When to recharge extender

[Hovtech]

How often should I recharge the extender. Is after 1 cycle of the main battery on the P2V too often or should I go much longer? I know it is not good for rechargeable batteries to just always be topping them off. You want them to go down to at least 20% before a charge, but I have no idea where this point is?

 

[iResq]

I recharge about every 4 batteries.

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[oztrk]

When it is full charge, the light doesn't go green, still yellow. Is it normal?