Expected voltage sag?

No voltage sag. just a small steady drain.

Tell us what it is and we'll tell you if it's ok.

ewarnerusa wrote:
I've got two 6 volt GC batteries. I have an inverter just powering a laptop. It's pulling under 100 watts AC. What kind of voltage sag would you expect that to put on these batteries if they were fully charged? What would the voltage read across the battery posts?

Depends on what else those batteries are powering inside the trailer at the same time. Regardless, voltage drop from inverter load will be greater with a pair of 6 volt GC-2s than would be the case with a pair of similarly rated 12 volt G31s ... and with a sufficiently heavy load that extra voltage drop can be just enough to trigger the inverter alarm or even shut it down when it might not when running on paralleled G31s. Contrary to popular opinion sometimes the ubiquitous GC-2 is not always the best choice - depends on the intended use. ;)

My 150 watt inverter will drop 1/4 to 3/4 volt depending on load.

Doug

My batteries got hammered pretty bad last season while the trailer was in the shop. Solar was disconnected and batteries were deeply discharged. I suspected they were shot but I've tried nursing them back with equalizations and being maintained with temperature corrected 14.8V charging profile. But I think they're damaged. After a full day charge, the inverter draw alone with the laptop charger will drop the voltage to 11.6V and hold steady. The voltage will often recover back to 12.5+V even after running this load for a while. Voltage reading is coming from the SCC voltage sense wires at the battery terminals.

OP again. For instance, I ran the inverter all night (~10 hrs) with only the laptop on and charging. No other AC or DC loads, I have eliminated most of my parasitic draws. Voltage remained steady all night at 11.6-11.7V. I shut the inverter off this morning and voltage recovered to 12.3V. Outside temp is -1C.

Quote:
No voltage sag. just a small steady drain
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At any amperage draw, there will be voltage sag. And w/ 100 watts off 2 full charged batts, I would think could be .1 to .2 volts.
Your voltage drop seems too low, unless something else is drawing.

A SG check of each cell is in order, should all be within +/- 0.005. I suspect at least one battery has a weak or bad cell. At 12.3 volts you are most likely below 50% SOC. Depending what the inverter plus laptop draws, ours is 3 amps with one laptop, it should be a few days before you get to that point. But battery temp is also a factor here at -1C.

ewarnerusa wrote:
My batteries got hammered pretty bad last season while the trailer was in the shop. Solar was disconnected and batteries were deeply discharged. I suspected they were shot but I've tried nursing them back with equalizations and being maintained with temperature corrected 14.8V charging profile. But I think they're damaged. After a full day charge, the inverter draw alone with the laptop charger will drop the voltage to 11.6V and hold steady. The voltage will often recover back to 12.5+V even after running this load for a while. Voltage reading is coming from the SCC voltage sense wires at the battery terminals.

That sounds like too much of a drop.
My experience with a heavier load (21.3A) was a drop to 12.0V, here:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/28437479.cfm

It does sound like too much of a drop to me as well. I don't recall what this load did to the voltage prior to this past summer's battery beat down, but I think it was more along the lines of barely a dip. I'm sure I would have a good recollection of voltage readings this low.

I always mean to get around to the SG check, I do have a hydrometer. But I just can't seem to get around to opening the cases up to check. Maybe this weekend...

12.3V this morning at -1C doesn't seem unusual to me. The Kill-a-watt on the inverter output hovers around 90 VA with this laptop on and charging load, so rough estimate of 9 amp DC draw by the inverter. Over 10 hours, that's 90 Ah which is nearly half of the 200ish Ah capacity.