Im busy designing a power supply system that makes use of a battery within its own charger. The supply is going to be used to power two PIC devices (3.3V and 5V).

My question is what would be the most suitable battery to use in such an application. I was thinking along the NiCad route. Any advice please?

 

You'd likely be better off using NiMH batteries. You'll get more charge/discharge cycles out of them, and they don't have some of the disadvantages that NiCD batteries do.

Use a charging IC designed for NiMH batteries. Lots of them on the market now.

 

Thanks, although one of my university professors suggested that using lead sealed acid batteries would be the more preferrable option. Is there any validation to what he says?

 

It depends upon what your application and requirements are.

Size, weight, cost, desired run-time, possible exposure to shock/vibration, temperature extremes, etc - everything needs to be considered as how relevant it might be.

If you define your system requirements well enough, your selection of power cell will become much easier.

 

NiCd's don't survive well if float charged. SLA batteries survive fairly well when float charged. NiMH probably float charge OK, but don't have the recharge cycle life of NiCd's. So, if float charged, they probably don't last very long either. If you are float charging, then SLA is probably best. If you are going to allow self discharge to something like 70% capacity, then recharge, you might consider one of the newer technologies, like A123 or Eneloop (NiMH batteries that don't self discharge nearly as much as even NiCd's do). LiPo's might be considered, but are more finicky. You would have to be sure to recharge them only when they had discharged to a certain point.

I don't get a penny from Sanyo (Eneloop), but this past Winter I did not touch my Eneloop packs. Last charge was mid-November 2009. I checked them over the weekend (04-17-2010) and they were still almost fully charged (i.e., >=90%). If that is typical, you could have a back-up power supply and recharge only twice a year, if that often. Even a 400 cycle recharge life would not be too bad then. In reality, they are supposed to have an even greater recharge cycle life.

John