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.

[SoundGuy]

ewarnerusa wrote:
I do appreciate your desire for more information before providing a response.

2oldman wrote:
Don't feel bad. Few OP's contain all information necessary. That's why it's beneficial to read all OP's posts before responding.

Sorry, but until you yourself provided a link to another discussion in which it was evident the OP's batteries are most likely pooched anyway he never said anything about this in any of his previous posts in this thread which I did read. Obviously you knew about this previous thread but I didn't ... we're not mind readers. :R

[2oldman]

ewarnerusa wrote:
So it's ironic that I've been chastised about not providing extra info to nuance the question.

You can't win. If you provide too much in one post, some skim over it and miss important info. If you don't provide enough, they don't read subsequent posts and chastise you for not saying that!

sigh!

[ewarnerusa]

2oldman wrote:

ewarnerusa wrote:
I do appreciate your desire for more information before providing a response.

Don't feel bad. Few OP's contain all information necessary. That's why it's beneficial to read all OP's posts before responding.

I have a tendency to get too wordy, so I have been trying to shorten my thread posts! So it's ironic that I've been chastised about not providing extra info to nuance the question. I figure that the discussion that hopefully follows is where one gets into nuancing the topic or getting into the weeds.

[2oldman]

ewarnerusa wrote:
I do appreciate your desire for more information before providing a response.

Don't feel bad. Few OP's contain all information necessary. That's why it's beneficial to read all OP's posts before responding.

[doughere]

If you are drawing off 90 AH and then bouncing back to 12.3 (about 70% charge), then when fully charged reading a steady 12.6; you're fine.

I would equalize with a 5 AMP charge watching with a hydrometer until all cells are fully up.

Doug

[ewarnerusa]

Not really. The question was what is a reasonably expected voltage drop from a 100 watt inverter load. I even used the term "just" which clearly implies no other loads. I do appreciate your desire for more information before providing a response.

[SoundGuy]

2oldman wrote:
OP explains why batteries shot

Would have made this discussion a lot simpler if the OP had mentioned this in his original post. :R

[DrewE]

ewarnerusa wrote:

DrewE wrote:
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.....

Yes it does recover immediately. Wouldn't taking my voltage readings directly from the battery posts essentially bypass any wiring issues? I get that I would be measuring voltage drop due to resistance in the wiring if I took measurements anywhere near the inverter. I did clean up my connections after getting the camper back home in the fall, there was corrosion present when there never was before. Wiring is quite fat and relatively short, so I don't suspect a problem there. I'll see what the other inverter does which shares the same wiring to the battery (2 gauge, 5').

If there's a poor connection for whatever reason in the jumper between the two 6V batteries, that would still be reflected in a voltage reading taken at the extreme battery terminals.

I'm not suggesting that the connections are necessarily bad, only that it is a possibility to consider and rule out. Taking a voltage reading between, for instance, the terminals of the batteries that the inter-battery jumper connects to would easily show whether you're getting an unacceptable voltage drop there or not. (Basically, any voltage drop would be unacceptable at 10A, or at least any drop above a very few millivolts.)

[ewarnerusa]

I also think it is a battery problem and not related to wiring or the inverters. After charging and allowing surface charge to dissipate, I can count on a voltage reading >12.6V. I can also count on it holding that voltage at least over night. Over the course of days I don't know because solar goes to work on them daily.

[AllegroD]

I will stand by the original statement. No sag. At least negligible to the point of non relevant.

11.6 to 11.7 volts in a battery is very, very bad. As someone said, they are probably shot. Recovery to 14.8 does not tell the whole story. after charging. turn off the charger, wait about two hours and retest.

If batteries are not holding charge, which seems very likely with given statements above, then they will again drop extremely rapidly. This is not an inverter problem. This is a battery problem.

[2oldman]

ewarnerusa wrote:
But wouldn't damage also result in decreased capacity? I mean it seems like they recover back to an acceptable voltage after drawing ~90 Ah from them last night (came back up to 12.3V at -1C temp). So capacity seems unaffected but they experience much higher voltage sag under load than they should.

You can limp along with them, but in the not-too-distant future they won't be able to maintain enough voltage to keep the inverter going.

Replace them.

[ewarnerusa]

DrewE wrote:
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.....

Yes it does recover immediately. Wouldn't taking my voltage readings directly from the battery posts essentially bypass any wiring issues? I get that I would be measuring voltage drop due to resistance in the wiring if I took measurements anywhere near the inverter. I did clean up my connections after getting the camper back home in the fall, there was corrosion present when there never was before. Wiring is quite fat and relatively short, so I don't suspect a problem there. I'll see what the other inverter does which shares the same wiring to the battery (2 gauge, 5').

Our weather has been pleasantly warm these past couple weeks (approaching 60F during the day) and the batteries behave the same. It just happened to be a bit below freezing this morning.

Another thought... I wouldn't doubt that they batteries are compromised and should be replaced if I want the full benefits of 2x6V GC batteries. But wouldn't damage also result in decreased capacity? I mean it seems like they recover back to an acceptable voltage after drawing ~90 Ah from them last night (came back up to 12.3V at -1C temp). So capacity seems unaffected but they experience much higher voltage sag under load than they should.

[NinerBikes]

Why don't you bring the batteries inside, warm them up to room temperature, and retest them for voltage drop /sag. Being at 0F or lower isn't helping the chemical reaction capabilities of those batteries being able to hold a load or rebound once shut off.

[2oldman]

OP explains why batteries shot

[CA_Traveler]

To much voltage drop but why? Have the batteries load tested to help determine weak batteries or other problems.