[draft]

This is not a rah-rah session for LFP; it’s about how in my particular case lithium (Li) is a good fit. And how I use and charge the battery.

my use case

I boondock full-time in the campervan. I previously had a FLA GC2 bank that provided excellent service at a good $/kAh cost. The bank started to decline after 1000+ cycles and my needs slowly evolved.

practical effects on my boondocking

For my present uses lithium offers these benefits over lead:

  • ability to camp in areas with limited sun. No need to get 6+ hours of continuous sun (to complete Absorption), no need for strong sun at particular times (Bulk charging in the morning to reverse soft sulfation) or keep good sun to sundown.
  • ability to run loads when I want. Since Li doesn’t care about sitting at partial state of charge you can “borrow against future sun” in a way that lead chemistries do not appreciate.
  • at my sub-C uses Li should last a very long time, and may be less $/kWh than Pb.

my bank

I have 1x 100Ah LFP battery from Rebel Batteries.

By spec my usable Ah decreased from 110Ah (FLA @ 50% DoD) to 80Ah (LFP @ 80% DoD).

overnight power use

I tend to average -30Ah in the mornings, so I’d need to go into evening at 50% (30Ah used + 20Ah in reserve at the bottom). My battery monitor counts amps so 50Ah on the meter should cover me.

A 50Ah Li bank would likely meet my needs, but would also constrain charge/discharge rates. And they tend to be more $$/Ah than 100Ah.

daytime power use

I have copious solar so most of my daytime loads run off the panel

how I charge

solar

  1. bulk - C/2 (50A for 100Ah LFP) is a common max charging current to ensure longevity. I prefer the more conservative C/5 (≤20A) when practical. I have a 50A controller but in practice 15A-25A going into the bank is more common, depending on Vabs and local conditions. Overpaneled configurations finish charging so early in the day that the system is rarely running full-bore.
  2. absorption - 13.6v until ~95% state of charge. This lower voltage usually results in ≤20A charging under normal circumstances. At this voltage the cells stay in excellent balance (4-7mV delta between highest/lowest) right up to 100% SoC. Charging to 14.0v would charge faster (not necessarily desirable) and negatively affect cell balance.
  3. quasi-float - 13.4v indefinitely (standby voltage, not float in the Pb sense)
  4. quasi-equalization - 13.8v for 20 minutes to reset the BMS “full” metric. This is monthly on my SCC.

  5. absorption restart - < 13.0v
  6. low voltage disconnect - 12.4v, ~80% DoD.
  7. temperature adjustment (lead-style) - off

shore

Same as solar only 10A because of my DIY converter. Maybe slightly lower Float.

isolator

Isolator running wide open until 13.4v, cut off by voltage-sensing relay). Manually disconnect with a switch on the VSR’s ground wire if charging rate threatens to stay above 50A. The ground wire is only for powering the VSR’s own circuitry and does not carry charging current.

My reading on the Promaster forum suggests the PM alternator charges 100Ah of LFP at 20-30A. If charging rates are too high I may insteall a 20A DC-DC charger like the Renogy.

Update: Isolator charging is going unexpectedly well. I am pleased to observe the following charge rates

  • 17A when bank was sitting at 13.3v
  • 29A when bank was sitting at 13.2v
  • 32A when bank was sitting at 13.0v

heating

Battle Born sells a 15W heating mat for $220 (!)

The BMS has charging cutoff at -2C and discharge cutoff at -10C. (defaults)

I picked up a 20w heating mat to turn on when it gets cold. I had a 25w mat that used to keep an open bucket of water unfrozen down to 9F so I have hopes this will work. The mat says it’s intended to hold 10F-20F above ambient, which would mean 12F-22F in my use. I don’t intend to camp in that kind of cold although I have been caught in freezing temps a few times.

I didn’t realize until later that it came with a thermostat. The thermo is for seed sprouting purposes and is settable between 40°F /5°C - 108°F/ 42°C. Right now I have it set at 50°F.

Update: I picked up a mat that fits the battery better, and is 16w.

teardown

The cells are single-stacked vertically (see pic) so each one would be getting heat from the bottom. During the day the mat would run off the panels. Overnight it would consume 8hr x 16 / 13v = ~9.9Ah. Meaning we’d need to go into night with 69.9Ah left in the bank to stay above 20%.

Theory:

  1. there is solid evidence that LFP’s charging rate should effectively stop at freezing (“cold shut-off”)
  2. there is also solid (but varying) evidence that LFP’s charging rate should decrease at lower temps
  3. there is some evidence that LFP is happiest when protected from thermal shock (big current at low temps causing rapid internal temp rise)

… so maybe it should be set a bit warmer like 50F?

Discussions on the DIY Solar forum suggest people are using minimum temps between 35F and 50F.

further reading

I’ve collected some information in this RVwiki article on drop-in LiFepPO4. Work in progress, as always.