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I have three AGM batteries (VARTA LA95, 12V, 95Ah) that I want to connect in parallel in a battery bank.

They were connected to a Victron charger for a long time and now that I turned it off, I began reading the voltage daily just for fun. They were all connected to the same charger output.

One of them is clearly behaving differently, but I can't figure out what that means.

My question is:

  • Is battery #3 "healthy"?
  • Bonus question: What happens if I use it in parallel with the other two?
timestamp batt #1 batt #2 batt #3
t0 + 24h 12.90 V 12.83 V 12.51 V
t0 + 48h 12.90 V 12.83 V 12.48 V
t0 + 72h 12.90 V 12.83 V 12.45 V
t0 + 96h 12.89 V 12.83 V 12.43 V
t0 + 120h 12.89 V 12.83 V 12.40 V

(t0 is when I removed the charger)

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  • \$\begingroup\$ To answer your bonus question: Don't connect batteries in parallel unless they've been charged in parallel (or by a charger designed for that use case). \$\endgroup\$
    – Sneftel
    Commented Sep 1, 2023 at 14:06
  • \$\begingroup\$ These ones were charged in parallel and they all are the same model, so I expect it to be fine. I was only surprised to find that one of them looses charge much faster than the other two. But they were all bought used and from different sellers... \$\endgroup\$
    – Pedro Ferreira
    Commented Sep 5, 2023 at 8:42

2 Answers 2

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AGM batteries have a self discharge rate of 1%-3%/month Assuming for the moment that self-discharge will be linear with time (probably as a worst case), extrapolating your 120 hours to 30 days, shows battery 3 to fall within the 1%-3% range.

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  • \$\begingroup\$ That's a good way to look at it; I forgot to estimate the self-discharge. But I get a rate of over 5% for battery #3 and less than 1% for the other two. It's probably better to replace it. \$\endgroup\$
    – Pedro Ferreira
    Commented Sep 5, 2023 at 8:49
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Bonus question: What happens if I use it in parallel with the other two?

Nothing spectacular will happen, maybe tiny sparks when connected together, and then they will even out. The batteries will work as one unit.

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  • \$\begingroup\$ This got me thinking and I suppose what will happen is that #3 will slowly drain from #1 and #2 and the whole SoC and voltage will be a sort-of-average of my values in the table. And the sparks will be due to the current flowing into #3. \$\endgroup\$
    – Pedro Ferreira
    Commented Sep 5, 2023 at 8:52

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