80V and only 5% of the maximum current to maintain the 13.80V. You should really only go to 14.2V if you cycle it regular.

When charging a SLA you need to limit the current otherwise if you get the battery to hot the gel will crystallise the gel inside and the battery will not hold the charge. There are quite a few designs out there, I used to build the SLA chargers for the golf buggies the golf trolleys and sell them has a product I only used the L200 2 Amp regulator with a few components, 2 Pots 1 for setting the maximum voltage (open circuit 14.4V) and the other to limit the current along with a couple of LEDS to let the user know the when it was bulk/float charging. Once it was set to carry out a full test and leave it charging sometimes a little adjustment was made to maintain the 13.8V.

I will try and see if I can find it (if I still have it) and upload it,

[showtime]

Hi chuck,

Thank you for charging ideas wihch i know. I want to know some details about mackrackit's booster charger.

Will I see normal charging values like conventional circuits. I'm trying to say step by step that is important for me:
-I assume I have boost circuit which is controlled by PIC or not. It boost's the voltage to 14.2V when PV give me i.e 9V. Therefore I always measure 14.2V without the battery. Because the circuit adjust the output via feedback.
-We sure we connect the battery to the PV, voltage is decreased. for exmple 10-12V. If this is OK

-Will always Our buck-bust circuit adjust itself to 14.2V constant voltage during charging?.

I know I'm your headache but forgive my experience and my bad English.

[mackrackit]

-We sure we connect the battery to the PV, voltage is decreased. for exmple 10-12V. If this is OK

Yes.

-Will always Our buck-bust circuit adjust itself to 14.2V constant voltage during charging?.

A buck- boost circuit would. The LM2588 is for boost, so when the PV voltage becomes higher than 14.2 the LM2588 will basically shut off. The voltage from the PV will go straight through the inductor and diode to the battery ( minus the inductor resistance and diode drop ). This is where the PIC could help. Some way to monitor the voltage at the battery. When the battery is "FULL", shut the whole thing off or switch to a maintaining "FLOAT" voltage. This all depends on what the battery is used for. Backup power - a float charge . Charge the battery to use daily - shutting the charger off until the voltage drops to 13? might be good for you.

[showtime]

I don't know how much i can thank you. I understood everything.
Last night I see MPPT charging technic. I was happier. They are famous style to get maximum efficiency. You are expert.

Best regards.

[mackrackit]

Glad you got it, but I am no expert. Thanks for the compliment though

[iw2fvo]

Dave,

is there any MPPT PV> battery Charger project available ?
Possibly using PIC and PICbasic !
Thanks
Ambrogio

[mackrackit]

I can not think of any on this forum. I seem to remember seeing one on the Home Power magazine web site a few years ago.

[Heckler]

here are several BUT none of them use a PIC micro or PBP...


http://www.repeater-builder.com/backup-power/pdfs/automatic-sealed-bat-chrgr.pdf

http://adventure-radio.org/ars/pages...ext/N7CEE.html

http://www.batteryspace.com/leadacid...sfloating.aspx

http://www.theheathkitshop.com/page4/page4.html

[Charles Linquis]

I haven't taken the time to read the whole thread, but you certainly CAN have varying NOT CHOPPED voltage (and constant current, too, if you have a current sensor in the path) with PWM. You simply have to make a 'buck' converter. You can find a schematic at

http://dl.dropbox.com/u/566712/PIC_BUCK.jpg

To be accurate, the circuit does require a small minimum load. You also have to measure the output voltage with an A/D channel and change the PWM value accordingly, but the output is a low-ripple DC voltage at up to 6 Amps.

[Scampy]

Why are you replying to a post that is almost 7 years old... it's all irrelevant now!

[pedja089]

It is bot...