Should voltage drop this fast?

[brianka]

Charged my 2 6v batteries up to 13 volts (under almost no load). I’m in my 2022 wildwood xlite. Only thing on in the rig is the water pump, fridge is on lp, and carbon monoxide detector I guess which says uses like .5 amps. In one hour the battery is down to 12.6. Is that normal? These batteries have 225ah. To go down .4 of a volt in an hour seems like a lot no?

[Rockwood27]

Yes! A fully charged 12V (twin 6V) battery should measure 12.6 to 12.7 volts. The 13V you were reading is the surface voltage from charging. When the battery voltage gets down to about 12.0 to 12.1 volts, then you're at around 50% state of charge (SOC). This is the point where you should start recharging the battery to prolong its life.

[brianka]

My battery went from 12.8 last night to 12.2 this morning. Doesn’t that also seem like a bit too fast?

[Bobby F]

Keep in mind that, in order to really know how far down your batteries are, you need to let them sit with no drain and no charging for an hour or more. I wake up to sub-12-volt readings often when it's furnace season, but if I let the batteries sit for a while, the resting voltage always comes back up to 12v+.

[Rockwood27]

Starting at 12.8V suggests that you still have some surface charge on the battery. Going from 12.6V to 12.2V means that you used about 100Ah from your batteries. What were you running all night long? Have you performed an energy audit to determine the amperage draw of each item? A furnace can be a large power hog. Were you using it overnight? Do you have an inverter? They have parasitic draw when turned on and not used.

[Mark_K5LXP]

It's highly error prone to use voltage alone to determine battery capacity, especially under charge and discharge conditions. Using a battery monitor that also factors current is much more accurate.

Mark B.
Albuquerque, NM

[Marine359]

Mark is spot on.
Once you have a neg shunt battery monitor, it’s easy to trace where your watts are going by removing all 12v fuses from your panel, and plugging them back in one at a time. Turn off all loads. Each time you plug in a fuse note the power (watt) reading on the monitor, and you’ll have a complete audit of 12v component consumption in no-load state. Of course you can’t turn off loads for alarm monitors, but you’ll know how much they’re using when you plug in their fuse. If you then turn on loads one at a time, you can note each load’s power draw.

[Iceclimber]

2 6v batteries is a small bank. Consider doubling.

[twinboat]

Charging until 13 volts is not full charge. Most deep cycle batteries need to be charged above 14 volts and then held at 13.8 to 14 volts for a few hours. Only then are you getting to full charge.

Most times your starting with full charged batteries, if your on shore power at home for at least a day. That second day, your not getting back to 100% charge because it takes hours of generator time, so you may be stopping at 80% charge. Every day you camp you will be using more then your returning from your batteries.

[wolfe10]

Originally Posted by twinboat

Charging until 13 volts is not full charge.

Correct.


To further explain, as soon as the amp output of a converter, charger or inverter charger is enough to raise voltage it will do so. It will continue to increase voltage until it reaches its preset target for bulk rate (if smart charger).


But the battery could still be 30-40% discharged with voltage at 13. Without an ammeter to show charge rate (amps/charge rate will drop off as the battery reaches full SOC) or more sophisticated monitoring device, SOC (State of Charge) is just a guess.

[Persistent]

Lead acid batteries have three basic voltage profiles. They are Charging, Discharging, and Static.

Static profile is when no charging or discharging has been occurring for 3 hours or so. Voltage settles slowly. Voltage can be used to estimate the state of charge. See attached file for static voltages and state of charge.

Charging profile always has higher voltage than Static. The faster the charge rate, the higher the voltage will rise.

Voltage starts at the static level and rises to what ever the charger is capable of delivering. As charge progresses, current decreases and voltage rises up to preprogrammed voltage. Typical voltages preprogrammed may be 13.2, 13.6, 14.4 volts. Voltage will typically stay at 13.6 for a long time while charging finishes.

Using a 50 amp battery charger on a 200 amp hour battery bank may rise to 80% charge in 4 hours. Voltage will typically reach 13.6 volts. It may go to 14.4 and then drop back to 13.6. It takes and additional 10 hours to reach 100% state of charge.

Discharging profile voltage is always lower than Static voltage. The greater the discharge, the lower the voltage. Voltage quickly drops to discharging voltage and rises back to static level when discharging stops.

[Triangle Drifter]

Get yourself a battery load tester such as one of https://www.amazon.com/OEMTOOLS-2433...7&sr=8-11&th=1

[twinboat]

Originally Posted by Triangle Drifter

Get yourself a battery load tester such as one of https://www.amazon.com/OEMTOOLS-2433...7&sr=8-11&th=1

That's for testing CCA of starter batteries, it says so right on the meter face.

Won't tell you anything about deep cycle battery capacity.
Need a complex tester or do a timed capacity test.

[Vintage465]

What I've read and noticed is, with flooded batteries anyway, you need like 14.6 volts to charge. You end up with a float of 13.5 or more at float stage. The voltage window between 12.7 and 13.5 is the surface charge or what I call the foam on the beer....Rootbeer in my case. This becomes pretty apparent when the sun goes down, and you are left with just the battery bank. You see, well, I'll say.......I see the surface charge go away pretty quick and then the real "body of the charge" holds solid in the 12.6-12.8 range for several hours depending on load. By morning in most every case I have 12.5 volts in the bank and the solar has it back fat and happy by 11:00-12:00.