ckamps avatar image
ckamps asked

Poor battery performance? New MPPT 100/20 in cold weather

Update: Thanks to @snoobler, I now understand that you cannot trust the measured voltage of a battery system when it’s under load or being charged. Battery specs pertain to measuring voltage at rest. Once I understood this electricity 101 aspect, per the advice of @snoobler, I began to focus on the state of charge (SOC) data to help me home in on the measured voltage that equates to 50% SOC for my system. Once I home in on that value, I’ll be able to refine my charge controller configuration to avoid defer turning off the load terminals until that voltage is reached.

I installed a new outdoor pole mounted off-grid solar system of the configuration shown below in late November 2021. I am trying to determine if the battery performance I’m experiencing should be expected during colder winter months.

The 200Ah dual 12V AGM deep cycle parallel battery system appears to power the ~1.4A/18W 12VDC total load for only a few hours before the MPPT automatically shuts off the load via a 12V relay. The load is reliably switched off and on based on the Load output settings shown below.

Even in our variable winter temperatures, it seems that the battery system should be able to power the load for more than just a few hours.

My goal is to enable this system to run for at least 2-3 days without solar charging.

Several Notes and Questions

  • Over discharge damage? Prior to configuring the Load output settings as shown below, I had been using the BatteryLife setting. However, with that setting, the MPPT did not appear to shut off the load during low battery voltage situations. Before I made the switch to the Load output settings shown below, I had seen the battery voltage drop to ~11.6V. I hope I didn’t damage the batteries by discharging them to that extent.
  • Depth of discharge: Is 50% the right target for AGM deep cycle batteries? I’ve read conflicting advice. My current load switch low voltage is set to 12.24V or 50% DoD. If 80% DoD is reasonable, then I would adjust this setting to ~11.9V. However, I don’t see how that would dramatically alter the battery performance of the system.
  • Potential battery damage due to charging in cold temperatures? We’ve had several sunny days during which the the outside temperature was in the mid-teens F. A few nights dropping into the low teens. It’s not clear to me if the AGM batteries could have frozen overnight and gotten damaged if attempting to charge them during the day. I do not yet have a battery temperature sensor attached via the SmartShunt, but am considering adding one so that I can configure the MPPT to avoid charging at low temperatures. I am not sure if the charging that has occurred to date during low temperature days. We’ve seen several days in the ~20F or so since I installed the system with lower overnight temperatures.
  • Temperature compensation: It’s not clear to me how I should convert the -4 mV/Cell/C as specified in the battery specs in the diagram below to the F unit that is apparently needed in the MPPT settings.
  • Translating battery spec absorption range: I used 14.4 as the rough midpoint of the battery spec’s 14.1-14.7V absorption number. Does that sound reasonable?
  • Purpose of bulk value in battery spec? How, if at all, should I factor the batter spec’s Bulk number into the MPPT configuration?
  • Load switch high value: I currently set this at 12.5V or roughly 75% SOC. Is this a reasonable setting?
  • Switch to lithium? 100ah? Perhaps 24V? I’d hate to have to switch out these new AGM batteries, but if that’s what’s necessary, then so be it. If that comes to pass, would I be best served by going the lithium route? Since lithium can deal with deeper discharges, I’m wondering if 100ah 12V would suffice. Since most of the system components can support 24VDC (I’d swap out the relay and the 12VDC-USB C 5VDC down converter for 24VDC compatible units), I’m also wondering if I should make the switch to 24VDC battery system.

Recent Day

Mid-30sF day. Data captured via MQTT from the VenusOS-based Raspberry Pi.



The fact that the system is entering bulk charging mode multiple times a day appears to point out a problem with the battery systems. I gather that with a properly sized and well operating battery system, bulk mode shouldn’t be needed more than once per day.


MPPT Settings


Battery Specs

System Overview


MPPT ControllersAGM Battery
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1 Answer
snoobler avatar image
snoobler answered ·

First and foremost:

It looks to me that regardless of setting, it's cutting off at 12.5V according to the charts. Insane to have your cut-off at 12.5V. The chart of voltages vs. SoC is for BATTERIES AT REST - no charging or discharging for at least 2 hours. You are likely cutting off at ~90% SoC.

discharge damage:

If you discharge them deeply, with regularity and fail to recharge them fully after each deep discharge, yes, damage can occur. A single deep discharge like that shouldn't hurt anything at all. Yes, it shortened the battery's life, but you'd be hard pressed to measure it.

Depth of discharge:

50% is generally the correct number for any type of lead-acid. Again, per the above, the voltage charts are for batteries AT REST. What you really need is a smartshunt or BMV to actually monitor SoC.

Potential battery damage due to charging in cold temperatures?

AGM charge just fine in low temps. In fact, they need to be charged at HIGHER voltages. The real damage would have occurred from freezing, but that didn't occur unless your batteries were at a very low SoC.

NOT charging AGM in cold temperatures is more risky than charging them.

Temperature Compensation?

-4mV/°C/cell means you should input -24mV°C because you have 6 cells in a 12V battery. If you aren't using temperature compensation in these cold temperatures, you are undercharging the battery. Example:

At freezing, that's a 25°C drop from the 14.7V specified at 25°C. -25°C * -.024mv/°C = 0.6V, thus you should be charging at absorption 14.7+0.6 = 15.3V.

Translating battery spec absorption range

I would choose 14.7V in this case since you're trying to maximize capacity. Once you get temp compensation sorted, 14.4V may be fine.

Purpose of bulk value in battery spec?

Bulk and absorption are often used interchangeably. Recommend 14.7V absorption.

Set equalization the same as absorption. Shouldn't equalize AGM batteries.

Load switch high value:

No. Once the load is removed, voltage will rise. Set to 13V. You don't want it coming back on until charging has occurred.

Switch to lithium? 100ah? Perhaps 24V?

Lithium can't be charged below freezing. 24V will buy you nothing vs. 2X 12V in parallel - same amount of energy stored.

Bulk multiple times per day.

This tends to indicate inconsistent solar or loads that exceed the solar. Under expert settings, there is a "re-bulk voltage offset" value - defaults to 0.2V. You might consider increasing that number to 0.4 or 0.6V.

Based on the chart, it looks like your solar output became insufficient to support the loads, and the battery voltage dropped to the re-bulk level.

Recommend a fixed 2-4 hour absorption time vs. adaptive in expert settings.

Recommend you lower load switch low value to 12.0V for the time being. Disable the charger in the morning and see what happens a few hours after all loads and charge have been removed from the battery. Check the 2-3 hour resting voltage against the chart.

If I read it right, you have 400Ah of 12V AGM. If that's the case, you need 40-60A of charging. Since you only have a 100/20 charger, you are short. Lead-acid work best when charged at 10-20% of their capacity. Charging at lower or higher current shortens their life.

Get a temp sensor for the smartshunt immediately and share the temp data with the MPPT.

Why aren't you using the smartshunt's SoC to determine where your 50% cutoff is?

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ckamps avatar image ckamps commented ·
Thanks for the detailed response. The system is 200Ah of 12V AGM (2 x 100Ah batteries in parallel). I’ll try out your recommendations over the next few days.
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snoobler avatar image snoobler ckamps commented ·

Okay, then disregard the charging current comment.

I see that now in the system diagram. At that point, I was dealing with information fatigue. :)

Concerning your goal of 2-3 days of power with no solar:

18W * 24h = 432Wh/day

12V * 100Ah (usable) = 1200Wh

1200Wh/432Wh/day = 2.78 days

Note that AGM and all lead acid lose capacity in cold temperatures. At freezing an AGM has about 70% of rated capacity.

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ckamps avatar image ckamps commented ·

RE: Recommend you lower load switch low value to 12.0V for the time being. Disable the charger in the morning and see what happens a few hours after all loads and charge have been removed from the battery. Check the 2-3 hour resting voltage against the chart.

I haven’t yet done this exactly as you stated, but in the meantime, I noticed from data this AM that the battery was at 12.8V or nearly 100% SOC first thing this AM before any load was applied and after the battery sat overnight with no loads and no solar charging. Which appears to reinforce several of your earlier points.


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snoobler avatar image snoobler ckamps commented ·
You can't trust a loaded or charging voltage to represent SoC.

Really recommend you use the shunt SoC to correlate to your loaded voltage, i.e., discharge until 50% on the shunt and then use the loaded voltage at that level as your cut-off.

Always worth double-checking all your connections - every single one - to confirm that they are secure and properly torqued. Bad connections can contribute to voltage drop.

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