Chandrapal Mehula

For something that gets hauled around and used for pumping, I would put vibration and cold tolerance ahead of price. Cheap 100Ah drop-ins vary a lot in how well the cells are clamped and how robust the terminals are, so open one up if you can or at least check reviews from people who move them often. A foam or padded box helps a lot. Pumps are usually a light load, so even a modest BMS rated for around 100A continuous is fine, but make sure it has low temperature charge cutoff. LiFePO4 must not be charged below freezing, and an unheated one will protect itself by refusing charge, which looks like a dead battery. If you charge indoors after each trip you are fine. Also use a proper LiFePO4 charger set to about 14.4V, not an old lead acid one.

The easiest one I have seen work is a thermostat-controlled resistive load on the surplus, mainly a hot water tank. A small diverter, or even a simple relay that kicks in when battery voltage reaches absorb and the panels are still making power, can send the extra into an electric element, and a tank stores a surprising amount: about 1 kWh heats roughly 15 litres by 50 degrees C. Do it on the DC or low voltage side if you can, so you skip inverter losses. Pumping water up to a header tank for gravity-fed taps or irrigation is another good one since the pump only runs when there is spare power. Whatever you choose, put the load on its own fused circuit and size the wire for the full surge.

Nice build, and thanks for the detailed write-up. A few things from my experience with 4S LiFePO4 boxes of this size that might be worth a look before you close it up:

1. Short-circuit protection. A 314 Ah 4S pack can deliver well over 10,000 A into a dead short, and the BMS MOSFETs will not reliably save you from that. Many breakers and ANL or MRBF fuses are only rated to interrupt around 2 to 6 kA at DC. For the main battery lead, a Class T fuse (rated 20 kA or more) mounted as close to the positive terminal as practical is the usual answer. Size it to protect the wire, not the loads.
2. DC breakers must be DC-rated for the voltage, and polarity-sensitive ones must be wired the right way round. The 60 A breakers on the charge controller inputs are fine, but check the datasheet voltage rating against your PV array's open-circuit voltage in cold weather (Voc rises about 0.3 percent per degree C below 25 C).
3. Cell compression. EVE 314 Ah prismatic cells last noticeably longer with light, even compression (roughly 300 kgf for the pack is the common figure). A rigid fixture or end plates with a thin foam layer is better than loose foam blocks if the box will be carried around.
4. Terminal torque and bus bars. Torque the M6 terminals to the cell maker's spec (around 4 to 6 N·m) and recheck after the first few thermal cycles, since loose terminals are a common source of heat. Use clean, flat bus bars with a bit of anti-oxidant paste, particularly where aluminium meets copper.
5. Charging. Set the charge controllers to about 14.2 to 14.4 V absorption with a short absorption time, float around 13.5 V or off, and temperature compensation turned off for lithium. Don't charge below 0 C unless the BMS has a low-temperature cutoff, because lithium plating is permanent.
6. Cabling in a carried box. Secure the heavy cables so they can't pull on the lugs while you carry it, and use ferrules or properly crimped lugs only.

My position is that the fuse is the item people most often skip, so I'd put that first on your list.

My position, for what it's worth: a capacity test on LiFePO4 is worth doing once when the battery is new (to confirm the rated Ah is real) and then only occasionally, because the BMS and a shunt already give you most of the answer.

A low-tech way to do it with gear you probably have:
1. Fully charge until the charger is in float and the tail current has dropped below roughly 0.02C (the BMS or shunt should read 100%). Let it rest for an hour.
2. Zero the shunt's amp-hour counter, disconnect all other sources, and run a steady known load through the inverter. Aim for roughly C/5 to C/2 so the result is repeatable. A resistive load like a space heater is ideal.
3. Stop at the BMS or inverter low-voltage cutoff, or at about 2.9 to 3.0 V per cell if you are watching individual cells. Don't push below that.
4. The amp-hours the shunt counted is your usable capacity. Compare it with the rating (and with your own earlier test) and log the temperature, since cold cells will read noticeably low.

Two cautions. A single-point capacity number can mislead if the shunt isn't calibrated, so check its reading against a clamp meter. Also, with LiFePO4 the voltage curve is so flat that you can't judge state of charge by voltage the way you can with lead-acid. The knee at the bottom comes suddenly, and in a parallel bank the weakest or lowest-resistance battery hits the cutoff first.

For ongoing health, a practical trigger is to watch for these:
- Cell imbalance at the top of charge (more than about 50 to 100 mV spread).
- Amp-hours consumed per day that no longer match your loads.
- Hitting the low-voltage cutoff earlier than before at the same load.

If you see any of them, do a full capacity test, and test parallel batteries individually if you suspect one is lagging. A fully charged pack that still reaches 80 to 90 percent of its rated Ah after a few years is generally considered healthy.