LI ION battery charger polarity protection - Using / Designing with Mosfets help

Hi guys ,
i dont use mosfets as much more than nice power switch as yet ,
but have need to use the body diode as a reverse polarity protection in a USB ( 5v supply ) LI Ion charger cct , and would like get some input into the mosfet selection design specs and assumptions i have , so far
Option 1
Attachment 7057


Option 2

Attachment 7058
I have not made this cct yet ,but waiting on the charger modules

The chargers are common around the place and use the TP4056 controller.

( a good review here ) http://lygte-info.dk/review/Review%2...4056%20UK.html

Unfortunately this controller has no reverse polarity protection on the output side , ( it does on the input side) so if you put the battery in reverse polarity it fries the charger/ controller chip , so reverse polarity protection is required.

option is to use low resistance on mosfets , using the body diode built into the mosfet ,such that the body diode is orientated in the direction of normal current flow , when looking from the battery to the changer and block the direction of the reverse polarity of incorrectly put in battery.

Also i have need to series the batteries as well as charge them individually to ensure a faster charge rate, and use the existing charger modules , without much additional circuit and costs

I thought of using a Pch as well to isolate for both reverse polarity protection of both outputs of the each module , but thought it not required ??? see cct option 2

Also i did not see a reason to isolate the input to the chargers if the power switch was on , as the chargers have current limiting , also the drain for this cct when on is less than 1A

Mosfet Selection

The choice of mosfets were based on following
Common requirements
1. Low On resistance at a voltage min of 10v
2. 1A or greater drain current ability
3. Low gate on voltage of less than 2v , ( so battery will still charge)
4. Body diode reverse voltage of min 10v
5. smd size
6. price , availability - no point buying chargers at $2 and paying $6 for mosfet polarity protection

N channel - Q2, Q4
http://www.nxp.com/documents/data_sheet/PMV30UN.pdf

P channel - Q1,Q3

http://www.farnell.com/datasheets/1698603.pdf

any comments on my planned cct on my mosfet selection or the quick design

looking at the picture in your link for the charger it appears that the gnd and bat- TERMINALS are directly connected. this will short out battery 2
you need a switching regulator with an isolated secondary to do what you are trying
ps why not jut use a low forward volt drop diode for reverse battery protection eg PMEG2010ER

any diode will have a voltage drop of at least .3v and this will over charge the the battery

but it does appear that the bat gnd and input gnd are joined

with a low vgs you may find the vds voltage drops across both mosfets may exceed the .3v from a low vf diode .(your vgs maximum is vcell how low will it be with a flat cell).either way the battery is likely to undercharge not overcharge .(the charger sees vcell plus 2x vds)or (vcell + vf)
how does this "charger" sense when the battery is charged , if it looks for the knee in the cell charging voltage then any series resistance between the cell and the charger may distort this process with less than ideal results

the on resistance is very low on these N channel Mosfets at 4.5v . should provide min volt drop when charging
[URL="http://au.element14.com/jsp/search/productdetail.jsp?sku=1858601RL"]

given the charge module gnd and bat - seem to be connected ( most likely this circuit)
Attachment 7059

Attachment 7060

the cct uses P ch
http://au.element14.com/jsp/search/p...sp?sku=1894738
or
http://au.element14.com/jsp/search/p...sp?sku=1858668