Charging / Battery circuit

[Platima]

Hey people,

This may seem like a basic question to some of you, but im trying to get my head around this and i cant seem to figure out if its 100% OK. It looks good, but i may very well be wrong.

What im trying to do is tie together these 3x 240VAC Li-ion battery chargers, and 3x 12VDC Li-ion batteries into one 240VAC IN -> Charge -> Supply -> 36VDC OUT kind of circuit (if that makes any sense. Haha).

Linked in is a basic circuit diagram of my best guess at how to achieve this. If my explanation above made no sense hopefully the picture will clarify it.

Thanks heaps in advance,
-Keith

http://portal.scopesystems.com.au/ChargerEtc.PNG

[audioguru]

The purpose of the relay and transformers is to keep 240VAC away from the batteries and away from the 36VDC output.
But you have them connected together which is a severe electrocution hazzard.

Lithium-Ion cells are nominally 3.7V each and are 4.2V when fully charged.
Three cells are nominally 11.1V and are 12.6V when fully charged, not 12V.

You have a 30A fuse for three batteries. They must be huge batteries to safely charge at the very high current of 10A each. If you have a charging current that is too high then the batteries might catch on fire.

[Platima]

audioguru,

If you look, you'll see the 240v is only connected to the batteries via the relay (which isnt internally connected), and the transformers, i've specifically put the relay in to isolate it and break the battery circuit when the AC (charging) power is applied.

Thanks for the info though, i knew roughly the voltages of the cells but not that specific, and i've decided to deal with the cells individually instead of in battery packs anyway.

Also yes, the batteries will put out a maximum of about 30A, but thats just a spike. And they charge at approx 1A each i believe (hence the 1A fuse up the top). The circuit connected to them normally only draws about 10A which is fine as each one is actually two cells in parallel, rated 6.5Ah each. In theory this means i could draw 36V @ 13A for an hour, and from experience i know these cells can each peak to about 16-17A for a second or two.

Correct me if im wrong though as it seems you know a fair bit more about Lithium Ion tech than i do :)

Thanks
-Keith

[audioguru]

I recently replaced the Ni-MH 8.4V (seven AAA cells) battery in my electric RC model airplane with heavier dual 1865 Lithium-Ion cells (18mm diameter by 65mm long). The lithium cells have a lower voltage in theory but provide much more and much longer duration power. The airplane can easily carry the extra weight, it simply flies a little faster.

I use a current-limited wall-wart driving an LM317 adjustable voltage regulator set to 8.4V. The LM317 and the wall-wart get pretty warm during the charging then get cold near full charge.
A full charge takes about 4 hours.

The Sony cells are poor (maybe they are worn out).
The Panasonic cells produce a lot of power at first but their charge doesn't last long.
The no-name brand Taiwan cells produce plenty of power for a very long time.
I got them free from a thown-away laptop pc, a DVD player and a cable modem.

If I win the lottery then I will buy powerful and expensive Lipo (Lithium-Polymer) batteries.