Passives for a Battery Management System: The Shunt, the Divider and the Isolation Barrier

Walsin Technology’s 10,428-part thick film low-ohm current sense line stops at 5 milliohm, and that floor is held by exactly two part numbers — both in the 0815 footprint, the rarest case size in the whole resistor catalogue. Every one of the 58 part numbers at 1 milliohm or below is metal foil, not thick film. For a battery management system shunt, the film technology is chosen for you by the resistance, not by preference.

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Venatech Limited, an authorised Walsin distributor in Hong Kong, holds Walsin Technology (Walsin) full chip resistor, thin film and MLCC parametric exports plus 26 MLCC and 2 chip resistor datasheets. This post is a selection checklist for the three places a battery management system (BMS) design gets decided: the current shunt, the cell and pack voltage divider, and the ceramic capacitors around the isolation barrier. Every part number below is real, and every count is a count of part numbers, not pieces.

What resistance does a BMS shunt actually need, and does Walsin reach it?

Yes, down to 0.5 milliohm, but only in metal foil. A pack carrying 100 A through a 1 milliohm shunt drops 100 mV and dissipates 10 W, so most BMS designs land between 0.2 and 2 milliohm. Walsin’s catalogue floor is 0.5 milliohm, on six part numbers.

Resistance band	Thick film parts	Metal foil parts	Match / mismatch
0.5 milliohm and below	0	6	Metal foil only — no thick film option exists
1 milliohm and below	0	58	Metal foil only
2 milliohm and below	0	114	Metal foil only
5 milliohm and below	2	317	Thick film appears, at its floor
10 milliohm and below	10	436	Thick film ladder begins in earnest

The two thick film parts at the 5 milliohm floor are WW15WR005FTL and WW15WR005JTL, both 0815, both 1 W. The next rung up in thick film is 10 milliohm — there is no 6, 7, 8 or 9 milliohm part anywhere in the 10,428-part line.

Why is there no thick film option below 5 milliohm?

Because the thick film low-ohm series simply does not publish one. This is worth stating plainly rather than inferring, because a parametric search that returns nothing reads the same as a search typed wrong. We measured the resistance column across all 10,428 thick film low-ohm rows: the minimum value is 0.005 Ω and the distinct values below 20 milliohm are 5, 10, 11, 12, 13, 14, 15, 16 and 18 milliohm. Nine rungs, and the first one is the floor.

The 0815 footprint that holds that floor has four part numbers in the entire catalogue — WW15WR005FTL, WW15WR005JTL, WW15WR010FTL and WW15WR010JTL — which is a point we have published before as the catalogue’s rarest case size. What is new here is that two of those four are the only thick film parts at 5 milliohm. The rarest footprint in the catalogue is also the sole holder of the line’s resistance floor.

For the broader comparison of the two film technologies at low ohms, see low-ohm current sense: thick film vs metal.

What TCR can I get at 1 milliohm and below?

≦±70 ppm/°C at best, and only at 1206 and 2512. This is the number that decides whether a coulomb counter holds its calibration across a pack’s temperature range, so it is worth reading the ladder rather than the headline.

Part number	Size	Resistance	Tolerance	TCR	Rated power	Match / mismatch
WW25AR0L5FTL	2512	0.5 milliohm	±1 %	≦±70	3 W	Match — lowest value, best TCR, highest power
WW25YR0L5FTL	1225	0.5 milliohm	±1 %	≦±100	2 W	Mismatch on TCR — 100 ppm at the same resistance
WW12YR0L5FTL	0612	0.5 milliohm	±1 %	≦±150	1 W	Mismatch on TCR — 150 ppm, the worst in this band
WW25RR001FTL	2512	1 milliohm	±1 %	≦±75	2 W	Match — 75 ppm at 1 milliohm
WW12RR001FTL	1206	1 milliohm	±1 %	≦±70	1 W	Match — 70 ppm in a 1206 footprint
WW12RR001FTLV	1206	1 milliohm	±1 %	≦±100	1 W	Mismatch on TCR — same body and value, 100 ppm

Two observations a selection tool will not surface. First, the TCR at 0.5 milliohm is a function of footprint, not of value: 2512 gets 70 ppm, 1225 gets 100 ppm and 0612 gets 150 ppm, at identical resistance and tolerance. Second, WW12RR001FTL and WW12RR001FTLV are the same resistance, tolerance, power and case size, and differ only in the packaging suffix — and the TCR moves from 70 to 100 ppm across that suffix. A packaging letter is carrying an electrical specification.

The metal foil line as a whole offers five TCR grades: ≦±50 on 612 parts, ≦±70 on 438, ≦±100 on 81, ≦±75 on 56 and ≦±150 on one. The tightest grade, 50 ppm, does not appear at or below 1 milliohm at all — it starts at 5 milliohm.

Which resistors suit the cell voltage divider?

A cell or pack voltage divider is a ratio problem, so tolerance and TCR tracking matter more than absolute value. Walsin’s two precision lines answer it differently, and the difference is worth seeing at a single resistance.

Part number	Line	Size	Resistance	Tolerance	TCR	Match / mismatch
WF06B1002TTL	Thin film precision WF	0603	10 kΩ	±0.01 %	≦±3	Match — tightest pair available
WF06Z1002TTL	Thin film precision WF	0603	10 kΩ	±0.01 %	≦±5	Match — one grade looser
WF12J1002BTL	High precision thick film TC25	1206	10 kΩ	±0.1 %	≦±25	Mismatch on tolerance — ±0.1 % is the tightest thick film grade
WF12K1002CTL	High precision thick film TC50	1206	10 kΩ	±0.25 %	≦±50	Mismatch on both — looser tolerance and TCR
WF25H1002BTL	High precision thick film TC100	2512	10 kΩ	±0.1 %	≦±100	Mismatch on TCR — 100 ppm is all 2512 offers

The thin film precision WF series publishes ±0.01 % on every row in its 0402 to 2512 exports, so tolerance is not a selectable field inside those files — the TCR grade is what you choose. At 0603 and 10 kΩ it offers eight part numbers across six TCR grades from 3 to 50 ppm.

For the full picture on which line to reach for, see thin film vs thick film chip resistors. The range sits on the thin film products page.

Which MLCC are AEC-Q200 compliant for the pack electronics?

Walsin publishes AEC-Q200 compliance on the automotive MT series, which carries 2,176 rows and 2,173 distinct part numbers across six case sizes: 0603 (585), 0805 (512), 0402 (485), 1206 (312), 0201 (204) and 1210 (78). Dielectric is NPO on 1,283 and X7R on 889, with three X8G parts.

Part number	Series	Case	Dielectric	Capacitance	Voltage	Match / mismatch
MT31N101J102	MT (AEC-Q200)	1206	NPO	100 pF	1000 Vdc	Match — isolation barrier, AEC-Q200
MT32N103J12E	MT (AEC-Q200)	1210	NPO	10 nF	1250 Vdc	Match — highest MT voltage in the export
MG31N101J102	MG (no AEC-Q200)	1206	NPO	100 pF	1000 Vdc	Mismatch on qualification — same electricals, no AEC-Q200
ST31B472K102	ST (AEC-Q200, soft term)	1206	X7R	4700 pF	1000 Vdc	Match — adds flex-crack margin

The MG part is the one to watch on a BMS BOM. It is automotive-grade and electrically identical on the published fields, but Walsin’s own series name is “Automotive Caps Without AEC-Q200 Certification”, so it cannot stand in where the qualification is the requirement. Walsin’s datasheets state AEC-Q200 compliance, not certification — the standard issues no third-party certificate. Request the qualification report for your exact part number and check its revision date. Venatech makes no qualification claim of its own.

Source: WTC_MLCC_Automotive_MT.pdf.

What does the shunt footprint cost me in board area?

More than the resistance suggests, because the low values live at the large end. Across the 1,188-part metal foil line, 625 parts are 2512 and 375 are 1206 — those two footprints are 84 % of the line. The remaining nine per cent spreads across 0805 (61), 1225 (47), 2010 (28), 0603 (20), 0612 (17), 0306 (10) and 0402 (5).

At 0.5 milliohm the choice is three footprints and three TCR grades, as the table above shows. At 1 milliohm it widens to 1206 and 2512 with 58 part numbers. There is no 0402 or 0603 part below 1 milliohm anywhere in the catalogue, so a space-constrained pack cannot have both the small footprint and the low value.

What do I check before the design review?

Confirm the shunt’s temperature coefficient over the pack’s actual range, not at 25 °C, because a 70 ppm part across 60 °C of swing moves 0.42 % and that lands directly in your state-of-charge error. Confirm the tolerance of either 0815 part with Walsin if you are using the thick film floor, because the letter and the column disagree. Confirm the AEC-Q200 report revision for every MT or ST part number on the BOM, not for the series. And confirm the precision divider’s long-term drift specification from the datasheet, because the export publishes initial tolerance and TCR and says nothing about drift.

What Venatech adds past the datasheet is the part a manufacturer’s spec sheet and a public cross tool cannot do: lot and date-code traceability on the parts you actually receive, a Certificate of Conformance on request, and original sealed manufacturer packaging. On a shunt feeding a coulomb counter, knowing which lot you measured matters as much as the specification.

The short version

Film technology on a BMS shunt is not a preference, it is a consequence of the value. Below 5 milliohm Walsin’s thick film line offers nothing; below 1 milliohm all 58 part numbers are metal foil; the catalogue floor is 0.5 milliohm on six parts. At those values TCR follows footprint rather than resistance, and a packaging suffix can move it from 70 to 100 ppm. On the divider, the thin film WF series fixes tolerance at ±0.01 % and lets you pick TCR; thick film stops at ±0.1 %. On the barrier, AEC-Q200 lives on MT and ST only, and the MG series is excluded by its own name.

Send us your BMS shunt value, pack voltage and TCR budget, and we will send back the part numbers that clear all three — and tell you where the catalogue has nothing. A single line is enough to start.

Check it against the catalogue yourself. Cross-reference accepts one part number or a whole BOM column and prints the mismatches as plainly as the matches — qualification stays with you, which is the honest division of labour. Ready stock covers the lines held in Hong Kong, as a dated snapshot rather than a live feed.

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Frequently asked questions

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Three ways to get this answered today

1. Cross a part, free. Paste up to 500 part numbers, or drop an Excel or CSV file, into the free cross-reference tool. It is matched against Walsin’s published catalogue and returns candidates field by field, with every difference from your original stated rather than hidden. Filter by commercial or automotive grade, or for soft termination, high temperature, safety certified, high Q / low ESR or low profile. There is no login and no charge.

2. Send the whole BOM. Upload a BOM and request a quote and we will come back line by line — including the lines we cannot help with, which is usually the more useful half of the answer.

3. Check the shelf before you commit. Ready stock shows what was physically in Hong Kong as at 23 September 2026. It is a dated snapshot, never a live feed, and quantities are never permanent.

Why take our word for any of this? Don’t — check it.

Venatech Limited (威納電子有限公司) is an authorised Walsin Technology distributor in Hong Kong. Confirm that with Walsin rather than with us. On the safety capacitor series we hold UL file E182369, which is searchable in UL’s own public database, together with TUV certificates R50381780 and R50195920 and CQC CQC20001280137 and CQC20001280136, against EN 60384-14:2023, IEC 60384-14:2023 and UL 60384-14. We hold 28 Walsin datasheets, 451 pages, and we will send the one covering your part number. Shipments arrive in original sealed Walsin packaging with lot and date-code traceability, and a certificate of conformity is available per shipment on request.

We will also tell you what we do not hold, which no competitor publishes. REACH, SVHC, conflict minerals, PFAS, China RoHS, ISO 9001, IATF 16949, PPAP, IMDS and a Declaration of Conformity appear nowhere in those 451 pages. You would rather hear that from us now than from your own supplier-approval team later. AEC-Q200 is compliant in Walsin’s own words and never certified, we hold no manufacturer’s end-of-life or allocation list for any brand, and qualification in your own circuit stays with you. We will not tell you a part is equivalent. We will show you where it differs.

Venatech Limited · Unit 207, 2/F, Sun Ling Plaza, 30 On Kui Street, Fanling, N.T., Hong Kong · (852) 2628 4777 · Hong Kong, Shenzhen, Singapore and Jiangmen, with Chengdu in preparation.