intro

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

My original bank was 220Ah of 6v GC flooded in series for 12v. When that died I switched to lithium.

  • v1.0 - 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 was outpeforming my old FLA bank.
  • v1.1 - 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 worked surprisingly well.
  • v2.0 - replaced the hodge-podge bank with a single SFK-275SE “275Ah”2 battery for the reasons described in this blog post. At $905 delivered it wasn’t as cheap as the value brands like LiTime, Chins, etc, but it is maintainable, has an active balancer, a BT enabled JBD-adjacent BMS, and I got to pick the cells I wanted (REPT 280Ah).

power use patterns

overnight

In summer tend to average -40Ah by morning

In winter longer nights mean more hours of lighting and colder temps mean running contact heat like the electric mattress pad. so maybe -70Ah in the mornings.

In freezing weather another ~10Ah will be needed for battery warming (see below).

In practice, it is a rare day when I do not hit my charging targets for the day.

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 0.05C3, or until 30 minutes elapses (average is 22 minutes). This results in a high-90s state of charge.

float stage

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

13.32v 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 was pleased to see normal current acceptance ~0.2C, with occasional forays to 0.32C. No need for DC-DC at this point. 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 15A (DIY converter limited to 15A).

heating

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

I picked up a 20w heating mat with a thermostat that keeps the battery at a toasty 50F in cold weather.

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.

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 battery is sold as 275Ah, is made from 280Ah cells, and tested at 295Ah. Why do they call it 275? Dunno, but I suspect they are trying to keep the lowest-cst battery from cannibalizing sales of the more expensive 304Ah versions. ↩

  3. the actual number is 0.05C + 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. ↩