Dual Lithium Cell Charger Common Earth Problem - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: August 03, 2014, 08:41:33 pm
content: Hi all thanks for reading my post.I've been searching around for a compact and cheap solution for in-line charging 2 x li-ion cells which are connected in series in a device. There are of course proper balance charger systems which it looks like I'll have to use. Basically I'll have to have have a 3 terminal connector on my device which connects to a proper lithium cell balance charger. This solution is quite costly however. So I was looking for solutions that could be built into the device itself and powered simply by the USB communication port that is already a part of the device. After some searching I came across this basic concept from a few places.The image above is very simplified just to get the point across but essentially the charger is a simple single cell usb charger solution, the output/sense and common of which are toggled between each cell. I imagine a final solution would use something like a 555 timer and a dual gang SSR or something. My problem with this solution is that it's all fine and dandy when the device and the charger circuitry do not share a common earth. However, the USB plug is also used for data transfer, so it requires the USB earth to be common, and that messes up the charger toggle concept.Am I stuck with having to use the 3 terminal balance charger approach and thus have to supply end-users with balance chargers and have another port on the device, or is there some clever way I can use this cell toggle thing without the common earth problem?Many thanksMarcus.

author: amyk
author_rank: Super Contributor
timestamp: August 04, 2014, 12:24:44 am
content: First of all, is there any reason why you want a series instead of parallel battery? Would a parallel configuration with a boost converter be better? A 2p pack does not need any special balancing.If you want to charge one at a time you'll also need circuitry to ensure that neither of the cells goes into undervoltage or overvoltage. If you really want to charge them sequentially and have thought of that, then simply leave the switch defaulted to the bottom cell, and the earth will be the same as that of the USB.

author: Unknown
author_rank: Unknown
timestamp: August 04, 2014, 07:03:35 am
content: Quote from: amyk on August 04, 2014, 12:24:44 amYes they need to be series. No other way of getting the current required. Sorry. Boosting 3.0V-4.2V to 7.4V @ 5A-8A is no mean feat.Quote from: BoltarSomething like thishttp://www.ebay.co.uk/itm/Micro-USB-1A-5V-Lithium-Battery-Charging-Board-Charger-Module-18650-Protection-/271498314169I don't think you understand the concept judging from what you've said, but thanks for the effort. The concept does not charge them sequentially, it charges them alternately using a single cell charger board. The battery end of the USB charger board is simply toggled between each cell I would imagine about every 30 seconds or so, there is no actual switch, the diagram was just to illustrate the concept, it's not meant to be taken literally. The board of course already contains cell voltage protection or it wouldn't be a lithium charger board. Don't worry though, I posted this at a few places and got a simpler solution elsewhere. Basically using a pre-made dual cell USB charger board which I assume boosts the USB 5V up so it can apply enough voltage to charge them in series and it can also keep them balanced.