A guide to understanding boat batteries part 1, lead-acid

11 Responses

  1. Wolfgang Jansen says:

    Nice article, waiting for part 2.
    Lately I was sourcing information regarding best type of batteries in my situation. I am curious if these articles will solidify my thoughts.
    I am wondering why you did not mention charging efficiency per type of battery? Maybe this is reserved for part 2 🙂

    • Luis Soltero Luis Soltero says:

      Good question.

      There is discharge efficiency, and there is charging efficiency.

      Charging efficiency corresponds to how many amps X must be generated by the alternator to store Y amps in the battery. Typically for lead-acid, the charge efficiency runs between 80-85%. So… to charge a 100 AmpHr battery, you will need to generate the equivalent of 115 (or so) Amp/Hr from the alternator keeping in mind that you want to observe the optimum charging curves discussed above. If you end up putting in more than the acceptance current, you increase electrolysis, decreasing your charge efficiency. So lots of actors at play. Again efficiency will vary as a function of the battery type, charge profile, and temperature.

      On the other hand, discharge efficiency measures the amount of energy you get out of a battery based on its rated capacity. For lead-acid batteries, discharge efficiencies are very sensitive to the rate of discharge and temperature. If you discharge at the battery in 20 hours, you usually get about 100% of the rated capacity (not taking temperature variations into effect). Faster discharge rates will give you decreased capacity. So… say 80% at 4-hour rate or 60% at 1-hour rate.

      The tech specs provided by the manufacturer provide all this information. Please take a look at
      for my Victron gel batteries. You will find discharge efficiencies but no charge efficiencies. Just know those typical charge efficiencies for a lead-acid battery run about 80-85%… your mileage will vary.

      Efficiencies for lithium-ion batteries is much higher for both charging and discharging. Typical discharging efficiencies are 100% for 20 hours, 99% for 4-hour rate, and 92% for 1-hour rates. Average charging efficiencies are 95% for these batteries.

      More on this in part 2.

      take care.


  2. Kai Curry Kai Curry says:

    Luis, you state the “industry standard for battery failure is when it will not accept more than 20% of its original capacity.” Can you please reference where this standard is published?

    I’ve reviewed many technical manuals and don’t remember seeing that 20% of original capacity clearly stated anywhere. I’ve seen anything from 50-80% of its original capacity in relation to warranty claims but can’t think of any that have gone as low as 20% before accepting a claim. When Eastpenn/Deka runs comparison tests with other manufacturers they run tests until original capacity drops to 50%. Trojan Battery states they consider the battery has failed when discharge runtime is less than 50% of the batteries’ rated capacity.

    Thanks for the thorough article.

  3. Gary H Hagstrom says:

    I don’t think you will be learning much about your batteries with a hygrometer. I suspect you mean a hydrometer.

    I am a user of Odyssey AGM batteries in my MaineCat P-47. I use them for the house and the 3 starting batteries. The batteries are now 10 years old and still functioning fine. Odyssey claims 400 cycles of life at 80% discharge. This is twice what your article says for the “typical AGM”. When two brands of the same type battery have a 100% difference in in their life at 80% discharge it seems wrong to call your quoted number as “typical” of the type.

    My house bank(630 ah @ 24 volts) has been severely discharged twice in its life. Once to less than 1 volt indicated and once to just over 9 volts indicated. Both due to user error when in winter storage. Anyhow the bank is still functioning fine and seems to still have about 90% of its original capacity and does a great job at supporting shorter term 4kva AC loads and long term 0,8 kva loads via the inverter. The house charging system is a Victron 5kva system from 2009/10 vintage with the correct charging profile specified by Odyssey when plugged into shore power or the genset is running and 200 amps of 24volt power from engine auxiliary alternators via a Balmar system again with the proper charging profile for the Odyssey batteries. In normal use the house bank is not discharged ever below 40% and usually not below 50%.

    The three starting batteries still show as 95% charged, based upon the resting no load voltage check after 6 months of winter storage with no charging. One starting battery was discharged once to about 5 volts indicated and 2 of them discharged to about 6 volts indicated. The other advantage of AGMs (at least Odysseys but I think all) is they tend not to fail catastrophically but generally fail with a slowly reducing capacity over months so it is unlikely that the starting battery will work fine one day and not at all the next day as can happen with a flooded cell battery.

    ANyhow, like so many things every case is different, but given my mistakes resulting in abuse to the batteries I am very happy with their long term performance/price/hassle balance.

  4. Evan says:

    Great article!

    Thanks for this in depth analysis.

    Looking forward to part 2.


  5. Joseph Pica says:

    Luis good article, however no mention of partial state of charge (psoc) vulnerability of agms with the exception of Firefly carbon Foam.


    • Wolfgang Jansen says:

      Probably because this “exception” is not true, in perspective of the chemical side.

      And the difference in this partial SOC aspect is mentioned in the article, I quote “This structure increases the surface area of lead, increasing efficiency while reducing lead sulfate’s harmful effects, proving exceptional longevity and high performance.”.

      While it is reduced, it is still there (and could not be different, because the electrochemical reactions are the same).

      Whil I cannot look ahead, probably in part 2 SOC and other aspects of batteries will be summarized, because lithium-ion batteries have their own issues in this aspect.

  6. Jelles Fonda says:

    Luis –

    Thank you for the excellent, quite thorough review given the length of the article (big topic!). Stressing the importance of limiting drawdown to 75%SOC is important to the liveaboard cruiser as well as periodic equalization to maintain battery health and longevity.

    I thought that your table on cycle life for AGMs vs gels was slanted towards gels. I have 5 Lifeline 8D AGMs house batteries. Cycle life reported to be 2000 at a 25% depth of discharge.

    Looking forward to part II.


  7. Jason Ellison says:

    “Like flooded batteries, gels and AGM batteries last longer if cycled lightly and frequently changed.”

    Was the last word meant to be “charged” instead of “changed”?

  8. Luis Soltero Luis Soltero says:

    sorry about that… Yes. CHARGED is the what was intended here.


  9. Luis Soltero Luis Soltero says:

    Note that lifeline AGMs are considerably different than EastPenn AGM batteries. They are milspec and have much better cycle life. Check out their user manual
    they do close to 2000 cycles at 30% discharge and 1000 at 50% dischage. Not quite as good as the Carbon Foam batteries but very good none the less.


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