Be Free wrote: ↑Sun Jul 28, 2024 12:21 am
Starscream,
Thanks for the kind words.
I don't have any experience with the battery you linked to or the company that makes it. It appears to be what is generally referred to as a "drop-in" lithium battery. They are meant to simplify the transition from the various lead based batteries to lithium. As I understand them you might consider them the "Macs" of lithium batteries. They have made some compromises in their design that makes it easier for them to live in the Pb ecosystem just like our boats have to make some compromises to be able to move between the sail and power worlds. The changes are relatively minor and don't make that much difference in the long run. If the ease of "dropping in" a lithium battery where a lead battery used to be is important then the design changes are worth it.
Depending on how long you have been switching your house and start functions on your battery bank this next part might be moot but it may be of interest to others in the future in any case.
One quick disclaimer: I'm going to use voltage as a proxy for the depth of discharge of the battery. This would only actually work if you let the battery "rest" with no load on it for a while (say an hour) before you measured the voltage. The voltage when the battery is under load is always going to be lower than the resting voltage (sometimes quite a bit lower).
The only way to accurately measure depth of discharge (DOD) is to measure the actual capacity of the battery (not easy), fully charge it, and then measure the amperage and time while it is under load. There are gauges that will give a good approximation of the DOD if you program them correctly but most people don't (program them correctly).
For something that is "close enough for government work" you can assume the capacity is what it says on the battery (de-rated if the battery is not new) and then measure the amps and time to get the Amp hours. 25 Ah out of a 100Ah battery is 25% discharged. Now, back to our story.
Lead acid batteries do not like to be deeply discharged and they really hate to stay that way. They also hate it when you don't charge them up all the way.
Every time you take your batteries down below 50% you shorten their life measurably. A fully charged battery is a little over 12.6V. You will hit the 50% mark right around 12V. Ballpark numbers: if you discharge your battery 25% (around 12.3V) and charge it back up immediately your battery will last about twice as long as it would if you discharge it 50% (around 12V). Around 25% is a good target if you want to maximize battery life and minimize cost per battery cycle (discharge/charge cycle). Below 50% and you are hurting the battery more than the extra Amp hours are helping you.
You said you were killing your batteries overnight so let's assume you were hitting (at least) 50% discharge. You also said that you would choose one battery for the firing squad --- I mean the house bank for the entire season. This is good if your plan was to be sure you had a battery to start the outboard the next morning but not so good for the victim. It is a "good" idea as long as you are not doing it to try to "even the wear" on the batteries or something along those lines. I made the same mistake when I started. If you pulled the selected battery down below 50% multiple times over the season you may have seriously decreased it's life expectancy.
Remember, the life of the battery is directly related to how deep you discharge it. If you take it to 50% it will wear out about 2x faster than if you take it to 25%. So, if instead of using the batteries one at a time, you used your Perko switch to combine them (ALL) the exact same load would pull down your combined battery to 25% instead of 50%. Your batteries now last twice as long under the same load.
"But wait!", you say. They last twice as long but I'm using twice as many batteries so I have not gained anything.
That's sort of true, but I have not told you the rest of the story. First of all, you will get a little better than a 2x improvement but that's not the important part. The other side is the fact that lead batteries don't like to stay discharged (see above) and they don't like to be partially charged on a regular basis.
To bring a lead battery from 50% to 80% in our hypothetical 100Ah battery would require 30Ah to be replaced. Because of the peculiarities of the chemistry, the battery really prefers to be charged at around 20% of it's capacity when it's deeply discharged. It is unlikely that you have an outboard that has 20A of charge capacity (some do, most don't) and even if you did you probably don't want to run your outboard for 90 minutes to charge the battery (assuming no losses). With the right controller you might get that from your solar panel (350W) on a good day.
Another peculiarity of lead batteries is that it will take a lot longer to charge the battery from 80% to 100% than it did to charge it from 50% to 80%. Fortunately, the battery will be mostly content if you can get it to 80% on most days. This implies that our minimum target should be to get the battery to 80% on most days and 100% when possible.
Here's the trick. Instead of taking one battery down to 50% then back to 80% (or more) you take two batteries down to 75% then back to 80% (or more). It takes the same amount of power (50Ah) to get them fully charged, but it takes 30Ah to get a single battery to the minimum of 80% but it only takes 10Ah to get two batteries to the same point. Now it only takes your 20A alternator 30 minutes to get your batteries charged back to 80% instead of 90 minutes to get the single battery charged.
On the plus side: Your batteries get back to 80% faster. You never get close to the 50% "cliff" where battery life really starts dropping off.
On the minus side: You need to have another way of starting your engine if you manage to kill both batteries somehow.
And on the major plus side: If your discharge is only to 40% (20 from each battery), you will increase your battery life to around 3X to 50% rate and your batteries never go below the magic 80% point.
Unfortunately, there may be a major minus as well: If you have been regularly killing one of your house batteries, even you you've been alternating it by year, you may have decreased their life expectancy. Your comments indicated that they seem to have decreased their capacity as well. Your 100Ah house batteries may be something less, maybe a lot less. It's a vicious cycle. The over-discharged battery loses capacity so even under a lesser load it still get over-discharged and looses more capacity.
Given their relatively small Ah rating I'm going to assume that these are "Marine deep cycle" batteries. If that is the case, they have a normal life expectancy of 3-5 years. Deep cycle batteries are considered "worn out" when they can no longer deliver at least 80% of their rated capacity. Best case: you need to de-rate your battery's capacity 4Ah per year. If it's 3 years old it is probably (at best) 88Ah.
It is very late here and I'm going to quit before I make a math mistake (assuming I have not already made one).
