Hello again
I've been using this circuit to charge a laptop 3s2p battery in the laptop. The AC charger uses 16.5v on this particular laptop. It's an older IBM Thinkpad. I was curious to see the built in charging circuit would behave when presented with less current than usual (and a variable one at that) but at the correct voltage.
I used a 15w solar panel and it was peaking at around 0.7A this late in the day. The charging circuit was accessed through the acpi monitoring installed in my GNU/Linux install (gentoo). I can monitor how the computer is seeing the charging process as I have compiled a full acpi suite of tools.
It works best when it is charging or discharging. That might sound strange but if the computer is discharging overall but there is some current going in it is fine. For example a 10w load and 7w of power in, the battery monitoring will say it's discharging 3000mW. If the computer is at the cusp of charging and discharging it is not so happy. That is the way the circuit monitoring the charging process deals with this is it dwells at "charged" even though it is either being charged or discharged. And if it's got enough charge to get past the base load it's fine as well as if it's in it's discharging state.
It was charging when it was sleeping (in standby). Then it stopped. I am not sure what happened there. The battery is well worn and might have met it's prerequisite charging voltage. I don't know.
It will not charge when the computer is off with only 0.7A available. I have no idea how it can sense there is only 0.7A as there is not the slightest bit of current going through, but there you go.. It might have sensed it had reached a charged state. These do have ICs and possibly battery tank chips inside the batteries themselves.
So, overall, my little experiment left me a bit nonplussed. It will work but really needs a decent amount of solar 30-60w and perhaps two of these little circuits in parallel to do it properly. That the laptop will not charge in an off-state has left me a little bit baffled but again, I aren't too fussed as the experiment told me what I needed to know. That charging directly off solar will work but there are many possible pitfalls with the process. The main one being how happy the laptop (or the battery) will be in accepting the amount of charge available or how happy the laptop will be in accepting a variable charging rate (even though it as the correct voltage).
I had in my possession an OLPC 1.75 for a week or two a month back and that laptop has built Maximum Power Point technology built into the charging circuit. LiFePo4 batteries and can handle voltages of between (11-40v from memory). It will also limit the amount of power the laptop requires depending on the charging profile. It really is the way to go for a fully solar charging laptop.