Updated on January 13, 2016
Electricity Part 2 – A bucket for your energy
Fancy switch: check. Shelf for battery: check. New wires: check.
Batteries: kind of… check?
In my last post, I talked about cleaning up a mess of acid and fixing up the storage area for our batteries. As I mentioned in that post, for our first year on Milou we used simple flooded lead-acid batteries. These are pretty good, but there are much better technologies available today when it comes to storing energy.
My concern at the time was that on the boat my daughter will be sleeping directly above the batteries. Traditional batteries are flooded with battery acid–pretty mean stuff. The acid is fine as long as it stays in the battery, but if the boat tips to much, acid will leak out under my kid’s bed. If the batteries get too hot; or over-charge; or if a lead plate on the inside breaks, causing an internal short, then the battery will vent acid in the form of hydrogen sulfide gas–a corrosive, flammable, toxic gas.
Because I love my daughter and the rest of my family, I wanted to find a better solution. Of course, better always exists, it just usually costs more. To replace the old shorted-out batteries with the same technology–two group 4D lead-acid batteries (4D is the size, they are big)–would cost about $700 for 400 amp hours.
Before we go any further down this road, let me explain amp hours. I like to use the water analogy for electricity. Amperage is analogous to flow–that is, how much water is going through the pipe. Voltage is analogous to pressure–that is, how much push the water has in the pipe. A battery is like a bucket for electricity. An amp hour is a way to measure how much water, or electricity, a battery can hold. A 400-amp hour battery can give 20 amps for 20 hours. For comparison, the biggest energy hog we will have will be our new, custom, super-efficient refrigerator (yes, that is another story, still not done with the fridge install). It will draw one to three-and-a-half amps. According to our friend and boat guru, Eric, 400 amp hours is what you want. In lead-acid, you don’t really get all 400 amp hours. Lead-acid batteries do not like to be discharged more then 50% or they get hurt; ideally, they would only be discharged to 70-75%, which means that a 400-amp hour lead-acid battery is really only good for about 100 usable amp hours.
So I wanted a safer battery that would not spill acid or vent poison gas, and that would also give us a lot of amp hours. There are two other traditional options: gel cell, and absorbed glass mat or AGM. Both are sealed batteries that still use lead and acid, but both are better at containing the acid. In a gel cell battery 400 amp hours costs about $1400, twice the cost of the old-school battery. In AGM, 400 amp hours is closer to $1700, but they are a little better–they last longer and can take a charge better.
Scope creep.
I thought, “Doesn’t Tesla make those awesome electric sports cars with lithium batteries, can a guy get those?” (And just for future reference, when your train of thought begins, “Doesn’t Tesla…” that’s a sign that things are going to get expensive.)
Then I ran into this beast of a Cruisers Forum thread. I tried to read the whole thing but I could not stay awake through all 4000+ posts. What I gained was there are people out there using Lithium Iron Phosphate batteries (LiFePo) in boats, and a lot more using them in motorcycles and motorhomes.
After some internet searching I found a company called Balqon (insert joke here). I went ahead and ordered 400 amp hours of LiFePo house batteries and a 12-volt LiFePo starting battery for around $1900.00. They fit in the space of the old lead-acid with room to spare and they weigh half as much, maybe less. The reason it took a year for us to use the new batteries is that when you order Chinese-manufactured batteries from the lowest bidder, it takes a year (ok, only 9 months) for them to get to your front door.
LiFePo is not a perfect drop-in replacement for lead-acid. Each cell has a voltage range of 2.6v to 3.8v. To get to a nominal 12v you have to string four cells together in series for a total of 10.4 to 15.4 volts. This matches up pretty closely with the lead-acid voltage range
of 10 to 14.5. The way a LiFePo battery accepts charge is also a little different. They are very good at sucking up electricity and equally good at pouring it out. Lead-acid batteries are not so eager to accept electrons, so engineers have figured out all sorts of fancy ways to charge them–things like temperature-compensated four-stage charging. With the LiFePo battery, I have to trick the various charging systems into simple one-stage, constant-voltage charging.
One nice effect of the different chemistry of the LiFePo is that their liberal-with-electrons nature means you get more usable amp hours from a given battery. Our new battery gives us 200 usable amp hours, twice what we had in the original battery bank.
This battery upgrade is actually more expensive that it looks on the surface. We needed to install a configurable alternator-to-battery charger that wouldn’t be necessary with old-school lead-acid batteries, and we sized up our alternator to make charging off the engine more efficient with LiFePo. But the trade off–doubling our useable amp hours, while saving weight and space, and making everything more efficient–seemed worth it to us. If we were starting out on a boat with a functional lead-acid battery set-up, we might have hesitated more; but designing a system from the ground up gave us more options.
Next up, the eagerly-anticipated Electricity Part 3: Managing and Filling the Bucket; or, How Does the Electricity Get In There?
Definitely the future, we’ll all be telling our grandkids about heavy lead-acid batteries. Now you need a Torqeedo outboard for your dinghy, and to replace the diesel aux with an electric motor, and…no, just go cruising.