Expected voltage sag?

[ewarnerusa]

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?
Thanks.

[2oldman]

ewarnerusa wrote:
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.

They're shot.

[wa8yxm]

At 100 watts (10 amps) very little loss of voltage less the wires are way too small between battery and inverter. Perhaps 0.1 or 0.2 max. Likely less.

[DrewE]

Does the battery voltage recover immediately when the inverter is turned off? If so, it may be a relatively high resistance spot in the circuit and not a battery problem. This may be a poor connection at a battery terminal, for instance, caused by corrosion or crud on the wire terminal, or a poor ground, or....

I have a somewhat similar problem on my battery, which I have tracked down to be the connection on the battery (in this case a "marine/RV" pseudo-deep-cycle battery) between the screw terminal and the large round stud--that is, on the battery terminal itself. You can find these sorts of things by measuring voltage drops along the circuit while its under load--or, if bad enough, by looking (carefully) for warm or hot connections.

Of course, the problem may be an actual battery problem and not a connection problem equally as well.

[ewarnerusa]

Battery bank is only seeing the inverter load. Batteries are still hooked up to the trailer, but the trailer is in storage mode at my house. All my parasitic loads (propane detector, fridge, TV antennae booster, stereo) are on one circuit, so I pull the fuse to that circuit and those parasitic loads are gone. Although the control panel does light up an LED indicating the missing fuse, so there is still a minor draw from that. Other than an LED light or two while I'm in the camper tinkering, no other DC loads. I confirmed the voltages with the LED lights off, too. ~11.7V under this load.

[SoundGuy]

pianotuna wrote:
100 watts = a 10 amp load @ 12 volts. That should produce minimal voltage drop--not at all what you are seeing.

In his original post the OP said he was powering "just" the inverter with his two 6 volt batteries ... if so, does this mean the batteries were disconnected entirely from the trailer itself? Since the OP wasn't clear his battery bank may not be seeing just a 100 watt load but rather much more, some from the inverter itself, some from the load it's powering, PLUS additional load from the trailer's own 12 vdc system, which could be considerable depending on just what may be running at any given time, particularly noticeable draws such as a fridge climate control system that hasn't been forced off. It's easy to overlook other draw but all draw contributes to voltage drop.

[pianotuna]

Hi,


100 watts = a 10 amp load @ 12 volts. That should produce minimal voltage drop--not at all what you are seeing.

[ewarnerusa]

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.

[brulaz]

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

[scrubjaysnest]

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.

[RJsfishin]

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.

[ewarnerusa]

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.

[ewarnerusa]

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.

[doughere]

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

Doug

[SoundGuy]

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. 😉

[2oldman]

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