Voltage overshoot with makeskyblue v119 SCC.

therealjw

New Member
I'm trying to determine why I am seeing a voltage overshoot of 19 to 20 volts in my 12v setup. It is a simple off-grid system with 1000 watts of solar, a Makeskyblue v119 60-amp charge controller and a 4S-100ah Lifepo4 battery with a 100a Daly smart BMS (common port). The only load device is a 12v Reliable brand 1500 watt inverter. I have the BMS set to a maximum cell voltage of 3.4 volts so the overall voltage tops out at 13.6 volts and the SCC is set to a maximum of 13.8v (the BMS ends the charging at 13.6, there is no float). With small loads, everything is fine. Testing the inverter with higher loads using a heat gun on a setting around 700 watts, I see a draw of 50 amps through the BMS while no solar charging and I can switch the heat gun on and off without issue and the voltage obviously drops a little under load and returns to approximately 13 volts when the load is stopped. However, when I am charging with about 20 amps of solar and running the heat gun, I see a draw from the battery of around 30 amps (20 from solar, 30 from battery) and when the heat gun is turned off, the voltage to the input of the inverter shoots up to around 20 volts temporarily and the inverter either alarms or trips off completely until reset. I've confirmed this voltage overshoot is definitely coming from the Makeskyblue SCC. The inverter negative and charge controller negative are both connected to the BMS P negative and the inverter positive and charge controller positive connect directly to the battery positive terminal. Has anyone else seen a voltage overshoot with Makeskyblue solar charge controllers when load suddenly reduces?
 

snoobler

Solar Honey Badger
Moderator
Welcome to the forum.

First, you shouldn't be using the BMS to terminate the charge. You should use the SCC to terminate the charge. Set to 13.6 and set the BMS to 13.8 if you really want to operate like that.

Second, 1000W/12V = 83A. A 60A charge controller may leave a lot of power on the table.

Sounds like a defective SCC.
 

therealjw

New Member
Thank you for the reply. The voltage is more stable when the SCC is set lower than the BMS, though I get a significantly lower charge current, around 10 amps, when the battery is around 13.5 volts and this can be a relatively low SOC for lifepo4 so would you suggest charging at 14v and setting the BMS max 3.55 per cell (14.2v) to try and maintain around 85% to 90% SOC and charge faster when there is sufficient solar? I will be adding additional batteries and charge at some point.
 

snoobler

Solar Honey Badger
Moderator
More questions:

  1. Is your battery rated for 60+ amps? Most cells are rated at 0.5C or 50A in your case.
  2. Is your BMS rated for 60+ amps? Most DALY have asymmetrical charge/discharge limits. 100A discharge unit has 50A charge limit.

I would set the BMS to 3.70 and manage the SoC via the SCC peak voltage.

In a solar power system, you want to float LFP. If you don't, you end up mini-cycling the battery and repeating bulk/absorption charges rather than just holding the battery at a higher SoC. LFP doesn't need to be floated as a chemistry, but as a solar power system source, it should be.

Picking a target voltage isn't a reliable way to establish an SoC range without taking the entire system into account.

I have personally seen 95%+ SoC at 13.6V and 99% at 13.8V, but as you say, the charge current is lower.

Charge rate influences voltage/SoC substantially.

If you want to hit a peak SoC, you should get a decent battery monitor and establish your own votlage/current/SoC/tail current relationship.

We're deep in the weeds here. Still sounds like your SCC shooting to 20V is an indication of a defective SCC.
 
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