I'm trying to float charge a 12v car battery with constant voltage charging set to 13.5v. At start the battery voltage was 12.65. After 2 days it's up to 13.2 which seems higher than it should be. What's full charge? I thought 12.7v. Should I lower the charging voltage? I'm mainly trying to offset parasitic drain which was measured at 14.6 ma. It's a solar set up so not charging 24 hours a day. Thanx
Battery University is a good resource for learning about the different battery chemistries and their particular needs.
I pulled the above chart from Battery University, it describes the charging profile for a single lead-acid cell. As you can see the cell is maintained at 2.25V. For a six cell battery like your typical 12V that comes to 13.5V exactly what your charger is set to already. 13.5V Looks like a good compromise between filling the battery to capacity and a reasonable use life.
What you plan to do is what some people call as Battery maintainer. In most cases it is a 1-stage charger, as it sets Voltage limit only; it is essentially a Constant Voltage (CV) supply. If you also limit the initial current (CC), then it becomes a “CV/CC power supply” to charge your battery.
So, a quick answer to answer your question: The Floating voltage set as 13.50V at 25Celsius is okay.
Some points I thought,to,clarify:
- Battery Charger/maintainer should slightly change the Voltage set-values based on ambient/battery temperature. At Summer temperatures of about +40Celsius, “13.5V” should be lowered to 13.20V. And Winter as 0Celsius increased to 14.40V . So, if thermal variations are large along the year, the charger shall have a thermal compensantion to maintain at 100% state of charge (SOC) without undercharging (sulphated battery) or overcharging (gassing and dryout).
- The battery voltage as nominal 13.5V is measured while in the charging process. If you measure without charging, a “skin” effect in the electrode plates might give you an abnormally high reading just few minutes from being charged.
- If you wish to remove the skin effect of charging, turn on a load - as car light-beams - for 5 minutes or so. Then measure the voltage and it might be 12.5V to 12.8V if at 100% SOC (and Amb. Temp.).
- You mentioned initially was 12.65V and after 2 days it was at 13.2V. If I understand you correctly and 13.2V was measured while charging, the charger has very low current capacity for your size of battery. This probably will create problems of undercharging = as battery ages, the internal self-discharge will increase and you battery charger might not be able to replenish the battery to 100% SOC.