must… resist… buying an SFK 275SE battery

my existing plan

My original plan was to run the existing 150Ah bank until 2028 when I start receiving early Social Security. SS sounds like a pittance but my monthly income will roughly double, allowing me breathing room, funds for some upgrades, and building up the emergency fund.

necessities and niceties

I do not need a bigger bank. I do want a bigger bank for a couple pracitcal reasons, explained below. 1

capacity smoothing

Most of the year my solar setup has the bank fully charged and quasi-floating by 11am or so. At the moment I’m not always getting it done that early because

  1. I am in the shade of the tree canopy to escape the heat; and
  2. monsoon season has begun in earnest. I harvested 15 gallons of rainwater yesterday, limited by the fact I only had 3x 5gal homer buckets.

Even in these conditions the setup is floating half the time:2

last few days

There were two days in the last month when I didn’t finish Absorption another two days where I almost didn’t. So about on 13% of the past 30 days I would have benefitted from carryover capacity.

This is not precisely the same concept as days of autonomy, which generally assumes there will be no charging for those days.

alternator charging

With both 100Ah and the enlarged 150Ah LFP bank I observe

  • 0.2C charging (30A for the combined bank) in the middle 80% of SoC
  • 0.32C max at low SoC
  • and 0.1C at high SoC

… which is in the ballpark with data that other direct-charging vandwellers report. These patterns wouldn’t change but the bigger bank (~300Ah) means more charge current from my underutilized alternator:

  • ~55A charging in the middle 80% of SoC
  • ~88A max at low SoC
  • and ~28A at high SoC

my weakness in the face of SFK’s offerings

I’ve been doing some assessment of my financial plans and I poked around a bit to see what was out there in the ~300Ah range. Damnit, Sun Fun Kits has a pre-built that has everything I want and nothing I do not want. And it’s on freaking sale. With free shipping.

what I want

  • ~300Ah of LFP. The cells in the batt above are rated 275Ah but consistently test ~295Ah.
  • maintainable (serviceable case and reasonable design)
  • active balancer - balance hasn’t been an problem because I charge at moderate voltages, but I’d like to have it on cells this big.
  • no internal heating
  • BMS sized to my needs. I don’t need a 300A BMS on a 300Ah bank. My heaviest loads are ~60A.
  • info on what the components are: the cells are 280Ah REPT. The BMS is a JBD-like BMS (made by the people who make JBD). The case is proprietary and the mountings are 3d printed by SFK. The only unknown is who makes the capacitive active balancer.

what I don’t want

Anything but the above. SFK let’s you pick features at additional cost, which IMO is perfect. Not custom, exactly, but buildable to order within a set of sane constraints.

Must… resist….

update July 2nd

Before I wrote this blog post I had emailed both SFK and a local UPS store to see if SFK would ship to an address other than the billing address, and if the shop would accept packages from travelers.

Yes, and yes (for $5/day holding fee). This made the temptation stronger. The kicker is that, quite unusual for me, I actually have the funds to buy it now if required.


When I had a SLA bank I guarded every Wh. When I went to LFP I was aware of every Wh but did not have to worry about each of them to keep the bank healthy. Moving to ~300Ah would be license, effectively, to do whatever the hell I want whenever I want, power-wise.3

So my reframe is this: starting last night and for the next week I’m going to consume as much energy as I want to see how the bank handles it. Hopefully it will survive without going <20% SoC. If so, I have all the benefits and the $1,000 still in my pocket. If it goes <20% I’ll have to ponder more. If it hits actual shutoff at 0% then the bigger bank would seem to what the doctor ordered.

We shall see.

update July 3rd

This is a bit nervewracking. 32% SoC this morning, and recovered to about 77% this afternoon. Going into sundown at 69%, which should put me at ~20% in the morning. Tomorrow is supposed to be sunny tomorrow and at least partly sunny the rest of the week. So the bank will recover.

  • main array: 1.72kWh
  • portable array: 0.51kWh

update July 4th

31% SoC this morning. Full sun so finished absorption by 1315. Going into sundown at 63.1%; did a lot of cooking.

  • main array: 1.91kWh
  • portable array: 0.9kWh <- w00t!

update July 5th

31% this morning. Unexpected partial clouds, did not finish Bulk. Going into sundown at 82.7%, highest yet.

  • main array: 1.85kWh
  • portable array: 0.72kWh

update July 6th

~40% this morning. More unexpected partial clouds. Less input from portables because I moved them in preparation for my upcoming trip to town. Finished Abs by 1145.

  • main array: 1.83kWh
  • portable array: 0.77kWh

Going into sundown with 77.3% SoC.

update July 7th

40% SoC this morning. Did not complete Absorption. Going into sundown at 84%.

  • main array: 2.06kWh
  • portable array: 0.51kWh

update July 8th

~50% this morning. Drove into town so alternator did the heavy lifting. 84% at sundown.

update July 9th

~50% in the morning. Moved the van more into the shade. 73% at 10pm.

  • main array: 1.35kWh
  • portable array: 0.72kWh

update July 10th

Final update on this post:

50% in the morning. In this position the mounted solar sees clear sun from 8am to 10am, then partially shaded thereafter. 77% at sunset.

  • main array: 1.3kWh
  • portable array: 0.7kWh
  1. unless I’m rationalizing. I don’t think I am. 

  2. the top segment (dark blue) is float. Bottom segment is Bulk and the think slice in betewen is Absorption. 

  3. luckily, what I want is fairly modest.