Explanation of how a switched-off battery can be charged

[rolygate]

Hey electronics/battery guys,

I'm looking for a good explanation of the following question, for use as required elsewhere on the forum, please:

How is it that a battery that is physically disconnected by a switch and shows no voltage at its terminals can be charged? This applies to manual batteries like a 510 or eGo, and auto batteries with a pressure switch, as well - they both have a switch in the circuit that disconnects them and therefore would seem to prevent them being charged.

Yes, we know that ordinary rechargeable cells cannot be charged if there is a physical switch in the circuit; and what we are really talking about here is batteries with an electronic package that functions as a switch, the actual switch being an illusion because it just operates the control circuit of a mosfet (or maybe a chip that does that job now, I'm not up to date).

But you can probably give the actual workings better than that

Thanks.

 

[mdocod]

Hi rolygate,

Probably the simplest way to do this is with a simple diode connected all the time that only allows current to flow IN to the "battery" at all times. Then current is allowed to flow out when a separate switch is activated.

A diode, as I understand, causes a voltage drop, and with some smart planning, that voltage drop can be used as a cheap way to get closer to the target charge voltage for the device from the standard 5V power supply. (Most diodes have a drop of 0.7V).

Eric

[edit in] This question may reveal more than you may have originally realized. If it is in fact a simple diode connected to the cell all the time, and the charge control is nothing more than the diode ramping up it's resistance as the cell voltages closes in on that 4.2-4.3V range (causing voltage differential between the 5V source and the cell to drop below the cut-in of the diode), then there may be a reason why so many manufacturers suggest leaving the charger on the cell for longer periods of time after the light has "turned green" to get a full charge. If this is all true, then not only are they cutting corners (expected), they are increasing the danger of the device by not using safe charging methods.

 

[Java_Az]

Pretty sure it is done with a mosfet which is a type of diode. They are electrical switches non mechanical. , But you have 3 pins on the mosfet a line in( source, battery charger), a line out (Load, battery) and a gate. Basiclly the line in and line out wont let electricity flow unless a voltage is applied to the gate. When you hook a charger up to a battery it sends a current to the gate pin on the diode thus letting voltage flow threw to the battery with out pressing the switch.

Not sure if thats very clear about as simple as i can explain it.

 

[Rocketman]

The battery control circuit (the Brain) has comparators that compare the voltage at the atty connection to preset internal voltage values. Charge and discharge connections are through FETS which act as very small resistors when turned on (like 25 milli ohms) or an open circuit when turned off. No diodes needed. The FETs drop very little voltage when conducting.


There is a comparator for overvoltage, undervoltage that disconnect the battery from the atty connector.
There are one or more overcurrent (overcurrent and short circuit) that disconnect the battery.
There is a comparator that detects a charge voltage greater than the shutdown voltage and turns on the charge FET.

No current will flow into or out of the battery unless at least one FET is turned on.

 

[mdocod]

Hi Rocketman,

Any idea how the charge is terminated?

 

[Rocketman]

I would think that the charge voltage is compared to an internal reference and the charge fet turned off when the voltage exceeds the reference.