AC200max solar charging is pitiful (sometimes)

Sorry in advance for the length of this post. Just curious if any of you have seen similar solar charging issues.

I’ve had the AC200max now for about a month. It’s been working great and does charge well most of the time (So Cal sun). However, about once every 3 days, the charge rate drops dramatically.

For example, yesterday morning, it started charging at sunrise at about 40w. PV charge was climbing until around 10:30 am and was producing 500w+. It stayed around that level for several hours, then as expected, it slowly dropped charge rate until the sun went down. That’s a “normal” PV charge day (no clouds and perfect winter sun all day).

Today started the same in the morning, then after about 30 minutes, charge rate dropped from 150w to 15w and stayed at that pitiful rate for about 20 minutes. No other indications as to why it dropped. I checked the panels and nothing interfering. I pulled the PV charge cable out of the AC200max and waited for the volts to drop to zero. Plugged it back and and after about 30 seconds the PV rate was back up to normal. After 10 minutes, it dropped down again to 15w. Pulled plug and repeated. It’s currently charging at 180w, so that’s a bit better giving the time of day. I presume this might just be the battery protecting itself, but it seems overly aggressive if that is the case (fans are not running, so it’s not hot).

BTW: I’ve also noticed that if I leave the PV cable attached overnight, it will read the wrong voltage the next day when the sun rises. I remove the cable and reattach it and then volts now read correctly.

Other tid-bits: 3 x 350w solar panels attached in series. VOC is 142 running around 8 amps and the PV usually reads about 130v input (that’s volts, not watts). Fans turn on based on AC usage or high charge rate, but not very often and it doesn’t appear to affect the charge rate. Firmware is up to date, and no alarms logged.

I typically use the battery between 85% charge, drain to 50% and let it charge back up – sometimes a bit higher or lower depending on the use case. Battery does seem to go into trickle-charge mode at around 90% charged.

I have a grid-tied solar system on my house, so I have some comparison data for charge rates based on current conditions. For example, if my house system is charging at 60% capacity, the Bluetti is usually around 50% (that’s OK because there is battery system overhead). This makes it easier to see what the Bluetti “should” be doing based on comparison to my house system.

Have you run into similar PV charging issues with your system?

I’m no expert on this so take this with a grain of salt, but you may have an over voltage situation. I was doing my research on how to “over panel” my AC200Max so I could get max wattage throughout the day. I stumbled upon the below article and learned about “temperature coefficient of VOC.” Depending on the ambient temperature, your panels may draw more than the stated VOC from the manufacturer. If so, the 350w panels in series may be over the max input voltage for AC200Max solar charging (145v). This happens with the temperature drops below the “standard test conditions” (STC) that the panel was originally tested at. This may explain the “wonkiness” in the morning hours which then clears up by the evening (after the panels heat up). Try running one of the panels in parallel. You may be over on amperage (15.2A +/- 0.3A max) but it won’t impair charging like being over on voltage (user manual says extra amperage is “wasted,” which to my lay person understanding means “not used”). See if two 350w in series with one 350w in parallel fixes the problem.

Hey Ceradinimp,

Thanks for the info and ideas. Interesting that you mentioned it going over the VOC limit and perhaps trying one panel in parallel (instead of all 3 in series). I thought about trying that, but I’m wasn’t sure how the charge controller would treat over amp’d input. I guess the worst that could happen (fingers crossed) is that the AC200m would shut down the PV charge port and log an alarm.

I’ll do a bit more research. If it looks safe to attempt, I’ll let you know how it works out.

MY AC200Max has six 260 watts panels tied to the input without any problem. Two sets of 3 panels are in series and then tied in parallel. I never exceed the Voc limit but the amp range is ok to exceed it. Some people call it “over paneling it”. I do the same thing with my EP500, with 1200 watts on a sunny day.
Have fun

Hey hey,

Thanks for confirming the over paneling works without issues. I’ll give it a go.

I’m also looking to expand the battery and PV input to 1400w. It should be a great setup for our over-abundant power-outages in southern California.

Hi @200maxer,

Sorry for the inconvenience.
Could you please provide your solar panels’ link? I will pass it to our technical team and tell you what to do. Also, could you please use two solar panels and see if it is normal?

Hi Admin,

Here’s the panel details (screenshot of back attached).

I did try 2 panels when I first noticed a bad charging day several weeks ago. It didn’t make any difference… in other words, it only reduced the low charge rate by 1/3, rather than improving it. Along those lines, I don’t think it’s butting-up against the VOC limit as 2 panels would have resolved that. However, I’m not sure if a complete battery shutdown would be needed to cool it and basically have it reset to begin fresh. Anyway, it’s been working great the last 2 days with 3 panels attached.

I am looking to expand the charging, perhaps attaching a B300 or using the DC CHARGING ENHANCER, or both! Either way, I’ll add panels and go with a mixed parallel/series setup. That may be a way down the road because those Bluetti items aren’t available yet.

Any ideas appreciated.


Hi @200maxer

Thank you for your picture. You mean the charge rate also drops dramatically when you use 2 solar panels?

Yes, on the “bad” charge days (3 panels), going to 2 panels makes no improvement. I have noticed something that might be an lead on the issue. When the unit is 100% charged (or very close), it stops charging as it should. If I then use up quite a lot of it, say down to 40% SOC, that’s when it has a bad charging day. It’s as if the full charge caused it to enter into trickle charge mode, which makes sense, and any charging after that is severely limited for some time (even though it’s now at 40% SOC). The next day charging is usually much better, so perhaps the BMS/charge controller is overprotective after 100% charge is achieved and then drained. Just an idea at this point, so I’ll try it again to confirm.