PLEASE be very careful with the advice you were given. He should have forwarded you their charging profile sheet.
Some things that were missing from his explanation:
They like to see a four stage algorithm like are used in big industrial or golf applications. The four stages are Bulk (CC/constant current) (current up to 10% of capacity), Absorption (CV/constant voltage) (14.4V to 97%+ of capacity), Finishing (CC/constant current) (15.3V @ 3% or less of capacity) & Float (CV/constant voltage).
The 15.3V they want to see as a finishing charge is constant current and current limited to 3% of the battery capacity and acts sort of like a mini-equalizing charge on each cycle. You WILL use a lot more water and in constant current the current can certainly push the voltage higher than 15.3V if you are not careful..
In marine applications we rarely have the time to wait for a charger that only pumps out 10% of the 20 hour capacity so the algorithm above likely changes. We also don't have chargers that know what the current is so it is tough to limit the current to 3% of C @ 15.3V.. We use CC/CV/CV chargers not CC/CV/CC/CV chargers.
In marine applications there are very few chargers that can do a current limited "finishing charge". Some may have the capability, but I don't know of one even deep into programming menus. Most marine chargers are voltage regulated not current regulated.. If you raise voltage, you also raise the current, unless you have a way to "limit it". Some limit current in equalize mode
but I can't think of a program on any of them that would allow you to include an equalizing voltage/current on each charge cycle.
Keep in mind that you will be cycling down to about 50% state of charge so BULK (below the chargers limiting voltage) will be from about 50% to 70%-85% SOC (depending upon charge rate) and ABSORPTION charging (CV) from that point on at 14.7V would ideally be held to about 97% - 99% SOC, depending upon your charger & how smart it really is.
If you set up a marine charger the way you were told you'd have a MUCH higher voltage, & potentially current, for a much longer time and could really do some damage to your batteries. Unless your charger can do two "absorption" stages, or "absorption" & a current limited "finishing" charge, then I would urge you to reconsider the 15.5V recommendation from Fred. It is also .2V higher than any voltage I remember US recommending except for perhaps an equalizing charge. The "finishing voltage" US recommends in their own lit is 15.3V or 2.55V per cell X 6 = 15.3V..
We charge using what US calls "two stage charging
" with "optional float
". This is what most marine chargers do. It is really just a CC (Bulk) to CV (Absorption) to CV (Float) charge profile. With this profile USB wants the absorption voltage at a max of 14.7V, then a float voltage, but remember this is all predicated on a charger rated at 10% of the 20 hour capacity. They recommend an equalizing voltage of 15.3V for this type of charging, but not every cycle.. If your charger is larger than 10% of C a reduction in absorption voltage may be warranted. All charging should be temp compensated.
Deep cycle wet cells in marine applications are usually charged at 14.6V - 14.8V, these voltages have continually crept upwards int he last ten years as battery makers finally began listening to what the folks in off grid/PSOC arena have been saying all along, and that is that 14.4V in a cycling application is simply too low. I have set up many regulators, such as Balmar's, that can do dual absorption stages, to push 14.8V then cut back to 14.6V - 14.7V but in each of these situations they batteries use considerably more water despite getting a helathier charge. Hydro caps are usually in order... The gassing voltage really begins at around 14.4V but the batteries really need more than 14.4V for optimal charging..
More information on this subject from 2015..
I have had long, long conversations with Fred W. USB's Senior VP of Engineering about this. The charge profile that original guidance was written around does not widely exist in the real world, CC/CV/CC
/CV and not at all in the marine market, but it did in their labs and does in some very specific industrial settings.. After giving that guidance for a while they reverted to a much more sane
US Batteries current charging guidance is here: US Battery Charge Recommendations
For CC>CV charging (USB calls this "two stage) 14.7V - 15.0V for absorption. Fred W. strongly urged using the 14.7V end for marine applications, especially if all sources are not temp compensated and where we are using a higher charge rate than 10% of capacity. He still wants to see 13.0V - 13.2V for float. Equalization is 15.3V -15.6V if temp compensated. If not temp compensated Fred recommends a maximum voltage of 15.0V for EQ.
Please note that "two stage with optional float
" is where you want to be. The USB three stage profile has a CC>CV>CC
>CV profile not a CC>CV>CV profile like our marine charge sources use. We use CC>CV charging in the marine market... Also remember that guidance was written around a 3-4 hour period in absorption.
If you are charging using a higher current than .1C (10% of Ah capacity) then you will have longer absorption times and the voltage should be at the lower end of the 14.7V to 15.0V recomendation. At 10% of C in charge current you will be in bulk for the majority of the charge but as you bump up to .2C (20%) or .25C (25%) you extend the time you will be in absorption because you will attain absorption/limiting voltage sooner in the SOC curve....