If you have an hp39gii you may find that its a little bit weak on the batteries. So therefore I modified mine snip and separate battery connectors into data two halves then wire across to give 2 in series and 2 in parallel. A 3v system then a diode on the positive end series to drop 0.6v , now you can use NiMh batteries which work much better about 1.8v goes to the calculator, now the hp39gii won't freeze up when writing to memory. Caution, don't do this mod unless you are 100 percent sure of your soldering skills. Don't use alkalines they just don't provide enough current.