I'd appreciate if someone can advise and give me an idea about the condition of my battery and my overall set-up.

I set-up off grid solar system about 2 years ago, for my parents in my hometown

Below are the following specs

  • Battery 12V- 180Ah (Sacred Sun Brand)
  • Solar Panel - 200W ( but was only producing 60W max ever since)
  • Pure Sine Wave Inverter - 800W (Seldom use)
  • PWM Charge Controller - 30Amps

I haven't checked this set-up for about 2 years. and just checked it now.

Right now, everything seems to be working perfectly fine since I left except the battery.

My battery seems to be staying at 10.5V when on standby mode. Weird thing is, when I hook my inverter, it is still producing power at around 50W-65W or maybe up to 80W (laptop connected) for around 4-6 hours, and the inverter won't beep for low input voltage. The inverter would previously sets of the alarm when the battery voltage reaches down to 11.5, but now, the inverter will only sets off the alarm if the battery voltage is below 10V (somewhere around 9.9V or 9.8V).

I find it really weird for both my battery and inverter to work like this.

I am also pretty sure the the voltage readings are correct, I use 3 different voltage tester to test the battery voltage.

This is what I notice on my battery.

  • If there's about 60W from solar panel, battery charge voltage starts at around 10.8V and would climb up to 11.5V or 11.6V

  • I never notice the battery charges from 11.9V - 12.9V

  • After few hours with only 30W-60W average charge, the battery voltage reaches around 13.5V up to 15V, and my PWM controller would indicate that the battery is fully charge.

  • After sun sets. battery voltage would drop back to around 10.5V

  • During load ( 40W ) battery voltage would drop to around 0.1V or 0.2V Voltage, until it reaches, 9.9V and inverter will sets of the alarm.

Can anybody advise the state of my battery, I'm also curious why my Inverter isn't beeping even when the input voltage is low.

Appreciate any help.


  • 1
    \$\begingroup\$ Sounds like a worn out battery. Can you charge it fully and log the voltage over a few hours or days with no load? If you PWM controller allows it to go to 15 V, it probably boiled off too much water. Never float to more than 13.5 V long term! \$\endgroup\$
    – winny
    Apr 29, 2018 at 18:02
  • \$\begingroup\$ @Winny Since Sacred Sun make AGM sealed batteries, boiling off should not be an issue. It sound like the charger/Inverter is faulty and not charging the battery to the correct terminal voltage. The indicator for this is that the low voltage alarms seem to be much lower than when new. \$\endgroup\$ Apr 29, 2018 at 18:29
  • \$\begingroup\$ AGM will recombine about 99 % of all oxygen+hydrogen back to water before it can escape. The downside is that you can’t refill them nor check the water level, so if they will see 15 V for let’s say a few days, they will be irreversibly damaged due to water loss. Well, if the undervoltage limit is too low, you are sulphating your batteries. You have a complex system with several variables. Please measure and log each one. Voltage is of primary concern. \$\endgroup\$
    – winny
    Apr 29, 2018 at 18:43
  • 3
    \$\begingroup\$ Losing 1.5V exactly sounds like a single cell has failed. \$\endgroup\$
    – pjc50
    Apr 29, 2018 at 20:08
  • \$\begingroup\$ Something is not right. Pull the battery out. Charge it with an off-the shelf charger. Test it with some sort of load to measure its capacity. \$\endgroup\$
    – user57037
    Apr 29, 2018 at 22:48

1 Answer 1


Assumptions: By matching 12V and 180Ah I find this SPG12-660W 12V180Ah

Your estimates of power draw on the laptop charger may be too high as that would only be used when the screen is on maximum brightness and the battery is charging at maximum current.

Your measurements are insufficient to measure State of Charge (SoC) or capacity. (Ah), remaining life (mo.) or % of new Capacity.

Look below to match up when you get better power measurements.


  • you don't have OVP (over voltage protection)if charger goes to 15V vs 14.4Vmax @ 25'C. The spec is given in per cell so you multiply by 6 cells.

    • Cycle use 2.40V(-3.5mV/℃/cell)
    • Float use 2.27V(-3.5mV/℃/cell)
  • charge battery voltage should be well above 12V at sunset so your load may be greater than I expect or you have a weak cell from overcharge fatigue. New, the fully charged battery should be able to produce 34W/cell x6=204W for 10 h.

  • you need more accurate data in a spreadsheet using a current shunt and any cheap DMM and more accurate data for P vs time.

  • if you charge 50W for 3 h or 150Wh with 100% (not) efficiency that would be 7% of new capacity of 180Wh*12V=2160Wh, so there may be 1 bad cell.

enter image description here

Capacity declines normall from a 10h dischrage rate. But for this battery the table says 34.4W/cell for 10h=2064Wh

  • \$\begingroup\$ The 15V reading when the charge controller indicate full charge might just be an inaccurate reading from multi-meter, I monitor the Battery temperature and it never exceeds 30C, battery temp usually stay at 27C-30C \$\endgroup\$ Apr 30, 2018 at 2:55
  • \$\begingroup\$ I'm going to set-up DMM with Shunt, but Im waiting for my orders to arrive, I really believe one of the battery cell might be weak or dead, but I wonder if one cell is dead or weak, can a battery still holds some AMPS? \$\endgroup\$ Apr 30, 2018 at 2:57
  • \$\begingroup\$ and I think I'm pulling around 100W from the battery during for couple of hours and around 20W overnight, I had 4 (5W) led bulbs running overnight, the laptop charger indicates 19V*2.37 amps, but since the process is 12VDC -> 220AC -> 19VDC and I'm using an inverter with 85% efficiency, I think Im losing 30%-40% of power while laptop is charging and in use. \$\endgroup\$ Apr 30, 2018 at 3:01
  • \$\begingroup\$ Im just quite surprise that a battery that has only 10.5V and would even reach down to 10.1V on load can still supply enough power for hours \$\endgroup\$ Apr 30, 2018 at 3:03
  • \$\begingroup\$ Heat. Is not great indicator of overvoltage for these cells if charged slowly. I should expect 50% life and capacity but not surprise if it is only 15% of 2kWh \$\endgroup\$ Apr 30, 2018 at 5:12

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