Get a active balancer. It transfers power from the high battery to the low battery to keep them all in series equally. Theres also non active balancers which use resistive loads to waste energy from the high voltage batteries and drop it down equally to the rest. But thats alot more wasteful and they dont have the current capacity thats needed.
Are they all in parallel? Then why not hook up one end to positive (battery 1) and have negative at the other end (battery 3). They will automatically balance better. Only the internal resistance of the battery will create a difference, but not the position of the battery.
Dont inwest more in lead acid battery They will last maybe 5y max if not just 2-3 before they start to die Real capacity of lead acid is like 70% max of nominal capacity And you can only use 1/10 amps to capacity (for example 100ah battery = 10A constant draw for 70% capacity , anything over it can end in overheat and capacity either way drop to 50% same life cycles ) Best option is to look for cheap lifepo4 , 100% capacity for next 10years :) And at least 1 to 1 amp draw to capacity (100ah battery= 100A constant draw possible easly (unless bms wont allow you to / battery start to heat up for some reason like to thin connections , hot outside , no cooling etc)) Edit PS You can also look for "used" scooter / car etc battery packs (liion or lifepo4) cheap and most of the time super easy to connect (just need to make sure to use BMS and set max voltagw of charger to max voltage of pack you use (if its 3s liion = 3x 4.2v = max 12.6v NO MORE or can end up in failure or fire (bms will shut down earlier)
Get a active balancer. It transfers power from the high battery to the low battery to keep them all in series equally. Theres also non active balancers which use resistive loads to waste energy from the high voltage batteries and drop it down equally to the rest. But thats alot more wasteful and they dont have the current capacity thats needed.
Are they all in parallel?
Then why not hook up one end to positive (battery 1) and have negative at the other end (battery 3).
They will automatically balance better.
Only the internal resistance of the battery will create a difference, but not the position of the battery.
Dont inwest more in lead acid battery
They will last maybe 5y max if not just 2-3 before they start to die
Real capacity of lead acid is like 70% max of nominal capacity
And you can only use 1/10 amps to capacity (for example 100ah battery = 10A constant draw for 70% capacity , anything over it can end in overheat and capacity either way drop to 50% same life cycles )
Best option is to look for cheap lifepo4 , 100% capacity for next 10years :)
And at least 1 to 1 amp draw to capacity (100ah battery= 100A constant draw possible easly (unless bms wont allow you to / battery start to heat up for some reason like to thin connections , hot outside , no cooling etc))
Edit
PS
You can also look for "used" scooter / car etc battery packs (liion or lifepo4) cheap and most of the time super easy to connect (just need to make sure to use BMS and set max voltagw of charger to max voltage of pack you use (if its 3s liion = 3x 4.2v = max 12.6v NO MORE or can end up in failure or fire (bms will shut down earlier)