• Have you tried out dark mode?! Scroll to the bottom of any page to find a sun or moon icon to turn dark mode on or off!

diy solar

diy solar

How to measure battery capacity/Battery health?

ThorCT

New Member
Joined
Nov 10, 2024
Messages
5
Location
ISA
I'm essentially an intermediate guy when it comes to solar systems, but I'm stuck with this. How do I check the battery capacity of my bank or essentially check their health?

I've searched here multiple times but can't seem to find the answer...please point me the way if this has already been answered before, maybe my searchfu is just crap.

Here is my situation.

I have two Oddesey Extreme group 31 batteries that I bought new in my van setup. They are 100ah each and I have 400watt of solar charging them. This setup has been working great for a while. I bought these batteries in spring of '22 so they have been cycling for essentially 2.5 years....I've never discharged them below 50% except for one time. I blew a fuze from the charge controller to the batteries, but my fridge kept pulling juice from the batteries (I probably should have not wired it this way but years ago I did so shame on me). I travel a lot for work so I don't know how long the fridge was trying to pull power from the non-charging batteries but it was at least 7 days...maybe 14 days. I've since added a low voltage disconnect so this doesn't happen again, but from that point forward I've always felt like the batteries would drain faster on overnight camping trips so I think I did damage them.

The last 2 months have been really bad. The batteries get down to about 75% capacity within 3hrs of the sun going down. The only constant drain is my fridge at about 4.5amps per hr avg and my Max Aire fan at lets say 1.5ah/hr and some lights at maybe 1ah/hr (I don't use them very much). So, despite this low load I've gotten below 12.4v before midnight (7hrs after sunset). So, I think my batteries are shot but how do I know for sure?

I disconnected the load and they have been sitting on a charger for about two weeks now but I'd really like to figure out their condition before I spend some big bucks on another replacement. How do I do that?

Jeff
 
I'm essentially an intermediate guy when it comes to solar systems, but I'm stuck with this. How do I check the battery capacity of my bank or essentially check their health?

I've searched here multiple times but can't seem to find the answer...please point me the way if this has already been answered before, maybe my searchfu is just crap.

Here is my situation.

I have two Oddesey Extreme group 31 batteries that I bought new in my van setup. They are 100ah each and I have 400watt of solar charging them. This setup has been working great for a while. I bought these batteries in spring of '22 so they have been cycling for essentially 2.5 years....I've never discharged them below 50% except for one time.

What said 50%? If it's not a shunt that knows the battery bank is 200Ah and its counting current and calculating SoC based on (Ah remaining) / (Ah total) %, it's not a valid number.

I blew a fuze from the charge controller to the batteries, but my fridge kept pulling juice from the batteries (I probably should have not wired it this way but years ago I did so shame on me). I travel a lot for work so I don't know how long the fridge was trying to pull power from the non-charging batteries but it was at least 7 days...maybe 14 days. I've since added a low voltage disconnect so this doesn't happen again, but from that point forward I've always felt like the batteries would drain faster on overnight camping trips so I think I did damage them.

The last 2 months have been really bad. The batteries get down to about 75% capacity within 3hrs of the sun going down.

What is reporting 75%?

The only constant drain is my fridge at about 4.5amps per hr avg and my Max Aire fan at lets say 1.5ah/hr and some lights at maybe 1ah/hr (I don't use them very much). So, despite this low load I've gotten below 12.4v before midnight (7hrs after sunset).

VERY roughly speaking, AGM 12.4V under a light load is > 75% SoC.

So, I think my batteries are shot but how do I know for sure?

I disconnected the load and they have been sitting on a charger for about two weeks now but I'd really like to figure out their condition before I spend some big bucks on another replacement. How do I do that?

Jeff

The only way to know for sure is to do a capacity test. The one I prefer is a reserve capacity test. Reserve capacity is the number of minutes a battery can deliver 25A and stay above 10.5V. The nice thing about this test is it doesn't completely drain the battery and minimizes the potential for damage as you have 10-15% of the 20h capacity remaining. A 300W load through an inverter is pretty close to a 25A draw.

A typical way to recover some lost capacity with AGM batteries is to run an "equalization" cycle where you just hold them at absorption voltage for an extended period. I've done this a few times and restored some capacity and improved internal resistance.

Are your batteries 32-PC2150 or 31M-PC2150? If so, these have an internal resistance of 2.2mΩ, a CCA of 1150, and a reserve capacity of 205 minutes.

You can also fully charge them and use a digital battery tester that can measure internal resistance. I have this one as the one I use for when things really matter:


and each car has one of these for a quick check of the battery. If it's in question, I use the other:

 
@sunshine_eggo thanks for the response. Let me answer these in sequence...and please, if I'm being dense or thinking about this wrong please tell me, I only know what I know.

I'm determining the % capacity based on the voltage as read by my charge controller...I've confirmed it is really close to the actual voltage when I measure the bank with a multimeter so I usually just go with that. I have a Renogy 40amp MPPT charge controller. From that voltage I look at the Odyssey battery chart (page 13 here - https://www.odysseybatteries.com/odyssey/docs/us-ody-tm.pdf) to determine the SOC.

To your comment on a capacity test, this is really my question. I know what it is, but I can't seem to find any specifics on HOW to do it. So can you point me the way on how to measure capacity?

Your comment on the Reserve Capacity Test is helpful. How do you get an accurate way to pull a 300W/25A draw? I don't have an inverter (everything in my van is 12v), but was thinking about getting one for another setup I'm building for my garage use.

And according to that Odyssey chart 10.5V is off the chart so I assume that means my battery is at 0%. I'm not comfortable going that low so what are my other options?

As to running an equalization cycle, is there a preferred type of battery charger you prefer to do that? The charger that has been on the batteries for the past 3 weeks is a Noco Genius GenPro10x1. It doesn't really have specific cycles. That said a couple days ago I ordered a Noco Genius 10 that does have multiple cycles. Should I do this on each better individually or use it on the bank itself?

My Batteries are the 31M-PC2150.

What does your recommended battery tester do that you can't do with a normal voltmeter? I don't know how CCA is measured but I'm assuming it some simple math. And the resistance can be measured pretty easily. Also, how is it calculating the battery health and what does that mean in terms of current vs new capacity?

And finally, I'm not really sure if any of this will tell me my last question there...how do I measure the capacity of a battery bank? I hear people refer to it all the time but little on HOW to actually do it.
 
@sunshine_eggo thanks for the response. Let me answer these in sequence...and please, if I'm being dense or thinking about this wrong please tell me, I only know what I know.

I'm determining the % capacity based on the voltage as read by my charge controller...I've confirmed it is really close to the actual voltage when I measure the bank with a multimeter so I usually just go with that. I have a Renogy 40amp MPPT charge controller. From that voltage I look at the Odyssey battery chart (page 13 here - https://www.odysseybatteries.com/odyssey/docs/us-ody-tm.pdf) to determine the SOC.

Key aspect of the chart:

"as long as the battery has not been charged or discharged for six or more hours."

If you're reading a voltage under load, your SoC is ALWAYS higher than the chart indicates.

If you're reading a voltage under charge, your SoC is ALWAYS lower than the chart indicates.

To your comment on a capacity test, this is really my question. I know what it is, but I can't seem to find any specifics on HOW to do it. So can you point me the way on how to measure capacity?

Your comment on the Reserve Capacity Test is helpful. How do you get an accurate way to pull a 300W/25A draw? I don't have an inverter (everything in my van is 12v), but was thinking about getting one for another setup I'm building for my garage use.

Fully charge.
Apply a 300W load, which is about 25A.
Time it in minutes until it reaches 10.5V in minutes.
Compare to rated reserve capacity, or "RC"

It's up to you to figure out how to get the 300W load. 5X 60W incandescent light bulbs gets you pretty close. You could buy a cheap inverter, a power strip and a light fixture. You could also get a bunch of 12V bulbs, or you could figure out what in your system consumes 300W/25A or close to it.

And according to that Odyssey chart 10.5V is off the chart so I assume that means my battery is at 0%. I'm not comfortable going that low so what are my other options?

The chart ends at 10%, and again, it's for a battery that has been sitting for AT LEAST 6 hours. The moment you remove the load at 10.5V, the voltage will climb.

Again:

The nice thing about this test is it doesn't completely drain the battery and minimizes the potential for damage as you have 10-15% of the 20h capacity remaining.

Also, from the data you linked:

1733692413391.png

Your batteries are rated for 300+ discharges to 0%.


As to running an equalization cycle, is there a preferred type of battery charger you prefer to do that? The charger that has been on the batteries for the past 3 weeks is a Noco Genius GenPro10x1. It doesn't really have specific cycles. That said a couple days ago I ordered a Noco Genius 10 that does have multiple cycles. Should I do this on each better individually or use it on the bank itself?

The chart I included above indicates a 16 hour charge at 14.7V. Do that.

IMHO, Noco is overpriced garbage. I'm not saying they're bad, I'm saying they don't deserve their reputation or price because there's no transparency. You need a programmable charger.

My Batteries are the 31M-PC2150.

What does your recommended battery tester do that you can't do with a normal voltmeter? I don't know how CCA is measured but I'm assuming it some simple math. And the resistance can be measured pretty easily. Also, how is it calculating the battery health and what does that mean in terms of current vs new capacity?

It measures internal resistance. If you know what you're doing, yes, you can do it with a voltmeter, but it takes a bit longer, and you must be able to measure accurate currents.

And finally, I'm not really sure if any of this will tell me my last question there...how do I measure the capacity of a battery bank? I hear people refer to it all the time but little on HOW to actually do it.

Did you miss this?

The only way to know for sure is to do a capacity test. The one I prefer is a reserve capacity test. Reserve capacity is the number of minutes a battery can deliver 25A and stay above 10.5V. The nice thing about this test is it doesn't completely drain the battery and minimizes the potential for damage as you have 10-15% of the 20h capacity remaining. A 300W load through an inverter is pretty close to a 25A draw.

Maybe you need the logic presented to you.

First, the capacity of a lead acid battery bank is variable. It is DEFINED as the C20 rate, i.e., how many Ah it will deliver in a 20 hour constant current discharge. If you draw more than that, you will have less capacity. If you draw less than that, you will have more capacity.

Specfically, YOUR battery can deliver 5A for 20hours. If you pull 10A, it will be for LESS than 10 hours. Pulling twice as much means you run for LESS than half as long.

From the battery datasheet:


The 20 hour rate is 100Ah (5A draw)
The 10 hour rate is 92Ah (9.2Adraw)

The reserve capacity is a 25A draw, 205 minutes @ 25A, 205min / 60 min/hour * 25A = 85.4Ah

See where I'm going here?

If you test to any of those specifications (10.5V cut off), you get a state of health.

If you test at 5A and get 19 hours, you have 19/20 = 95% health.
If you test at 9.2A and get 9 hours, you have 9/10 = 90% health.
If you test at 25A and get 195 minutes, you have 195/205 = 95% health.

In retrospect, since you have two batteries in parallel, you need to supply a 600W/50A load.
 
@sunshine_eggo thanks again for the detailed reply. There is a lot here so I need to read this a time or 10 more and to make sure I understand it it all before I reply.

That said, I'll concede my Noco Charger is not ideal. Is there a programable one you can recommend to me? The Noco one has not arrived yet so when it does I can always send it back. I would like something better but I don't know how to qualify a good or bad programable charger.
 
@sunshine_eggo thanks again for the detailed reply. There is a lot here so I need to read this a time or 10 more and to make sure I understand it it all before I reply.

That said, I'll concede my Noco Charger is not ideal. Is there a programable one you can recommend to me? The Noco one has not arrived yet so when it does I can always send it back. I would like something better but I don't know how to qualify a good or bad programable charger.

Victron IP22 are expensive and fully programmable. They can also be set to operate as a constant voltage source.


1733791235274.png

I think the ones with the (3) in parenthesis have multiple output, i.e., you can charge three separate batteries.


The 20A is probably the best match for your batteries - about 1/10th of their total capacity.

Sample settings that would work for your battery:

1733791674984.png
 
Victron IP22 are expensive and fully programmable. They can also be set to operate as a constant voltage source.
OK, holidays over, sales meetings over, family drama over, bike projects over, so now back to fun stuff - electrons.

Thanks again for this recommendation, I did return my Noco last month and am about to order an Victron IP22. I'm thinking about the 30amp 3 plug version because I plan on upgrading my battery system with my next vehicle build and figured I should get the best. I also just bought two 12V 230AH Li Time battery so need to give those a good charge as well. So, is my desire to go with 30amp good logic or should I just get the 20amp version?
 
Last edited:
Fully charge.
Apply a 300W load, which is about 25A.
Time it in minutes until it reaches 10.5V in minutes.
Compare to rated reserve capacity, or "RC"

It's up to you to figure out how to get the 300W load. 5X 60W incandescent light bulbs gets you pretty close. You could buy a cheap inverter, a power strip and a light fixture. You could also get a bunch of 12V bulbs, or you could figure out what in your system consumes 300W/25A or close to it.




Did you miss this?



Maybe you need the logic presented to you.
First off I've never been able to do multilevel quote like you have been doing, but let me say this...

Your long post above, of which I just put a few lines in my reply, is exactly what I've been wanting to read for years. I have probably read that process and logic a dozen times in various ways and various places,but it never sunk in until you spelled it out above.

Yes, I absolutely missed it the first time you described the process in this thread, and YES YES YES, I did need the logic presented to me. So thank you for putting in the time and effort to write out that educational piece for me in a clear and simple manner. It is so helpful for me and in a way opened a door to a whole new level of understanding about batteries. So thank you again.

I'm sure I'll have more questions when I actually get around to doing the test in a few weeks, but at least now I know how I should be doing the testing.

Jeff
 

diy solar

diy solar
Back
Top