Solar powered (charging) circuits with LiPo/LiFe/Li-ion battery

[Narkki]

Hi fellow diyaudionists. I've got a plan for next summer and it is solar powered boombox. The idea is that you don't usually use boombox 1) inside 2) when it's bad weather => there is the sun available to power the boombox. In this thread I won't and don't want to discuss the general build and elements and such, just about the solar panels and batteries.

I can get 61mm*61mm*6mm solar panel for 5,90€ a piece and its rated for 4,5 volts and 100mA. 3 of those would put out (at maximum) 13,5V, meaning the maximum power is 1,35W with in serial circuit. 3-4 of those in parallel and I would get 4-5 watts.

I want to keep the build light so I'm planning to use LiPo battery. How should I make the circuit? I can get 11,1V and 14,8V LiPo batteries easily. The size and capacity is kinda irrelevant but I'm thinking something between 2-4Ah.

I've red some articles about float charging Li-ion batteries

Float charging lithium ion cells - 2/1/2006 - Electronics Weekly
Battery Chargers and Charging Methods
How to design a Li-Ion battery charger to get maximum power from a solar panel (good one)
Battery Charging (scroll down a bit)
Float charging Li ion cells
Float Charging NiMH Cells [227] | General Electronics | Electronics

I'd like to attach the battery and solar cells in parallel so that with low volumes and bright sun solar cells would charge the battery thats giving power for amp(10,5-13,5V like amp6). Will this work or do I miss some point? Should there be overvoltage/current protection circuit or special charging circuit?

[Saturnus]

Ok, my first reply got screwed up by the server maintenance. So let me just short form my points.

- At €13/W for those cells you're getting ripped off. Current solar cell retail price is around €1/W, and generally less than €4/W for completed panels.

- Use amorphous/thin-film panels for boombox use. Mono/poly-crystalline panels are not suited for use where you can't always make sure you have optimum angling and weather conditions.

- Do not use or try to use li-po/li-ion cells without a proper charge controller. This is a serious warning for your own safety.

- SLA/LFP cells can safely be used for boombox use without a proper charge controller

- "12V solar systems" have Voc~20V-22V and Vop~16V-18V. Please be aware of that. Not only why it needs to be this high voltage but also note what effect it may have on your amp should you accidentally connect you solar panels directly to your amp without a battery.

- Further information in my Boominator thread

[Narkki]

Thanks! I've red about your boominator and thats the inspiration for my 1st boombox anyway (made one as dissertation in vocational school).

The reason I'm looking Li-ion batteries is weight.

If I make simple circuit that cuts off the battery from solar panels when the battery reachers like 90% of its charge and reconnects when load starts to drain current? In those articles they float charged Li-ion batteries without charging them to full charge.

And where (in EU) you can get those cheaper solar panels?

[Saturnus]

You can always search on ebay.

But I'm guessing you have been looking at ebay li-ion CCTV battery packs, and their capacity claims are overrated by 50% or more. Just so you know. In fact their claimed capacity is physically impossible given the technical limits of todays battery technology.

Btw, you can skip the whole problem with charge controllers and most of the weight issue as weel by going with LFP cells.

For example LP12V7AHP - 12V/7Ah - GWL/Power - Your Complete Power Solutions - EV-Power.eu is €70 (including VAT) for a direct replacement for a standard 12V 7Ah SLA battery in the exact same size but at half the weight but with built in battery management system.