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My solar charge controller allows me to set a cut-off voltage, so that the battery charging is stopped when the battery reaches that voltage. The value I set will probably also be the maximum voltage at which the batteries are charged by the controller.

My battery says:

enter image description here

My question is... is it safe to set the cut-off to 14.40V in the charge controller? I am worried because, once the battery is full, the charger will continue to apply 14.40V which is greater than the popular floating charge voltage recommendation of 13.6V.

My usage of the battery is cyclic. It gets almost entirely discharged in the night, and is fully charged by 1PM the following day.

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    \$\begingroup\$ Probably OK BUT you need to read up on LA charging to understand the various aspects. A fair amount of black magi. | Note that you use two conflicting concepts in one sentence. You say "popular FLOATING charge voltage ..." BUT you are not using the battery in FLOT mode. You are using it in very dep discharge mode (which is far worse for it than holding it at 14.4V. To use in FLOAT mode where it is not usually discharged much at all but only occasionally is you use the "standby use" voltage. Your 13.6V is in the 13.5-13.8 range specified. BUT for deeper regular discharge the higher range ... \$\endgroup\$
    – Russell McMahon
    Apr 25, 2014 at 9:29
  • \$\begingroup\$ ... applies. | BUT if you discharge a "normal" LA battery "almost entirely" all of most nights it will last typically 50-200 cycles - and 50 would not be a surprise. If you use double the capacity and only half discharge it (or use two the same in parallel) you will get FAR more than 2 x the cycle life. See www.batteryuniversity.com \$\endgroup\$
    – Russell McMahon
    Apr 25, 2014 at 9:31
  • \$\begingroup\$ Wow, I never thought like this before.... adding batteries to my battery bank can pay back by extending the number of cycles. \$\endgroup\$
    – Zuhaib Ali
    Apr 25, 2014 at 10:04
  • \$\begingroup\$ By "almost entirely discharged' I mean a voltage of ~11.8V after which my inverter stops using the battery. \$\endgroup\$
    – Zuhaib Ali
    Apr 25, 2014 at 10:06
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    \$\begingroup\$ 11.8V is "OK" - the inverter makers did not want to totally thrash the batteries. Full discharge ~= 10.5V but you REALLY want to avoid discharging that low. Two batteries of original capacity with same load will usefully increase your battery life with a decrease in long term battery cost. Better is probably a single battery of 2x or 3x original capacity and you then do not need to worry about balancing etc. Battery university suggest that 50% DOD gives somewhat more than 2 x cycle life but 30% DOD gives substantially better than 3x increase - so being substantially overprovisioned is good. \$\endgroup\$
    – Russell McMahon
    Apr 25, 2014 at 12:46

3 Answers 3

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This is the tradeoff using a cheap charger with no float mode. Capacity vs battery life.

Your options are ;

  • Get a float mode when charge current drops low.
  • Get more battery capacity. Now. Or...
    • Replace the battery sooner, than expected

If your charge periods are short term, use 14.4 if it charges up always more than you can store, use 14.1

If there is no load during charge and only used at night, use 14.1, otherwise higher.

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The float voltage you mention is a voltage which is kept for a prolonged time. If your charger cuts off the battery completely when done or lowers down to a float level, this won't be a problem.

Check this out: http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

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  • \$\begingroup\$ That's the issue... my charger doesn't lower down to the float level. \$\endgroup\$
    – Zuhaib Ali
    Apr 25, 2014 at 10:03
  • \$\begingroup\$ Aha, do you have a datasheet or similar of the charger? \$\endgroup\$
    – Dejvid_no1
    Apr 25, 2014 at 10:39
  • \$\begingroup\$ No it do not. :'( \$\endgroup\$
    – Zuhaib Ali
    Apr 26, 2014 at 4:43
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If you read datasheets of lead-acid battery charger ICs (e.g.; BQ2031 and BQ24450), you will see that they have internal voltage references of 2.2V and 2.3V. They are for taking feedback from 1-cell battery (though multiple cell batteries can be connected by "fooling" the IC with a voltage divider network). These kind of ICs stop fast charging when the battery voltage reaches to 2.2V, and apply a special slow charing method until it is 2.3V. Datasheet of BQ2031 summarizes three different charging algorithm.

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  • \$\begingroup\$ This does not really address the question - and you should idally relate your answer to the voltages mentioned by the question and not the reference voltage.|| HOWEVER the BQ 24450 datasheet that you cite contains some excellent application material re LA battery charging. \$\endgroup\$
    – Russell McMahon
    Apr 25, 2014 at 9:25

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