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Batteries overcooked(?) with BlueSmart IP65 12/25

On our boat we have 4x 6v 225Ah deep-cycle FLA batteries, wired together into a bank of 12v x 450Ah. This is attached to a Victron BlueSmart IP65 12|25 charger using the 'Normal' preset.
The batteries were purchased around July 2021. The system had been working fine up until recently. Sure, occasionally the batteries would dry out a bit if we didn't keep on top of checking the electrolyte levels, and we ran the recondition cycle a couple of times (and I mean no more than twice).


The past few months have been an exceedingly hot summer, and temperatures in the boat would regularly reach 95F (35C) during the middle of the day. The batteries are squeezed together in an enclosed compartment which, while it has a vent to the outside, retains a lot of heat. We were running our freezer off the batteries all the time, and we had the charger on all day, connected to shore power. We found that the batteries dried out quickly in these conditions and we had to top them up regularly with distilled water.


A couple of weeks ago our CO alarm went off - first time it's ever done that. We quickly established that the cause was off-gassing from the batteries (hydrogen and other gases can set off some CO detectors). Thinking that the problem was the batteries were getting too hot charging during the day, we opted to start charging them at night when temperatures were cooler. Due to this alarm we also started paying closer attention to the charger status on VictronConnect, and we noticed that although we'd leave the batteries charging all night, they never reached Absorbtion stage. In fact, watching VictronConnect, I could see the battery voltage during Bulk creep up towards the absorbtion voltage (14.4) but never quite make it there. Usually they would reach between 14.2 and 14.25 volts, and then the voltage would start slowly dropping down. At the same time I could see that the charging current had decreased from 25 to 22 or 23 amps, which I assumed was due to the temperature compensation mechanism in the charger.


Despite multiple charging sessions with all loads turned off, we could never get the battery voltage above 14.35V, at which point they started off-gassing and the CO alarm went off. We pulled one of the batteries out of the compartment and found that it was starting to bulge - not as bad as some photos I've seen but definitely bulging a little bit. This I understand means they'd been overcharged.

My questions then are the following:
- What causes a battery to reach a condition where it cannot be charged to Absorbtion voltage without dangerous off-gassing?
- Was this a problem with the charger, or our own fault for not setting a maximum bulk cycle duration or for attempting to charge when ambient temperatures were too high?

- In cases like this, is it safe to set a lower Absorbtion threshold voltage?
- If we replace the batteries, what can we do to make sure this doesn't happen again?

battery charging
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JohnC avatar image JohnC ♦ commented ·
Hi @Maddy Markovitz

These sound awfully like GC2 pattern batteries, can you confirm? Also, from a previous post, you may have solar and a shunt too?


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JohnC answered ·

Ok, thanks for that @Maddy Markovitz. You may mean Century 105's, which I use in a main 48V bank, but also have a 12V aux bank just like yours of >7yo survivors (one is >13yo). They're tough, heavy plated things, and as long as you don't sulphate them up they last a good time.

But they do vent a lot. I find that consistent all year, and I take particular care not to overcharge them. My aux bank is lightly loaded, so I give it an automated daily dose at a flat 13.9V for a few hours. Regardless of Temp, which here has reached 36 degC for the past 4 days (ambient was higher).

The 48V cycling bank gets the full solar treatment (figures converted to 12V). 14.4V Abs, 13.6 Float. Absorb rarely exceeds 1 to 1.5 hours, and is terminated by Tail at 3.5 - 3.7%. Temp Comp -25mV/C, but seems to work better in the heat at -30, this a lot higher than Victron default.

Gassing comes with the territory, excessive gassing from too high a V or too long at Absorb. It's certainly safe to use a lower Absorb setting, and if you have the luxury of time I much prefer it. Gentle does it. You won't need new batts. But do everything you can to ventilate them as much as possible.

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Maddy avatar image Maddy commented ·
Thanks for your input @JohnC. I am a bit concerned as to how I should treat the batteries now that they seem to be overcharged, but not capable of reaching absorption on the standard settings. It seems you're suggesting to set the Absorb threshold lower, and that they should get into Absorb then. Or if not, what's the long-term effect of only charging in Bulk?

I'm unsure how to set "a flat 13.9V" on the charger. I can adjust the charge current, and the voltage setpoints, but not the voltage that the charger pumps into the batteries during Bulk phase.


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JohnC avatar image JohnC ♦ Maddy commented ·
@Maddy

I'm not sure you understand that the Absorb setting you make is first Temperature Compensated before the charger applies it, So the standard 14.4V becomes (say) 14.2V in hot weather, the charger then targets that. This corrected value isn't shown anywhere, but the charger State should show Absorb once it's reached, rather than Bulk. 'Bulk' State is Victron terminology for attempting, but not yet reaching Absorb state.

I haven't actually recommended you anything firm, just examples of what I do in varied circumstances. If you're not sure what do do, easing back is the best way to look.

In the Expert settings of the charger you can set many things to do what you want. Create your own charge profile as you will. You just need to determine what that is. If your batts are venting excessively, even getting hotter than they should, then reduce V. Or set a Fixed Abs time, etc.

This type of battery can go all the way to 100% charge at say 13.5 V, slower but safer than at 14.4V. Somewhere in between is yours to choose..


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