A couple of weeks ago I read a particularly interesting article over at Pambo about the risk of trashing lithium batteries over a winter when the boat is stored in a cold location.
I highly recommend reading the original article in it’s entirety, but the key things I took away were:
- LiFePO4 batteries do self discharge over time. 2-3% a month is a good estimate.
- Parasitic loads that remain connected to the batteries can dramatically increase that discharge rate to the point that the batteries, if not regularly recharged, can be toast in just a month or two.
- Most lithium batteries will be damaged if charged with the temperature below about 5C (41F), not very cold, making things doubly difficult to manage while the boat is stored in a cold place.
- However, lithium batteries can be stored at much colder temperatures without damage, for example Victron Smart Lithium can be stored down to -45C (-49F), which should work in most places.
Clearly the problem here, at least over say a six month period, is not self discharge. For example, the battery could be charged to say 80% in the fall and six months later would be still a bit over 60%, assuming 3% (worst case).
Rather the real culprit is parasitic loads from the BMS as well as any connected monitoring systems, which can flatten the battery to destruction in a month or so.
John: I agree entirely with the suggestion to consider disconnecting everything from a lithium bank. Several winters ago I disconnected the load bus from the lithium bank but I left connected an external BMS and some other components that were directly wired to the bank. I set a recurring calendar appointment to remember to check on the state of charge monthly. You guessed it, I missed one check and after 60 days the cells were trashed. The parasitic loads included a WiFi module for broadcasting info from the BMS, the BMS itself, and the microcontroller in a charge controller that was separate from the BMS (plus god knows whatever else i missed). Those loads killed the bank. Great lesson. Luckily it was not too expensive a blunder because the bank had been assembled from bare cells and all the cost+labour of the battery box, contactors, BMS, wiring, fusing, backup lead acid (your system for this is fab!) was preserved. That said, I am not at all sure about the upside in leaving the batteries for six months, starting at 80% SOC. Somewhere I have read (Eric Bretscher perhaps) never to store for the long term above 50% SOC. Even a 20% self discharge over 6 months would leave your batteries at a very healthy 30% in the Spring. I have done this 3 winters and my bank still shows the 99% State of Health with which they came, new, and the same internal resistances as when new. So it has worked fine for me in Toronto Canada winters.