Lets say I have 4 12v, 7ah SLAs that I want to charge in parallel.
I don't have a decent smart charger yet.
Can I do a 3 stage charge MANUALLY by using a variac and a car charger?
I use two multimeters -- one to monitor volts, one to monitor current draw (its rated for 10 amps and run in series with the charger. The little ammeter needle in the car charger doesn't have enough resolution and seems to be pretty inaccurate compared to the multimeter).
I'm following this chart from Battery University
Stage one. Initially I let the car charger run wide open on mains. Current does its initial ramp up bit, plateaus, and starts to drop slowly. I let it run like this with the voltage slowly climbing until it hits somewhere around 14.35v or so (not sure whether I ought to aim for 14.30 or 14.40).
Stage two. This is where the variac comes in. The car charger would happily push the voltage waaay up over 15v. It's dumb. Basically I just keep urning the variac down to keep the voltage between 14.3v and 14.4v. The current continues to drop.
End of charge
Alright, so I'm really only interested in the first two stages. These things aren't sitting there unused much so they don't need float.
Once the current has dropped to 3% of the combined rated current of the batteries, I consider the charge complete and disconnect everything. The way I'm figuring this is that, in parallel, four 7ah batteries have a capacity of 28ah, so 28*.03=.84mA, so when it drops under that I stop the charge.
Then I test each 12v 7ah battery individually by hooking it back up to see that it has reached its 3% of capacity (.21ma). They usually have. If not, it goes back on.
So far, I think this has been working well. I don't really trust myself to know for sure when the charge is done, but I think I'm figuring it right. It sounds like a lot of work, and it sort of is. At work its easy -- I just look over every once in awhile and tweak the thing. At home in the evening its a bit of a ball and chain.
I am in the market for a 12v smart charger, but I don't really know which one will be able to handle all my batteries at once and have the correct cutoffs for SLAs. Oh, its also got to be very affordable. $50 would be nice. Until I figure that out I'm trying to use what I have.
Is this way off base?
I don't have a decent smart charger yet.
Can I do a 3 stage charge MANUALLY by using a variac and a car charger?
I use two multimeters -- one to monitor volts, one to monitor current draw (its rated for 10 amps and run in series with the charger. The little ammeter needle in the car charger doesn't have enough resolution and seems to be pretty inaccurate compared to the multimeter).
I'm following this chart from Battery University
Stage one. Initially I let the car charger run wide open on mains. Current does its initial ramp up bit, plateaus, and starts to drop slowly. I let it run like this with the voltage slowly climbing until it hits somewhere around 14.35v or so (not sure whether I ought to aim for 14.30 or 14.40).
Stage two. This is where the variac comes in. The car charger would happily push the voltage waaay up over 15v. It's dumb. Basically I just keep urning the variac down to keep the voltage between 14.3v and 14.4v. The current continues to drop.
End of charge
Alright, so I'm really only interested in the first two stages. These things aren't sitting there unused much so they don't need float.
Once the current has dropped to 3% of the combined rated current of the batteries, I consider the charge complete and disconnect everything. The way I'm figuring this is that, in parallel, four 7ah batteries have a capacity of 28ah, so 28*.03=.84mA, so when it drops under that I stop the charge.
Then I test each 12v 7ah battery individually by hooking it back up to see that it has reached its 3% of capacity (.21ma). They usually have. If not, it goes back on.
So far, I think this has been working well. I don't really trust myself to know for sure when the charge is done, but I think I'm figuring it right. It sounds like a lot of work, and it sort of is. At work its easy -- I just look over every once in awhile and tweak the thing. At home in the evening its a bit of a ball and chain.
I am in the market for a 12v smart charger, but I don't really know which one will be able to handle all my batteries at once and have the correct cutoffs for SLAs. Oh, its also got to be very affordable. $50 would be nice. Until I figure that out I'm trying to use what I have.
Is this way off base?
