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Battery woos
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Author:  laurie54 [ 06 Jan 2019, 09:14 ]
Post subject:  Battery woos

I have come across a perplexing problem. The aim is to use and charge a Li-ion battery to power a cct and also charge the battery and run the cct from a USB 5V. Looking at many cct's they all seem to have the same type of topology. A batt followed by a P-ch mos used as a switch with USB feeding the battery charger and threw a protection diode feeding power to the cct. However in all the cct's I see the mos is drawn without showing the internal bypass diode. As shown the mos is turned on by a gate res to gnd so the batt runs the cct threw the mos. Then when the USB 5v is plugged in it feeds the charger IC and shuts off the mos by supplying 5v to the mos's gate, then a protection diode to allow the USB 5v to power the cct while charging the batt. Good in theory but in practice the internal bypass diode located inside the mos case means the USB 5v is actually doing the charging (threw the internal diode) and the battery charger is doing nothing. I have added a series diode between batt+charge IC and the mos's source input but that took some doing to find a diode which does not allow back current flow to get threw as well. The BAVT70 diode pair (I have some) seems to be the best allowing only 12mv to creep threw but even that seems just wrong. The battery charger I am using is the LTC4053-4.2 which is dual mode CC and CV charging IC and even it's data sheet shows cct's like a USB or 5v Wall adapter protecting the USB's output by way o fa mos again configured the same way to shut the mos off when the Wall adapter is plugged in. Again the internal diode is missing and when drawn in means the wall adapter will supply the charger IC and cut off the USB supply but will not stop the wall power from feeding back threw the mos's internal diode and blow the USB supply.
Anybody have any ideas? What am I missing?

Author:  laurie54 [ 07 Jan 2019, 21:42 ]
Post subject:  Re: Battery woos

I have solved the delema using a SPDT version of the same SPST switch I now use.. In one position the battery is connected and powers the unit. Due to the mos battery protection if USB power is plugged in, the battery is disconnected and charged. In the second position the battery is disconnected so batt life is what it would be if sitting on the shelf (3 years I believe for a Li-ion). But in this position if USB power is connected, the batt is charged, and, the unit can still be turned on and will run on USB power.
Strange how a good nights sleep brings solutions to the mind.

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