Teardown: Brentronics BB-390A/U (Nickle Metal Hydride battery, military)
Last Update: August 15th, 2026
Table of Contents
Introduction
When I said I'd deep-dive into more military batteries in the "near future", I didn't think it would take me two years. Yeesh. This is a teardown and analysis of a Brentronics BB-390A/U (P/N BT-70290, NSN 6140-01-419-8187).This particular example was purchased on eBay in like-new condition. It appears to have been built in May of 2000, and appears to have been "inventoried" twice in 2001, once in 2002, and possibly once in 2006 because it was (presumably) sold as surplus. It doesn't not appear to have ever been opened, nevermind used. The datasheet can be found here (local copy). The battery is designed to offer either 12 V and 24 V output, and has a rated storage capacity of 86.4 Watt-hours. This part was replaced by the Brentronics BB-390B/U some time in the early 2000's (Website was updated in 2004/2005.)
Teardown Photos
Top of battery.
In this view, the PCB is on the side facing down.
Note the gold pads for thermister connection.
Front and back of battery (based on PCB placement, see below).
Box that it came in.
Primary connector.
The silicone mastic make it disturbingly similar to cracking a hard-boiled egg - including the tiny shards of 'shell'.
Note the charge level monitor and LCD driver PCB, just barely visible at the top.
The "cut out" for the primary connector is visible on the right side.
View of the mastic and PCB on the back side.
Some cells were clearly leaking.
Better view of the charge level/LCD PCB.
Detail of joint between top and case.
Detail of charge level/LCD PCB and primary connector.
Assembly "spun" around verticle axis.
Who or what is "DI"? Quality control station, maybe?
L to R: Cell pack detached. Backside of PCB. Front side of PCB.
Better detail of the control circuit.
All four of the Nickle Metal Hydrid (NiMH) cells strings seperated.
Cell markings. Note the "HR" stamp.
The heatshrink was def green, not blue. My phone camera "color corrected" it automagically.
Schematic
The part numbers in the schematic should match the silkscreen names.
Click for PDF
Theory of Operation
Energy Storage
A battery is a string of electrochemical cells, linked in series and/or parallel to reach a desired output voltage and storage capacity. In this case, this battery achieves the needed voltage (~12 V) by wiring up 10x "A" size (17x50 mm) cells in series. (Yes, the "A" size is where the "AA", "1⁄2AA", "AAA", and "AAAA" sizes/names are derived from. Contrary to popular belief, no "AAAAA" or longer exists, as the IEC determined that those names were regularly confused with a person screaming.) The cells sport a green heatshrink tube, a "Made in Japan" label, and strings "EBH" and stamped with "HR". These days, the "HR" mark is used in the part numbers of todays NiMH cells from Panasonic (or used to, they recently updated their part number nomenclature). Note that Panasonic acquired a majority stake in Sanyo 2009, and then absorbed it entirely in 2013.It's this writers opinion that these cells were likely made by Sanyo, and are part of the famous NiMH line (arguably the highest quality NiMH cells in the world) now managed by Panasonic. That said, I've been unable to locate an era-correct datasheet or catalog to confirm this, likely because pre-2000 marketing material was typical physical/printed. One major oddity with the battery is they (the people designing the system) connected two 10x strings in parallel - which is highly unusual: NiMH cells have a relatively small voltage swing over the charge cycle (a flat charge/discharge curve). Two cells wired in parallel will charge at different rates (due to minute differences in internal resistance) but it would be impossible to tell (even if the cells were removed from the parallel setup) because the resting voltages would be very close to effectively identical. Over multiple charge/discharge cycles, one cell would never be fully charged and would eventually be driven below it's discharge cut-off voltage, and thus be damaged and eventually fail entirely. For comparison, Lithium Ion cells have a voltage output very closely related to it's state of charge (SoC). If one connects two cells in parallel that are at disparate states of charge, the "higher" cell with discharge into the "lower" cell until the voltage reach equilibrium. This is why large Lithium Ion packs can be built from smaller cell sizes (i.e. 18650, 21700, or 26650), while NiMH (and NiCad) packs must be made from physically larger cells. For a classic example of this, see the NiMH packs found in most Toyota Prius vehicles. So it's really kind of a conundrum how they made this work. It's clearly evident on the PCB that the strings are permanently tied in parallel, no switching tricks here. My assumption is they took the unusual step to test and match-up cells with very similar internal resistances so both strings would charge in almost the same amount of time. This is a time-consuming process (and probably would generate a stream of "waste" cells, that were hard or impossible to match) but this is a military product - presumably with the typically associated astronomical price-tag - so Bren-Tronics could literally afford to do this. Even then it's just delaying the inevitable, and I assume these batteries failed (read: lost a significant amount of capacity and became useless) earlier than strictly necessary. So why do this in the first place? My assumption here is the US military issued a battery specification they needed, including voltages, minimum energy storage capacity, probably a minimum number of required charge/discharge cycles, and the physical characteristics (i.e. size of the case), and this is what Bren-Tronics engineers came up with to hit that target. It doesn't matter if the battery fails earlier than strictly possible, as long as the cycle count and end-of-life capacity is higher than the spec.
Keeping that in mind, note that the battery is built from 40x cells with a nominal rating of 2700 mA at ~1.2 volts, or 129.6 Watt-hours, and the nameplate capacity is listed as about 86.4 Watt-hours. That works out to a difference of 43.2 Watt-hours or an extra 50% (!) over the listed capacity. That is a lot of margin.
That said, the string of 10x cells is defined by the voltage output requirements, and the cell capacity is defined by what is available from manufacturers. That extra 50% is probably more of a happy accident than a deliberate decision, but surely helps offset the premature failure issue. The only other thing to note is this battery design must support both 12 V and 24 V operation. Apparently some military electronics require 12 V and some requires 24 V and the solution is to have two 12 V strings of cells in each battery, thus requiring a total of 40x cells in the battery (four strings of 10x cells, A1 and A2, and B1 and B2). The end system can chose to connect these two strings in series for 24 V or tie them in parallel for 12 V - which then makes the premature failure problem worse. I'd be really interested to know if Bren-Tronics matched the internal resistance at the cell-level or the string-level.