# Float charging 12v lead acid battery

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

• Pb car batteries usually charge to about 13.8V, so you should be good. Jun 2, 2021 at 19:13
• A healthy, 100% charged, lead battery at rest (not being charged) is about 12.7V. Float charge should be about 13.5-13.8V (well below gassing voltage). Jun 2, 2021 at 21:13
• Ok, thanks. I checked the battery again this morning.It went from 13.2 last night to 12.7 this morning. Why would it drop?Will it usually drop from 13.5 down to 12.7 at night when it's not charging? Jun 3, 2021 at 14:36

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:

1. 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).
2. 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.
3. 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.).
4. 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.