Charging 6V SLA batteries

[gopher]

Im creating a portable radio system that runs on 6V DC.

i have 3x 6V 4.5Ah SLA batteries connected in parallel
Its says on the battery

Constant Voltage Charge at 20 deg C

STANDBY USE
Voltage Regulation 6.75 - 6.90V
Initial Current 0.675A

CYCLIC USE
Voltage Regulation 7.20-7.50V
Initial Current 1.35A

The Load on the batteries draws 6V at 1.6A
So i calculate thats 9.6Watts
Total battery Ah = 13.5 so thats 8.4hours of runtime.

Someone correct me if im wrong with the above.

I went to the local electronics supply. They had a nice selection of SLA batteries, and a few chargers. The only 6V one they had was just a simple wallwart transformer with selectable 6V/12V setting, and alligator clips for connection. No cutoff circuit, just cheap dirty voltage. Its rated at 500ma @ 6V. I guess this will work but i have no idea when to cutoff the charger.

My other thought is to create a custom circuit that will take a wider input range DC voltage say 6V-12V and charge the batteries, cut off the charger when full, and have a selectable switch to run the load on DC adapter power or batteries.

The power supply that originally came with my device im powering is a switching regulated 6V @ 3A. My question is can i use this power supply to charge the SLA batteries? or do i risk damaging them?

Ideas, thoughts, what should i do?

 

[MrAl]

Hi,


The main idea is to limit the current while also limiting the voltage.

You can look up the proper voltage limit on the web for your batteries. The voltage limit is the maximum voltage you should apply to the battery regardless of the current.

The current limit is there just to prevent too much current from flowing through the battery when the voltage is low enough to accept a larger current. If you limit it to 500ma for one battery that would probably be ok, but the exact limit is found on the data sheet for the particular battery and may vary from manufacturer to manufacturer.

The simplest charger is a wall wart and if the current is limited then you can stop the charge after the voltage reaches the max value allowed (look on the web for this voltage i think it is 2.45v per cell for fast charge and 2.35v for slow charge, and there are 3 cells in a 6v battery and 6 cells in a 12v battery).

A slightly better charger uses an LM317 or similar IC chip which makes a fairly decent charger despite it's simplicity.

I charge mine with a 30v 10 amp power supply but set the current limit to around 1 amp and voltage limit to 14.2 volts (12v batteries).