What one do I trust, Midnight Sun Charge Controller or Sunny Island State of charge?

[peakbagger]

My solar trailer with two 500 AMP 48 volt forklift batteries appear to use a Midnight Solar charge controller to control the 2500Kw PV array 24/7 when the Sunny Island inverters are powered down. The trailers stacked Sunny Islands are currently powered down for the winter barring emergencies. When I check the Midnight Solar display, the readout indicates Float charge on the Midnight Solar when the sun is out, and it has done do for several weeks. The only loads on the batteries would be two de-sulphinators and the midnight solar device. The trailer can send power to my house and create AC coupled microgrid with my other solar arrays, but I still need to run additional cabling and possibly change parameters on the unit to tie the Inverters to grid power and leave the inverters running. I managed to break my ankle a month ago and it is going to be another 6 weeks before I can put any weight on it so working on the trailer is on hold. Unfortunately weather does not wait for broken ankles so yesterday morning I lost utility power. The expected duration was longer than I really wanted to wait so hobbled around and swapped over to the trailer power for the duration of the outage. When I turned the Sunny Island Inverters on they reported a 51% state of charge for the same two batteries. There is 7 KW diesel with limited fuel, so I cranked up the generator and let it run.for about an hour. When I got notice the power came back I went out and the Inverter reported 71% SOC. The batteries are roughly 500 AMPs each in parallel. I do not know what the house load was but a guess would be not more than 2 KW. In theory that meant that the Inverter was sending 5KW (104) amps from the diesel into the batteries for one hour. If I assume a straight line charge curve in this range at best the charge should have gone from 510 amps (1000 *51%) to 614 Amps or 61%. There was no solar gain (dark and foggy). So the question of the day is which one to trust to determine battery charge, the Midnight Solar being in float or the Sunny Island that had just been turned on?.

The Sunny Island documentation claims that the SOC reading is not just a battery voltage lookup table, it claims that the inverter is constantly testing the battery to determine its actual capacity and adjusting the SOC so it is reflecting the true SOC based on cthe urrent battery condition. I have seen it up to 100% earlier in the fall once or twice but have also seen he mismatch between Midnight Solars Float mode (implying full charge) and the Inverter SOC when its been powered down for awhile. Ideally I would equalize the batteries and see what happens but crawling around the trailer with crutches and one foot is not filling battery cells is not going to happen for few months. I have equalized in the past on the Midnight Solar by turning on the equalization mode and letting it run on Solar input only but did not pay attention to SOC on the inverter. The Sunny Island is supposed to equalize the batteries on some internal timer or algorithim but I have not seen it try to initiate one.

[Ampster]

If you are using Pb batteries you should be able to tell which reading is consistent with SOC by looking at voltage.. I use Lithium batteries and ocassionally the Coulomb Counters on my Inverter and my BMS get out of sych. With Lithium, voltage is only a general indication. I am able to reset the BMS and the Inverter by doing a full charge which resets everything to 100%.

[Mike90250]

Flooded batteries can be accurately measured with a hydrometer ( slow and painful for 2, 48V banks )
Otherwise, a voltage measurement must be made when the batteries have been idle ( not charging or discharging ) for several hours. This would mean disconnecting the SI inverters so their idle current does not mislead you. Sulphated batteries will charge up quickly and read good voltage, but will have lost capacity and discharge quickly when under a load


A single Midnight Classic controller, cannot deliver enough power to charge 2, 500ah batteries properly. It can do a mild charge, and possibly ( depending on the battery health ) get enough amps to gas ( bubble ) the cells to de-stratify them. If the cells get a couple days of good, full Absorb, it may be possible to Equalize the batteries, or you may have to do one bank at a time.

2,500 watts of solar, charging a 48V bank at 55V, would be lucky to get 45A of charge current on the very best sunny day. 1,000ah of batteries needs 100A to properly charge and de-stratify the cells

[peakbagger]

Thanks, Sound like when I am mobile again, I will need to fill up the bulk tank and run another long equalization charge with the diesel and keep an eye on the electrolyte levels. Then check the SGs . Once I get the other cable to the inverter from the house, I can use house current to charge the battery instead of the diesel as its set up for 50 amps at 240 volts which should be plenty of amps once it goes through the inverter. Of course, I need to figure out what the Sunny Island is set for charge current. My guess is its set up for the 7 KW diesel. Once I am done with that then I can look into the mismatch between the Midnight solar and Sunny Island. Really given my ankle I am just trying to minimize the damage to the battery over the winter until I can move around. I really need to bury the new cable to protect it from a snowblower and the ground is already frozen plus I need to put a 50 Amp circuit and twist plug on the outside of my house to plug it in. I got the parts but not the mobility.

When I have charged my Rav 4 (16 KWH) overnight, a rough correlation between the power draw and SOC seems to line up with reasonable battery charge capacity. I just upgraded the charger to a 240 volt unit, pre broken ankle, which will pull twice the power which I can use as a dump load. Then again since I cant drive for 6 weeks at best, I will not need to charge it