Lena, f you can find a junked emergency lamp, may ckt dun na nag o off pag fully charged na yan 6v4AH batt mo. DO NOT USE CONSTANT CURRENT. Use constant voltage. You can use a 7806 and you can increase the voltage to 7V or find a 7807 kung mayroon. The rule we use for "safe" charging kahit naiwan ay .1C, so 400ma. kaya na ang 7806 yan. U C , as the internal resistance of the batt increases as the V goes up, then current tapers down kapag constant V. That is what we want kapag standby ang use ng batt mo. Now, kung kilangan mo mag charge mabilis, use a timer. Ideally sana if you can put a voltmeteracross the battery or a an LED indicator(zener lang) or use the LM3914 voltage displacy IC. Yung China digital,mura lang. Whatever you use, it will be an indicator when to wind up.
Rule of thumb:Put in 50% more of what has to be replaced sa batt. SLA ya, sealed Lead acid. am sure. problemo, we do not know what has been depleted. Crude way is to graph voltage to capacity left in your battery if you want to be serious.The best way to know your batt is to measure its discharge time at a given load at hindi yun charge time kasi baka di nya natatangap lahat ang na inject. Cge, put a lamp on your fully charge batt. You know fully charge kapag peak and voltage reading mo 6++. Use 4 amp and see kung aabot ng 1 hr. Kung impatient ka, use 8 amps and see if it will last 30minutes.(parallel kalang ng bulbs or use a huge resistor(s). Measure usable capacity. Now, habang bumabab ang V, log down the remaining time ok pa. Then you have chart showing V to rea=maining capacity, di ba? Yan kilala mo na yung battery mo. So, kung may V meter ka , yoy can tell anytime ano pa natira dun sa batt, di ba? now if you wish to charge, yung capacity ng batt mo(di yung rated, yung natest mo) minus the remaining capacity, yung ang ibabalik mo x a factor of 1.5.
Example: you found out na ang batt mo 3 AH ang usable sa purpose nya. After taking a V reading, say 1AH and natira. Eh, di, 2AH ang nagamit or na self discharge. How do you put back the 2AH? Well, u have to put back 2x1.5 o 3 AH. Its up to you on how to do it. Kung nagmamadali ka use .3C or 900ma. Ok lang 1amp, kahit 1C AS LONG AS YOU HAVE A TIMER.
hanapin mo yung 3 hr mechanical timer ng nga dating electric fan. Or yung 8 hr electric motor timer ng 3D box fan. Sa 3A, one hour, sa 1 amp, 3 hours. SAPOL yan. in this case, constant current ang config mo sa 7806 at kailangan mo ng bypass ransistor to go beyond 1 amp, max ng ic or even less to be conservative.. (heatsink mo ha). kapag constant V, habang chinachrage, bumabagsak and I.