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Todd Wallace avatar image
Todd Wallace asked

MPPT 100/30 exits absorption early on cloudy days. BMV reports -14Ah when goes into float

I know this has been discussed alot in the forum, but my problem is my 300 watt panels can only put out max of 2 amps or so on a rainy/cloudy day. If the battery is not deeply discharged, it is possible for a few mins of brighter conditions can just barely hit absorption set point bringing controller into absorption mode, but then im back to 2amps or less max output from the panels, which results in being under the 2amp tail current and kicking me out into float. I know you cant change the tail current on the MPPT which is frustrating, but my battery is not getting charged properly because of that. what can i do? it is a lead acid battery and NEEDS proper absorption charge, and as the BMV shows, the SOC isnt even close to 100% when it goes into float. please help

MPPT ControllersBMV Battery Monitor
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Mark avatar image
Mark answered ·

Just after a full charge cycle, maybe try to manually sync your BMV to 100% SOC to provide a fresh 'baseline' - then see how it reads on the next day. That will help understand if it's only a BMV long term Ah tracking issue or real.

Also it will only switch IF the absorption voltage setpoint is maintained while the charge current drop's below the 1A threshold. If the battery is not fully charged or very close to it, then the voltage will drop if there is not enough charge current & the phase switch should not occur.

It is possible that it's actually switching to float based on the calculated absorption time limit being reached.

If you have a read of the online manual you will find that a multiplication factor is applied to the 'max absorption time' that you specify, based on the battery voltage when the charge cycle commences. With a high battery voltage in the morning this can result in a calculated absorption time limit of only 1/6 the max time specified (so 1h for 6h max).

You could also try to increase/double the 'max absorption time' for a few days and see if this changes things. If it is really switching to float based on the current threshold then it should have no effect at all.

Also note that in the upcoming MPPT & VictronConnect firmware releases this logic will become more user configurable (including configurable tail current & the option of fixed absorption time).

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Todd Wallace avatar image Todd Wallace commented ·

Thank you for clarifying some things. I don't remember reading anything about a multiplication factor and I read the manual pretty carefully. So if I set a maximum absorption time of 12 hours, then it will change the MINIMUM actual absorption time to 2 hours instead of 1 if the start voltage is above 12.6 (for 12 volt system)? Also from all the information I read, my 100/30 amp model, the tail current is 2amps. Under 30 amp controllers were 1A. I read about Victron releasing a firmware update and I got excited, but then discovered they pulled it back. Any idea when it will be available? I would LOVE to be able to configure things more precisely since every battery system is different. In fact, just today I ended up in float again with -12Ah out of my 100Ah AGM battery still remaining on my BMV so it's very frustrating.

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JohnC avatar image
JohnC answered ·

Hi Todd. I can't help with the 2A tail, and I guess if your batt is quite small this could be an issue.

The easiest workaround would be just to raise your Float V target up to or near the Abs level. I'm sure you'll know to take care with that..

Bear in mind that though that Abs isn't a cast-in-stone requirement to charge batts, and even if in Bulk or Float they may still be charging well.

BMV for your SOC? Also consider the Peukert factor entered there is to compensate for a high charge rate, and if you're doing it much more gently the SOC derived may well be underestimating the true charge level of your batts. A run of short/cloudy winter days will accumulate that error.

In practice it's probably rare that BMV's are set up totally correctly, and even if they are should be synced regularly.

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Todd Wallace avatar image Todd Wallace commented ·

after spending over 200 bucks on a charge controller, I feel like I shouldnt have to "cheat" and periodically edit the float voltage, but its a valid workaround suggestion and I appreciate it. absorption IS a cast-in-stone requirement for lead acid batteries though in terms of their lifespan. a few days without it isnt a huge deal, but it needs 14.4 volts to break up sulfation which occurs during normal operation. without that, lifespan and total capacity can degrade quickly. my Peukert factor should be correct in the BMV since the values were straight from the battery datasheet. my BMV is also linked to the MPPT for accurate voltage reporting. maybe Victron should recommend a minimum battery capacity for a given MPPT charger if the tail current is going to be unchangable.

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