The original battery is a 6.0v 4.5ah SLA. I am looking to replace the SLA with nimh because I don't see any reason to continue to use a SLA battery.
A couple of reasons to carry on with the SLA:
You save the money for NiMH
You save the money for a NiMH charger
You save the time you'd have to spend doing the switch.
Besides, your 'bot may be designed for the weight of the battery being used to stabilize it and set the COG as low as possible and a switch may lead to unstable movement - I don't know your 'bot though, so I just mention this as a possible outcome.
My plan is to just build a C-sized 5 cell nimh pack (nominal 6.0v) and wire it in place of the SLA. Not sure what the nimh capacity will be (haven't bought the cells yet), but hopefully it will be ok. I think I might be able to use the original charger (6v, 300ma) to slow charge the nimh pack in place.
Lead-acid is charged to around 7.2V and quickly fall to around 6.9V - considered (really) flat at around 5.8V.
A 5 cell NiMH setup will charge to ~7.25V, When removed from the charger it will fall to around 6.75V and for most of its discharge will be close to 6.25V - will be considered flat at, depending on current drain, between ~4.5V and ~5.5V.
This differences may seem subtle, but apart from the 'bots needs (which may or may not comply), charging one as the other, is a safe way to ruin it ahead of time.
As an example, just look at the poor chargers that comes with most battery driven power tools (hand drills etc). They're designed for low manufacturing cost to just sell the goods, but will ruin the battery prematurely by undercharging or more likely overcharging, cooking the battery out if it's left in the charger and by the weak current and lack of balancers, can be considered tools of destructionj.
BTW. Most cell phone sales people will still tell you to go home and charge your new phone for 24 hour, which would ruin the battery or make it burst into flames, if it weren't for the fact that nowadays, cell phones contain clever charge circuits, making it impossible to charge the battery after a few hours at most.
This just goes to show that the battery knowledge of Joe Random and even those who ought to know better since they do it for a living, really haven't got a clue and don't spend a bit of time getting up to speed, but just chant on, the lesson learned some decades ago for a different chemistry.
Any thoughts or better ideas? I was also thinking of using a more standard 7.2v nimh pack, but would probably need some kind of regulator on the output and more complicated charging.
Keep the SLA or if it's about at its end of life, replace it.
Your 'bot was designed for it and by switching it, you're trying to say that you know more than the person or team that designed it (and if you did, you wouldn't really have to ask).
A 6 cell NiMH will charge to 8.7V, quickly fall to ~8.1V and be at around 7.5V for the majority of a discharge cycle. It will be flat at (again depending of current draw) between 5.4V and 6.6V.
On top of the other things, you'd have to make sure that the 'bot can live with the higher voltage.
If adding a regulator to keep the voltage down, you'd need it to be a switcher of either buck/boost, SEPIC or CHUK topology. This will present further work and/or outlay, so my best advice is still to keep the SLA.