[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. [Update: I did the math and the breakeven appears to be 2,462 cycles, about 6.75 years. Check back on Saturday, July 8, 2028. :-) ]

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). But in reality the 100AH LiFePO4 is outpeforming my old FLA bank.

Chins sidecar

In Feb 2023 I was given a 50Ah Chins LFP when the previous owner upgraded to the 100Ah version.1 I paralleled it with the original 100Ah and it seems to do fine. It does mean that at my max 0.4C comfort rate for charging the bank can take 60A instead of 40A.

The only anomaly I observe is that under heavy loads/charging the 100Ah moves a few percent more current. For example, if the shunt shows combined charging at 30A I might expect the 100Ah to be accepting 20A and the 50Ah 10Ah. But the big one’s BMS reports 21A which leaves 9A for the smaller one.2 In relatively calm periods the big batt’s draw approaches parity (2/3 + 1/3).

power use patterns

overnight

In summer tend to average -30Ah in the mornings, so I’d need to go into evening at with ≥50Ah in the bank (30Ah used + 20Ah in reserve at the bottom).

In winter longer nights mean more hours of lighting and colder temps mean running contact heat like the electric mattress pad. so maybe -55Ah in the mornings. This means I need to go into evening with ≥75Ah in the bank% (55Ah used + 20Ah in reserve at the bottom).

In freezing weather another ~10Ah will be needed for battery warming (see below). So I need to hit sundown with 85Ah.

In practice, it is a rare day when I do not got into sundown at >95% State of Charge (95Ah on the original bank, 142.5Ah on the combined bank).

daytime power use

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

how I charge

solar

Voltages are taken from the battery with Victron’s Smart Battery Sense. It’s annoying that it’s a $40 BT device rather than a $5 wire, but that’s how Victron does things. Accurate voltage-sensing is a major upgrade in my system.

bulk stage

C/2 (50A for 100Ah LFP) is a common max charging current to ensure longevity. I prefer the more conservative 0.2C (≤20A) when possible and <0.4C as a normal max. I have a 45A 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.

absorption stage

13.8v until trailing amps fall to 0.10C3, or until 30 minutes elapses (average is 22 minutes). This results in a high-90s 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.

float stage

13.4v indefinitely (standby voltage, not float in the Pb sense)

13.35v in summer when bank temps are slightly higher.

EQ

I have EQ defined as 14.0v for 20 minutes in case I want to reset the amp counter, etc.

rebulk

Bulk restarts <13.2v

misc setpoings

  1. temperature adjustment (lead-style) - off
  2. low temp cutoff - 34F, although the battery warmer will likely mean this won’t happen.

isolator

I camp in place most of the time so the Battery Doctor isolator (VSR type) doesn’t see much action. But when it does it works fine. Yes, isolators can charge lithium.

The Promaster’s OEM 180A alternator puts out ~14.2v, which is acceptable for LiFePO4 charging. My reading on the Promaster forum suggested the PM alternator charges 100Ah of LFP at 20-30A. That sounded about where I wanted the current to be so I gave it a shot before investing in a DC-DC charger like the Renogy 20A.

I used the battery’s bluetooth app to watch charging current as I drove and it was well in my comfort zone. I observed 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

The isolator was already was on an ON/OFF switch from the original FLA bank so I can disable it at will.4 It is easily accessible from the driving position without looking. I turn if off at ≥13.7v or when combined solar+alternator charging is >0.4C.

shore

13.4v at 10A (DIY converter limited to 10A).

heating

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

The Rebel’s JBD 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. In winter I keep it set to ~60F. In summer I set it to 50F to help reduced average cell temps.

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 under worst case conditions it might consume 8hr x 16 / 13v = ~9.9Ah. Meaning we’d need to go into night with 59.9Ah left in the bank to stay above 20%. In practice, even in near-freezing conditions the pad runs so little it has no real impact on battery Ah.

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 I set it on the warm side, 40F-50F.

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

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

Update Dec 2021: water frozen in the dogbowl this morning and warming mat held batt temp steady.

further reading

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

  1. they didn’t have physical room do 150Ah in their space ↩

  2. The Chins has not BT so I can’t see what its BMS says. ↩

  3. the actual number is 0.10C + normal background loads, since I don’t have a smart shunt or similar to tell the controller about net charging. ↩

  4. The Battery Doctor has heavy +IN and +OUT terminals and a slender ground wire. The ground is so there so the isolator can run its own electronics and solenoid; it is not carrying charging current. ↩