Calculating minimum trickle charge power requirements

I'm looking to buy a solar panel to trickle charge my phone + other devices while out and about. Obviously I don't want a panel that is excessively large/heavy but also I want something that won't allow the battery to drain even after potentially many hours of usage (not just idle).

So is the following a method of working out how much power I need?

I know from experience that using the satnav on my phone the phone will last at least 2 hours from a full charge. If I also know that the phone can gain a full charge from empty in 2 hours when powered off by using a 5v, 1amp charger. Does this mean I can simply half this power requirement to retain whatever charge level the phone currently has indefinitely?

Or more generally can I apply this formula:

Pc    Pr
-- =  --
Tc    2*Tu


where: Pc = power output of charger, Tc = time take to charge from empty in powered off state, Tu = time to full drain battery from full when in required usage scenario. Pr = required power to maintain a usable charge level indefinitely.

Obviously in reality I'd probably add a 10% error/safety margin.

Thinking about it I'm pretty much sure that the 2 in my formula is wrong. Also I'm wondering if there is some overhead on the charger. i.e. a minimum power level required before charging even starts. Also I'd probably have to add an overhead for any required voltage regulation etc.

• You can use a variable power supply and meter to see how your phone responds on low energy sources. It may be possible to charge a capacitor to 5V and then connect it to the phone, let it transfer what it will and then repeat. Do not exceed 5V! Jun 9, 2014 at 13:39