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For a 70 Ah 12 V lead-acid battery with an unknown number of cells (likely 6 or 9 plates), what is the ideal cutoff voltage to maximize its lifespan? Additionally, at what voltage does this battery reach its full charge capacity?

I currently stop charging the battery when it reaches 13.8 V. Is this voltage safe for optimizing its lifespan(cycles life)?

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    \$\begingroup\$ This depends on the chemistry and number of cells, not the capacity. Please edit your question to include this information. \$\endgroup\$
    – Attie
    Dec 9, 2023 at 15:12
  • \$\begingroup\$ it depends what you mean by 'optimise' \$\endgroup\$
    – Neil_UK
    Dec 9, 2023 at 16:41
  • \$\begingroup\$ question update please review it thanks \$\endgroup\$
    – JOHN
    Dec 10, 2023 at 19:17
  • \$\begingroup\$ A 12 volt lead-acid battery will have 6 cells, and likely many more than six or nine plates per cell. \$\endgroup\$ Dec 10, 2023 at 20:40

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A search for "ideal charge voltage for lead acid 12 V battery" returns this result that says "13.5 to 13.8 V at 25 °C".

A search for "ideal cutoff voltage for 12 V lead acid battery" returns this result that says "any lead acid battery, be it a Gel Cell, AGM or flooded batteries such as DCBs, should be cut-off at 11.6 volts.".

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For led-acid chemistry, it’s 2.2 volts nominal per cell. In monoblocks that means 6 cells. Number of plates is probably irrelevant to your question.

Any given specific manufacturer’s product will advise exact cell voltage for low-charge and high/full-charge cut-off thresholds.

Given (a) the need to ensure all cells within a monoblock are fully recharged, and (b) lead-acid’s forgiving tolerance to over-voltage during recharge (tho not so high that you experience high levels of out-gassing of H2), it’s much easier to attain 100% charge on all cells by allowing per-cell voltage to rise to relatively high levels, above 14 volts. This strategy is even formalised with periodic ‘cell refresh’ and ‘rejuvenation’ recharge cycles, to help with other lead-acid chemistry quirks.

Best cell longevity is achieved by not *discharging cells below datasheet-specified states-of-charge, often in the realm of 50%, maybe not even that low(!!). This has been lead-acid’s Achilles Heel since its beginning, over a century ago. There’s a major class of lead-acid batteries dedicated to this reality of almost never being deeply discharged, but also capable of brief very high current output: the ones used for starting IC engines in cars, trucks, generators, etc.

This is quite different to newer Lithium rechargable chemistry, which can tolerate much lower states-of-charge, but which have much stricter limits on maximum cell voltage, making the need to perform ‘balanced cell charging’ necessary, in order to keep all cells in a series string as close in state-of-charge as possible (see other answers in my history detailing this).

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  • \$\begingroup\$ Is it okay if I charge my battery at 13.8 volts? I checked, and it has 6 cells mono block. \$\endgroup\$
    – JOHN
    Dec 11, 2023 at 11:09
  • \$\begingroup\$ How i determine my battery is overcharged? \$\endgroup\$
    – JOHN
    Dec 11, 2023 at 11:10
  • \$\begingroup\$ Did you miss the 2nd sentence? 13.8V recharge maximum voltage will probably get the job done, but it's the datasheet for that brand and model battery you need to consult. There's several types of lead-acid, and they all have their own care-&-feeding constraints. batteryuniversity.com/article/… \$\endgroup\$ Dec 11, 2023 at 18:06
  • \$\begingroup\$ How’s your situation going, @JOHN ? If an answer here was correct and addressed your situation, we’d appreciate you marking it ‘answered’. \$\endgroup\$ Dec 19, 2023 at 20:42

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