They are Sanyo NiMh packs.
In my opinion, the best they make for RC use.

Your 50ma charger will charge them, but it will take about 33 hours. Therefore, you might want to pick up a Sirius Charger or something that can put out 150-175ma. To reduce your charge time.

Did you read the article Dave wrote concerning the charging of NiMh batteries? If not, take a look at it. It is on his web site.