Battery questions on understanding specs and performance...

[Jennifer Rose Towing]

I previously made a post about multiple batteries but now looking for help deciphering the battery specs.
I purchased a 35ah MightyMax gel battery. I think I am more confused after reading the spec sheet and would love some clarification.

The main questions are...
1, at what voltage is the battery full?
2, at what voltage is the battery 50% depleted and what voltage is flat dead?
3, running a 3 amp load, how much time should I expect to use it before it gets to a 50% discharge.

Here is where some of my confusion comes in. In looking at the spec sheet, it gives 4 capacity hour ratings for different amp draws. What I do not understand is the voltages listed vary wildly and weather that is a safe rating or presumably completely dead (why/how 11.6v and 9.1v can be the same amount of dead) I am perfectly fine with, and aiming for up to a 50% discharge during use then recharge.

Another point of confusion is The discharge current and power charts. Each give 5 endpoint voltages per cell, I do not understand the point of listing 5 while stating what % of discharge that is. The other thing is massive discrepancies in the watt and amp charts. Using the 1.8 volt at the 1 hour mark, the amp chart says 16.7 amps but the watt chart says 39.7watts. At 12 volts, 16.7 amps would be 200 watts. 3.3 amps would get the 39.7 watts. I must be completely missing something or totally wrong in my comprehension.

The biggest question I have is what voltage is this battery half dead/50%. The info I am finding online indicates that the levels are different for flooded, AGM/gel batteries. I would prefer to understand how the conclusion is those numbers rather then just being handed a number. Been doing a lot of research and getting a lot of conflicting answers. In my mind, under perfect conditions, I should be able to run a 3 amp load on a 35ah battery for about 5 hours and 50 minutes to drain the battery 50%. I know that's not real world but that's the math. I do not know what that would translate to in voltage be able to know where 50% is.

I did send Mighty Max an email asking what a safe depletion voltage level is for this battery but all they replied with was the attached, blurry spec sheet.

I have a decent understanding of starter batteries, and charging scenarios but not deep cycle or gel. Feel free to add anything, post articles/documents ir point me in the rite direction if you have the time.

Thank you.

 

[RS485]

w/r/t amps vs watts charts, check out:

Constant Power Battery Discharge | Big Mess o' Wires

www.bigmessowires.com

The per-cell watts number (39.7) needs to be multiplied by 6 (cells in a 12V battery) to be comparable.

I also found:

BatteryStuff Tools | Determine Run Time for Specific Load Calculator

BatteryStuff Knowledge Base Calculator to help you find the duration of time your battery will last under a specified load amount. To find accurate results, you will need to know the Capacity Ratings, typically Amp Hours (AH) for your battery.

www.batterystuff.com

which calculates and applies "Peukert's Law" in the form of an online calculator which after plugging in numbers from your battery's specs, and your 3amp load, yields a possible answer to your question #3:


Peukert's law explained here:

Peukert's law - Wikipedia

en.wikipedia.org

 

[Jennifer Rose Towing]

Thank you @RS485 for looking it all over. I did mistake the amp/volt calculations and overlook that it was per cell. Strange that they list it that way vs the whole battery. (First thing I thought of is measuring miles per individual cylinder usage vs miles per gallon).

I will definitely dig into those links later when I have more time. They might explain it but how would I go about knowing where 50% is based off voltage? The only way I can think of at the moment is to run a specific load for the pre calculated time and use the voltage at that point as a bench mark.

I will review more later but definitely appreciate your time replying above. Thank you.

 

[RS485]

This is a pure guess based on the blurry chart (and nudging the numbers on the graph hoping to line them up better -- I hope I got them right):

and I'm not sure this even makes intuitive sense...but it if we use the curve closest to the 3amp load goal, we'd expect about 12V (2.00 V per cell) at the 5 hour mark (1/2 way or 50% of discharge).

If you have a chance to run your experiment, I'd be real curious what voltages you observe at the 5 hour mark, under load, then immediately after removing the load, then again at say...the 1 hour mark (allowing the battery to rest for an hour).

Also, this might be of interest:

Under-Load Battery Voltage vs. SoC - Marine How To

After 20 Hour Capacity Test = 95.69 Ah Capacity For this test I wanted to: Show the relationship of loaded voltage to SoC on a popular AGM lead acid battery. Utilize a battery not in perfect health to represent real

marinehowto.com

This was an experiment drawing 5.25A off a 105Ah battery, measuring voltage underload along the way.

 

[Jennifer Rose Towing]

Thank you. Still doing the install but will do a bench test to run those numbers. Hoping to dig into those links tonight or tomorrow.

 

[Jennifer Rose Towing]

I just set up to do the test but my cheap meter is not up to it. When trying to get an initial reading I am getting more then two tenths of a difference each time I am trying to get a reading. Anywhere from 12.98 to 13.26. I don't use meters a lot and it was a freebee so I can't complain I guess. My battery charger has a meter but only to the 10ths so it could be 13.00 to 13.99. I am gonna skip the test for now until I can find a more accurate measurement.

 

[RS485]

Thanks for trying...no worries.

 

[Jennifer Rose Towing]

@RS485
I was able to run the test tonight.
I ran the test for 5 hours running a 35 watt GE 4411 bulb. I tested taking readings with 2 different generic multi meters. The battery has only been in the house at roughly 70° and charged last week. Cheap charger so not sure how accurate it is. Here are the results for the Mightymax 35ah gel battery listed above...

13.01/13.02 before connecting
12.52/12.54 connecting the bulb.
12.55/12.56 1 hour in
12.48/12.48 2 hours in
12.39/12.39 3 hours in
12.30/12.30 4 hours in
12.19/12.20 5 hours in, end of test.
12.28 <5 seconds of disconnecting the bulb
12.36 1 minute after disconnect
12.44/12.44 1 hour after disconnecting the bulb.

 

[RS485]

Thanks for testing!
I predicted ~12.00V @ 5 hours but I was off a bit. Though really zooming into the blurry charts shows the crossing point to be a little higher than 12.00V.


For the sake of battery life, the depth of discharge should be as shallow as possible:

Not sure I helped much but I really appreciate you're having asked questions. I never realized how complex lead-acid batteries were! So many tradeoffs and qualifiers.

 

[Jennifer Rose Towing]

You definitely helped, thank you! I am not looking for exact time/numbers, just trying to get a guideline to operate at.

Do you know what 50% discharge voltage for this battery should be? And is that voltage the running voltage in my test or the resting voltage an hour later?

Some of my use can be based off time as a gauge but I may occasionally use it for other reasons and those draws would vary and differ.

The deep cycle and gel battery world is new to me and I just want to have as much info as I can. unfortunately the info I am finding online varies greatly with even less about the smaller AGM gel.

Definitely appreciate your time and replies RS485! If you have or get any more info, feel free to share whenever.

 

[RS485]

No single voltage number can be used to indicate charge state because different discharge rates (loads) produce different under-load voltage curves over time...ie:

Given a fixed load, we can at least use this chart to "ballpark" an under-load voltage measurement.

Still, it seems to me (from what I've read) that a more reliable voltage measurement (covering both fixed-load use as well as otherwise) to indicate charge state is the open-circuit voltage. However, I have two problems with that:
1. It's not yet clear to me how long the battery should be rested (achieves internal equilibrium). I've seen numbers between 30minutes to 24 hours!
2. There seem to be so many choices! I googled "lead acid soc from voltage", clicked on (images) and there seemed to be many slightly different numbers indicating "50%" from 12.04V to 12.23V...although I may have glossed over specific details (i.e. flooded, gel, etc).

I can understand why the under-load voltage could vary so much. I don't understand why its so hard to get a simple open-circuit number (you mentioned you reached out the your battery's maker and they replied with the blurry sheet -- which doesn't even seem to address open-circuit measurements).

I gather this was much simpler in the "old" days when you could pop the cap off a cell and measure the gravity of the electrolyte. _That_ seamed like a direct measurement of the "fuel" of a lead-acid battery.

 

[Jennifer Rose Towing]

That all seems rational. I was questioning off a 3 amp load. With your info and the varying discharge voltages for amp draw along that curve, I presume that is under load vs having rested. It seems like the resting voltage (whatever length of time that is) should be the measurement used as depth of discharge vs the load voltage.

I did email them again asking for a non blurry copy but the 2 attached pages are what they emailed me back the 1st time. Yes there is a lot of varying info out there. If I get better charts or more info from them I will post it here.

 

[JohnMarcson]

A little off topic, but still fun....
Lead acid batteries are pretty cool. They have been a reliable tool for a long time. As tribute to the lead acid battery, this is a fun video. Less informational as it is an explosive over the top demonstration in honor of the technology.

 

[Jennifer Rose Towing]

 

[JohnMarcson]

View attachment 247380

At least they admitted it..... but kinda disappointing