Stopping battery current feedback

Thread Starter

Nmjb18

Joined Sep 13, 2016
15
Regarding the circuit in post #16.
I have difficulty seeing how T2 can disconnect again?

When the charging circuit is connected then both T1 and T2 is on.
When the charging circuit is disconnected after the charging, then T2 still provide battery voltage back into the "+ Ve Charge" terminal.
Yes your right, I think the key is to use the uC to measure voltage drop over Q2. It would need a soft control, as once the charger has finished and terminated Q2 Vf would drop to 0 and the uC would turn off Q1 and in turn Q2 would close. The charger would then see the cell had disconnected and increase it's output voltage to sense the next battery for charging. This increase in voltage would be seen across Q2 and in turn make the uC believe the charger had just been plugged in - not a great scenario for a battery.
 

Thread Starter

Nmjb18

Joined Sep 13, 2016
15
Have you considered inductive charging? Making reliable electrical contacts in salt water will be problematic.
Thanks for your feedback.

yes I have considered it and have a couple of prototype loops here with me. The issue is that I am tight for space and 5 Amp chargers tend to kick out a fair bit of heat which I don't have the space to deal with. N.B. charging will not occur under water, rather the UAV will surface for charging.
 

crutschow

Joined Mar 14, 2008
34,285
As I noted previously, your circuit suffers from the problem that it is difficult to detect when the charger is removed since he battery voltage will appear at the charge port through the ON MOSFET when you remove the charger.
Given that, how do you plan on detecting when the charger is removed?
 

Thread Starter

Nmjb18

Joined Sep 13, 2016
15
As I noted previously, your circuit suffers from the problem that it is difficult to detect when the charger is removed since he battery voltage will appear at the charge port through the ON MOSFET when you remove the charger.
Given that, how do you plan on detecting when the charger is removed?
As I said to Kjeldgaard, I now plan to measure Vf across Q2
 

crutschow

Joined Mar 14, 2008
34,285
But how will you measure this small voltage difference?
Can the uC measure that voltage difference when it is riding on the battery voltage?

It might be easier to use an N-MOSFET in the negative (ground) side of the connection to disconnect the charger (unless the battery negative is connected to the chassis).
You just have reverse the control voltage polarity.
That way you would be measuring the small voltage drop at ground level, which generally is easier to do.
 

Thread Starter

Nmjb18

Joined Sep 13, 2016
15
But how will you measure this small voltage difference?
Can the uC measure that voltage difference when it is riding on the battery voltage?

It might be easier to use an N-MOSFET in the negative (ground) side of the connection to disconnect the charger (unless the battery negative is connected to the chassis).
You just have reverse the control voltage polarity.
That way you would be measuring the small voltage drop at ground level, which generally is easier to do.
That's the question, Vref is 2.5V and I'm using a 10bit ADC, so that should give me a resolution of 2.4mV. I would expect the V drop to be around 30mV dropping to 0 at the end of the charge. I could always include an Opp Amp if it was a significant problem.

Yes I will have a think about your N-FET on the other side idea.
 
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