Bobbie,
You are getting a higher reading because the batteries are not being tested/read under load (detector operating) and therefore can read up to a volt higher than the detectors true battery reading.
Just alternate them.....change over when they first "squeal". Then charge for approx 12 hours (or 14 hours when first new as it takes newer rechargeable batteries longer to reach full capacity). Don't be tempted to charge them whenever you like...ideally they will be charged fully when the detector says they are discharged and squealing. You will have a good lifespan on the pods if you stick to a proper charging procedure. If you keep a mental log of how many hours you have detected from a fully charged battery pod then you will know when the 10 hours total is coming up.
Also, it is important to manually turn the detector off and change pods when you first get the low battery squeal....don't wait for another 5 or 10 minutes for the detector to turn itself off automatically as you can run the risk of over discharging the batteries....this can lead to "reverse polarity" which is a good way to harm your batteries.
Regards,
Tony.