Running out?

[csp99]

Hi everyone. First time posting, pretty new to solar.
We ran out of battery at our recreational property while we were gone for the week, and I'm wondering if someone might be able to point me in the right direction.

Setup is...
- 4x 100w panels, south facing
- 40a MPPT controller
- 4x 200ah 6v batteries in series
- 1100w inverter

Had been turning Starlink off when not in use but I left it on last week while we were gone to see if the system could handle it full-time. It did not. Left on Saturday and came back Friday to dead batteries. It was a pretty gloomy week, fair bit of rain and clouds, but I had figured I had more than enough battery.

My thinking was...
- Batteries are about 200 ah each, so in series that makes 400 ah usable?
- 400 ah x 12v = 4,800 wh.
- I believe Starlink takes about 20 watts per hour when not in use. It's set to sleep, I believe, 6 hours a night but lets say 4 for a round number.
- 4,800 wh ÷ 20 w/h = 240 hrs (10 days) the batteries ought to be able to run Starlink...? We lasted somewhere under 5.5 days, not including whatever was being generated during the partially-sunny week.

I must be wrong somewhere in there, just not sure where?

 

[Mattb4]

First off unless you measure the Starlink for a day you really are only guessing at how much a days power is. You also have to account for the inverter idle current and other conversion losses. Say your inverter was drawing 1amp/hr idle current for all of the 24 hours.

 

[Supervstech]

4 6v batteries in series would be 24V, correct?
If so, then the Ah stays the same.
If you put two in series, then parallel them with another two in series, you have 12V and yes, 400Ah.

 

[Craig]

I'm betting starkink takes more than you think. Also is the antenna heater turned off? It has 3 settings off auto and pre heat.

 

[csp99]

4 6v batteries in series would be 24V, correct?
If so, then the Ah stays the same.
If you put two in series, then parallel them with another two in series, you have 12V and yes, 400Ah.

Sorry, yes, this is correct - 2 in series and then parallel.

 

[MichaelK]

From what I'm reading, you have a 12V system with your four batteries wired 2S2P. Is that correct? These are golf-cart lead-acid batteries? Most likely they will like charging best at about 1/8 of C, and if C is 400Ah, then that is 50A. Assuming that in the real world your panels only put out ~85% of their nameplate rating, that's really only about 27A max. Slightly more than 1/2 what they would like.

What your batteries really need is [(400Ah/8) X 12.5Vcharging]/85% = 735W of panels. What I would do is buy an additional four panels, wire them 4S2P, and replace the MPPT controller with a higher amperage model. If you don't want to replace the controller, then get just two additional panels, and hope that they can keep up. Wire them 3S2P.

If you are going to be away for a while, just shut the inverter off. The inverter itself is likely to be consuming as much power as the Starlink dish does.

 

[Supervstech]

From what I'm reading, you have a 12V system with your four batteries wired 2S2P. Is that correct? These are golf-cart lead-acid batteries? Most likely they will like charging best at about 1/8 of C, and if C is 400Ah, then that is 50A. Assuming that in the real world your panels only put out ~85% of their nameplate rating, that's really only about 27A max. Slightly more than 1/2 what they would like.

What your batteries really need is [(400Ah/8) X 12.5Vcharging]/85% = 735W of panels. What I would do is buy an additional four panels, wire them 4S2P, and replace the MPPT controller with a higher amperage model. If you don't want to replace the controller, then get just two additional panels, and hope that they can keep up. Wire them 3S2P.

If you are going to be away for a while, just shut the inverter off. The inverter itself is likely to be consuming as much power as the Starlink dish does.

Agreed, another option is getting another controller, and another 4 panels.

 

[LakeHouse]

Hi everyone. First time posting, pretty new to solar.
We ran out of battery at our recreational property while we were gone for the week, and I'm wondering if someone might be able to point me in the right direction.

Setup is...
- 4x 100w panels, south facing
- 40a MPPT controller
- 4x 200ah 6v batteries in series
- 1100w inverter

Had been turning Starlink off when not in use but I left it on last week while we were gone to see if the system could handle it full-time. It did not. Left on Saturday and came back Friday to dead batteries. It was a pretty gloomy week, fair bit of rain and clouds, but I had figured I had more than enough battery.

My thinking was...
- Batteries are about 200 ah each, so in series that makes 400 ah usable?
- 400 ah x 12v = 4,800 wh.
- I believe Starlink takes about 20 watts per hour when not in use. It's set to sleep, I believe, 6 hours a night but lets say 4 for a round number.
- 4,800 wh ÷ 20 w/h = 240 hrs (10 days) the batteries ought to be able to run Starlink...? We lasted somewhere under 5.5 days, not including whatever was being generated during the partially-sunny week.

I must be wrong somewhere in there, just not sure where?

A few things:

1. Count on Starlink drawing more like 50W in heavy use, maybe 30-40 on idle. Turn off heating for the summer, it'll push that up to more like 200W intermittently. I'm guessing the cabin is in Canada, so overnight low temps could be enough that heating activates.
2. Don't neglect the power consumption of your inverter. This can vary wildly between inverters. The 1000W inverter that came with my cabin drew 40W with nothing connected. The 3500W inverter I replaced it with draws about 50W.

So, let's say 40W for the Starlink for 18 hours a day, plus 40W for the inverter for 24 hours per day, and we're at 40W * 18h + 40W * 24h = 1680Wh per day. With no sun you'd be lucky to get three days.

 

[csp99]

First off unless you measure the Starlink for a day you really are only guessing at how much a days power is. You also have to account for the inverter idle current and other conversion losses. Say your inverter was drawing 1amp/hr idle current for all of the 24 hours.

That's true, Starlink might be taking more. But with no use, I wouldn't think that much more?

I'm betting starkink takes more than you think. Also is the antenna heater turned off? It has 3 settings off auto and pre heat.

Good point on the heater, I did not have that turned off (did that over the weekend) and that might have added some as well.
I guess I'm just surprised that my math was so far off. It didn't do 5 days of the 10 I was hoping for.

From what I'm reading, you have a 12V system with your four batteries wired 2S2P. Is that correct? These are golf-cart lead-acid batteries? Most likely they will like charging best at about 1/8 of C, and if C is 400Ah, then that is 50A. Assuming that in the real world your panels only put out ~85% of their nameplate rating, that's really only about 27A max. Slightly more than 1/2 what they would like.

What your batteries really need is [(400Ah/8) X 12.5Vcharging]/85% = 735W of panels. What I would do is buy an additional four panels, wire them 4S2P, and replace the MPPT controller with a higher amperage model. If you don't want to replace the controller, then get just two additional panels, and hope that they can keep up. Wire them 3S2P.

If you are going to be away for a while, just shut the inverter off. The inverter itself is likely to be consuming as much power as the Starlink dish does.

Ah ha. That makes a lot of sense. Thank you!

 

[MichaelK]

Agreed, another option is getting another controller, and another 4 panels.

Didn't think of that. With the option of a second controller, you're free to diverge from the panel type you already have. Shop around, and you'll find that larger high-voltage residential grid-tie style panels give you more bang for your buck.

In my area, I'm getting 250W grid-tie panels for ~75$, cheaper I think then what you can get a 100W panel for.

Even without a second controller, you might still be able to put in high-voltage panels. Assuming your 100W panels put out 18Vmp, and you wired four in series to get 72VDC, you could parallel those with two 72-cell high-voltage panels putting out 36V each. Each parallel string would be putting out 72V, so they are compatible.

 

[csp99]

Actually have a 60a controller on the shelf I wasn't sure if I would need. So I gather that if I were to pop that in and then replace the panels with 200w (x4) in parallel, that would give me enough for an effective battery charge now @ 2S2P, but would prevent me from expanding by 2-4 batteries (3S3P/4S4P) down the road?

 

[MisterSandals]

but would prevent me from expanding by 2-4 batteries (3S3P/4S4P) down the road?

If you put your batteries 3S that will be 36V. 4S will be 48V.

Adding 2 batteries to 2S2P would need to be 2S3P to stay at 12V.
Adding 4 batteries to 2S2P would need to be 2S4P to stay at 12v.

At 3P and 4P, you need to make sure to make the parallel connections in a balanced manner to have equal charging and discharging.
Proper wiring will make the currents to and from each battery (2S pair) the same (or close enough).

 

[csp99]

If you put your batteries 3S that will be 36V. 4S will be 48V.

Adding 2 batteries to 2S2P would need to be 2S3P to stay at 12V.
Adding 4 batteries to 2S2P would need to be 2S4P to stay at 12v.

At 3P and 4P, you need to make sure to make the parallel connections in a balanced manner to have equal charging and discharging.
Proper wiring will make the currents to and from each battery (2S pair) the same (or close enough).

Ugh, that's right. My brain farted there.

So if I were wanting to go 2S3P or 2S4P in the future, having those 4x 200w 12v panels would not be enough, correct?

 

[MisterSandals]

having those 4x 200w 12v panels would not be enough, correct?

Enough? 1x 100W panel is enough given enough time. The question (answer?) is how many Wh you need/want to replace each day.

6x 6V x 200Ah = 7200Wh (3600Wh usable if lead acid)

3600Wh / (4x 200W) = 4.5h to recharge

 

[Supervstech]

Enough? 1x 100W panel is enough given enough time. The question (answer?) is how many Wh you need/want to replace each day.

6x 6V x 200Ah = 7200Wh (3600Wh usable if lead acid)

3600Wh / (4x 200W) = 4.5h to recharge

He is talking about keeping the charge rate high enough to get the fla batteries charged correctly.
An earlier post said he would need .5C to adequately charge the batteries. Around 50A

 

[MisterSandals]

Yep, I was not considering minimum charge current, I was incorrectly only looking at watt hours.

An earlier post said he would need .5C to adequately charge the batteries. Around 50A

.125C ?

I won't disagree with the 50A number but that just sounds like a lot to just "adequately" charge. I never used an AGM.

will like charging best at about 1/8 of C, and if C is 400Ah, then that is 50A

 

[Supervstech]

Yeah, SLA and FLA or AGM batteries need to overcome their internal resistance. Charging is a bit trickier than with lithium.

 

[KITROBASKIN]

We do not know the age and health of the batteries. This talk about adding more batteries to a lead acid arrangement is not advised. Was that done in the past so that there is a pair of old batteries in this mix? This will draw down the newer cells.

Are the wire sizes sufficient for the system? Are all connections snug? Any other components (switches, breakers, fuses, surge protection) that might have an adverse affect?

 

[csp99]

We do not know the age and health of the batteries. This talk about adding more batteries to a lead acid arrangement is not advised. Was that done in the past so that there is a pair of old batteries in this mix? This will draw down the newer cells.

Are the wire sizes sufficient for the system? Are all connections snug? Any other components (switches, breakers, fuses, surge protection) that might have an adverse affect?

All batteries are marked 2019 so they might be getting due?
I don't think the battery wires were sufficient when I took the system over, but I've since replaced them with 2/0. Not sure about the others, however. Water levels are good and all connections clean and tight.

No switches or breakers. There is one surge protector (Starlink not plugged into it, and was off for the week). And 1 fuse from batteries to inverter.

 

[MichaelK]

All batteries are marked 2019 so they might be getting due?
I don't think the battery wires were sufficient when I took the system over, but I've since replaced them with 2/0. Not sure about the others, however. Water levels are good and all connections clean and tight.

From what I'm reading here, the fact that you can check the water levels means these are NOT AGM, but flooded lead-acid.

Which is perfectly fine. The advantage of flooded is that you can read the specific gravity of each battery cell and determine the battery's true state of charge. You can also use the SG to evaluate the battery's health.

Do you have a battery hydrometer? I use this one, because it has temperature compensation. Basically the gold-standard for flooded battery health/charge.

Deka East Penn 00231 Glass Temperature Correcting Battery Hydrometer | eBay

Deka East Penn 00231. Glass tube.

www.ebay.com

Do this. Get the bank fully charged, and after completing the charge, take a hydrometer reading of each cell. A fully charged battery will have an SG of ~1.165-1.175. Maybe a bit higher for a new battery, and maybe a bit lower for older batteries. The hydrometer is also great for spotting a defective cell. Each cell should not vary by more than ~0.01SG. Maybe 0.005SG for a new, healthy battery.

If you've fully charged the bank and the cells have a significant variation, cell to cell, or if the SG average is significantly less than 1.150, I'd say it's time to replace them.

If it is time to replace the batteries, I'd suggest it's also a good time to pull out that bigger 60A controller, so you can charge the new bank with more amps. Remember, chronic undercharging is the death of lead-acid. Keep them at a respectable charging rate, and they will last a long time.