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That's a big NiMH - twice as big as the one in a Civic Hybrid, iirc. Where'd you get the cells?
I'm pretty sure you'll need a BMS. You need to keep an eye on every cell to ensure it doesn't get too close to 100°C - Honda does this by monitoring twelve cells at a time with a PTC resistor. You also need to ensure that no cell has a polarity reversal - voltage less than zero. Again, Honda keeps an eye on the voltage of 12 cells at a time, and stops discharging at some predefined voltage around 1 VPC.
You can get a dirt-cheap DC/DC converter out of a wrecked Insight or Civic. In fact, I'm using a Civic DC/DC + Inverter as a decoration on an end table in my living room. There's also a great selection of MeanWell DC/DC's in the Jameco catalog, but they're not free either.
You said your whole array weighs 25lbs... by my math, your battery alone should weigh 90lbs just for the cells, plus more for a housing for 90lbs of battery.
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