If i have to charge each module, would it not be better to buy a 24v charger of some sort?
Usually a 24 V charger would be suitable for lead acid (charging to 28.8 V), which also suits 8S LFP. But that sort of voltage works better for 7S NMC, which is not convenient.
I hadn't thought about charging several at once as i wasn't sure what the voltage per module maximum was. I don't know if i was to link in parallel the + and the -'s would the modules balance the voltage between all of the modules?
That will tend to equalise the modules, but not the cells within each module. It also takes a long time, and the voltage drops make it less than ideal. Much less. Unless you use copper bars and bolts. I would not recommend it.
if i took my 48v hybrid inverter, and connected 4 modules in parallel, i would be able to set the voltage to 50v and then i could charge at a rate of anything from 10 up to 150 amps. Maybe thats a better way??
That would be a quick way to charge the modules fully, yes. But NOTE: you need pairs of modules in series, and you can parallel a few strings if you like. I would use 49.9 V as the target voltage (nearest to 499 / (18/2)). But I'm nervous about charging so high a voltage with known unbalanced cells, and no battery management system in place.
What you could perhaps do is charge them up to about 23 V per module (46 V total), but unfortunately the inverter will probably think that your 48 V battery is at a dangerously low voltage.
I recommend starting with the lab power supply, as the cells are in an unknown state, and 5 A is nice and gentle, yet will get things moving.
If you can make up a pair of leads with banana plugs at one end and an 8 mm (?) eye terminal at the other, and bolt the cables to the module, that will at least minimise cable resistance. Clip leads can drop several tenths of a volt at five amps. Preferably use at least 1.5 mm² wire. The low resistance will allow for more current near the power supply's set point, allowing you to more confidently and quickly get the module to a desired voltage.
As a point of interest, have you used a multimeter to measure the resistance from battery terminals to metal case? While it's not as definitive as the 500 V megger, it will give you a good indication as to whether you are reducing the leakage, or more toothbrush work is needed. Voltage measurements might also be useful, especially if you find that you get a solid reading.
I wouldn't connect them in parallel while they are at different voltages, the current flow could be too high.
In my experience, it's not a problem, but my experience is mainly with LFP cells, which have a much flatter voltage curve.
So yes, probably wait with paralleling until the voltages are pretty close, say 0.2 V difference maximum.
but everyday is a new adventure and we never stop learning so I can tick this off the list.
