another view on level of battery discharge

This Old Boat by Don Casey is all about how to rehabilitate, repair and improve an old fiberglass sailboat, and teaches the needed skills. It has a section on electricity which includes a view on depth of discharge of batteries that is different from the standard one. It is in a context of the primary recharge of the batteries being accomplished by running the engine, but I think it is interesting enough to share with other battery-dependent people.  I'm interested to hear comments from others about what they think of drawing batteries down below 50% discharge. There's also a section about monitoring battery state of charge by reading voltage.

From This Old Boat second edition by Don Casey, pages 271-2

HERESY . . . AGAIN

We are about finished with the subject of batteries, but before we move on, let's kill a sacred cow, shall we? In cycle-life testing, deep-cycle lead-acid batteries typically deliver the most lifetime amp-hours when discharge is limited to about 50% of capacity, so this is the maximum depth of discharge normally recommended. If maximizing the “value” you derive from your batteries were the controlling issue, the 50% discharge rule would, well, rule, but value isn't really what you want from your batteries. What you want is convenience. You want plenty of light, fans when you need them, pumps that don't require muscle, radio communications, movies, and the ability to recharge your laptop battery. And you want to run your engine as infrequently and as little as possible. The best solution to the generating capacity/storage capacity equation is solar and/or wind-generating equipment, which we will come to later, but what can you do to maximize convenience on the storage side?

We have already noted that the decreasing rate of charge acceptance will not allow us to efficiently charge deep-cycle batteries above around 90% of capacity, so restricting discharge to 50% limits usable battery capacity to just 40% of rated capacity – 180 Ah from a 450 Ah battery bank made up of four golf cart batteries.

What if you ignored the 50% rule and discharged your batteries to (drumroll here) 80% Wouldn't this shorten the life of the batteries? Yes it would, but having a bacon and egg breakfast probably isn't doing your life span any favors either.

There is little doubt that discharging your house batteries to 80% rather than 50% reduces the number of cycles the battery will deliver, but that is just half the truth. Discharging the bank more deeply also has the effect of reducing the number of times you cycle the battery. If the deeper discharge reduces the number of cycles your batteries will deliver by 40% but also extends the time between recharges by 40%, guess what? The reality is not quite this good, but probably better than you can imagine. Test data from Trojan Battery Company reflects a reduction in total lifetime amp-hours delivered of just 6% when their industrial batteries are discharged to 80% rather than 50%. Much less than you thought, right? For this nearly negligible reduction in battery life, your 450 Ah battery bank now delivers 315 Ah between charges rather than 180. You get this additional 135 Ah of convenience without changing your charging equipment in any way. Even accounting for the reduction in battery life, the net effect on per-amp-hour cost is likely to be positive because batteries accept charge at a higher rate at deeper levels of discharge. This means the alternator operates at the upper end of its output capacity for a greater portion or recharge, which translates into fewer total engine hours, saving fuel and wear and tear. You also run the engine less often. If you have been running it every other day, you can now go almost 4 days between charges.

If you have lots of solar and wind-charging capacity, baby your batteries. However, if you rely on your engine alternator for recharging, you are likely to get better service from them drawing the batteries down more deeply. If you discharge good-quality deep-cycle batteries to an at-rest voltage of around 12 volts (but not below) between charging cycles, the positive effect on your quality of life should far over-shadow any negative impact on the batteries.

Have cereal with skim milk for breakfast if you like. I'm going for the bacon.

from pages 269-70

Voltage

Battery voltage is about the closest you will come to having the equivalent of a gas gauge for the charge level of your house bank. At-rest voltage is a pretty good indicator of charge level, but unfortunately the batteries on a boat in use are rarely at rest. If the battery is being discharged – say the refrigerator is running – it will register lower than the true voltage. If it is being charged, the voltage reading will be higher. Even if neither is occurring at the moment, batteries “recover” for several hours after discharge. The voltage also rises for some time after charging stops. For an accurate assessment of charge level based on battery voltage, the batteries should have rested for 24 hours. In practice, your will get a fairly accurate reading of battery condition after a couple of hours of rest. A good time to check the battery voltage on a cruising boat is just before sunrise. The cycle time for electric refrigeration will be at its longest, there are likely to be few other loads, and any fixed solar panels will still be dormant.

Between fully charged and 50% discharged, battery voltage declines by less than 0.5 volt, so assessing the state of the batteries based on voltage requires a digital voltmeter. A voltage reading above 12.6 suggests that a 12 volt battery is nearly 100% charged. When 25% discharged, battery voltage will have declined to around 12.4 volts. A reading of 12.2 volts corresponds to the 50% discharge (or half-charge, if you like) level. Not allowing battery voltage to decline below this level is generally considered to maximize the total number of lifetime amp-hours a battery will deliver. A reading of 12 volts indicates that the battery (or bank) is around 75% discharged, and the battery is effectively dead when the voltage declines below 11.8.