Lithium (LiFePO4) Batteries, Part 1
04 May 2017
Jerry
I've been very interested in lithium batteries for years. When Vida Dulce was struck by lightning last year it created the opportunity to look more closely into them since the lightning, although causing no obvious damage, will reduce the life of our existing batteries.
This is the first of a multi-post series on my rational for moving some or all of the house battery bank to lithium, and my experience in doing so.
Part 1: Why lithium?
Let's look at some numbers.
We currently have 5 batteries (AGM) for a total of 1050 Amp hours of capacity. That's how much energy they can store. But the usable capacity is quite a bit less. You can only really use 30% because If you discharge the batteries to less than 50% of their capacity you damage them, and on the charging side, once they get above 80% of their full capacity their charge acceptance rate drops. To charge above the 80% of capacity you have to run the generator for a long time because the batteries won't let many amps come in to recharge them.
Lithium batteries are different. They will essentially take all the current that you can throw at them. However, if you over charge them or excessively discharge them you will destroy them. I'm starting with a fairly conservative approach and will use 70% of their capacity, between 15% and 85% of full charge.
This has important consequences. My current batteries (AGMs) have a useable capacity of about 315 Amp hours. I plan to replace that 1050 Amp hour bank with a 600 Amp hour lithium bank (6 x 100 Amp hour batteries) which will give me (conservatively) over 400 Amp hours of useable capacity.
Looks pretty good, and it gets better when you consider weight. The AGMs weigh 125 lbs each for a total of 625 lbs. The Lithiums weigh 28 lbs each for a total of 168 lbs. I can reduce the weight of the batteries on the boat by over 450 lbs. That's really good for many reasons.
A third factor is the cost of the batteries. The AGMs retail for $795 each for a total of $3975. The Lithiums retail for $625 each for a total of $3750. On the surface, it looks only marginally better however when you add in the lifetimes of the batteries, the numbers dramatically favor lithiums. Lead acid based batteries generally have a lifetime of 500 charge-discharge cycles. Lithium's have discharge cycle ratings above 1500. So the cost per recharge cycle for each battery bank is:
Lithiums $2.50
AGMs $7.95
Usable capacity, weight, cost and the expected lifetimes all favor the lithiums.
But there is a dark side.
Lithiums do burst into flames. You know that from all the cell phone horror stores you've heard. However, there are different lithium batteries chemistries. The ones I'm using are lithium ferrous phosphate (LiFePO4). They don't have the energy density that other lithium ion batteries have but they are far safer. They don't burst into flames. Their energy density is far better than lead acid batteries so I'm happy with that.
I alluded to the issue that if you are not careful managing your lithium batteries (charging or discharging) you can destroy them. They are not as forgiving as lead acid batteries. For this reason you need a "battery management system". And you need to look closely at the chargers that are currently installed and how compatible they are with lithium batteries (mostly they are not).
I'll address these issues in more detail in future blog entries, but for now a preview of my transition and testing strategy. I've installed one 100 Amp hour lithium (LiFePO4) battery that I'm using to power our heaviest load, our freezer. I've limited it's charger to a max of 80 Amps. When the charger goes on that battery sucks in those 80 amps all the way up to the 85% (my conservative approach upper-limit). As a comparison, an hour later the AGMs are still not fully charged (80%).