More of a puzzler, than repair.
Item in question is Ryobi cordless drill 7.2V. Bought it with one battery and charger. Added another 7.2 V battery from home depot. All's fine some far. When I charge the battery, the charger gets somewhat warm, but seems normal.
Later I found batteries on sale at HD and bought two more. These are physically the same and came with short adapter cable for the old charger, which had bigger plug. These batteries have a differnet p/n but according to Ryobi have the same capacity in mAh and clearly the same voltage. When I charge these NEW batteries, the charger gets much hotter.
Called Ryobi and confirmed all numbers, both batteries are 1300mAh. The charger recommended with these new batteries has a different p/n and also has lower current rating (200ma vs 400ma original).
Why would batteries of the same capacity and design, overheat the charger, while others do not. TS at Ryobi is not very helpfull. If anything a higher cap charger should be less likely to overheat, and it's behaving "backwards".