question

joshseal avatar image
joshseal asked

Why does battery charge state go into Float, even when my battery is low?

I have a MPPT 75/15 connected to 2 x 100W Flexable Solar panels, all powering my Renogy 12V 170AH Lithium-Iron Phosphate Battery. In the Victron app I'm using the battery preset for Lithium Iron Phosphate.

After a long trip away, my battery was very low (about 15% this morning) - I know this because I have a battery monitor.

At around 2pm today I when to check how charged the battery was, it was reading about 35%, but when I looked at the app to check solar charging, I noticed only 50W was being produced. My panels are in full sun. Then I noticed that the battery state on the Victron app was saying it was in Float mode, and was limiting the voltage to 13.5V.

I have a circuit breaker between my solar panels and charge controller, so I switched it off, and then turned it back on again, and now the battery is being charged in Bulk mode again at around 140W.

So my question is, why would the controller go into Float mode, when the battery is still low? How do I prevent this from happening again?

Thanks, Josh

MPPT Controllersbattery chargingfloat
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klim8skeptic avatar image klim8skeptic ♦ commented ·

Hi Josh,

Next time you look at the app check the history for details about the charging states achieved, time duration for each charge state, yield and max/min battery voltages. Useful stuff, report back.

What firmware is the mppt running? How do you measure SOC?

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joshseal avatar image joshseal klim8skeptic ♦ commented ·

I'm running v1.39

I have a battery meter that reports how charged it is, and I've found to be very accurate.

As you can see from the screenshots below, I was able to generate significantly more power today than any previous day, even though my battery has never reached 100% for the last week or so (I've been off grid)

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klim8skeptic avatar image klim8skeptic ♦ joshseal commented ·

Hi Josh,

It looks like the mppt has charged the battery to ~14.1v with a full absorb cycle most days. That "should" indicate that the battery has been fully charged. If you touch one of the history vertical bars, the app will give a break down of time spent in each charge state.

The only thing I can think of that would interfere with the mppt's voltage reading (hence batt soc) would be voltage drop between the mppt and battery. It would be good to measure the battery and mppt batt terminal voltages, to see if there is any variance. Check the tightness of terminals/crimps/fuses.

Upgrade of the firmware to v1.42 will help with the variation of absorb voltage, and will give you a "fixed" absorb time.

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joshseal avatar image joshseal commented ·

Same thing happened again today. My battery only charged to 78%, but the controller only spent ~6h in bulk, when it was sunny for much longer.

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2 Answers
nebulight avatar image
nebulight answered ·

I don't think you understand your system well enough. If you are floating at 13.5v, your battery is full and your battery monitor is most likely not configured correctly. Are you using a BMV or another battery monitor? If you are using a BMV, please post a screen shot of your battery settings of the BMV so we can take a look.

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joshseal avatar image joshseal commented ·

It is fair to say I'm no expert, but I don't agree with your assessment. I don't have the victron monitor so no screen shots, but after I disconnected then reconnected my solar panels, the charge state changed from float to bulk and stayed charging all day. In float the panels were only putting 50 watts into the battery, but when the controller switched to bulk, it went up to 140 watts for hours. My battery went from 35% to ~50% by the time the sun went down. I've found the battery monitor I have to be quite accurate, and I have the correct battery capacity configured.

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adev avatar image adev joshseal commented ·

There are certain conditions that can trick the controller into going into float mode early - there are a few threads on it here somewhere. In such cases it won’t return to bulk charge until the next day normally, but if you unplug/replug the panels, that fools the controller into thinking it’s a new day and it will start a new charge cycle.



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inhabor avatar image
inhabor answered ·

I think you need to adjust the size of the battery in your BMV. Looks like the one you have right now is way bigger than 100ah.

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inhabor avatar image inhabor commented ·

If you don't have a BMV-712 you should get one.

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joshseal avatar image joshseal commented ·

My battery monitor is configured to 170ah (what my battery is) and I've verified it is correctly reporting battery level over the past few weeks of operation.

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inhabor avatar image inhabor joshseal commented ·

I have the same 170ah Renogy battery, two of them and I can tell you that 13.5 volts at float mode is a fully charge battery. What kind of battery monitor are you using? I use to have a trimetric meter that was extremely confusing, at least for me. Now I have a Victron BMV712 and changed my life.

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joshseal avatar image joshseal inhabor commented ·

It seems like the victron is going into float mode and limiting the incoming voltage to 13.5V. If I manually change the float voltage in the settings, the power flowing into the battery increases. I don't believe this would happen if the battery was actually full. So something doesn't seem to be working correctly. The problem isn't that the battery is full, and I just don't realize that.

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inhabor avatar image inhabor joshseal commented ·

I just look at the screen shots of your charger. I see 14.15 volts in the history. That's a full charge. The lithium battery won't stay at 14.15 when it goes from absorption to float and that's why you see 13.5. You're system is working perfect. Now if you really want peace of mind get a BMV 17 SMART

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inhabor avatar image inhabor joshseal commented ·

Are you in USA?

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