Voltmeter/battery bank help

[Craig.Gorder]

Hi there -- I've been running my solar system for a few months now and it seems to be giving me some trouble/anxiety. I have a total of 400ah in my battery bank. However, whenever there is no solar input (e.g. nightime) and I am pulling from the battery bank, the voltmeter drops substantially. I'm not sure if there is an issue here, or I just don't understand what the volmeter represents (it's battery bank life remaining, right?).

For example: My fridge pulls more power than anything else in my rig (about 6-7 amps for about 10 minutes per hour). Before my fridge kicks on, my voltmeter will read 12.7V. As soon as the fridge turns on, it will drop as low as 11.8V. Obviously, this shouldn't be happening, considering that suggests I'm going from 100% battery to 0%. Once the fridge turns off, it will bump back up to 12.7V. It worries me that I am somehow damaging my battery bank and pulling below the 12.2/12.3 range.

A few questions:

1) Is this typical for the voltage meters on other peoples' set ups? It seems like the voltmeter is correlated to the current pull, rather than battery bank remaining. I have two voltage meters in the system, and they both share similar readings. Even my fridge has a voltage display, and that is inline with those voltage meters -- so I assume the three of those are not all off.

2) Is there a better means of actually measuring the percentage left in the battery bank? Or should I just do the rough math in terms of amp hours used? I certainly want to baby my batteries.

3) Do you all think this means something may be messed up in my system? If so, any ideas?

Any help is appreciated! This was my first solar project, so feel free to talk to me like I'm a child

My setup:
4 x 100W solar panels
4 x 100 ah deep cycle AGM batteries
200 amp relay isolator
1000W inverter (although every permonent components go through this, they are all DC)
EPEVER 40A MPPT Solar Charge Controller

My battery wiring is in parallel (I wanted to post some pictures, but I can't seem to get them to upload).

I've also tried the "perfectly balance drain" wiring, but I switched it, fearing that was the cause of the issues. I assume battery wiring isn't the culprit, since both wiring efforts produced the same results.

 

[Craig.Gorder]

Hah! Well, I guess I did upload some photos, even if they are sideways

 

[john61ct]

Volts have a very loose relationship to SoC.

You want a battery monitor for that.

Merlin / Balmar SmartGauge is most accurate and user friendly.

Victron BMW-702 is a good AH-counting type.

Many use both, as the latter also gives details on power usage.

SoC can also be read via hydrometer if you have an FLA bank.

 

[john61ct]

It is normal for volts to drop when the bank is under load, higher the load greater the drop.

After no load, you'll see volts recover a bit.

Reverse is true after receiving a high-amp charge.

True "resting voltage" is after being isolated for 48+ hours.

 

[john61ct]

wrt wiring, ​http://smartgauge.co.uk/batt_con.html

 

[Joe917]

http://www.bogartengineering.com/products/trimetrics/
Highly recommended.

 

[john61ct]

Yes a great AH counting monitor, but all these, unlike SmartGauge, do need periodic recalibrating.

Excellent details here, just search for "battery monitor" among the post titles: http://www.pbase.com/mainecruising/boat_projects

 

[Haf-E]

Where is the volt meter connected? is it connected with its own wires all the way to the battery or is it connected to a fuse panel which also supplies the DC loads? If so, then its most likely just the voltage drop you are seeing in your wiring.

Its best to have the volt meter connected directly to the battery with its own wiring, fuse and holder.

For a battery that large, I would not expect it to drop that low with the frig running.

 

[Haf-E]

Also should mention that the voltage when the solar is charging can be confusing - for example, if the battery was really low (like 11.8 vdc in the morning) and the sun was charging - the voltage will be pushed up to a higher level (such as 12.7 vdc). When the frig runs, if it uses more power than the solar produces, then the voltage will drop back down to a level which is similar to what the battery voltage was without the solar charging.

Bottom line is you should only really trust the voltage of the battery when there is no load or charging sources connected. Also, Battery voltage doesn't indicate battery "life" - just is an approximation of the state of charge level - i.e. how full the battery is.

 

[jonyjoe101]

Maybe your batteries aren't being fully charged. The 12.7 volts might just be the surface charge. With solar sometimes you might not get your batteries fully charge. Since your fridge is the major power user is it a 12 volt model, maybe you are using more power than you can replace.

I decided to run a test using my old kinetik 102 ah agm that no longer holds a good charge. It was connected to solar all day. After unplug from solar it read 12.7 volts, I plugged in my roadpro (uses 11 amps) I only ran it for 3 minutes, the voltage drop to 12.1 volts. It only drop that low because the battery no longer holds capacity. When new it might drop to 12.5 volts.

With your 400 amp hours, your voltage shouldnt be dropping too much, maybe the lowest should be 12.3 volts with a modest 7 amp load. 7 amps is insignificant as far as loads go, and if it's a fridge it might max out at 7 amps when the compressor starts and then settle at 4 amps.

When you are charging with solar, what is the highest voltage you see? you should see 14.4 volts for several hours at a time. A battery bank as large as you got, should rarely be floating. If possible just set your float voltage to 14.4 volts also.
That is what I had to do to my battery, the mppt solar controller I use, kept going from bulk, straight to float, skipping the absorb stage. When the battery reaches 14.4 volts it need to stay there for several hours.
This won't damage your batterybank, the batteries like to stay at 14.4 volts all day long, and with solar you will never overcharge them.

But chronically undercharging lead acid batteries will damage them permanently. Also some agm batteries need to be charged at high amps from time to time to keep them healthy, solar panel can never reach those types of amps they need.

 

[Craig.Gorder]

Well, one of the voltage reading is coming directly from the charge controller, so I guess in a way that is coming directly from a battery. Would you agree? The other is from the fuse block (it's part of a USB outlet that I have on the block).

It sounds like an AH monitor would be good in this situation rather than trusting the voltage reading. This is super helpful, thank you!

 

[rayra]

seconding the question about how the voltmeter is wired in and whether it is hot / displaying all the time, too. You'd do best to have the voltmeter on its own direct circuit. Or only pay attention to its reading when the bank isn't charging or under another load. After a surface charge dissipates.

Voltmeter isn't a real good snapshot of battery bank status during active discharge. Or even active charging. Good enough to tell you power is coming in or going out.

I added two voltmeters to my aux power setup, as well as several USB charging ports with LED indicators, always hot. I soon noticed the parasitic drain was costing me about a tenth of a volt every day or two. I wound up putting an intermittent switch on my voltmeter. I still need to swap out my USB ports and/or put them on a switched circuit.

 

[Craig.Gorder]

When you are charging with solar, what is the highest voltage you see? you should see 14.4 volts for several hours at a time. A battery bank as large as you got, should rarely be floating. If possible just set your float voltage to 14.4 volts also.
That is what I had to do to my battery, the mppt solar controller I use, kept going from bulk, straight to float, skipping the absorb stage. When the battery reaches 14.4 volts it need to stay there for several hours.
This won't damage your batterybank, the batteries like to stay at 14.4 volts all day long, and with solar you will never overcharge them.

But chronically undercharging lead acid batteries will damage them permanently. Also some agm batteries need to be charged at high amps from time to time to keep them healthy, solar panel can never reach those types of amps they need.

Do you think the isolator would be charging it with high enough amps? If not, how else would you charge it?

As for the float/absorb idea. I am in and out of my van, so I haven't carefully tracked it, but it looks like I'm at 14.4 most of the day (I'm in central oregon in the summer right now). Would it be more in the 13.8 range for the absorb stage? I notice that number regularly as well.

 

[jonyjoe101]

Depending on how long the isolator is feeding the batteries. If you are running the engine for several hours it might be giving them a good enough charge. It has to be able to keep the battery at 14.4 volts for several hours. The high amps will get it up to to about 80 percent, but then the the final 20 percent will take hours. Also you can check for a loose connection somewhere that might be preventing the battery from getting all the amps being sent to it.

The only time I see 13.8 volts is if thats the float setting or it's a cloudy day and my panel isnt able to put out enough amps to get the battery to 14.4 volts. The default float voltage for most agm is about 13.7 volts, but as low as 13.3 volts.

Myself all I use to measure how much power I got is the voltage, I also like to see the amps the solar is putting into the battery, If I see 8 amps an hour going into my battery for the past 3 hours thats about 24 amps, and I know the previous night I used about 20 amps, then I can guess that I put back what I took out. If in the morning my battery reads about 12.4 or 12.5 volts after normal use, too me that means I have a good battery.
When my kinetik 102 ah agm started to go bad, I could tell just by how much the voltage started to drop. After charging all day on a sunny day, the voltage showed it was fully charge, but as soon as I put loads on it the voltage drop quickly to under 12 volts like your battery did. What happened is I took the battery down below 11 volts about 3 times and after that the battery lost capacity. It charges quickly to 14.4 volts but it maybe only has 30 ah of usable capacity.

This is the 90 volt 30 amp combo meter I have connected between the controller and battery, it's the only thing I been using for the past 4 years to tell me the condition of the battery.

 

[AndrewP]

Some voltage sag under load is normal. In the extreme case, when starting, you'll transiently see 10 volts or so. Yours sounds like a lot, especially with such a big battery bank. I;,d look for a high resistance connection somewhere between the battery and the measurement. Clean the battery terminals, take apart and clean every connection.

I am assuming it's 6-7 amps DC not 6-7 amps AC from the inverter.

What does your battery recover to when not running? 7 amps for 10 minutes is a tiny load, and the battery should recover to full voltage in between. If it does, everything is fine.

But with a big battery bank, I think a real battery monitor would be helpful.