Invertor question

[Offgrid]

I saw on a post a night or 2 ago,someone mentioning that batterys need to be matched to invertors.Something like the maximum a 100AH battery can handle is a 400w invertor....what is that all about? I mean I thought you can use any invertor as long as the voltage is correct.I have a 100AH battery and a 650W invertor (Surge watts included) is that a problem?

[Sunking]

Batteries have internal resistance. As you use more current, you loose voltage. Inverters trip off line when the voltage dips below a preset point like 11 volts.

A 100 AH FLA 12 volt battery has an average internal resistance of about .05 Ohms and goes higher as it discharges. When you draw 20 amps off the battery that is 100% fully charge the voltage drops to about 11 volts.

Guess what happens to the inverter when the voltage dips below it trip point?

Think of it like this. You can lift 150 pounds right? Can you carry it up 5 flights of stairs without stopping or help?

[Offgrid]

BEEEEEEP BEEEEEEEEEP BEEEEEEEEEP *Your own personal load shedding* *Darkness*....I have a confession to make that I didnt tell you because I didnt want to sway your input hehehe....Well you know I have been struggling with this dang storage problem and the battery not keeping a 15w light bulb on for something like 16 hours (You calculated) and more like only getting 3 hours before 12.42V.....Well,I have been thinking...I got this battery for half its going price (yes it was still new) and Im wondering if there is a reason for that.hehe

I am only getting about 19 AH (maximum 38watts ever seen) replaced according to the whatsapp meter but still that should be enough as its 20% of the battery and STILL I get crappy results.I know the battery is "full" because 3 days of solid charging with no loads in the evening or day and the CC 14.5V blinking pretty much constantly indicating the battery is near full.

We have eliminated pretty much everything except the battery.

After a evening of use,3 hours,the voltage is 12.42V.I go to sleep,wake up and its slightly higher.I put a load on and straight away goes back to 12.42V.So the DOD is correct.

Its not the CC as input current = output current.

We have now discussed the invertor but my 15W bulb is a far shot from 20 amps.

This is so annoying,learning and wanting to go on but cant because I havnt worked out a basic problem lol.

In your professional opinion...What do you think is the problem?

[Sunking]

After a evening of use,3 hours,the voltage is 12.42V.I go to sleep,wake up and its slightly higher.I put a load on and straight away goes back to 12.42V.So the DOD is correct.

Then the battery is still near 100% charged.

Remember SOC is only valid after the battery has rested several hours.It cannot have anything connected and set for several hours. You cannot determine much of anything if the battery is charging or discharging.

Perhaps an extreme example is necessary. When you are at the auto parts store look at starting batteries. They will have some number printed on them like 550 CCA.

CCA = Cold Cranking Amps. That means if the battery is 100% fully charged, brand new, it will deliver 550 amps, for 30 seconds, at 0 degree F, at 7.2 volts for a 12 volt battery.

Now think about that for a while. a 12 volt fully charged battery at 7.2 volts. Why is that?

[Offgrid]

The battery will have lower volts during the cranking in order to supply the power to crank?

[Sunking]

The battery will have lower volts during the cranking in order to supply the power to crank?

You are getting warmer. It has to do with Ohm's Law. Batteries have internal resistance.

Resistance x Current = Volts
Volts / Current = Resistance

Based just on that information we can conclude

The internal battery resistance = 7.2 volts / 550 amps = .013 ohms.

[Offgrid]

Dont give me beat down here because I just read your statement close to 20 times to try get what you saying...

That internal resistance 0.13 ohms prevents the battery from producing or cranking all 12v from the battery and only manages to get 7.12V? The battery is resisting the current draw?

Remember SOC is only valid after the battery has rested several hours.It cannot have anything connected and set for several hours. You cannot determine much of anything if the battery is charging or discharging.

For those 8 hours that Im sleeping,there is nothing turned on except the wattsup meter attached and CC.Nothing else is drawing current.Invertor is turned off.

[Mike90250]

.... The battery is resisting the current draw?

yessss 'e getsss itsss

[Offgrid]

LOL Mike!

...and because its resisting the current draw,its showing lower voltage but theres still plenty amp hours in the battery,more than what the voltage is telling you?

[Sunking]

LOL Mike!

...and because its resisting the current draw,its showing lower voltage but there is still plenty amp hours in the battery,more than what the voltage is telling you?

Correct. That is what I am trying to tell you. SOC measurements are meaningless. The battery has to be rested for several hours.

You are trying to perform SOC measurements on a working system. It will not work

[Offgrid]

*Click* Thanks for your patience Sunking,Mike. So basically on a sealed lead acid.You pretty much have to rely on your sizing calculations to make sure you dont go below the 20% DOD.In other words... work out the watts you can use and for how long and stick to it! because doing a voltage test and then resting batteries for 7 hours then adding a small load is not always viable on a active system to get SOC as will be disruptive. Once in a while do AC charge to top off the batteries.

If this was not a sealed battery it would be a little simpler because you could do a lab gravity/hydrometer test that Sunking keeps telling people is the only real way to get a proper SOC status.

[Offgrid]

Oh and I forgot to mention.On a new battery, it seems the higher the DOD the previous night,the better the capacity after a full recharge.Does that sound right?

If so,you just solved my 2 problems!!!!!!

1.SOC is not accurate unless battery has no loads and rested for 7 hours.
2.A new deep cycle should be charged up and given a deeper discharge once or twice to get correct capacity!

[Sunking]

Yes any new battery needs a few cycles to reach full capacity.

[Offgrid]

Any recommendations on DOD when cycling a new battery?

[Sunking]

50% ought to do it. Just make sure you recharge immediately.