Charging NIMH 12V with/out arduino?

Hello, I am working on this "Robot presence" which is a robot much like one of thoose https://www.google.fr/search?q=robot+presence&espv=2&biw=1375&bih=761&source=lnms&tbm=isch&sa=X&ved=0CAYQ_AUoAWoVChMIo5W6_o3nyAIViUoUCh3hZwE3 except for the graphic design maybe :(

The robot stays on all the time.

Most of the time it is plugged to the wall, but when someone wants to wander around the house of course we switch to the battery.This is the diagram i made with the welcomed help of OldSteve and others here. |500x233

Now what i would like is to have a circuit to recharge (cleverly) the battery knowing that it stays on the wall most of the time (maybe 95%) The battery i have for now is a cheap 3800mah NIMH. I might change for a better quality one but it will stay arround 3k-4K mah.

There is loads of resources online about how to charge those battery going from a simple resistor to a whole circuit detecting heat on the battery.

I am not sure what direction take. I can detect heat easily enough with the arduino but i doubt this is a good idea ( what happen if arduino goes down?) Maybe the simple trickle charger with a resistor would do just like on this page

http://www.talkingelectronics.com/projects/ChargingNiMH/ChargingNiMH.html

What do you think?

Well the charger would have to have a supply greater than 12 volts to charge a 12V NiMh pack, the full charge voltage would probably be something like 14 volts, you could use an AC powered charger but you would have to figure out a way to switch it off when on battery power so its not trying to drive the load.

jcallen:
Well the charger would have to have a supply greater than 12 volts to charge a 12V NiMh pack, the full charge voltage would probably be something like 14 volts, you could use an AC powered charger but you would have to figure out a way to switch it off when on battery power so its not trying to drive the load.

That is true, the home supply will be 14 or16v.
Otherwise the mosfet will have trouble to trigger too.

Any other thought?

glub0x: Hello, I am working on this "Robot presence" which is a robot much like one of thoose https://www.google.fr/search?q=robot+presence&espv=2&biw=1375&bih=761&source=lnms&tbm=isch&sa=X&ved=0CAYQ_AUoAWoVChMIo5W6_o3nyAIViUoUCh3hZwE3 except for the graphic design maybe :(

The robot stays on all the time.

Most of the time it is plugged to the wall, but when someone wants to wander around the house of course we switch to the battery.This is the diagram i made with the welcomed help of OldSteve and others here. |500x233

Now what i would like is to have a circuit to recharge (cleverly) the battery knowing that it stays on the wall most of the time (maybe 95%) The battery i have for now is a cheap 3800mah NIMH. I might change for a better quality one but it will stay arround 3k-4K mah.

There is loads of resources online about how to charge those battery going from a simple resistor to a whole circuit detecting heat on the battery.

I am not sure what direction take. I can detect heat easily enough with the arduino but i doubt this is a good idea ( what happen if arduino goes down?) Maybe the simple trickle charger with a resistor would do just like on this page

http://www.talkingelectronics.com/projects/ChargingNiMH/ChargingNiMH.html

What do you think?

What do you need?

a) A "Fast" NIMH Charger, which takes about 1 hour b) A "Slow" NIMH Charger, which takes about 24 hours

Trying to detect the "End-Of-Charge" voltage for a NIMH battery is not easy. Perpetual over-charging will reduce the life of a NIMH battery. If your heat sensor or Arduino fails then charging should stop or automatically self time-out. Consider a charging circuit that will "Fail-Safe".

Slow charging is no problem for that project. What imports is reliability over time.

Actually as a side option i wrote this diagram

with a single charger i could charge my 3 cells equally and when the robot leave the home station all the relay trigger and i got a 3S battery back...

Probably there is a better way to do. What if a relay fail? sounds like dangerous...

Now, your drawing is showing 3.7 Volts batteries??? That is Lithium-Ion technology.

Previously, you said NIMH battery. NIMH is only 1.2 volts per battery.

I think, the NIMH market has peaked. Lithium-Ion type batteries are now the new standard for Power per Weight.

Charging (NIMH or Li-Ion) batteries in series or parallel is not that simple ...

Yes i am sorry that is confusing, i am exploring various options ... I would prefer nimh for safety and weight is no problem ( i even need weight :) )

That being said i will go to the simplest option, and this is the first schematic i managed to produce.