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Axpert Inverter Discharging Batteries


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Hi All

I have a problem with the Axpert/Mercer MKS Plus 3kV. The inverter seems to discharge the batteries(200Ah Lead Crystal x 2) at about 05h30 in the morning. I noticed that this is the time the voltage from the panels(260W x 4, 2 strings of 2) are around the 30v level. There is a relay that clicks on and off as the voltage goes above and below the 30v level. This seems to cause the battery capacity to suddenly drop from 70% to 40 %. Once the panels input voltage to the inverter remains above 30v, the system continues charging the batterries to 100% in about 5hrs.

Any help to resolve this issue would be appreciated.

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Welcome Doc.

We can help. Can see two problems already, but first a few very important question: What is the load you are running at nigh, and for how long?


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Hi Doc, welcome to the forum. Can you give more details on your settings? Would be helpful to figure out what's happening. There are a lot of Axpert experts on here so I'm sure they will be able to assist you.


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For your consideration.

To power 300w for 12 hours, you need 12 x 200ah batteries on 24v system, if you run them to 20% DOD (Depth of Discharge) with 1 days backup.

To 50% DOD you will need 4 batteries.

And down to 80% DOD, the 2 x 200ah batteries you have.

You will need a minimum of 3 panels to just recharge the batteries, no load, in 5.5 good sun hours.

Do you have a BMV battery monitor?

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Yes, I worked on a average to give you an idea. The batteries are being strained. The inverter needs some settings adjusted.

if you can, please get a BMV Monitor.

Here: https://www.powerforum-store.co.za/collections/accessories/products/copy-of-bmv-702-battery-monitor

Or ask Camel on the forum also.

Why? With a BMV you can better monitor the batteries thereby prolonging their life expectancy by using them optimally.

I am not familiar with lead Crystal batteries, but the investment you have in them is substantial and a BMV will help you manage it better.

Eskom power is cheaper than batteries in the cities.


Right, the other guys will help you with the settings on the inverter for I suspect they are also wrong.

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Hi Doc


Do you have a datasheet for your batteries? The only settings I see where you can make changes are charging . You should not be charging lead acid batteries at greater than 10% of you Ah capacity i.e. in your case 20A. Lead crystal can handle deep discharges and faster charging but without a datasheet one plays safe.

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Thanks Chris

I have been in contact with Mecer via email. Still waiting for a decent answer from them. So far they've passed the blame onto the batteries (without justification). Will have to continue prodding them to get a technician out to look at the problem.

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Lets start here

Doc, get each battery tested. 

Option 1: Take them to a car battery shop.
Option 2: Borrow a tester for one evening from a battery shop.

Testing the volts is useless, been there, done that. Test must be done with a huge load like in your 200ah battery, put a 600amp load on it for few seconds.

If a battery has a problem it will not be able to handle the current for a few seconds and red line on the tester, replace it under warranty or buy a new one.

Note 1: The batteries must be fully charged when tested.
Note 2: IF you move the batteries yourdself: Do.Not.Let.The.Sides.Touch.Your.Clothes.


@superdiy Jip, wife was smoking mad the 3rd time I did that quite innocently. :D

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7 hours ago, Doc said:

Disconnected the solar input this morning. Batterries did not discharge at 05h30. Reconnected again at 06h30 when panels voltage was about 60v, then the batterries discharged again???

Battery Capacity 20161026.jpg

@Chris Hobson, you know these Inverters better than me, but I saw similar behavior on my inverter. Just remember, the Axperts Capacity indication is related to the Back to AC voltage setting and 0 Capacity means it has reached that point. As @The Terrible Triplett, pointed out, 2-300watt load will result in a pretty deep discharge (Low voltage) 

Looking at the time on the Graph, the PV production could not have been enough to satisfy the load and probably the load was still taking some power from the batterys even lowering the Voltage some more, indicating a further discharge even though some PV is available. 

@Doc , Can you maybe re-post this graph taken over a longer period, lets say till 12H00. If my theory is right, the Capacity should start to increase once the PV production exceeds the Load. 

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My first though on the 1st post was the inverter switches off due to min volts level of bats, forcing inverter to switch off.

Solar starts showing some volts, inverter sees that, starts up again.

Load starts, volts drop immediately, inverter switches off.

Cycles starts till there is enough solar.


My plan of Action here:
1) Testing the batts rules out any damage to them and satisfies Mecer's point of view.
2) Then the BMV is purchased, installed and software to control the inverter is installed and configured.
3) Then we have data, batteries are protected, and the load can be tuned to the batts.

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