Hey everyone! I've been working on my first DIY off-grid solar system and I've run into some issues I need help with. I'm using an MPPT charge controller set to 48V for my LifePo4 100aH battery. Everything seems to be working fine until the battery hits 100% charge (which I believe is around 51-53V). Once it reaches this level, the charge controller throws an error message saying the battery settings are incorrect, and then it seems to allow the full panel voltage (which can be over 77V) to go directly into the batteries. This is really worrying since it could burn out my inverter if I don't shut it down in time. I always thought that when a battery is fully charged, the charge controller should stop supplying power and maintain a floating voltage. Am I expecting too much, or do I have a faulty charge controller? How does a typical charge controller behave in this situation?
3 Answers
You’re right to question that behavior; a charge controller should cut off charging when the battery is full and keep it in float mode. If it’s sending 77V through, you really need to check your settings. For your LifePo4 battery, ideally, it should be around 51.75V when fully charged. The BMS should protect against overvoltage, too, so it’s concerning if it’s not doing that. Definitely run through your connections and settings again—or consider talking to the manufacturer if it's a known issue.
It sounds like your charge controller is definitely malfunctioning. If it's outputting 77V when it should be in float mode, that's a major red flag. Normally, a functional charge controller would reduce the voltage going to the battery to avoid damage after reaching full charge. You should disconnect it right away to prevent any damage to your setup!
Yeah, your charge controller is broken. When mine switches to float, it barely pulls from the panels and keeps the output steady at the set float voltage. You shouldn't be seeing that high of voltage coming through at all. It’s crucial to check your controller and possibly get a replacement before it harms your expensive batteries.