[bronsonj]

There is a lot to digest in your initial post, so I'll pick a few of the ones I can add my $0.02 to pretty easily.

1) 70% is generally thought of as remaining capacity. As a battery is used the capacity slowly diminishes. If the battery is abused the capacity quickly diminishes. BUT... Ebay people say 70% life because that isn't a clearly defined term and you can't go back on them for it if you didn't clarify with them before hand. It might (and should) refer to capacity, but they intentionally chose to list "70% life" instead. Also be careful of the cells that are measured, some people measure from 4.2vpc to 2.5vpc others measure from 4.17vpc to 3.0vpc. Obviously the second seller is more honest in their approach than the first. So if you get cells with a measured capacity check out what the start/end V was when they measured the cells.

2) There are many options on the BMS. Some use CANBUS communication to the charger and remote displays and foot massagers (I made that last part up) others are low powered nonadjustable and pretty useless. You want a BMS that will handle the number of cells you are putting together. You want one you can adjust the max V and the min V. Any decent BMS will open the contactor if the pack V gets too high or low (even when regen braking). Some BMS units can supposedly shift the charge so cells that are fully charged then don't get further charged and wait for the rest. Other BMS (the one I just bought does this) put a small resistive load on cells that are high and then when any single cell gets beyond the parameters (say above 4.17v) it shuts the entire charge process down. Still other BMS without balancing function don't use the resistive load and just shut it down when outside the params. Yes, use the fuse in addition to the contactor. The contactor is there to save the pack according to the BMS. The fuse is there in case the cable rubs raw against the frame.

3) the voltage should match your pack max V in theory. I am using a 48V solenoid on my 57.6v pack. It may help that I have a 300 amp controller though.

4) The charger I have is an aftermarket one that I can set the output V to a specific value. Some people use chargers with CANBUS communication so the BMS tells the charger directly that it's time to stop. Most use a dumb charger that they can specify the final V and the BMS will will act as a backup and disconnect the charge when the V gets too high on any single cell.

5) Lithium Pouch Cells (like the Leaf and others) need to be under compression. So you'll need threaded rod and nuts to tighten them down, but how you attach it in the cart and how you make it look is entirely up to you as this is a relatively niche product there are not a great many parts out there to dress up DIY lithium installs. Getting it to be quickly removable but also entirely secured in case of a rollover may be difficult. My pack is secured and it'd take me 45 minutes or more to remove. There are many really pretty installs here. Mine isn't one of those... :)



Then again....... You have a 72V controller/motor/solenoid. If you went with an 20S pack the max V at 4.17vpc would be just under 84V. What is the Max V your controller is good for? Your current lead acid pack is 72.6V at 50% when it's time to charge. If you get your lithium settup going with a max V at where your current pack is basically dead you may not like how it runs... A 20S leaf pack (10 leaf modules since they are double) is between the cost of your proposed 7 module and 14 module packs.

Also on the regen braking front, your cart will weigh a lot less so it will take a lot less regen to slow the cart down. You won't need 250 amps of regen anymore, 130 will probably suit you fine. My cart is set to 86 amps for max regen here in Flat Florida and it feels like I put the brakes on. I have a 94ah pack and I know I can charge at greater than 1C I still like to keep the max at 1C for charging, just my preference.