Originally Posted by Cyclaholic
What do you mean by "robust" when you say that SLA is less robust than NiMH or NiCd? in my experience with deep discharge applications I found SLA to be a more reliable chemistry, and cheapest per Ah, although it's the lowest energy density.
BTW I like how you paralleled two NiMH R/C car battery packs, I'm going to give that a try.
SLA doesn't like deep discharge at all. SLA can be discharged to around 1.75 V per cell from 2V (around a 12% reduction in voltage). NiCd and NiMH can go to around 0.9V from 1.2V (a 25% reduction in voltage) without problems. Nickel chemistrys, especially NiCd, even withstand some cell reversal which happens when you get to very low cell voltages.
As for cost, look
here. On a dollars per cycle basis, the NiCd beats the pants off of any other chemistry...including SLA...mostly because it can to to 1500 duty cycles which is 3 to 5 times that of the other batteries.
I've been doing the RC battery thing for years. It works very well and I'm seeing lots of 4000+mAhr batteries around now for pretty cheap.