NiMh or LiPo batteries for unattended charging/usage

Question

I'm in the planning stage of a summer project: A WiFi controlled robot for remotely monitoring a shore home. The robot will consist of a 4-wheel platform (4 motors, 1 A stall current each, 7 V), Raspberry Pi Model A + WiFi dongle, and Raspberry Pi camera module.

Since this robot will be operated while I'm not at the house, I need to be able to remotely recharge it. The plan is to implement something similar to the Roomba's drive-on charging base:

My issue is that I'm unsure which battery chemistry to choose.

LiPo has the benefit of being lighter, but since their nominal voltage is 3.7 V, I'd have to put two or three packs in series to be able to run my motors (which need 7 V, as I mentioned). But, I know this not recommended because then charging the batteries in series is complicated/unsafe.

NiMh seems attractive: wide range of voltage/current ratings, not as prone to fireballs, albeit heavier. My thinking is I could build a simple charging base that trickle charges the robot's NiMh battery. No fast charging is needed (the robot will spend most of its time on the charging base, when not in use anyway).

Summary of requirements: rechargeable battery for unattended charging/usage, somewhere in range of 8-12 V, several amp-hours capacity (4-6 Ah, maybe).

Is my evaluation of this correct - that NiMh seems the way to go? Or should I consider LiPo?

Answer 1

I've built several products around different chemistries. I have found LiPo the easiest to use since there are already specialised charger IC's that do all the work however, like you said, having them in series is not that straight forward. I believe the NiMH is a better solution if you don't have a lot of experience with chargers or if you don't find a good solution for charging LiPo in series.

Just one note, do not trickle charge NiMH indefinitely, use a timer at least to avoid overcharging the battery too much(even when you trickle them, they overcharge). You could also use, like Pwocky suggested, an LM317 to charge at a constant current and monitor the change in temperature in the battery pack, once the delta T becomes big enough (the temperature changes more rapidly) the battery is charged. You could also mix this method with delta V and a safety charge timer. This is what I do and it works perfectly. I charge my batteries fast and never overcharge them.

There is a lot of info online with graphs showing how the temperature changes when charging a NiMH battery.

Answer 2

For a hobbyist project, I'd go with a NiMH option. They are significantly easier to charge (you can charge them with a commodity linear regulator (LM317) and a few passive components), and safer. The weight difference will likely not matter much- your robot doesn't seem to need to change directions very quickly, and it's not something like a quadcopter where every ounce counts.

Answer 3

Charging the battery safely is a primary concern.

I typically like to DIY; however when safety is a factor, it would probably be better to buy a product from a quality company. So to give you some ideas, this is a "smart" charger you can set for various current and voltage settings (up to 9.6 volts); it will also turn off automatically. It will also charge NiMH. Here is a link from one of my favorite websites: http://www.servocity.com/html/super_brain_charger.html

Also, the user's manual for the charger: MCR Super Brain Charger Manual

For your battery, I suggest getting one of these NiMH: http://www.servocity.com/html/9_6v_batteries.html

Also, may I suggest a method to implement the charging station? After I built my Reginald project, I saw what this gentleman did to elegantly recharge his battery remotely:

Basically it's a positive and negative spring post that makes contact with some metal tabs.

EDIT: Link to Reginald: http://www.instructables.com/id/Reginald-a-UDP-surveillance-bot-control-via-the-/