Does anybody know what a reasonably acceptable charging current for a 6 volt 4.5 aH SLA battery? Searching google, I found that a good voltage is 2.3v per cell and about 1 amp for the first stage of charging. I'm using an unregulated wall wart that reads ~6.6 v when connected to the battery terminals and I measure .15 amps which seems kind of low. (the wall wart is capable of 2+ amps). Also, I'm about 90% sure the battery is bad because it hasn't been charged for 2~3 years.
So you've got a kna**ered battery and a wallwart that only outputs 6.6volts with a 150mA load current. I don't think you are going to get there.
If you want 2.3volts per cell ,then the open circuit of the power supply should be at least 7 volts. If you use an unstabilised unit then you don't want to go much above the 7volts because you're bound to forget to disconnect it when the battery is on charge.
General rule on charge rates should be somewhere around C/7 maximum (preferably a bit less) so for a 4.5AH battery I'd say around 500mA is more than enough. There's nothing wrong with 150mA - it'll just take longer to get a discharged amount back into the battery.
If the battery has been discharged for 2 years or more, it will be badly sulfated, and wont be able to be charged.
At the very least you'll need ETDA and a desulfator and lots of luck, lead acids are supposed to be stored fully charged and topped up every month or so. The longer the sulfate's been there the harder it becomes (it recrystallizes slowly to an insoluable form). Even if you recover the battery the capacity will be more like 0.45Ah, not 4.5
The 150ma is not necessarily a bad indication. I find that charging my 12 volt 7.2ah SLA batteries, the charge may start off high if they are discharged but will drop to 50 ma as they approach full charge.
Using a killawatt hooked up to a 12v 2a ac/dc converter, ran through a boost converter up to 15v, into a morningstar sg4, i noticed after discharging a healthy 12v 18ah agm overnight during a power failure (%50 soc), the killawatt showed ~10w being used. As it charged this wattage droppped, down to 500mw measured by the kill a watt. Basically, this shows the pwm switching of the charger as it tops off the battery to full capacity uses less and less current. Eventyally dropping down to almost nothing. The 12v 2a adapter not connected to a load plugged into the killawatt shows 0w consumption.
You can get a current sensor for an arduino, or killawatt for a ac/dc converter, and using a pwm charger, it will show you how much watttage is being pushed into the battery. As it approaches 0w, you know its charged fully. But this is with a sg4 smart pwm charger. Might not work with less advanced chargers that are only 2/3 stage, or just using a wall wart. Worth a try though imho if you want to by a killawatt or one of those current sensing sensors for an arduino instead.
I'm going to do this with a 12v unknown ah (prob 100ah+ new) that was badly overdischarged, 10.7v when i found out it was shorting in the car it was attached to. The battery was new, and only in there for ~10mo. I think there was a short that discharged it pretty bad, but perhaps it can be desulfated (white film on plates).
Just a thought.