Hey folks,
I have an electric bike, actually 2 bikes and an electric tractor. Most recently, I lost the charger to my main cargo bike and a replacement is on the slow boat from China. In the meantime, I have a 48V pack that needs to be charged. --I am quite anal about my SLA's --Left uncharged for any length of time is death to these batteries.
In addition, I am about to "winterize" the spare e-bike and electric tractor and bring thier batteries inside.
In total, I have about 9 12v 12Ah SLA batts to maintain --including (4) of those batteries comprising a 48v pack for my bike (that desperately needs charging.)
Question:
I have a fairly nice (huge) wall-wart charger for 12v Sla batteries. I have used it for years to charge one 12Ah batt at a time. In terms of "sucking amps" etc, is there any reason I can't hook up 4 (if not all 9) of my batteries in parallel and charge/ maintain them all at once? --I know I will be trading a lot of time here, say 1 amp of output from the charger; I will be going from a 12 hour charge for one battery to 4.5 days to charge the whole lot. Correct? --Or do the batteries in parallel increse thier "amp sucking ability" and my little charger will be fried?
Everyone here is incredibly awesome.
Thanks Gents, you have given me the courage to procede…
In the past few days, I have been charging my beloved battery pack one by one (just as a “in the meantime” precaution). Put simply, the fire is out for now and all batteries have been topped-off.
At this point, I think I am on to long-term (winter) trickle charging --which, by default, a crapload of Ah’s and a low-current-output charger is. With the confidence to continue, I am starting the job of crimping a ton of spade-connectors to a bunch of “parallel” wires. Yup, we’re going for it. Going to do 4 in a row to start, then add the other 5.
I will shoot video.
Thanks, guys.
An aside
I heard once about a guy that hooked a low current charger up to the light in his garage door opener. Everytime he opened/closed his garage door his lawn mower battery would get a few minutes recharge.
I know it is off topic a bit, but, it seems like the right place for the short story. 
Charger
Chris, back in the old days, (before sophisticated 1-chip charge controllers), the typical small SLA charger, (like for alarm systems), just used a light bulb between the battery and the charger. When the battery was low and drew more current, the light would get brighter and limit the current, as the battery charged, current draw was less and the light would dim. Besides being a cheap and effective limiter, it also gives a crude visual indication of what is going on. I would use an ammeter in series with the bulb initially to pick a bulb to use.
Cheap limiter
Chris, this is a cheap circuit I have used quite often for charge controllers in my robots. If you determine what the "float charge current" is you wish to maintain on a particular battery, you can use ohm's law to determine R's value so that .6V at x current equals R resistance. You then determine R's wattage by your "worst case" scenerio but it's seldom more than 5 watts. The relay can be replaced by an opto-isolator or a 1K resistor and 5V zener and can become an input to a chip.
As long as the battery is charging above the float value you have calculated for the value of R, the transistor remains turned on, as soon as the battery is charged and current drops below the calculated value, the transistor turns off. Simple but elegant.
