Let's Make Robots!

In circuit charging of NIMH pack

I'll be putting together a 14.4v 5ah battery pack out of 12 solder tabbed C cells.  The raw voltage will be fed into a 100w step up DC-DC converter to supply 18v to a single board computer, and also to two 80w step down DC-DC converters to provide 5v to an Arduino sensor shield, primarily for 5v servos, and 7.2 volts for the motors of a 4wd Rover 5.

It's my intention to charge the pack with an "intelligent 2.4v-14.4v NIMH charger."  What I'm not sure about is whether it would be safe to plug in the charger while the robot electronics are connected.  I THINK that the DC-DC converters would keep the output voltages where they belong, but I'm not sure if the voltage converters would interfere somehow with the charging voltage/current.

Can anyone explain how that would work?

 

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Ultimately, (someday) I'd like to set up automatic charging.  But I suspect that's not going to happen with this charger.  I suppose I'll put the computer in hibernation, and disconnect the power going to the dc-dc converters.  At least I wouldn't have to remove the batteries.

When you say "while the robot electronics are connected" do you mean connected to the battery pack and drawing power from it, or connected but not actually consuming any power?

The two areas I can think of where you might run into trouble are:
• If the DC-DC converters are drawing power from the batteries while you're charging them, this will increase the load on the charger. If the current required to charge the battery pack and supply the circuit at the same time is too great, then either the charger will fail (probably turn itself off) or the batteries will simply not get charged. Even if the charger is able to supply enough current for both the robot electronics and charging the battery pack, the charging rate will suffer for it.
• As the charger uses the -dV/dt method, it's looking for a sudden drop in battery cell voltage to indicate the end of the charging cycle. If the robot electronics are drawing current from the charger/battery pack, especially if the load is varying up and down, then the charger might get confused and stop charging early, or not stop at all.

The DC-DC converters should maintain the output voltage to wherever you've set it, regardless of the increased supply voltage from the charger.

The charger you've linked to can only put out 1A max according to the specs, which at a guess will not be enough to keep the SBC and Arduino both running while also charging the battery pack.