The Battery Debate (AGM, LA, LIFEPO4) Post 1


Sigh… YES we are still in St. Anne, Martinique, with no plans on leaving soon.  Although some of our boat projects have kept us here, that’s not the whole story. Frankly we really like it here and have been falling back into our cruising groove. We really have no place to be and none of us wants to leave. So Saint Anne, Martinique it is for a while longer.


Now back to those boat jobs and the title of this post. Batteries!


CAUTION…. Serious technical talk ahead. Like really serious somewhat boring technical details. Its going to be long and cover multiple posts! Since I know this won’t be for everyone may I suggest a trip over to for the people not interested in technical things. If you go there and don’t say AWE once, you are dead inside.


PLEASE – Do not bring up the safety issues about Lithium batteries. What I call lithium is actually LifePO4. There are TONS or resources on the internet showing how these batteries are different and inherently safe. I am absolutely %100 convinced this technology is safe for any house battery bank.


Ok… If you are still with me then lets get into it. 2 years ago when I was refitting Party of Five one of the top things on my list was the battery bank. The batteries that came with the boat were of poor quality and had been brutally punished during the years chartering in south Cuba. A complete replacement was in order. Upon researching I knew I had 3 choices. I could put in tried and true lead acid deep cycle (100 year old technology), or I could go with more dense, but WAY more money AGMs, or I could go super exotic and install bleeding edge lithium batteries (technically LifePo4).  Although I SO wanted to install a lithium bank it just was not feasible with all the other work I had. You see a lithium battery bank would have required me to design the entire system (including a bunch of electronics to manage the batteries). Really, in our price range the only viable option would have involved me actually designing a circuit board and having it manufactured in China (most likely). Just not possible on the time I had.. Hell, I needed to sleep sometime. In the end I choose the 100 year old lead acid technology which I could buy at Costco for only $600USD. Cheap and cheerful!


Now fast forward 2 years and I’m contemplating the battery replacement situation again. Although that lead acid bank I installed is still functioning fine its beginning to show the abuse of almost 2 years worth of full time cycling (not to mention the lightening strike as the batteries were the only electrical device that still survives). We aren’t exactly easy on our bank as we run a fridge, freezer, 2 laptops and all the kids gear. This equates to about 180-200AH of power draw per day (our bank is 660AH capacity, although a lead acid bank should NEVER go below %50). Since we NEVER visit marinas our only charging sources are solar and wind. This means the batteries rarely get back to %100, which is a true killer of lead acid batteries. We have now begun to run our generator about 2 hours every 2 days (or 1 hour a night). Every time I start the Honda, I secretly wonder when our bank is going to “fall off the cliff” and I will wake up with the batteries at 11V (bad bad bad).


That brings me to the meat of our issue. I can’t really control or predict when that failure will happen (hell I may get 16 more months with our current regime). So do I head it off at the pass and pull the trigger on a new bank early in a place where I can do it. or wait it out and deal with it later? and what are my options 2 years on! Lets first look at the options, then deal with the question of “Should I”.


I had written literally paragraphs about the options, but ended up deleting them and replacing all that with this simple table. It’s a very complex thing and I wanted to simplify it as much as possible.

Technology Price Weight Cycle Life Usable AH (DOD) Recharge efficiency
Lead Acid $1200-$1500 528lbs 600-800 330 (%50) %75-%85
AGM $1800-$4000 440lbs 800-1100 308 (%70) %80-%85
LifePO4 $3000 (ish) 121lb s 1800-2000 320 (80%) %98



Now you money grubbing types will look at the above options and immediately pick “Lead Acid”. Stick in comparable batteries, get 2 more years (maybe more as you know how to care for them better now), and be done with it. Bam, clean hands, ice sore bum for paying double (US prices), done!

Of course this was my initial thought too, but I eventually realized that path really ignores many other important data points. If we just do the simple math of cycle life per dollar we can see the lithium batteries WIN big time. Of course that is IF I get the rated cycles. While its not a guarantee since it’s a very new technology, the current “internet” wisdom says 1800 is quite achievable. Now the astute among you will also point out another thing. That cycle life only really matters if we plan to keep Party of Five for 6 more years (who knows what the future brings).


How about the other numbers. Of course the obvious standout is weight. Switching to lithium would drop over 400lb off the boat…. SERIOUSLY 400lbs. You have no idea how much that is for a small catamaran like ours. It will make a massive difference to our sailing performance.  Hell that might even make up for my shitty sail trimming techniques! If not I will lie and say something else if wrong Winking smile


The other important number is recharge efficiency. This number is a little hard to explain and quite gray (the percentages are argued), but I will try. Traditional lead acid batteries suffer from a phenomenon where their ability  absorb energy is not linear. In other words when they are discharged to %50 they accept much more energy than when they are %90. This makes that last %10 take a very long time to push into the battery. Many days we don’t get that last %10 into our bank so we really only live with %40 of our bank being usable most of the time. Conventional wisdom also says this causes a shortening of life on the batteries (reduces the cycle life). Since AGM is still a lead acid technology it also suffers from this phenomenon, but not to the same degree. On the other hand lithium does not have this issue and can accept EVERYTHING you can pour into them at all stages.  Cool, but what does this actually mean in the real world. Well with our current lead acid bank we are %90 recharged by about 11:00am (on sunny days). That happens to be when the sun is strongest and we are throwing away TONS of power since our battery just can’t accept it. Of course we don’t really waste all of it, as that is the time we plug in all our gear and let the solar controllers deal with the extra draw (if I’m paying attention). However, this is a big problem if we choose to run the generator in the evening (maybe watch a movie). Since our bank is close to full the generator runs but puts almost nothing into the bank as its close to %90 full already. A big waste.


Now lets talk about the final number. It may not be completely obvious as I tacked it onto the AH number. That number is “depth of discharge” (DOD). This is a hard number to wrap your head around coming from our modern world. Believe it or not but battery manufacturers ALWAYS have this number on their products. Its almost never seen by mere mortals since all those fancy electronics in your devices mask it. When your Iphone or Android device says its battery is dead, its not really COMPLTELY dead, its just hit that DOD number and the “Battery Management” (BMS) circuitry has cut it off.  No battery technology can survive very long if you pull it to %100 dead (well maybe NiCD or NiMH, but they are not suitable for house banks). Now why is this number important? Well lets look at it simply. How much of that battery you just purchased can you REALLY use. Sure, a lead acid bank only costs $1200 but I can only use %50 (at best) of its capacity.  That means I have to buy WAY more battery than I actually need to cover our needs. I only need 2/3 as much lithium capacity to equal lead acid. Less capacity usually means less components and less chance of failure.


You have probably read the above and thought, huh, sounds like lithium is the way to go, or maybe you didn’t. So for the ones still on the fence I will throw out a  few more upsides to lithium.

– No physical maintenance. I need to add distilled water to our current batteries once every 3 weeks (a huge PIA).

– No voltage sag. Lithium batteries have a very linear voltage curve. The voltage remains constant no matter how discharged the battery is. This mean things like winches, windlass, bilge pumps and fridges work MUCH better.

– No need to take the bank back to %100 EVER. Lithiums cycle life is not shortened by only charging to %70, %80, %90 or any other percent with the the range.

– No off gassing. Our current lead bank produces a variety off gasses during recharge (hydrogen is primary).  Lithium produces nothing.

– No temperature compensation. Lead acid and AGM do not charge as well as the temperature rises and thus you need to “push harder”. This requires special settings in your charger and even “temperature probes” to get it right.  Lithium does not have this issue.


I can hear you now.. OK DUMMY, JUST ORDER THE LITHIUM BATTERIES….. However I have withheld a VERY key piece of information. Remember up above where I mentioned Iphones and Android devices have a battery management systems. That system is the only reason that those devices can use these batteries and get a long battery life (in cycles). Lithium batteries are VERY sensitive to overcharging or undercharging (it kills them quick) and such a system is required to ensure their life. Unfortunately there is no Apple or Google that is making such a system for marine/rv house banks in our price range. That means Travis would need to design, build and test such a system. A huge job fraught with pitfalls, but right up my alley.


However… That is a story for the next post. Stay tuned, where I detail what that system would look like and if its even possible to build it down here (an important consideration).


P.S. Please don’t comment about Tesla…. I have inquired, their products are not supported in a marine environment. Nor are they made in a size or configuration that would fit in a boat.

  10 comments for “The Battery Debate (AGM, LA, LIFEPO4) Post 1

Leave a Reply

Your email address will not be published. Required fields are marked *