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Kodak inverter not charging to bulk/absorb voltage


openmind
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Hi Everyone,

Great forum, thanks for all the brilliant content!

I think I have an issue with my KODAK Inverter VMIII 3kW 24V (OG 3.24) - it does not seem to charge to the bulk/absorb voltage (28.8V) - it stops at the float voltage (27.6V).

I don't have solar panels, just the inverter with input from Eskom (when it's there) and using the batteries during load shedding for lights, fridge and a few other occasional items. I have four CSB HRL12390W 12V batteries connected in two parallel strings of 24V. Pics attached. The batteries are brand new.

My understanding is that when Eskom power returns after load shedding the inverter should charge the batteries at the bulk/absorb voltage and then once the current demand from the batteries drops off, (i.e. when they are full) the inverter should drop the voltage down to the float voltage. I have now monitored my system during three load shedding cycles and the same happens each time it charges - it only goes as high as float. I assume that the battery voltage reading on the display is the actual voltage being sent to the batteries (it seems that way - every time I check the batteries with a multimeter that reading is correct). So when the power comes on and the inverter begins the charge cycle, the battery voltage climbs slowly to the float voltage setting (at the max Amp setting, 60A during charging) and stays there, even once charging is "done" and the charge current is 0A. 

I'm worried that I'm undercharging my batteries. I suppose I could manually set the float to the bulk level and set it back down to the actual float once the batteries are changed, but I doubt that should be necessary. 

Anyone know what's going on? 

Thanks

 

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8 hours ago, openmind said:

I think I have an issue with my KODAK Inverter VMIII 3kW 24V (OG 3.24)

I don't have a recent VM III firmware, so my comments below refer to an older firmware (version 20.59).

Quote

- it does not seem to charge to the bulk/absorb voltage (28.8V) - it stops at the float voltage (27.6V).

That sounds like the premature float bug, but I don't think it is in this case. Usually the premature float bug doesn't take effect with utility settings, unless you choose unusual settings. If you are using 60 A for setting 11 (maximum utility charging current), then that doesn't seem to be the case. (VM III firmware seems to differ from other firmware; other firmware would consider only the maximum charge current, setting 02).

Quote

I have now monitored my system during three load shedding cycles and the same happens each time it charges - it only goes as high as float.

the battery voltage climbs slowly to the float voltage setting (at the max Amp setting, 60A during charging) and stays there, even once charging is "done" and the charge current is 0A. 

It's expected that the battery will stay at the voltage in the float setting (setting 27) after charging is complete.

Quote

I'm worried that I'm undercharging my batteries.

That is a quite legitimate concern.

I think what may be happening is that you've once charged your battery enough, and now it's staying in float stage. Does the charge (CHG) LED flash after a load shed? If not, and it stays on solid, then it's in float stage.

It looks like there are two ways to exit float stage to begin a new bulk charge (again, per old firmware).

The first is if the battery voltage falls below 24.0 V for about 1 second, no exceptions (50 samples). I hope that rarely if ever happens, although it could happen under load.

The second is if the battery voltage falls 2.0 V (on a 24 V system) below the float setting voltage. For you that would be 27.6 - 2.0 = 25.6 V. I forget how long it has to stay there.

Once either of these happens (with your settings, only the second case will happen), the CHG LED should go out for some 15 seconds (no charging at all), then it should start flashing (bulk or absorb charge stage in progress). It could take another half minute or so for the charge current to ramp up to the value in setting 11. Charging should continue until the charge current falls to strictly below the maximum utility charge current divided by 5. So for you, when the charge current reaches 11 (strictly less than 60/5 = 12). It won't care what the battery voltage is at that point, because of the premature float bug (which for you should not cause the float stage to start prematurely), as long as it's no lower than the float setting voltage (27.6 V).

If you find the CHG LED is flashing yet the charging stops at 27.6 V, can you check the net charge current? I would expect it to be well over 11 A when the battery is only at 27.6 V. If 11 A or less, then that's the problem.

Edited by Coulomb
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Hi Coulomb, thanks so much for that detailed reply. I've learned a thing or two.

Specifically, I now understand that the inverter will not go to bulk/absorb charge every time the batteries are used a bit, but only when there has been significant discharge. 

I tested this today by switching off the supply to the inverter and letting it run for about five hours until the battery voltage was about 25.2V under a small load (±250W). Towards the end, I put a big load on it (hairdryer!) to nudge it under 24V under load for a few secs just to be sure that bulk charge would be triggered. Sure enough, when I put the mains supply back on, the inverter quickly ramped up to 60A charging current and the voltage slowly crept up past float and up to the bulk voltage setting. Relief. An hour later the fans were still running like a jet engine and the current was is still at 60A so I assume a good charge was in progress. A short while later I noticed the current start to drop and, as you predicted, when it hit 11A the charge process stopped (CHG LED stopped flashing) and the voltage slowly dropped down from bulk to float. 

Thanks again, it's good to know what's actually going on! I feel properly ready for load shedding now :) Bring it on, Eskom.

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