Solar charge lithium ion and run load - Page 1
Forum section: electronics / projects

author: mm0hai
author_rank: Newbie
timestamp: May 16, 2012, 06:55:36 am
content: Hi,I need to power a system from a solar panel. The load will vary from less than to more than the solar panel can provide. I would like a battery to charge when there is excess power and discharge when there is not enough. When there is not enough power, I do not want the load to switch only to the battery - the power that is available from solar should still be used.This type of system would be very easy with a lead acid battery - just connect the battery and load in parallel. It seems to be much harder with lithium ion batteries though because of their charging requirements. I need a lightweight battery because this system is for my bicycle. I will also use a dynamo but I don't think that will complicate it too much - the difficult bit I am finding is designing the charger system.I thought I would use a battery the same as that in my laptop because this gives me a pre-made battery pack with protective circuitry and I could also put it directly in the laptop. It is 14.4V, 70Wh. Solar + dynamo should give about 10W. I will be powering a laptop (Thinkpad x60s, ~15W), iPod, camera battery charger, bike lights...The difficult bit seems to be charging and running the load at the same time. The MCP73871 as used in anadafruit designseems to do this but it's for charging a single cell.Any ideas on a good way to do this?Thanks

author: markus_b
author_rank: Regular Contributor
timestamp: May 16, 2012, 09:13:18 am
content: There are plenty of solar charge controllers out there, also for li-ion batteries.You can treat li-ions in many respects the same as lead acid. The cell voltage raises in line with the stored capacity and to prevent overcharging you just stop charging / limit the charging voltage to the maximum cell voltage.The main difference between li-ion and lead acid is how they react if abused: Overcharge/Overcurrent, etc. A lead acid will slowly deteriorate, while a li-ion has the potential for a violent burn-out/explosion. That is why we have the protection circuits.One problem for you may be that the protection circuits and cells are selected and designed for the use-case. For example your Thinkpad battery pack is designed to supply 30W over 3 hours and the cell designe and protection circuit reflects that. So if your load differs, for example you want to provide motor power to your bicycle, then such a pack will not work.For example I've replaced a 9.6V nicd pack for a RC car with a battery holder for three 16850 li-ion cells. The car is unusable of I use standard li-ion cells with a built-in protection circuit, because the overcurrent protection trips all the time. If I use 'naked' cells, ripped from a laptop battery pack, then it works fine (of course thene I have no protection and I have the risk of 'smoke signals' in case of a failure).You may be better off buying a li-ion pack designed for modelling (see hobbyking.com), the are not that expensive and designed for high currents. In modelling you typically suck your battery dry in 10-15 minutes. The cell design used in these packs allows for that.

author: sonicj
author_rank: Frequent Contributor
timestamp: May 16, 2012, 03:25:13 pm
content: What's wrong with just feeding DC into the factory power adaptor port and letting the laptop handle the charge? Your laptop only needs 15W?-sj