Originally Posted by T1 Terry
Not sure where you got that information from Dave. What cell voltage do you call a float voltage?
Its not a question of voltage, Good practice is that no long term impressed voltage is left on a Li cell after charging.
Floating is a concept
from LA tech. It really doesnt have relevance in Li as the self discharge is minimal. ( which is what float charge was designed to counter). Hence after charging in Li, the charger
should be disconnected.
The next issue is when to reconnect the charger
What we often have in boats is not float charging but load sharing. This is particularly problematic in Li, as it can generate lots of mini cycles. Two scenarios exist (a) Ie the charger resumes supplying current to the boat and hence impresses a charge voltage on the battery. or (b) a reconnect voltage is set so that when the battery discharges to that point the charge cycle begins. ( thats often what used in integrated IC circuits).
The correct way to share a Li battery and a load, is often referred to as Power Path Switching. For longest life, the Li battery should supply the load via a Diode sharing path ( either ordinary or MOSFET ideal) Once the power supply/charger load current is within the limits, No voltage is impressed on the Li battery. If the load is greater then the charger, both charger and battery share the load, with the battery slowly discharging.
Separately , behind the diode splitter, a Li charge circuit kicks in at a predetermined voltage threshold to recharge the battery and then disconnect. That recharge point should ideally be set at the low discharge point.
Thats what is in well designed Li power circuits in consumer devices that have to be operated while being charged ( iPhones etc)
Of course in a boat , using an existing charging schema, this is difficult to arrange ( which is why personally I would recommend a complete system redesign ) then all you can do is select a float voltage that reduces voltage stress in the Li battery. The problem comes when load sharing occurs and many chargers , come out of float mode and raise the voltage and supply the load and also stress the already charged Li battery.
No easy answer using existing systems.
PS A lot of this research
is coming from GSM devices that use Li to handle the large current pulses ( 2-3A) that occur in class 10 devices. This causes lots of mini cycles and has lead to reduced life times , often unexpectedly so.