question

You need to increase the absorption voltage to 29.2V, not decrease it.

What tells you the balancer isn't working?

My system does go out of balance a little under load/charge. But nowhere near this much. It also comes back to about 0.1%. This can take some time due to the low balancing current and battery size.

Which battery is low? I suspect it's the older one.

If you don't have wiring problems, like high resistance/poor connections, unequal cable lengths, it's likely that the older battery is causing the imbalance and should be replaced.

@Scott SD Is it a Victron Battery Balancer ?
I have never bought or seen one used. the spec sheet says it only balances at 0.7 amps
that's not a lot for a pair of 200Ah batteries

I don't think the AmperTime have Bluetooth or a BMS App to see what each cell is doing.
is it the older battery that is low ? (you say 2 years old... but how much use/ cycles has it had)

I have 4 x 100Ah Epoch batteries as a 48V system. to start with my mid point was always high around the 4%, and the BMS on the highest battery would alway shut off, before the others were charged
with your AmperTime batteries, one of them might be turning off the Charge Mosfets, and you would never know with out access to a BMS App.

I fixed mine by buying the ECO-WORTHY 48V Battery Equalizer as it balances at 5 Amp per battery
their 24V version balances at 10 Amp https://amzn.to/40kH8Q4

my batteries were all new at the install, with the 5 Amp balancer fitted, now my mid point never moves above 0.3% and drops to 0.0% as soon as the bulk charging stops

plus now, if one of the batteries BMS turns it's Charge Mosfets Off, it's not for long, as the balancer quickly starts draining it, to bring the others up to voltage

Thank you both for taking the time to answer my question.

@kevgermany I did not know if the balancer was functioning because it kicks on at 27.3v and I only had the system in absorption a few times, but I did check it yesterday and it is in fact functioning. It was flashing a warning light that one battery has higher voltage. When I charged both batteries prior to install, the older battery was reading 0.1v lower. Concerning the absorption setting, I heard that lithium does not need a the charge stages like a lead acid battery. I got the suggested voltages from the diy forum.

@IamMotorHomeless I am not having issues with the battery BMS kicking in and shutting the system down, it might be due to my lower absorption setting. Now that I know the balancer is working, I don't think it can handle that amount of imbalance. I will take your suggestion on the EcoWorthy balancer. For $28, it is worth a try. I just don't want to order another battery. I told my wife that I was done spending $$....

What is the actual voltage on each battery, measured with a meter?

True, lithium does not need absorption. But it does need time close to fully charged for the BMS to balance the cells internally.

Are these batteries specced for use in series?

Have you measured the length/resistance of the connections from balancer to batteries?

If one battery was lower than the other when you precharged, it suggests that either it's old/tired, or cell imbalances caused it to cut the charge, or that the BMSs are set differently.

Assuming a 14V fully charged voltage (makes numbers easier), 4% means a voltage difference of 0.56V. This does not agree with your measured differences. But does imply wiring issues if the 0.1V still holds. You can measure the voltages seen by the balancer at its terminals. Giving those and the actual battery voltages all measured with the same meter would help a lot.