What does a 100A BMS mean for a 48V pack?

A 100A BMS on a 48V pack allows 100A continuous = ~4.8kW. That is the safe sustained discharge. Peak is usually double (200A) for a few seconds. If your inverter is 5kW you are at the edge — choose 125A continuous to leave headroom, or the BMS trips during surge loads like a well pump.

Active vs passive balancing — which do I need?

Passive (50-100mA bleed) is enough if cells are grade-A and the pack cycles gently. Active (1-2A transfer) recovers faster when you parallel dissimilar banks or deep-cycle daily. For a home bank that sits most of the day, passive is fine; for daily heavy cyclic or multi-bank parallel, active pays off.

Why does my inverter show wrong SOC with a new BMS?

Wrong communication protocol. The inverter reads SOC from the BMS over CAN/RS485; if the profile is wrong (e.g. a Pylontech-profile battery on a Victron expecting a different frame) the inverter guesses and cuts at 30%. Set the inverter BMS protocol to match the battery — for most Chinese 48V packs that is 'Pylontech'.

How to Size a BMS for a LiFePO4 Pack

A BMS is not a fuse you bolt on at the end. It sets how much current the pack can ever deliver, what voltage the inverter sees, and whether your cells stay balanced. Size it wrong and the inverter trips on every motor start, or the top cell cooks while the BMS sleeps.

1. Cell count first

LiFePO4 is 3.2V nominal per cell. 12V = 4S, 24V = 8S, 48V = 16S. The BMS must have exactly that many series channels. A 15S BMS on a 16S pack never balances the top cell — it drifts up every cycle until the BMS cuts on over-voltage. We build 48V packs as 16S; the BMS channels must match.

2. Rate for surge, not just continuous

Continuous current is your normal draw. Surge — pump motor, compressor, power tools — is 3-4× for a few seconds. Size:

5 kW inverter (48V) → 100A continuous / 200A peak BMS (use 125A for headroom)
10 kW → 200A continuous / 400A peak
15 kW → 300A continuous / 500A peak

The failure we see most: a 100A BMS on a 5kW inverter that trips every time the well pump starts, because the pump surge hits 180A. Bump to 125A continuous and it stops.

3. Balancing current decides recovery speed

Passive balancing bleeds 50-100mA from the high cell — fine for grade-A matched cells that barely drift. Active balancing shuttles 1-2A between cells and pulls a drifted bank back in one cycle. If you parallel multiple banks or deep-cycle daily, active above 1A saves you.

4. Protocol must match the inverter

The BMS reports SOC and limits to the inverter over CAN or RS485. Most Chinese 48V packs speak the Pylontech CAN profile — native on Deye, Growatt, Luxpower. Set the inverter's BMS protocol to match, or it reads SOC wrong and disconnects at 30% with a full pack sitting there.

5. Thresholds must agree with charge voltage

For 16S LiFePO4: over-voltage cut 3.65V/cell (58.4V), under-voltage cut 2.5V/cell (40V), absorption 56.0V. The inverter's charge profile and the BMS cut must line up. When they fight, the inverter drops the load mid-day and you lose the sun.

Not sure which BMS your pack needs?

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