I have read online many slightly different methods for charging flooded lead acid (FLA) batteries. My real situation is this: I have six 6V FLA batteries wired in series in a 36V golf cart. They are all of medium age and medium health. The cart can make 18 holes ok but then seems to get sluggish. However, a normal charge using a 36V 21A (max) charger takes 4 to 8 hours to complete the entire charge cycle. I want to be able to fast charge the battery bank in 1 hour while I take a break for lunch, enough to play another 18 holes. What I am wondering is suppose after the first 18 holes the batteries are at a 30% state of charge (SoC) (70% discharged). I read online that as long as the charge voltage remains less than 2.4V per cell, a FLA battery can take any large amount of current but once that critical charge voltage is exceeded, the charge current MUST be slowed significantly. This is somewhat analogous to filling the gas tank on a lawn mower. When the tank is almost empty, you can begin to fill it quickly since it will accept it. However, as the tank becomes closer to full, you have to pour the gas in progressively slower as it may splash out otherwise.
So here is what I am proposing and I am asking here if it will work. After the first 18 holes, I put the batteries on a 3 rapid chargers (12V each), each capable of 55A. The six 6V batteries are rated at about 200Ah each but since they are medium age and medium health, I would say they have no more than about 150Ah of actual capacity so 55A is a little more than 1/3rd of the actual capacity.
Potential problems I have read about doing this are that FLA batteries have chemistry that takes time for the charge to spread. Therefore, there may be an excessively high charge in the electrolyte near the battery terminals but not yet uniformly distributed throughout the battery. This may "trick" the charger into wrongly thinking the battery is in a higher SoC than it really is in and as a result, wrongly shorten the charge cycle. If there was an effective (and safe) way to agitate the electrolyte during the rapid charge cycle, that might help, but I am not sure how to do this. Perhaps I could just shake the golf cart every 15 minutes or so to help?
So assuming the 3 chargers can each put 55A into the batteries for 1 hour, it seems that should be enough for maybe 9 more holes, possibly 18 more. I will be able to get a great price on three 12V 55A chargers soon so it is much cheaper for me to do this rapid charge method than to just purchase higher capacity batteries. It is also more fun.
One thing I have to be careful of is each 12V 55A charger will draw about 1000 watts so I can only use 1 per outlet here which means I will have to run 2 "beefy" extension cords from 2 other outlets not on the same 120V 15A breaker so some additional precautions will have to be taken. 3000 watts total of charge power is a little scary.
So will this method work or is charging at about C/3 (where C is the capacity of the battery in Ah at the 20 hour drain rate) risky for used FLA batteries in medium health? Remember I am not trying to rapid charge back to 100%, just speed thru the bulk charge phase in 1 hour is my goal. What I am thinking is if I had healthy new batteries, they would likely accept this fast charge method, but because my batteries are used and of medium health, the charger may sense this and quickly reduce the current from 55A to something the battery can better manage such as 40A or even less, thus somewhat defeating the purpose of the fast chargers. I do know the batteries I have WILL accept the full 21A charge from the 36V charger as the ammeter on it sweeps up to 21A and the charger sucks about 900 watts out of the wall (I measured it), so the batteries are still in a decent state of health.
Also I should mention that I don't need to do this rapid charge often. It is only when I want to play a LOT of golf in the same day and don't want to wait 2 hours to get 40Ah of charge back into the battery bank.
Also, just for fun, since the 12V 55A charger is on wheels, I could tow it to the nearby golf course I play on (only 1/4 mile away) and lock it up there. Before taking my 1 hour lunchbreak/rest in the clubhouse, I could quickly rewire the 36V bank in series/parallel for 12V and just apply the 55A to all, using the courtesy outlets there. That is better than nothing. If after the 27th hole it gets sluggish again, I can take a 2nd 1 hour break and repeat the charge. AN even easier way would be to charge each 12V pair for 20 minutes during my break. That would require no rewiring of the 36V bank so after the 1 hour I just disconnect the charger and play more golf.