Battery Overdischarge

[Bob_Dole]

Hi Guys,

New here. Details of my setup are in the signature. But I overdischarged my battery this weekend. My inverter has overdischarge prevention but it still took it too low. Is there any other method to prevent overdischarge? I think I was consuming too fast (about 1000w) and my 10Ah battery drained too quickly. Thats all I can think of.

Any thoughts?

 

[Hogheavenfarm]

Some Inverters run too low before they shut off, mine shuts down at 10.5 volts, I never want to take the battery that low. If running anything of high wattage I keep one eye on the rate of discharge and the voltage.

 

[DThames]

Hi Guys,

New here. Details of my setup are in the signature. But I overdischarged my battery this weekend. My inverter has overdischarge prevention but it still took it too low. Is there any other method to prevent overdischarge? I think I was consuming too fast (about 1000w) and my 10Ah battery drained too quickly. Thats all I can think of.

Any thoughts?

What type of battery and what voltage battery? How low did it discharge? Also, was that the voltage after the load was removed?

 

[Vigo]

I guess my question is, is the problem that you have a drained battery with a bms that now won't 'wake up', or is the question how to implement an external low voltage disconnect functionality that doesn't rely on the one in the inverter?

Or maybe it's both!

 

[Bob_Dole]

12v Lithium battery, dropped below ~11.0v. Not sure about the voltage after load removal. I ran a high load on the 10Ah battery for about 7 minutes or so and then the inverter clicked off. The solar controller then went in to a slow charge recovery mode and charged the battery like that until a certain level. Now the battery has been behaving normally since.

Vigo - Yes, a low voltage external disconnect to not rely on the inverter or battery BMS. I don't want to have to manually as be observing the battery voltage. It would be nice to have an external device that tripped at say 11.9v and then had to be reset to use again.

 

[Vigo]

Ok. You mention having a solar controller. What model is it?

Usually solar controllers have some kind of load ports with configurable voltage setpoints. If you have that, it is possible to use those load ports controlling either the simple on-off switch circuit of your inverter (would have to open it up and do wiring inside) or controlling a large relay/contactor which would simply disconnect the battery from the inverter. Given that your usual loads are well under 1kw (aka <100amp) it would be pretty cheap and simple to install a large relay between the battery and inverter and have the charge controller's load ports controlling it. The inverter would turn off when you hit the solar controller's 'load off' voltage and turn back on when battery recharged to 'load on' voltage.

 

[MisterSandals]

Details of my setup are in the signature.

Found them in your profile, not your signature.

>Small setup - 100w panel, 10Ah Lithium, Renogy 20A MPPT, 1200w PSW Inverter

A 1200W inverter draw on a 10Ah battery is WAAAY too much. How many amps do you think you were pulling? How many do you need to pull?

1200W / 12.8V nominal = 94A that's 10C discharge.

12.8V x 10Ah = 128Wh

128Wh / 1200W = .16h (.16h x 60m/h = 6.4minute runtime)


Oops, found it:
> I think I was consuming too fast (about 1000w)

1000W / 10Ah = .1h (10 min)

 

[DThames]

12v Lithium battery, dropped below ~11.0v. Not sure about the voltage after load removal. I ran a high load on the 10Ah battery for about 7 minutes or so and then the inverter clicked off. The solar controller then went in to a slow charge recovery mode and charged the battery like that until a certain level. Now the battery has been behaving normally since.

Vigo - Yes, a low voltage external disconnect to not rely on the inverter or battery BMS. I don't want to have to manually as be observing the battery voltage. It would be nice to have an external device that tripped at say 11.9v and then had to be reset to use again.

Some BMSs will shut off at 2.5v per cell or maybe 2.7v. So 11v would be about right for a BMS shutdown. Like other have said, you are pulling way too much current from such a small battery.

 

[Bob_Dole]

Ok. You mention having a solar controller. What model is it?

Usually solar controllers have some kind of load ports with configurable voltage setpoints. If you have that, it is possible to use those load ports controlling either the simple on-off switch circuit of your inverter (would have to open it up and do wiring inside) or controlling a large relay/contactor which would simply disconnect the battery from the inverter. Given that your usual loads are well under 1kw (aka <100amp) it would be pretty cheap and simple to install a large relay between the battery and inverter and have the charge controller's load ports controlling it. The inverter would turn off when you hit the solar controller's 'load off' voltage and turn back on when battery recharged to 'load on' voltage.

It is a Renogy Rover 20 amp MPPT Controller. It does have the load ports, but I am not sure how to use them.

 

[Bob_Dole]

Found them in your profile, not your signature.

>Small setup - 100w panel, 10Ah Lithium, Renogy 20A MPPT, 1200w PSW Inverter

A 1200W inverter draw on a 10Ah battery is WAAAY too much. How many amps do you think you were pulling? How many do you need to pull?

1200W / 12.8V nominal = 94A that's 10C discharge.

12.8V x 10Ah = 128Wh

128Wh / 1200W = .16h (.16h x 60m/h = 6.4minute runtime)


Oops, found it:
> I think I was consuming too fast (about 1000w)

1000W / 10Ah = .1h (10 min)

Yes it was consuming at about 1000w. Yeah I think part of my problem was pulling too hard on the small battery.

 

[Bob_Dole]

Some BMSs will shut off at 2.5v per cell or maybe 2.7v. So 11v would be about right for a BMS shutdown. Like other have said, you are pulling way too much current from such a small battery.

Yup. Got it. Makes sense.

 

[Vigo]

Well, if you want to implement the plan i mentioned then either study the manual or explore the settings for the SCC and see how low of a voltage you can set for turning the load ports off. If you can get it down to 11.0 then it will have a pretty good range of adjustability. You may not actually even want to go that low as a cutoff point, but with such a small battery you have to account for the fact that its terminal voltage will drop more with load, than that of a larger battery. So when pulling a heavy load on the battery, it may sag to 11.0, and when it cuts off the terminal voltage jumps right back up to 11.7, making that your 'real' cutoff voltage in that case. Unfortunately it's not easy to make a low voltage disconnect that accounts for the voltage sag which is proportional to load. That is an altogether more complicated device. But it's easy enough to determine the voltage sag for the largest load you'd intentionally run, subtract that from the voltage you want to disconnect the battery at, and set the load ports to turn off at that voltage. 11.7 - 0.7 = 11.0 = set 'load off' to 11.0, as an example.

 

[Bob_Dole]

Well, if you want to implement the plan i mentioned then either study the manual or explore the settings for the SCC and see how low of a voltage you can set for turning the load ports off. If you can get it down to 11.0 then it will have a pretty good range of adjustability. You may not actually even want to go that low as a cutoff point, but with such a small battery you have to account for the fact that its terminal voltage will drop more with load, than that of a larger battery. So when pulling a heavy load on the battery, it may sag to 11.0, and when it cuts off the terminal voltage jumps right back up to 11.7, making that your 'real' cutoff voltage in that case. Unfortunately it's not easy to make a low voltage disconnect that accounts for the voltage sag which is proportional to load. That is an altogether more complicated device. But it's easy enough to determine the voltage sag for the largest load you'd intentionally run, subtract that from the voltage you want to disconnect the battery at, and set the load ports to turn off at that voltage. 11.7 - 0.7 = 11.0 = set 'load off' to 11.0, as an example.

Do you have a wiring example of using the load terminals? I would have to hook my inverter up to the load terminals? I thought I read in the manual to never do that. Am I misunderstanding?

 

[Vigo]

The load terminals themselves are not capable of flowing the entire inverter current through them. But you can use those terminals to control a large external relay, and power the inverter from that.

Here is some MS Paint for you. Possibly not my best work, but the price is right.

 

[MisterSandals]

But you can use those terminals to control a large external relay

Just out of curiosity, what logic does a load terminal use in determining whether the relay is open or closed?

 

[Ampster]

Just out of curiosity, what logic does a load terminal use in determining whether the relay is open or closed?

He explained that in post #12 above.

 

[Bob_Dole]

The load terminals themselves are not capable of flowing the entire inverter current through them. But you can use those terminals to control a large external relay, and power the inverter from that.

Here is some MS Paint for you. Possibly not my best work, but the price is right.

View attachment 110736

 

[Bob_Dole]

Something like this?

 

[Vigo]

Yes, or a 'continuous duty 12v solenoid' of the appropriate amp rating.

I'm slightly dubious of those cheap really large plastic case relays (but i have one! Haven't tested it yet..) BUT, i will say that it's a lot easier to pop the plastic housing off a giant relay to inspect the guts, than it is to get into one of those crimped-together metal can solenoids. Better for curious minds, and those who would rather clean a set of contact points with a bit of sandpaper, than buy another $20 relay.

 

[Bob_Dole]

So based on that relay and its terminal numbering, I should be wired like this?

 

[Vigo]

No sir.

Should be like this:


Usually 85 and 86 are the 'control side', made up of an electromagnet that pulls on the switch when powered up.
30 and 87 are the switch for the 'load side' which is the thing you are trying to power/control with the relay. So charge controller controls the relay, relay controls the inverter. The point of the relay is basically as an 'amplifier'. You put a small signal into it and get a large signal out of it, larger than what you could directly power with your load port.

Relays aren't always labeled that way, but it is pretty common. Sometimes they are just 1 2 3 4 or A B C D, but if so they usually have a diagram on the side that makes it clear which terminal does what.

 

[Bob_Dole]

So an update - I now have a 1500wh lifepo 12v battery installed in the same setup. I ran an 820 watt load for about just shy of an hour and the bms shut off the discharge.

How much efficiency loss is happening? I thought it would run an 820 watt load for close to an hour and a half, not under an hour. What should I expect on a 1500 wh battery actually outputting?

 

[Ampster]

I think there is a disconnect between Watthours and Amphours. Try using one or the other and it may become clear. If you insist on using them interchangeably, at least give the reader a clue about the voltage do one of us can help you.

 

[Bob_Dole]

I think there is a disconnect between Watthours and Amphours. Try using one or the other and it may become clear. If you insist on using them interchangeably, at least give the reader a clue about the voltage do one of us can help you.

My last comment only spoke in watts and watt hours. I didn't say amp hours. And I did say the battery voltage.

Are you drunk?

 

[hankcurt]

If you are looking for someone who has been consuming alcohol to help you, I'll do my best.

So watt hours give you a rough idea of how much energy is in your battery, and a rough idea of how much power your load will consume. Kinda like saying "I bought a 12 pack of brews for my uncle, and he was sober three hours later when he should have been plastered all night."

With watt hours, much like alcohol in my uncle, not all of them go to the place where they have an effect. Some watts (alcohol) gets consumed by the load (the brain), however, if the uncle in question is a 300lb Irish mobster, he probably metabolized a good share of it in other places in the circuit (his liver).

However, if you measure amp hours, you are measuring how much alcohol went down the hatch, and that gives you a true sense of how much the 12 pack was able to deliver.

Also, unlike a true 12 pack, if the cans aren't evenly full, a battery will only be able to deliver the amount that is in the least full can. So it is important to know that the battery cells are properly balanced when fully charged.

Of course, depending on how reputable the vendor is that you purchased the 12 pack from, they could have sold you some O'Douls. But I hope that is not the case.