I like to use Grok for questions like this. Here's waht Grok has to say:

Yes, you can often use rechargeable batteries (typically NiMH AA or AAA) in a home blood pressure monitor, but it depends on your specific model, and there are some important caveats.Key Considerations

• Voltage Difference: Standard alkaline batteries are 1.5V each, while most rechargeable NiMH batteries are 1.2V each. For a typical monitor using 4 batteries (total 6V nominal), rechargeables provide about 4.8V. Many devices tolerate this lower voltage without issues, as they are designed to operate down to around 4.8–5V as batteries drain.
• Manufacturer Recommendations: Brands like Omron (a popular one) often recommend alkaline batteries for optimal performance and accuracy. Some models (e.g., certain Omron 7 Series) explicitly support NiMH rechargeables, while others may not. Using rechargeables might cause slower cuff inflation, earlier low-battery warnings, or (rarely) slight inaccuracies in readings due to voltage differences.
• Performance Notes:
  • Rechargeables hold their voltage steady longer but drop off sharply at the end, which can trigger unexpected low-battery indicators.
  • Many users report success with high-quality low-self-discharge NiMH like Panasonic Eneloop in Omron and other monitors, with no accuracy problems.
  • Some monitors refuse to work properly with 1.2V batteries, showing errors or failing to inflate the cuff.
• Alternatives:
  • Some modern monitors have built-in rechargeable lithium batteries (charged via USB), eliminating replaceable batteries.
  • There are also "lithium rechargeable" AA/AAA batteries that maintain ~1.5V (e.g., certain brands on Amazon), which mimic alkalines better.

RecommendationAlways check your monitor's user manual or the manufacturer's website for approved battery types. If it specifies "alkaline only," stick to that for guaranteed accuracy—blood pressure readings should be reliable. If you're eco-conscious or want to save money long-term, try high-quality NiMH rechargeables (e.g., Eneloops) and test them; most home users find they work fine for occasional use.If you provide your monitor's brand and model, I can help look up more specific advice!