Rapid SMF Lead-Acid Battery Charger with 16F873A/18F1320

Hi all,
I have a 12V 7.5Ah sealed maintenance-free lead-acid battery lying around. Till now, I've been charging it for 14 hours. Now I want to charge it within 4 hours or quicker if possible without damaging the battery too quickly.

Here's what I've got:
•To charge the battery quickly I need to provide at around 15v. My question, if I provide 15v to the battery, will it automatically charge quickly?
•For controlling the voltage, I am thinking of using PIC16F873A/PIC18F1320. There is a 25v dc supply, which I will convert to 15v with PWM and MOSFET. I will sense the current flowing to the battery and when current falls below 0.01C, I will assume charging is done and decrease PWM duty cycle to supply around 13.2V.

Is what I have stated correct and sufficient for charging it quickly? Don't worry about the programming, I think I can manage that. I just want advice about the required hardware.
If not, what should be done. I've had a look at some battery technical handbooks from different manufacturers where I've read about charging at 2.4v/cell and 0.4C current for fast charge.

Now, I'm going off to sleep. I'll be waiting for your responses, ideas and input.
Thanks everyone for any input.

Really SLA batteries are charged by constant voltage and you limit the current, Well you set the to say 2/3amps so when the battery if flat it would not charge more than this and the battery will only draw the required current to charge it. What you have to watch out for when charging them quicker is the battery tempitaure ig you get them to hot then you will scrap your battery, you would need around 18 volts to charge a 12 volt battery.

So, if I apply 18v, the battery will charge quickly. I'll add temperature monitor then. And that's all I need?

well it's not has simple has that, To charge any 12V battery you would require 18V any way, The current would be what your transformer is rated at, You would need to control the voltage aswell has the current, You would also need to monitor your tempitaure if the battery starts to get to hot you would need to reduce the current flowing to the battery.

Hi,
Questions:
1. I'll control the voltage based on the battery charge level, ie how much charge the battery already has, like during bulk charge 15v, during trickle/float 13.8v. Right?
2. Controlling the voltage should control the current, right? Or which other method should be used?
I think I get the temperature monitor thing, that's no problem, I'll be able to implement that.
Waiting for your reply.

Here are some links that you may find useful and explain more
http://www.atmel.com/dyn/resources/prod_documents/doc1659.pdf
**broken link removed**
http://www.powerstream.com/SLA.htm

These should explain it a bit better than I could, The first one is controlled by an AVR but should give you some good ideas

I had a book some where on battery chargers and power supplies which had a fast SLA charger, I will have to go through some discs I see if I can find it has it was useful for me.

Thanks for the links.
I'll go through them and let you know if there is any problem.

hi tahmid,
iam doing a project on 18f1320 for battery charging system. can i get the program for pic 18f1320, if u have.

Tahmid said:

Hi all,
I have a 12V 7.5Ah sealed maintenance-free lead-acid battery lying around. Till now, I've been charging it for 14 hours. Now I want to charge it within 4 hours or quicker if possible without damaging the battery too quickly.

Here's what I've got:
•To charge the battery quickly I need to provide at around 15v. My question, if I provide 15v to the battery, will it automatically charge quickly?
•For controlling the voltage, I am thinking of using PIC16F873A/PIC18F1320. There is a 25v dc supply, which I will convert to 15v with PWM and MOSFET. I will sense the current flowing to the battery and when current falls below 0.01C, I will assume charging is done and decrease PWM duty cycle to supply around 13.2V.

Is what I have stated correct and sufficient for charging it quickly? Don't worry about the programming, I think I can manage that. I just want advice about the required hardware.
If not, what should be done. I've had a look at some battery technical handbooks from different manufacturers where I've read about charging at 2.4v/cell and 0.4C current for fast charge.

Now, I'm going off to sleep. I'll be waiting for your responses, ideas and input.
Thanks everyone for any input.

Click to expand...

Microchip's AN626 shows how to charge a 12v 7ah SLA batter as well as plenty of design notes for changing the voltage.

The original 14C0000 chip is not recommended for new designs although it is still available. Microchip suggests using the 16F833 chip instead. I am wainting for a same of PIC16F883-I/SP - 1 so i can port the software over and build the same thing for a current project.

Here's the link to Microchip AN262 in pdf form as well as the firmware source in C which is written for the 14C0000. I have not looked yet but there should be a migration document on the microchip site for the conversion. I would think this could be used in any chip as long has it contains an Analoge input and programable voltage reference. I don't believe the new chip suggested by microchip has the built in temperature sensor but I am planning to implement that using a 1-Wire Temp sensor such as Maxim's DS18S20 precision unit. Maxim is pretty good for free development samples as well, just go to their website.

AN626
**broken link removed**

DS18S20
**broken link removed**

kaleemk said:

hi tahmid,
iam doing a project on 18f1320 for battery charging system. can i get the program for pic 18f1320, if u have.

Click to expand...

The AVR450 may give you an idea. Although it's based on the AVR, you might be able to make your own program if you understand the design.

www.atmel.com/dyn/resources/prod_documents/doc1659.pdf

Hope this helps.
Tahmid.

Hi,

First the batteries are not supposed to be charge fast as there is a term called "gravity" associated with all batteries. All 12v batteries are charged with 13.2 to 14V with varying current. The more current you apply the faster it charges. I think this is the thumb rule for batteries (Please correct if I am wrong). For 12V 7Ah the applied voltage is 14v and 1Amp or less. So the it requries about 14 hours to complete the charge. This is the most ideal curent that is aplied to 7ah batteries.

If you charge with 1amp the battery will last for more than 13-14months. On the other hand if charge faster that is applying more current let's say 4amp then the battery will finish charging in just 2 or less hours. But you can see practically that there will be a bulge on the batteries if you frequent charge cycles and finally the battery last for not more than 5-6 months. This is where the term gravity comes into picture. It is the holding power of the battery. The best example will be any UPS where the batteries usually last for 13 months.

For the small batteries like AA or AAA types you can see the manufacturer's specs for 120mA charge current. So the batteries are to be charges with low currents and constant voltage.

You have trade on the life of the battery or charge time.