discharge deeper = longer battery life

I had a realization recently that you get more battery life if you discharge the battery further (up to about 65% DoD)
Everybody always says the opposite; am I thinking about this wrong??

The chart below is for my batteries (full river 224-6).
For simple math, I have a 100Ah battery bank. If I discharge 67%, that's 67Ah - I'd get 1000 cycles before they die, so 67,000 total Ah.

If I choose to discharge much less, let's say 28%, that's 28 Ah - I'd get 2000 cycles before they die. That's 56,000 total Ah. Notice: THIS IS SIGNIFICANTLY LESS THAN ABOVE.

Deeper discharge: 67,000 lifetime Ah, shallower discharge, 56,000 lifetime Ah. What gives? I'm at anchor right now, could charge up every day (about 30% discharge), or every day (60% discharge), I'll get more life out of my batteries with the deeper discharge (not to mention less charge time).

For comparison I looked at the Trojan battery chart, and it makes no difference if you discharge 60% or 30%, the total lifetime Ah for their batteries is the same.

The takeaway: if I'm looking at this correctly, it's best for my batteries to discharge between 50% and 70%, not the 30%-50% range everybody tends to suggest. Also, none of this to mention the ability to have a smaller bank, which other people brought up in a different thread. Deeper discharge is less $$ and more convenience all around.

When I was maintaining equipment with Trojan 105 batteries, the rep told me to not discharge past 50%. I don't know what is right. I would ask the battery manufacturer what they suggest.

The point that will give the most total amp hours over the battery's life will be different from one battery model to another. Generally there's a range of discharge depth where the difference is pretty small. And 50% is often within that range.

I always took it to mean “days”. If I size the battery bank and
or adjust the loads so that I don’t draw it down more than x I will get many more days of use.

Ken

You may get more amp hours but less cycles

Meaning each night you could use more appliance watts.....

But for less calander days....meaning you replace batteries more often.

Thus the thought is to size your battery bank so your use between charging never dops the bank below 75% to 50% for flooded lead acid

You are very much correct. To a close approximation, the lifetime Ah throughput of a battery is a constant in the 10% to 80% DOD. At the fringes of very shallow cycling and very deep cycling is drops off, but in the typical range it's pretty much a constant.


But we have all had 50% drilled into our heads so hard that few people can see beyond it. But it's based on somebody's ideal of how many years a battery should last. And that's with no basis for how many times it's cycles in those years, so it's really a completely meaningless number.



Economically, you are probably better off with fewer batteries that you cycle deeper, and replace more often. If nothing else, by cycling deeper and replacing more often, you have less opportunity to trash the batteries yourself before it's time to replace them.



But the flip side is that battery replacement is work, and nobody likes work, so having to do it less often does have some appeal.

Battery banks should be sized to your needs. One should not worry about maximizing amp hrs or cycles. I am rarely on the hook for more than 24hrs before I need the engine or the generator for some other purpose. I boat in the PNW were it is considered rude to run a generator after 9pm or before 8am. I run lots of lights and entertainment. I sized my bank to be at 50% after 24hrs. In my case I consume 650 amp hours in a 24hr period so I have installed a 1400 amp hr bank.

That’s a nice laboratory generated graph. If your engine alternator charges the bank or you have solar you will never consistently reach the target DOD. So then who knows what you will get.

One part of the equation that the graph doesn't include is diminished capacity over time or cycles. To use your example, with the 100 AH battery, if you pull 67 AH out when it's new, you drain the battery by 67%. As batteries age, they loose capacity. So, after 800 cycles of your battery, it might only have a capacity of 80 AH. In essence, you would be discharging the battery by 84%.

My guess and it's only a guess, is that if you pull 67 AH out of your battery each time, the cycle life will be closer to 500 cycles based on the above example. The other way to look at it is that if you pull 67% of the capacity out of the battery each time, it may last 1,000 cycles, but the AH per cycle will significantly diminish.

My final thought is that many manufacturers (such as Trojan) give you a maximum amp draw rate. If you use a smaller bank with the goal of 1,000 cycles, you may exceed the amperage draw rate on things such as an inverter running a microwave, which will further diminish AH capacity and / or number of life cycles.

Ted

Another question is weather you might need a large amount of amps , say to bring up the anchor or to run a bow thruster or a big inverter ,when mostly discharged.

The large amps ability is usually poor when the batts are near dead.