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I know you are pretty savvy with the batteries. Question?
I got a U1 35AH SLA. I am charging it with an 8A bank charger. My understanding about charging is that you should not exceed 20 % of the rated AH. I am a little over this. What is the safe fudge factor on charging?
That's probably slightly too much charger. The manufacturer says to limit current to 6.6A. You may be okay if you keep the battery cool, but I'd use a 5A charger and not leave it hooked up after full charge unless the charger is designed for those small sealed batteries. Those batteries have a lower float current than the big ones so it might cook if left hooked up too long after it is fully charged.
Ok. I will check the battery to see if it is heating up. I always leave the battery compartments open when charging and I don't leave the battery to trickle charge. Don't want to add another charger if I can use the onboard 8 amp bank.
Easier to go to a bigger battery. Just heavier.
Always something.
Battery manufacturers specify charging current as a fraction of battery AH capacity, which they abbreviate as "C". Usually, they consider C/5 to be the really safe level.
In your case, C/5 would be 7A, but the manufacturer has gone further and specified 6.6A. Most of the time, most batteries can handle C/3 safely, but sealed batteries and gells aren't as good at getting rid of internal heat as liquid electrolyte batteries.
I have a cheap motorcycle charger that I use for small batteries. Wally World special for $20 or $25. It won't float charge, but I usually have a pretty good handle on when they need charging. Just watch the battery voltage and make sure to shut the charger off somewhere between 14.5V and 14.8V and you'll be good.
Some really high performance batteries have internal thermistors that provide feedback to the charger of the battery's internal temperature. Since charge rates are limited by a guess by the manufacturer about the battery's operating environment, this allows very high-rate charging that can cut charging time by 75% in moderate temperatures. They also have internal monitoring points for cell voltages to avoid fire risks from shorted cells and to provide instant feedback as to battery condition. These would be nice on a boat battery.