WZRELB Reliable Inverters Low Voltage Cutoff Mod How To

[FredyPND]

You really do not want BMS to disconnect battery as the primary low voltage detection. There are several potential issues doing this, relating to surge currents on restart. If you have a battery monitor for AH tracking you may also lose the cumulative AH info.

If you have an adjustable inverter low battery cuttoff, and you are unable to get an acceptable low battery inverter shutdown, it is usually caused by too much battery cable voltage drop on peak current demand (too small battery line wire gauge) or insuffcient battery size to support peak current demand.

With LFP batteries with a low cost BMS, the BMS MOSFET series switch resistance also comes into play during peak surge current voltage slump. Their switch resistance relates to their max current rating. Many of the low cost BMS current ratings are based on their peak current capability before they self destruct and should not be used with a continuous current greater than about half their specified current rating. They have an overtemp shutdown but that does nothing for surge current voltage drop across the BMS.

If inverter has a low battery voltage trip delay timer adjustment setting, it can be used to ignore momentary peak current DC input voltage slump.

If you allow batteries to get significantly out of balance it can impact on how low battery detect reacts. You will get significantly less capacity for battery array so original battery monitor AH capacity setting will likely be far off. When cells get to less then 20% state of charge their voltage slump under heavy load current will be greater, particularly if current peak last longer then 20-30 seconds.

 

[FredyPND]

Thanks for the reply. Actually that Is the problem i have - the wzrelb inverter does not have a lowvoltage cutoff setting nor delay. That Is why i would like to try And make this mod. Also i believe that the cutoff kicks in too soon. IT should cutoff at 10 volts but IT does more around 11.7 or so. That Is why i wanted to make this mod to just lower the cutoff -2.5 Volts as the Author of this thread said.

I also have a solar regulator which should be looking after the battery voltage for me And cutoff in case IT gets too low. For the cables i have discussed those with my electrician and he said they are almost overkill, but could be a bad connection somewhere. Will look into that.

My bms Is 60a And tbh i dont know how much current the fridge takes but i believe IT Is not more than 20.

 

[RCinFLA]

My bms Is 60a And tbh i dont know how much current the fridge takes but i believe IT Is not more than 20.

It's surge current that is the culpret. The wire gauge should support the surge. I would not expect refrig would be more than 10 amps startup surge at 120vac so that is about 115 amps at 12vdc input. A 60 amp BMS is not going to help with voltage slump.

 

[DanTM]

I know this is an older post but wanted to share some info that helped me complete this modification. The bottomline is I was not able to make the cutoff point change using a 10K trimmer pot. When I researched what value a 5102 (shown in picture) surface mount resistor was I got 51K from https://www.hobby-hour.com/electronics/smdcalc.php. So I think that OP meant using a 100K trimmer pot. Once I did this i could adjust the cutoff as intended.