so if i try and charge a 3v cell with 4v (and 20ish amps) the charge voltage will drop to (around) 3.2 and work its way up until the battery voltage reaches the charge voltage. So in a way the voltage will adjust to what i need it to be, to charge the batteries?
Yes, but
only if the charger or other power source is inherently current limiting. If you just connect a 144 V pack across a 120 V pack with a contactor, the current won't limit, and both packs would be damaged, and possibly the contactor as well.
If you connect a generator that doesn't have current limiting designed in, it could possibly attempt to charge at too high a current, and stall the engine as well as give a pulse of high current.
I'm not very technically minded so what ever i come up with has to be fairly simple. the generator can only put out so much current, and i plan only use it while i drive when i can watch what is happening. as long as the battery voltage doesn't go to high the generator shouldn't produce enough current to do any damage right?
I don't know the characteristics of a generator. It might be OK to just connect to a battery and it may be self limiting, but my suspicions are that it won't work and will just stall without suitable current limiting circuitry.
I could put a 30amp fuse between the generator and batteries or find a way to shut off the generator automatically if the voltage or current went to high, would these options be a suitable failsafe?
That should save the batteries, yes; one or two pulses of high current won't do much damage. But if the generator's engine stalls every time to try to charge when the pack is low, that's no good either. Batteries aren't like light bulb loads.
what happens if i floor it and the battery voltage sags while i'm charging, i'm guessing the current will just go into the motor, and not into the batteries
Yes, that's pretty much how it will work.
like what dc braveheart said i'll be in the middle of the cell range to increase safety, i'll also have a decent BMS.
I'm not a fan of limiting the charge voltage as a proxy for cell safety. Sure, if you only have 6 cells in a module, like with a car starter battery, then you usually don't have to worry about one cell going too high in voltage (though lead acid car starter batteries have other equalisation methods, involving gassing). But a typical pack is at least 30 cells in a series string. Even if you limit the volts per cell to say 3.4 V, which is pretty conservative, it doesn't take much imbalance (say ten cells down 0.1 V from the average) for one weak cell to suddenly have an extra volt across it, and 4.4 V will ruin a cell quickly.
You recognise the need for a BMS; that's great. I say don't limit the average volts per cell as a sort of "extra insurance"; give the cells what they need (and a little more, if the BMS has bypass capability), so you don't needlessly prolong charge times. I work on 3.65 VPC, since our cells (Sky Energy/CALB) are listed as 3.60 V maximum. Our BMS can bypass 1 A, and communicates with the charger to limit charge current to 0.9 A when a cell goes over-voltage.
sorry to bombard you with questions but i guess that's the curse of knowing what you talking about.[/quote]
