Battery resting voltage vs load voltage

[mjalar51]

After 3 years using our solar panels and 2 golf cart batteries in our van, here is a question that I have and have not yet found a clear answer.

We have 200w solar panels on the roof that are always connected to and feeding the batteries via an mppt controller. To measure the battery condition, we have a simple digital display connected to the fuse panel which is close to the batteries. Another voltmeter, measures the input from the solar panels.

To have long-lasting deep-cycle batteries, I am supposed to keep them from discharging below 50% (about 12.3 volts).

Our voltmeter typically shows 12.6-12.7 volts at the batteries when nothing of significance is running (maybe an led light or two or charging a phone.) When the sun is shining, of course the voltage is up around 14 volts. So far, so good. 

Here is what I don't understand. When the Whynter fridge kicks on, the meter typically drops 0.3 or 0.4 volts while it is running (for 15 minutes at a time) then the voltage normally recovers back to where it was pretty quickly  after the fridge shuts off. Voltage drops are less when there is input from the solar panels. When the panels start at 12.7 (resting) volts (or higher) and drop to 12.3 or 12.2 volts, that seems fine. When the (resting) voltage is at 12.3 and then temporarily drops to 12.0 or 11.9 volts as the fridge or other larger load kicks in, is this a problem?

Is this temporary volt drop considered discharging down to 25% or even lower if it is only when the batteries are under load?

I want to keep things simple without any more instruments than necessary. I hope that knowing the normal characteristics of our charging and storage system should be enough to keep it happy and healthy. I hope the question make some sense. Thanks.

 

[tx2sturgis]

Voltage under load will always 'sag'. 

What you need to consider is 'resting' voltage as the main parameter, and of course a battery electrolyte tester (hydrometer) will help you determine actual state of charge. 

As lead acid batteries age, the internal resistance goes up, and is partly a reason for the apparent 'sag' or drop under load. Keep mental notes on these figures as they will eventually get lower and the batteries will slowly decline as they age, but it sounds like you have some usable life remaining with them.

 

[MrAlvinDude]

To have long-lasting deep-cycle batteries, I am supposed to keep them from discharging below 50% (about 12.3 volts).

hmmm... I would say this is only half the story.

The other half of the long-lasting story is: keep them as charged as possible, and fully charged regularly.

You can also read more about why lead acid batteries "grow old" here: https://batteryuniversity.com/index.php/learn/article/sulfation_and_how_to_prevent_it
And note the mention of soft-sulfation and hard-sulfation stages/events.

Here is what I don't understand. When the Whynter fridge kicks on, the meter typically drops 0.3 or 0.4 volts while it is running (for 15 minutes at a time) then the voltage normally recovers back to where it was pretty quickly  after the fridge shuts off.

A somewhat slow/sluggish chemical reaction is involved in pulling energy out of a lead acid battery. This is why you see a voltage drop while the refrigerator is running, and why it can recover when after the fridge  shuts off.

The slowness in the chemical reaction is also why the datasheets of some high end batteries will have different charts for how many Ah you can pull from the battery, depending on how many A you pull at any one time.
The industry standard is, that the Ah number they print on the side of the battery, is when tested with a 20A load.

Voltage drops are less when there is input from the solar panels. When the panels start at 12.7 (resting) volts (or higher) and drop to 12.3 or 12.2 volts, that seems fine. When the (resting) voltage is at 12.3 and then temporarily drops to 12.0 or 11.9 volts as the fridge or other larger load kicks in, is this a problem?

Is this temporary volt drop considered discharging down to 25% or even lower if it is only when the batteries are under load?

I want to keep things simple without any more instruments than necessary. I hope that knowing the normal characteristics of our charging and storage system should be enough to keep it happy and healthy. I hope the question make some sense. Thanks.

Voltage is not a very accurate way to measure charge-% 
At best, measuring the voltage is merely a "rule-of-thumb" indicator. 

A higher voltage drop under similar a load, indicates that there is less ability left in the battery, for the chemical process that releases the energy of the battery to happen. 

There are several factors that influence the ability of the chemical reaction to happen.
Some of them are:
- state of charge of the battery
- temperature
- the correct mix/balance of acid/chemicals 
- how much hard-sulfation have settled on the plates

So to more precisely know the %-of-charge-state, you need to combine with other measuring methods as well. 
One way could be an energy meter, that combines voltage and amp in/out - summarized over time.

 

[mjalar51]

Thanks for the help. Maintaining batteries seems as much art as science. I have just got a hydrometer and will give it a try. Seems like another low tech way to test things.