I used to use rechargeable batteries but arrived at the same conclusion. By placing the rechargeables in the dry cell pack the voltage was borderline. Adding up 8 batteries missing .3 to .4 volt each adds up to a borderline low voltage in the minelab. I have a whites 5900 that uses 4 C. size battries. I have always used rechargeables in it, but remember that is only .3 times 4 batteries. They may not last as long as alkalines, but they will last at least one day no problem. For the past few years for the minelab I have just used alkalines. I have found them at Walmart in packages of 48 for as little as $12. That is a lot of detecting hours from one package of batteries. Art is right. If you want to use rechargeables, you need the factory one as it is designed to give you the extra voltage.