Best approach to build an FLA battery bank ?

[adaviel]

Most of my experience with FLAs is killing 12V ones in my boat and buying new ones (charging them with a regular car alternator then running them flat when anchored). But for most of that time, the technology didn't exist to do much better. In my collection of mostly-dead batteries, I note the cells often get out of sync - e.g. I have 4 good cells, one 80% cell and one 60% cell so that on load the voltage drops in steps 12,10,8.
I'm wondering how one avoids this effect in a battery bank - dealing with bad cells. E.g. whether a battery management unit would be a good idea. I'm looking at a 48V or 72V bank to power an electric motor; one BMU I looked at is switchable between 6V and 12V but not to 2V for per-cell control.
Is it better to charge the entire bank in series with the same current, or to use a BMU to equalize charge, or to charge individual batteries in parallel with isolated smart chargers ?

[Sunking]

Most of my experience with FLAs is killing 12V ones in my boat and buying new ones (charging them with a regular car alternator then running them flat when anchored). But for most of that time, the technology didn't exist to do much better

Well that is a sure fire recipe to destroy a battery real dang fast. You never ever want to discharge the battery more than 50%, or ever leave them less than 100% fully charged. As soon as you discharge them, recharge immediately. The technology existed before the battery, it is called a Protein Computer, Volt Meter, and Hydrometer. That is not a technology problem, that is operator error.

I'm wondering how one avoids this effect in a battery bank - dealing with bad cells. E.g. whether a battery management unit would be a good idea. I'm looking at a 48V or 72V bank to power an electric motor; one BMU I looked at is switchable between 6V and 12V but not to 2V for per-cell control.
Is it better to charge the entire bank in series with the same current, or to use a BMU to equalize charge, or to charge individual batteries in parallel with isolated smart chargers ?

Forget the BMU. The issue is you, not equipment. You gotta lot to learn.

First thing you have to do is determine the size of the battery required to do the job. Do not do anything until you are sure you have nailed it down. If you fail to do that, you will destroy the battery. Determine how much power in WATTS your load device uses like a light bulb uses 60 watts. Then determine how many hours in a day you will run your load device. Once you know the Watts and the Hours you will know how many Watt Hours you use in a day. Watt Hours = Watts x Hours. Example 200 watts x 6 Hour = 1200 watt hours. Once you now Watt Hours multiply by 5 so as to limit daily discharge to 20%. This will maximize cycle life and get you the best value for your money. Using our example of 1200 watt hours, 5 x 1200 wh = 6000 watt hours. OK to find the right size battery capacity in AMP HOURS = WATT HOURS / BATTERY VOLTAGE. So in our example let's assume 48 volt battery. 6000 watt hours / 48 volts = 125 Amp Hours. You go shopping for a 125 AH battery. That is going to be either 4 or 6 volt batteries. No such thing as a 12 volt 500 AH battery. Lead Acid batteries are 2 volts, not 12.

Next thing you are going to need is a charger. I will save that for later. What I will tell you is for a 500 AH battery is going to be a 40 to 60 Amp 48 volt charger.Size and type depends on how the battery is used.

[mapmaker]

OK to find the right size battery capacity in AMP HOURS = WATT HOURS / BATTERY VOLTAGE. So in our example let's assume 48 volt battery. 6000 watt hours / 48 volts = 500 Amp Hours.

I think your calculator needs a new battery --mapmaker

[Sunking]

I think your calculator needs a new battery --mapmaker

You can say that again, the ole battery is 60 years old. Corrected text. Transposed 12 with 48 volts.