Float charging, question about ma

[joecool85]

From what I understand, you can float charge a 12v sealed lead acid battery at 13.6v indefinetly. And, from what I gather, the quickest you can charge float charging is 8 hours or so, I'm ok with that as I'm fine even with 24 hour charging. Here is the question, does it matter how many ma the transformer I'm using is? I was going to use a 300ma and it will be charging a 3.2ah battery, but if I use a larger transformer, would that be a problem? Also, can I use this style of float charging to trickle charge a car battery?

 

[joecool85]

Also, would it be a good idea to have a switch to go from 13.6v to 14.4v, and then before use charge at 14.4 for a little while (30 min maybe), I read that by charging only via float, the battery will never get a "full" charge.

 

[Hero999]

You really need a resiustor or voltage regulator with current limit to keep the current to a safe level.

What transformer are you using?

Could you post a schematic?

 

[joecool85]

Its going to be real basic, a 12.6v transformer (either 300ma or 450ma), into a bridge rectifier, power filter cap, lm317 set to 13.6v, then into the battery.

Here is the schematic:
**broken link removed**

 

[hyedenny]

A lead-acid battery should be charged at 10%-30% of its rated capacity - larger batteries tending to the low end of that range. So, your choice of 300mA is good for your battery. 13.6-14.2 volts for charging. When the current falls to about 40mA it will be fully charged, and a float charge can be applied. 13.5v would be an ideal voltage for "float" charging at 25degrees C. If you use a low voltage (float) to charge from discharge, make sure its FULLY charged before you use it to prevent sulfation of the plates which will decrease its life. A higher current transformer will be no problem. Use current limiting and voltage regulation in any charger you constuct!

 

[Russlk]

You don't need filter caps in the battery charger, pulses are good for the battery. Automotive regulators are set at 13.8 volts and the batteries last for 5 years or more in a severe environment.

 

[joecool85]

So I wouldn't even need power filter caps?

**edit**
Also, asuming I use the 300ma transformer and set the voltage to 13.5v, if I left it for 24hours, it should be charged right?

 

[Russlk]

correct, you don't need any power filters, except as required by the regulator (see the data sheet). 13.5 volts is not full charge, 13.8 is recommended. Some chargers use a higher voltage but once the battery reaches the high voltage, the charge voltage is reduced to 13.8.

 

[hyedenny]

13.8 volts is ideal for charging - NOT a float charge... IF youre going to float, dont go above 13.5 volts.

 

[joecool85]

Ok, I would like it to float, but I need it to charge, thats possible right? Wouldn't it just take a little longer? From what I understand, you can charge a battery at 13.5v to its full capacity, and its actually good for it. It just takes a while. I was guessing somewhere in the 20-24hour range for charging a 3.2ah battery with a 300ma 13.5v float charger. Please, someone tell me if I'm way off base.

 

[Oznog]

You definitely DO need filter caps. The reg may not be all that stable without it. Also without it, even if it does come out stable, you're only charging a small percentage of the period.

The pulsing thing is not very well defined. There are a lot of pages on it but much of it is pretty questionable hack "reseach". Without a specific pulse shape and amplitude in mind, it's fairly unlikely that such a thing would happen to do something useful by chance.

 

[hyedenny]

Youre not off base at all!! Your 3.2 aH battery is tiny - not a car battery, so 300mA at 13.5v is plenty. Anything over 1.5 amps is too much. It should take about 10-24 hours for a FULL charge if the battery is fairly new and the temp is around 25C (77F). It varies a lot depending on the lead alloy used in the plates, its age, temperature, abuse, etc.

 

[joecool85]

Sweet. So, I'm going to use a 2200uF cap for power supply filtering. And also, you said in 10-24 hours it should be charged, so if I charge it for the 24 hours I was planning on, I should be golden.

**edit**
I was also wondering if I could use this to trickle charge a car battery.

 

[hyedenny]

The car battery would be fine - it just may take several days. Its good to apply a short topping charge (14.0 - 14.3v) periodically if float charging is the only way youre going to be charging your battery. This ensures a longer life and a battery that will accept a full charge.

 

[joecool85]

Ok...now lets say that I use this battery once week and drain it to 50% every time I use it. Would I want to do a 14v topoff charge every time before I use it, or like...once a month or something? Also, how long would I want to leave it at 14v? Maybe an hour or something? I appreciate all the help with this guys!