Battery charge differs between solar controller and BMS - Why?

Grumpy_G

themcgillcrew said:

Well, after a couple weeks I'm back to square one. We had a couple days of rain then a bright sunny day and once again the solar controller is up to its old tricks. The SC sees the batteries as full and therefore stays in absorption while the BMS shows the batteries need bulk charging. If I disable the solar controller then re-enable it, the SC shifts immediately to bulk. Maybe this is just the way it works and I'll have to keep turning the SC off and on again to get to bulk.

This is the exact same behavior that a friend of mine struggled with for quite a while. Battery would charge after restarting the solar controller as well. See here: https://tab-rv.vanillacommunity.com/discussion/comment/213029
The TL;DR is that voltage drop between the solar charge controller and the battery makes the charge controller see a higher voltage than the battery. The charge controller drops out of bulk mode and lowers the voltage which in turn is too low to charge the battery. 
Generally LiFePo batteries do not need any fancy charge algorithms as the built-in BMS takes care of managing the charging of the cells. The chargers from LiFePo manufacturers are typically just a straight up 14.4-14.6V power supply with short circuit protection. One possible workaround is to set the charge controller to put out 14.6V for bulk/absorption/float which mimics the external chargers.