Background.

I have a legacy battery system that is comprised of twenty-four 12V Li-ion battery modules (self-protected), enclosed in a shell with the charging connection provided as a common ground and 24 individual non-isolated charger connections. The 24 Li-ion modules are connected in parallel. Part of the circuit is the high current draw (4A) electronics running from 12V directly from batteries, with additional electronics running at low current (300mA) 5V and (200mA) 3.3V. Because of the cost of battery certification and testing, I must use the existing 12V modules. The enclosure shell imposes a height constraint on PCB+component of less than 0.5".

Current Challenge.

The high power consumption electronics is being upgraded, and can now run at up to 48V. The existing 12V modules were tested in a series connection, and they can output 4A without issue still. However, connecting the batteries in series would not allow the batteries to be charged with the existing system (due to common ground chargers), and we cannot have a "permanent" 48V Li-ion system because of the previously mentioned certification process.

Proposed approaches.

1. Create a re-configurable battery system where a 12V battery module connections can be managed with back-to-back MOSFET's when the charging system is connected. When the charging system is not connected, the battery modules would be disconnected from the charging ports and reconnected back into the predetermined configuration (4S6P). The losses from the MOSFET's operating in fully enhanced mode would be fairly small, with low Rds(ON) in the mOhm range. The component count added with this approach would be at least 24x(4xMOSFETs+2xGate drivers+4passives) = 240 SMD components or more.

2. Leave the battery modules connected in parallel with the same charging circuit and add a boost circuit to increase the voltage output to 48V and 4A. From most online resources, 90% efficiency or slightly less would be achievable. The issue here is lack of SMPS boost circuit design experience on our part, and most of the advice online is to stack batteries in series to achieve the high voltage, and buck that as necessary. There is also concern of fitting large boost circuit components into the height restriction.

I'd like to ask for advice on choosing between the two proposed approaches or introducing an alternative approach.

posted over 3 years ago

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