I get asked this question quite often and the answer is almost always a resounding, yes. There are always caveats to everything however..
In the video below I have two group 27 batteries, in parallel, consisting of 160 Ah's at the 20 hour rate. The batteries were recently equalized, specific gravity checked and tested. They are in excellent health for their age. Once equalized and fully charged they were left in float mode over night. Room temp in my shop was about 72F..
As you will see in the video below the current accepted or needed to maintain 14.4V is just 0.2A.
TWO TENTHS OF ONE AMP AT 100% FULL TO MAINTAIN 14.4 VOLTS !!!!!
Two tenths of an amp is all these batteries will take when full at 14.4V without over shooting 14.4V. They will remain taking this 0.02A for days or weeks at a time. With an accurate enough charger or power supply that can deliver mA current levels they will accept 0.02 - 0.08A continuously at 13.6V or float voltage. Most switch-mode chargers don't have the accuracy to do this so they "pulse" on and off when they get to currents below 0.2A or so. For older feroresonant chargers they begin pulsing on and off at much higher currents to maintain a voltage..
This "acceptance rate" is a far cry from the misguided and mostly incorrect information spread around the net that a "full battery will take 2% of its capacity". This information likely stems from resetting battery monitors when a bank is taking less than 2% of "C". It does not mean the battery is 100% full at 2% of capacity it just means that this is a "good enough" spot on a boat to call "full" for resetting a battery monitor.
There is also lots of information out there suggesting that a solar panel of 10% of capacity does not need a charge controller? yes, in many cases where batteries are used or cycled daily or even sometimes every third or fourth day this "unregulated charging" can
work and can be a sort of "truth".
Conversely when batteries are left to sit for weeks at a time on a solar panel with no regulation it can become a dangerous situation to battery health and quickly becomes an "untruth"... With many boats they often sit for days or multiple weeks between use with all loads turned OFF. In these situations the batteries can still get to 100% full even with just a 10% of "C" panel. When they eventually get to "full" they will
have the voltage pushed well beyond where it should be.
Having charged hundreds & hundreds of batteries and watched the "accepted current" at varying voltages these comments always made me cringe. The other day the question came up again and I decided to use my bench top power supply to illustrate this.
A diminutive 10W solar panel can produce about 0.59 -0.6A in good sun. 12V nominal panels have voltages from 16V to 18+V. So even with a small panel if the bank is left on charge for multiple days or weeks at a time, with no loads, as is the case with many boats, you can
over charge your batteries if you are not careful. It has happened to a number of my customers most recently a bass boat owner whom I installed a fish finder for. He toasted a very expensive AGM battery....
While that difference from 0.2A, what the battery is willing to accept at 14.4V, to 0.59A, what a 10W panel is capable of, may not sound like much the difference between what the batteries actually need at FLOAT, 0.02A to 0.08A, can be quite a dramatic increase.
Remember a 10W panel can produce, about 0.6A, and this is actually a 637% increase in current from a float current of 0.08A that the batteries "accept" to maintain 13.6V. Unregulated, when the batteries approach full, will
cause the voltage to rise and the electrolyte to boil off.
The general guidelines are usually stated that a panel of 10% of capacity or less would not need a controller. 10% of 160Ah would be a 16W panel but a 16W panel could produce nearly 1A of charge current even more than a 10W panel.
REAL WORLD FACT:
Just 1.0A of current (same as a 15W solar panel) at 100% full on this 160Ah bank pushes the voltage beyond 15V !!!!!!
Beyond just the potential to over charge your batteries, if left on charge long enough, a small panel like this won't recharge your batteries very quickly, but when it does, and does not have a controller, you could run a risk of over charge.
Something to think about anyway and if you do run unregulated please be careful.
I made a video seeing is this is much easier to see than to explain..